Comprehensive nuclear materials 5 02 water chemistry control in LWRs

Comprehensive nuclear materials 5 02 water chemistry control in LWRs Comprehensive nuclear materials 5 02 water chemistry control in LWRs Comprehensive nuclear materials 5 02 water chemistry control in LWRs Comprehensive nuclear materials 5 02 water chemistry control in LWRs Comprehensive nuclear materials 5 02 water chemistry control in LWRs Comprehensive nuclear materials 5 02 water chemistry control in LWRs Comprehensive nuclear materials 5 02 water chemistry control in LWRs

5.02 Water Chemistry Control in LWRs C J Wood Electric Power Research Institute, Palo Alto, CA, USA ß 2012 Elsevier Ltd All rights reserved 5.02.1 Introduction 18 5.02.2 5.02.2.1 5.02.2.2 5.02.2.3 5.02.2.4 5.02.2.5 5.02.3 5.02.3.1 5.02.3.2 5.02.3.3 5.02.3.4 5.02.4 5.02.4.1 5.02.4.2 5.02.4.3 5.02.4.4 5.02.5 5.02.6 References BWR Chemistry Control Evolution of BWR Chemistry Strategies Mitigating Effects of Water Chemistry on Degradation of Reactor Materials Radiation Field Control Fuel Performance Issues Online Addition of Noble Metals PWR Primary Water Chemistry Control Evolution of PWR Primary Chemistry Strategies Materials Degradation PWR Radiation Field Control Fuel Performance PWR Secondary System Water Chemistry Experience Evolution of PWR Secondary Chemistry Strategies Chemistry Effects on Materials Degradation of SGs Control of Sludge Fouling of SGs Lead Chemistry Chemistry Control for FAC in BWRs and PWRs Water Chemistry Control Strategies 19 19 20 23 26 27 27 27 29 33 35 37 37 40 43 44 45 45 46 Abbreviations AO AVT BOP BRAC BWR CGR CRUD DMA DZO EBA ECP Axial offset, referring to localized flux depression in reactor core caused by buildup of boroncontaining deposits Originally called AOA for axial offset anomaly All-volatile treatment, suing ammonia for pH control in steam generators Balance of plant BWR radiation and control, referring to designated standard points in BWR reactors for radiation field measurements Boiling water reactor Crack growth rate Corrosion product deposits on fuel element surfaces Dimethylamine Depleted zinc oxide (BWRs) Enriched boric acid (PWRs) Electrochemical corrosion potential ETA FAC GE HWC HWC-L HWC-M IGA IGSCC LWR MOX MPA MRC NDE NMCA NWC OD IGA/SCC Ethanolamine Flow-assisted corrosion General electric, the vendor for BWRs in the United States and some other countries Hydrogen water chemistry HWC (low) with 0.2–0.5 ppm hydrogen HWC (moderate) with 1.6–2.0 ppm hydrogen Intergranular attack Intergranular stress corrosion cracking Light water reactor Mixed oxide fuel 3-methoxypropylamine Molar ratio control (PWR secondary side) Nondestructive examination Noble metal chemical addition Normal water chemistry (BWRs) Outside diameter IGA/SCC in steam generator tubes 17 18 Water Chemistry Control in LWRs OLNC OTSG PAA PbSCC PWR PWSCC SCC SG SHE On-line noble chemistry Once through steam generator Poly acrylic acid Lead assisted stress corrosion cracking Pressurized water reactor Primary water stress corrosion cracking Stress corrosion cracking Steam generator Standard hydrogen electrode (for ECP measurements) 5.02.1 Introduction Other chapters of this comprehensive describe the various degradation processes affecting the structural materials used in the construction of nuclear power plants (see Chapter 5.04, Corrosion and Stress Corrosion Cracking of Ni-Base Alloys; Chapter 5.05, Corrosion and Stress Corrosion Cracking of Austenitic Stainless Steels; and Chapter 5.06, Corrosion and Environmentally-Assisted Cracking of Carbon and Low-Alloy Steels) This chapter describes the influence of water chemistry on corrosion of the most important materials in light water reactors (LWRs) In particular, alloys susceptible to intergranular attack (IGA) and stress corrosion cracking (SCC) are significantly impacted by water chemistry, most notably, sensitized 304 stainless steel in boiling water reactors (BWRs) and nickelbased alloys in pressurized water reactors (PWRs) Excellent water quality is essential if material degradation is to be controlled In the early days of nuclear power plant operation, impurities in the coolant water were a major factor in causing excessive corrosion Chlorides and sulfates are particularly aggressive in increasing intergranular stress corrosion cracking (IGSCC) and other corrosion processes Transient increases of impurities in the coolant that occur during fault conditions (e.g., condenser leaks and ingress of oil or ion exchange resins) proved to be particularly damaging Thus, water chemistry was traditionally regarded as a key cause of material degradation Initial efforts to improve water quality brought about a slow but steady reduction in impurities through improved design and operation of purification systems Not only were the average concentrations of impurities reduced over time, but the frequency and magnitude of impurity ‘spikes’ from transient fault conditions were also diminished However, excellent water chemistry alone was not sufficient to control corrosion Hence, programs to modify water chemistry were introduced, including minimizing oxygen to reduce the electrochemical corrosion potential (ECP) in BWRs, and oxygen and pH control in PWRs More recently, additives to further inhibit the corrosion process have been developed and are now in widespread use As a result, water chemistry advances are now an important part of the overall operating strategy to control material degradation Primary system water chemistry also affects fuel performance through the deposition of corrosion products on fuel pin surfaces, and influences radiation fields outside the core Core uprating through increased fuel duty has reduced margins for tolerating corrosion products (CRUD) on BWR fuel pin surfaces In PWRs, increasing fuel cycle duration has increased the challenge of controlling pH within the optimum range At the same time, regulatory limits on worker radiation exposure are tending to be tightened worldwide, putting pressure on the operators to reduce radiation dose rates Successful operation of PWR steam generators (SGs) and the remainder of the secondary system demand strict water chemistry control in secondary side systems if corrosion problems are to be avoided Other operating parameters also influence the optimization process, for example, life extension (to 60 years) has emphasized the importance of controlling degradation of circuit materials Therefore, although control of structural material degradation remains the highest priority, water chemistry must be optimized between the sometimes-conflicting requirements affecting other parts of the reactor Advances in water chemistry have enabled plant operators to respond successfully to these technical challenges, and the overall performance has steadily improved in recent years.1 Plant-specific considerations sometimes influence or indeed limit the options for controlling water chemistry, so we see different chemistry specifications at different plants This is especially true internationally and significant differences between countries are noted The US industry started developing water chemistry guidelines 25–30 years ago, and these now provide the technical basis for guidelines in many other countries The early editions of these guidelines presented impurity specifications and required action if limits were exceeded When advanced water chemistries were developed and qualified, the guidelines evolved into a menu of options within an envelope of specifications that should not be Water Chemistry Control in LWRs exceeded Guidance is now provided on how to select a plant-specific water chemistry strategy.2 The basis for water chemistry control was discussed in detail by Cohen.3 The remainder of this chapter describes more recent water chemistry developments for BWRs, PWR primary systems, and PWR secondary systems including SGs, with a short section on flow-assisted corrosion (FAC) in both BWRs and PWRs 5.02.2 BWR Chemistry Control 5.02.2.1 Evolution of BWR Chemistry Strategies BWR water chemistry has to be optimized between the requirements to minimize material degradation, avoid fuel performance issues, and control radiation fields These factors are depicted in Figure 1,4 which also includes the main chemistry changes involved in the optimization process Plant-specific considerations sometimes influence or indeed limit the options for controlling water chemistry, so we see different chemistry specifications at different plants This is especially true internationally and significant differences in chemistry strategies between countries are noted Design features are an important reason for these different chemistry regimes, to which must be added the effects of different operational strategies in recent years For example, a key issue facing BWRs in the United States concerns IGSCC of reactor internals, as discussed in other chapters The occurrence of IGSCC resulted in the Clad corrosion crud deposition: Limits on feedwater zinc Impurity control: Monitoring/analysis required implementation of hydrogen water chemistry, with or without noble metal chemical addition (NMCA), to ensure that extended plant lifetimes are achieved German plants use 347 stainless steel, which is less susceptible to IGSCC than sensitized 304 stainless steel used originally in US-designed plants Some Swedish and Japanese plants have replaced 304 stainless steel reactor internals with 316 nuclear grade material to minimize potential problems, as this material is less susceptible to IGSCC As a result, many of these plants continue to use oxygenated normal water chemistry, whereas all US plants control IGSCC through the use of hydrogen water chemistry (HWC) with or without normal metal chemical addition to improve the efficiency of the hydrogen in reducing ECP Second, BWRs in United States undoubtedly have greater cobalt sources than plants in most other countries, despite strong efforts to replace cobalt sources This resulted in higher out-of-core radiation fields, leading all US plants to implement zinc injection to control fields, whereas only a small number of plants of other designs use zinc Third, the move to longer fuel cycles and increased fuel duty at US plants, while having major economic benefits, has led to new constraints on chemistry specifications in order to avoid fuel performance issues Figure depicts the changing chemistry strategies over the past 30 years, showing the focus on improving water quality in the early 1980s and the move to educing chemistry to control IGSCC, which in turn resulted in increased radiation fields, subsequently controlled by zinc injection Materials degradation and mitigation Water chemistry guidelines Fuel performance Chemistry control issues Figure Boiling water reactor chemistry interactions 19 BWR internals IGSCC, IASCC: HWC or NMC required Radiation exposure Radiation fields crud bursts: Zinc required 20 Water Chemistry Control in LWRs Increasing concerns about core internals cracking led to the need to increase hydrogen injection rates, which in turn resulted in the introduction of NMCA to reduce operating radiation fields from N-16 Figure shows the rate of implementation of HWC, zinc and NMCA, and online noble metal addition (OLNC) The rationale and implications of these developments are discussed in greater detail in subsequent sections The goal for BWRs is therefore to specify chemistry regimes that, together with the improved materials used in replacement components (e.g., 316 nuclear grade stainless steel), will ensure that the full extended life of the plants will be achieved without the need for further major replacements At the same time, radiation dose rates, and hence worker radiation exposure, must be closely controlled, and fuel performance must not be adversely affected by chemistry changes The first requirement of plant chemistry is to maintain high-purity water in all coolant systems, including the need to avoid impurity transients, which are beyond the scope of this paper The performance of all plants has improved steadily over the years, as shown by the trend for reactor water conductivity for GE-designed plants, given in Figure This figure shows that conductivity now approaches the theoretical minimum for pure water In fact, deliberately added chemicals, such as zinc (discussed in the following section), account for much of the difference between measured values and the theoretical minimum The conductivity data are consistent with the reactor water concentrations for sulfate and chloride In fact, sulfate is the most aggressive impurity from the viewpoint of IGSCC, and much effort has gone into reducing it 5.02.2.2 Mitigating Effects of Water Chemistry on Degradation of Reactor Materials 1977: Neutral, oxygenated water Corrosion, radiation buildup issues 1980s: Purer is better IGSCC was first observed in small bore piping using sensitized 304 stainless steel fairly soon after BWRs started operation Laboratory studies showed that impurities increased IGSCC rates, and in fact water quality in BWRs gradually improved in the early 1980s However, the same studies found IGSCC in high-purity oxygenated water typical of good BWR operations The key parameter affecting IGSCC was found to be ECP, as shown in Figure In this laboratory test, the change from oxidizing conditions typical of normal water chemistry (NWC) operation Chemistry guidelines Late 1980s–1990s: HWC, zinc Controlling IGSCC, radiation buildup 2000s: Noble metal chemical addition Core internals cracking control with lower fields Promising new option 2006–2008: Online Noblechem Figure Evolution of Boiling water reactor chemistry options from 1977 to 2008 40 Number of BWRs 35 Zn injection NMCA HWC (no NMCA) OLNC 30 25 20 15 10 1983 1988 1993 1998 2003 2008 Figure Implementation of zinc injection, hydrogen water chemistry, noble metals chemical addition, and online noble metal at US boiling water reactors Water Chemistry Control in LWRs 21 0.40 0.35 EPRI action level Conductivity ( µS cm–1) 0.30 0.25 0.20 0.15 0.10 0.05 Theoretical conductivity limit, 25 ºC 0.00 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Figure Boiling water reactor mean reactor water conductivity at US boiling water reactor 250 0.4950 0.4945 200 2.7 ϫ 10−8 mm s–1 ϫ 10−6 mm s–1 150 0.4935 0.4930 0.4925 100 CT2 #7-304SS dpa Constant load, 19 ksi√in Dissolved O2 Outlet cond: 0.30 μS cm–1 50 Inlet cond: 0.27 μS cm–1 Na2SO4 0.4920 0.4915 1488 Dissolved oxygen (ppb) Crack length (in.) 0.4940 1508 1528 1548 1568 1588 1608 Test time (h) Figure Laboratory results showing the effect of reducing oxygen concentration on crack growth of 304 stainless steel to reducing conditions greatly reduced the rate of crack growth Furthermore, hydrogen injection was effective at reducing the ECP in BWRs, as shown in Figure In this figure, it can be seen that crack growth rates (CGR) for Alloy 182 were low in hydrogen water chemistry (HWC), but increased greatly when the plant reverted to normal water chemistry (NWC) These results indicated that continuous hydrogen injection was required to fully mitigate cracking Examination of extensive inspection data from several plants indicated that no IGSCC was observed with an 22 Water Chemistry Control in LWRs 901.00 900.00 HWC ECP = −510 mV (SHE) NWC ECP = +110 mV (SHE) Crack length 174 miles year-1 HWC < miles year-1 899.00 898.00 897.00 < miles year -1 Alloy 182 896.00 895.00 800 900 1000 1100 1200 1300 Time (h) 1400 1500 1600 1700 Figure Effect of hydrogen water chemistry on crack growth of Alloy 182 ECP of À230 mV or lower, using a standard hydrogen electrode (SHE) This is the basis for the À230 mV requirement used by US plants for IGSCC control In BWRs, the radiation field in the core decomposes water to hydrogen and oxygen species, most of which immediately recombine back to water But some remain as oxygen or hydrogen peroxide, because some hydrogen is stripped into the steam phase before it can recombine These same radiolysis reactions cause hydrogen to react with oxygen or peroxide to reduce ECP These reactions occur mainly in the downcomer, and relatively low hydrogen concentrations are effective at lowering ECP in out-of-core regions of the system More than half the BWRs in the United States adopted low hydrogen injection rates of 0.2–0.5 ppm (called HWC-L), which, coupled with the replacement of recirculation piping using 316 stainless steel, mitigated IGSCC of recirculation piping In the 1990s, concerns about the cracking of core internals increased, but the low concentrations of hydrogen used to protect out-of-core regions were not sufficient to reduce ECP enough to mitigate IGSCC of in-core materials, because of the radiolysis of water occurring in the core As a result, it was necessary to increase hydrogen concentrations to 1.6–2.0 ppm to lower the in-core ECP sufficiently to provide protection in the reactor vessel (termed HWC-M for moderate concentrations of hydrogen) Although this approach was effective in protecting core internals, it also increased radiation fields in the steam side of the circuit, including the turbines, as a result of carryover of nitrogen-16 under reducing chemistry (Under the oxidizing conditions of NWC, most of the N-16 remains in the water as soluble species such as nitrate, and only a small percent is transported with the steam.) In some plants, local shielding of turbine components has reduced the impact of the gamma radiation to acceptable levels, but the projected 4–6-fold increase did in fact curtail plans for increased hydrogen injection rates at many plants Note that these N-16 radiation fields are a problem only when the plant is at power, as rapid decay occurs at shutdown because of the short halflife of N-16 (By contrast, out-of-core radiation fields from Cobalt-60 persist after shutdown and impact on maintenance work during outages.) NMCA was developed to increase the efficiency of hydrogen in BWR cores, to avoid high N-16 fields In this process, a nanolayer of platinum ỵ rhodium is deposited on the wetted surfaces of the reactor These treated surfaces catalyze the hydrogen redox reaction, converting oxygen back to water When the addition of hydrogen to the feedwater raises the molar ratio of H2 to O2 to or higher, the ECP of the treated surfaces drops to the hydrogen/oxygen redox potential, which is about À450 mV (SHE) This can be achieved with hydrogen concentrations of only about 0.2 ppm, and under these conditions, the main steam radiation level is not increased to an unacceptable level The first plant used NMCA successfully in 1997, and over 25 plants have already followed, with excellent results Field measurements show that NMCA has been effective in providing mitigation against IGSCC by lowering the ECP below the À230 mV (SHE) threshold with relatively low hydrogen injection rates The NMCA process is typically applied at refueling outage, before new fuel is inserted into the core, Water Chemistry Control in LWRs additional benefit with NMCA on the upper, outer shroud regions, as indicated by the additional shading in the left-hand side of the figure5 It is estimated that noble metals protect slightly more of the outer core region than does moderate HWC (HWC-M), but the difference is not significant Figure shows the dramatic benefit of noble metals in reducing the rate of stub tube cracking at Nine Mile Point since the application in 2000 Before 2000, several stub tubes had to be repaired or replaced at each outage, but since the application, only one tube leaked, and this was believed to have already cracked before NMCA Recently, attention has been focused on the online application of noble metals, with the first application at the KKM plant in Switzerland By April 2008, there were four applications in the United States This is discussed in a later section HWC protected regions NMCA protected regions and is effective for about three fuel cycles, before reapplication is necessary The regions of the reactor vessel internals that are protected by HWC-M or NMCA are shown in Figure While both techniques offer significant areas of mitigation, there is an 5.02.2.3 Radiation Field Control Corrosion products deposited on the fuel become activated, are released back into the coolant, and may be deposited on out-of-core surfaces Both soluble and insoluble species may be involved, the latter tending to deposit in stagnate areas (‘crud traps’) The chemistry changes to control IGSCC resulted in increased out-of-core radiation fields, and the implementation by most plants of depleted zinc injection to Figure Mitigated regions of the boiling water reactor core Number of stub tubes identified with IGSCC throughwall cracking based on leakage 12 10 Noble metal applied mid cycle may 2000 1984–1985 1986–1987 1988–1990 23 1991 1993 1995 1997 1999 2001 2003 2005 2007 RFO-11 RFO-12 RFO-13 RFO-14 RFO-15 RFO-16 RFO-17 RFO-18 RFO-19 Year Figure Mitigation of stub tube cracking at Nine Mile Point Unit 24 Water Chemistry Control in LWRs control dose rates, as discussed later in this section During shutdowns, the major radiation source for personnel exposure is activated corrosion products, deposited on primary system surfaces Exposures are generally accumulated at high-radiation field locations where maintenance work is frequently needed Although improvement of maintenance equipment and procedures, reduction of maintenance requirements, increased hot-spot shielding, and control of contamination dispersion have significantly reduced total exposure, further reduction of radiation fields is a major goal in programs for minimizing occupational radiation exposure The primary source of radiation field buildup on out-of-core surfaces in BWRs is 60Co, which in mature plants usually accounts for 80–90% of the total dose 60Co has a relatively long half-life of 5.27 years The higher the soluble 60Co concentration in the coolant, the more 60Co is incorporated and deposited on out-of-core systems and components, resulting in higher dose rates on recirculation piping, the reactor water cleanup system, dead legs, and other crud traps in the system Other activated transition metals such as 54Mn, 58Co, 59Fe, and 65Zn contribute the remainder of the dose 51Cr also contributes significantly to the piping dose in some NMCA plants The radiation fields commonly measured in a BWR at the straight vertical section of recirculation pipes are considered to be more representative for the purposes of radiation buildup trending and comparison with other plants These measurements are done in a prescribed manner developed under the EPRI BWR Radiation and Control program and are called BRAC point measurements These measurements represent primarily the incorporation of soluble 60Co into the corrosion film on the piping surfaces and tend to be a fairly good predictor of drywell dose rates The deposition of particulate oxides that contain 60Co and other activated species can also contribute significantly to outof-core radiation levels in BWRs, especially in hot spots The particulate oxides, which vary in size, originate primarily from corrosion of the steam/condensate system and are introduced via the feedwater The sole precursor of the gamma-emitting 60Co isotope is 59Co 59Co is present as an impurity in the nickel in structural alloys used in BWRs (e.g., Type 304 stainless steel) and is the main constituent of wear-resistant alloys (e.g., Stellite), used as hard facing in valves and other applications requiring outstanding wear resistance Corrosion and wear lead to release of 59Co into the coolant from these sources, which is transported to the core and incorporated into the crud that deposits on the fuel rods The 59 Co is activated to 60Co by neutron activation, released back into the coolant, and incorporated as a minor constituent into the passive films that form on components that are inspected, repaired, and replaced by maintenance personnel Components in the neutron flux (e.g., the control blades) directly release 60Co Cobalt source removal is clearly important if radiation fields are to be minimized Another gamma-emitting isotope, 58Co, is formed by the activation of nickel from stainless steel and nickel-based alloys 58Co has a shorter half-life and is not as major a contributor to radiation fields as 60Co in BWRs, but is much more significant in PWRs Shutdown drywell dose rates increase when coolant chemistry is changed for the first time from oxidizing (NWC) to reducing (HWC) conditions This results from a partial restructuring of the oxides formed under the oxidizing conditions of NWC (Fe2O3 type) to a more reducing spinel type oxide compound (Fe3O4 type) The oxides affected are the fuel deposits, the corrosion films on stainless steel piping, and out-of core deposits This results in an increase in the chemical cobalt (and 60Co) concentration in the oxide because of the higher solid-state solubility of transition metals in the spinel structure The presence of a higher soluble reactor 60Co concentration released from fuel crud while this conversion is occurring only aggravates the situation The processes are depicted in Figure The net result at most plants is a temporary increase in reactor water 60 Co, both soluble and insoluble forms, which leads to significantly increased shutdown dose rates because of both the increased reactor water concentrations and the increased capacity for transition metal uptake by the spinel phases.6 Oxide stable under normal water chemistry Fe2O3 (containing 60Co, 58Co, 54Mn, etc.) • Corrosion films • Vessel crud • Fuel crud Restructuring under HWC conditions Fe3O4 form of oxide Small insoluble particles containing 60Co, 54Mn, etc Soluble 60Co, etc released during restructure Figure Boiling water reactor oxide behavior under reducing conditions Water Chemistry Control in LWRs 25 0.8 Before Zn addition After Zn addition RxW 60Co (Ci kg −1) 0.6 0.4 0.2 Brunswick-1 Brunswick-2 Dresden-2 Figure 10 Hydrogen water chemistry plant RxW 60 Duane Arnold Fitz patrick Monticello Pilgrim Co response to zinc addition As mentioned earlier, zinc addition reduces radiation field buildup The mechanism of the zinc ion effect is complex, as release of 60Co from fuel crud is reduced, and deposition out-core is also reduced Overall, reactor water 60Co is decreased significantly after zinc addition, as shown by plant data in Figure 10 Aqueous zinc ion promotes the formation of a more protective spinel-structured corrosion film on stainless steel, especially when reducing conditions are present Second, both cobalt and zinc favor tetrahedral sites in the spinel structure, but the site preference energy favors zinc incorporation Thus, the available sites have a higher probability of being filled with a zinc ion than a cobalt ion (or 60Co ion), and hence the uptake of 60Co into the film will be significantly less if zinc ion is present in the water The 60 Co remains longer in the water and is eventually removed by the cleanup system The zinc was originally added to the feedwater as ZnO, but it was quickly found that the 65Zn that was created by activation of the naturally occurring 64Zn isotope in natural zinc created problems With the use of zinc oxide depleted in the 64Zn isotope, called depleted zinc oxide (DZO), this drawback was eliminated Because of the high cost of DZO, feedwater zinc injection was not implemented widely until HWC shutdown dose issues emerged For the case of plants treated with NMCA and injecting hydrogen, the oxidant concentration on the surface of the stainless steel is zero (due to the Pt and Rh catalyzing the reaction of any oxidant with the surplus hydrogen) The net result is that the ECP is at or very near the hydrogen redox potential, typically about –490 mV (SHE) for neutral BWR water This low potential causes a much more thorough restructuring of the oxides to the spinel state than observed under moderate hydrogen water chemistry (HWC-M) Feedwater iron ingress has a significant influence on the effectiveness of zinc injection As discussed in the next section, deposits on fuel cladding surfaces (called ‘CRUD’) are mainly composed of iron oxides, with other constituents Therefore, reducing iron ingress from the feedwater has the benefit of minimizing crud buildup, which is important for fuel reliability (next section) For these reasons, extensive efforts have been made to reduce iron ingress, with significant success Furthermore, fuel crud has a large capacity for incorporating zinc and is in fact where most of the zinc ends up The lower the amount of crud on the fuel, the greater the proportion of zinc that remains in solution and can subsequently be incorporated in out-of-core surfaces Therefore, at plants with low feedwater iron, less zinc is captured by the crud on the fuel, so a relatively greater amount remains in solution and is available to control out-of core radiation fields This is very important, as zinc injection rates are limited by fuel performance concerns, and hence lowering feedwater iron is essential for maintaining lower radiation fields 26 Water Chemistry Control in LWRs 5.02.2.4 Fuel Performance Issues Fuel durability has improved over the years, and failures have declined, helped by improvements in water purity In operation, zircaloy fuel cladding develops a thin oxide layer (ZrO2), which typically does not adversely affect performance However, an increase of deposition of corrosion product deposits (‘crud’) on this oxide film is undesirable because it can reduce heat transfer and increase fuel pin temperatures, with resultant increased corrosion of the fuel cladding, ultimately increasing the risk of fuel failure Moreover, the addition of additives to control corrosion may increase the risk of crud buildup on the fuel For example, zinc and noble metals in BWRs tend to increase the adherence of crud deposits on the fuel, which can result in undesirable oxide spalling in higher-rated cores In fact, corrosion-related fuel failures occurred at four plants in the United States between 1999 and 2003 Although the precise root cause of fuel failures is often difficult to determine, it is clear that excessive crud buildup played a role in these failures With progressive uprating of fuel duty in both PWRs (and BWRs), the margin to tolerate crud has been reduced and additional care has to be taken in specifying the water chemistry to avoid undesirable fuel performance issues Despite these more demanding conditions, fuel failures have decreased in recent years Concern about the possibility of adverse effects of NMCA on fuel has prompted imposition of a strict limit on the amount of noble metal that can end up on the fuel and guidance on the injection of zinc Plant data indicate that spalling of the corrosion layer from fuel cladding, which is often regarded as a precursor to cladding failure, is prevented if the cycle average feedwater zinc is maintained below 0.4 ppb in NMCA plants (0.6 ppb for non-NMCA plants) More recent data indicate that quarterly averages may be as high as 0.5 ppb for NMCA plants, without occurrence of spalling.5 These feedwater zinc data are the basis for limits in the water chemistry guidelines The 2008 chemistry guidelines7 retain the cycle average feedwater zinc limit of 0.4 ppb (0.6 ppb for non-NMCA plants) but enable a slight increase in the quarterly average to 0.5 ppb, which may allow flexibility in controlling radiation buildup in parts of the cycle The tighter control of water chemistry in recent years has been successful in controlling crud formation on fuel cladding, and Figure 118 shows failures from pellet–clad interaction causing SCC, fabrication defects, debris, and crud/corrosion Note that there have been zero crud/cladding related fuel failures in US BWRs since 2004 (although assessment of 2007 failures is not yet complete, crud/corrosion is not believed to be a factor here) Analysis of recent plant data confirms that control of feedwater iron ingress has the positive benefit of reducing the amount of crud on the fuel Control of copper, which generally originates from admiralty brass alloys, is also beneficial; not only can copper have detrimental effects on the fuel, but it also limits the ability of hydrogen to reduce the ECP, and it also leads to higher radiation fields As a result, most US plants have replaced condensers containing brass tubing Number of failed assemblies 30 25 20 PCI-SCC Unknown Fabrication Debris Crud/corrosion 15 10 2000 2001 2002 2003 2004 EOC year 2005 2006 Figure 11 US boiling water reactor fuel failures by mechanism for each end-of-cycle (EOC) year 2007 Water Chemistry Control in LWRs reverse U-bend specimens, whereas the upper line shows crack growth data over a similar concentration range Thus, the lowering of hydrogen appears feasible However, the relative importance of crack initiation and crack propagation is very dependent on material and plant conditions In the United States, concern about increased crack propagation at low hydrogen and low temperatures, as shown in Figure 19, has resulted in moving to higher hydrogen being preferred to the alternative of reducing hydrogen Several factors combine to make higher H2 the preferred way to mitigate SCC, including the importance of bottom-head penetrations (which are exposed to $290  C water) and the recent observation that the CGR in coldworked Alloy 600 is not mitigated at low H2.11 The preferred strategy in the United States is to gradually increase hydrogen to the upper end of the existing range, with the potential to move higher (say to 60 ml kgÀ1) when the ongoing qualification work is completed This will include evaluation of the effects of dissolved hydrogen on radiation fields and fuel performance, although any such effects are expected to be minimal.16 5.02.3.3 PWR Radiation Field Control Corrosion products released from out-of-core materials (primarily SG tubing) deposit on the fuel and become activated, are released back into the coolant, and may be deposited on out-of-core surfaces Both soluble and insoluble species may be involved, with the latter tending to deposit in stagnate areas (‘crud traps’) In addition to the chemistry items discussed later in this section, it must be stressed that other factors are important to the goal of reducing radiation fields In particular, the success of the later German-designed plants in eliminating cobalt sources in hardfacing alloys, thereby achieving very low radiation fields, demonstrates the benefits of cobalt source reduction With many plants replacing SGs, a correlation between recontamination rates and surface finish of the new SG tubing has been noted by Hussey et al.17 Typical PWR fuel cycles start with a relatively high boric acid concentration, which gradually reduces to zero at the end of the cycle Lithium hydroxide is added to maintain an approximately constant pH As the duration of fuel cycles increased, more boric acid was required at the start of cycle, which in turn necessitated increased LiOH to maintain the desired pH (Figure 21) As mentioned earlier, radiation field buildup can be controlled by minimizing corrosion product 33 transport and activation Initially, coordination of lithium hydroxide with boron to maintain a constant at-temperature pH of 6.9 was recommended, based on the minimum solubility of magnetite In fact, the prime constituent of the crud turned out to be nickel ferrite, requiring a pH of $7.4 for minimum solubility Fruzzetti et al.15 have recently reviewed the data on elevated pH, which provides a number of benefits including decreased general corrosion (and thus reduced corrosion product transport to the core) Field-tests of pHs greater than 6.9 confirmed that radiation fields were lower Although no adverse effects were observed on the fuel, many plants were slow to abandon a 2.2 ppm limit, established to avoid excessive zircaloy corrosion However, there were indications of heavier crud formation after long periods operating below pH 6.9, and as fuel concerns relaxed, a gradual move toward a maximum of ppm lithium resulted Moreover, pHs in the range 7.1–7.2 became more popular in the late 1990s, with 7.3–7.4 eventually gaining favor Figure 22 shows the maximum lithium concentrations reported by US PWRs in recent years It can be seen that 95% are now using greater than ppm at full power: a significant change from earlier in the decade A demonstration of elevated Lithium/pH is in progress at Comanche Peak PWR.18 The goal was to reduce radiation fields and reduce susceptibility to the Axial Offset Anomaly (AOA) by reducing crud buildup This test involved increasing the primary system pH from 7.1/7.2 to 7.3 and then two cycles at 7.4 No significant adverse trends have been noted, either in the area of chemistry or core performance Radiation fields measured have shown a modest but continued improvement On the basis of the positive trends and absence of any negative effects, Comanche Peak has established elevated constant pHTave 7.4 as the primary chemistry regime for both units Without the increases in pH/lithium that have taken place, radiation fields would have been expected to increase significantly for longer fuel cycles The increase in boiling in localized regions of the core (called subcooled nucleate boiling) in PWRs resulting from power uprating has resulted in higher crud buildup on the upper fuel surfaces, and there is growing evidence from US PWRs that radiation fields are indeed higher for the highest rated cores Enriched boric acid (EBA), that is boric acid enriched with B-10, enables a given pH to be achieved with less lithium hydroxide, as the required concentration of B-10 can be obtained with less total 34 Water Chemistry Control in LWRs Constant pH 7.2 Lithium ‘Li high limit’ ‘Li low limit’ Li target = 6.0 E−7 B2 + 0.0023B + 0.4413 Lithium (ppm) 3.5 ppm limit 2.2 ppm limit Start of 18-month cycle Start of 12-month cycle 20 80 140 200 260 320 380 440 500 560 620 680 740 800 860 920 980 1040 1100 1160 1220 1280 1340 1400 1460 1520 1580 1640 1700 Boron (ppm) Figure 21 Lithium concentrations required to maintain pH 7.2 for different fuel cycle lengths boric acid EBA is used at several plants in Europe, typically to increase shutdown margin when using mixed oxide fuel (MOX), but has not been applied to date in the United States However, consideration is being given to using EBA at some plants that will use MOX fuel in the future Despite the transition to the use of EBA in operating plants, designing for it in new plants is recommended.19 As discussed earlier, the motivation for the initial applications of zinc in most US PWRs was to control PWSCC of SG tubing However, German-designed PWRs and a few US plants used $5 ppb depleted zinc for radiation control, mostly with depleted zinc to avoid zinc-65 formation A recent paper ‘‘Understanding the zinc behavior in PWR primary coolant: a comparison between French and German experience’’ by Tigeras et al.20 provides a European perspective on this topic This paper concludes that ‘zinc injection seems to present the most positive and clearest results: in all the units injecting zinc, a dose rate reduction has been detected after a certain period of exposure without leading to any negative impact on plant systems, components, and operation.’ Thus ‘zinc injection should be considered as a strategy with benefits in short, medium, and long term Its application as soon as possible in the life of nuclear power plants and especially before SG replacement and fuel cycles modifications seems to be an excellent decision to contribute to ensuring the passivation process of new components, the fuel performance, the full power operation of the units, and the long life of materials and components.’ Figure 23 shows the effect of zinc in reducing radiation dose rates at several plants It can be seen that the reduction factor approximately correlates with the cumulative zinc exposure in ppb months (the product of the average zinc concentration and the duration of zinc addition) As little as ppb zinc has been shown to reduce radiation fields by 35–50% at operating plants, based on zinc exposures of !700 ppb months There is relatively little difference between plants with Alloy 600/690 SG tubing and those with Alloy 800 tubing, but plants using depleted zinc show greater benefit than those using natural zinc, as shown in the figure Water Chemistry Control in LWRs 35 Percentage of units within range 70 3.5 ppm 60 50 40 30 20 10 2000 2001 2002 2003 2004 2005 2006 2007 EOC year Figure 22 Maximum reported coolant lithium (full power) at US pressurized water reactors Cumulative dose rate reduction fraction 1.2 Alloy 800 w/depleted zinc Alloy 600 and 690 w/depleted zinc Alloy 600 and 690 w/natural zinc Log Alloy 800 plants Log Alloy 600 and 690 w/depleted zinc Log Alloy 600 and 690 w/natural zinc 0.8 0.6 0.4 0.2 0 200 400 600 800 1000 1200 1400 1600 Cumulative zinc exposure (ppb months) 1800 2000 Figure 23 Effect of zinc injection on radiation dose rates 5.02.3.4 Fuel Performance With progressive uprating of fuel duty, the margin to tolerate crud has been reduced and additional care has to be taken in specifying the water chemistry to avoid undesirable fuel performance issues Figure 24 shows the root causes of PWR fuel failures since 2000, including failures from pellet–clad interaction causing SCC, fabrication defects, debris, grid fretting, and crud/corrosion In contrast to the BWR situation, shown in Figure 11, very few failures in recent years have been attributed to crud/corrosion (the exceptions to this comment are discussed in a following section) A phenomenon called axial offset (AO) has caused concern over the past 10 years.21 AO is a measure of the relative power produced in the upper and lower parts of the core and is normally expressed as a percent, with a positive percent indicating that 36 Water Chemistry Control in LWRs Number of failed assemblies 120 Unknown Debris Crud/corrosion 100 Fabrication PCI-SCC Grid fretting 80 60 40 20 2000 2001 2002 2003 2004 2005 2006 2007 EOC year Figure 24 US pressurized water reactor fuel failures by mechanism more power is produced in the upper part of the core AOA occurs when boron concentrates in corrosion product deposits (crud) on the upper spans of fuel assemblies undergoing subcooled nucleate boiling, causing a reduction in neutron flux AOA has affected at least 20 PWRs in the United States, as well as several in other countries Clearly, fuel crud is involved in the AO phenomenon, and water chemistry effects must be considered in controlling AO Besides their axial asymmetry, the composition of fuel deposits in boiling cores is different from nonboiling fuel The nickel-rich deposits on boiling cores tend to be removed much less effectively by conventional chemistry shutdown evolutions than the nickel-ferrite deposits on nonboiling cores Alternative methods are therefore required for removing corrosion product deposits from reload fuel from highduty cores, including ultrasonic fuel cleaning An important difference exists between plants with Alloy 600 or 690 SG tubing and those (such as German-designed plants) with Alloy 800 tubing The latter have a much lower proportion of nickel in fuel crud and have not experienced the AO phenomenon.22 Early work showed that lithium increased zircaloy oxidation rates, although the adverse effects were reduced in the presence of boric acid As a result, a limit of 2.2 ppm lithium was generally imposed to reduce zircaloy corrosion, although excessive crud formation at low pHs was likely to be more detrimental to the cladding than higher lithium concentrations, especially as the resistance to corrosion of zircaloy improved This was confirmed by one of the few failures in recent years that was uniquely attributed to crud buildup In this example, a move to a longer fuel cycle necessitated increasing the boron concentration at start of cycle; however, the 2.2 ppm lithium limit was retained, resulting in the pH being well below 6.9 for the initial period of the cycle This in turn caused heavy crud formation, to which subsequent fuel failures were attributed The move in the past ten years to greater fuel duty, with operation of fuel at higher temperatures (with localized subcooled nucleate boiling), has caused crud-related problems to reappear, particularly the localized flux depression as a result of buildup of boron-containing crud, which were discussed earlier This in turn has renewed interest in elevated pH/ lithium to minimize corrosion product transport, the use of EBA and the more immediate mitigation that can be obtained from fuel cleaning Fuel performance is always a concern with changes in water chemistry, such as zinc injection On the basis of current experience, the impact appears to be minimal for the majority of plants, but insufficient data exist for plants with the highest fuel duties to allow application without postexposure fuel inspections Data from US plants suggest little or no fuel concerns for coolant zinc levels up to 40 ppb for plants with less-highly rated cores Extended experience at these plants, over at least 10 years of operation, indicates no adverse effects on fuel at zinc concentrations from 15 to 25 ppb However, there have been no data Water Chemistry Control in LWRs available until recently for higher zinc concentrations in higher duty cores where significant subcooled nucleate boiling occurs on the fuel clad surface.23 Perkins et al.24 comment that fuel performance must be considered prior to injecting zinc and additional monitoring and fuel surveillances to understand and evaluate the impact and the role of zinc may be required in some circumstances 5.02.4 PWR Secondary System Water Chemistry Experience 5.02.4.1 Evolution of PWR Secondary Chemistry Strategies The objectives of PWR secondary water chemistry control are to maximize secondary system integrity and reliability by minimizing impurity ingress and transport, minimizing SG fouling, and minimizing corrosion damage of SG tubes Since secondary side corrosion damage of SG tubes is primarily caused by impurities in boiling regions, where high concentrations of impurities occur in occluded regions of the SG formed by corrosion product deposits, new approaches are continually sought to control corrosion product transport to and fouling within the SGs.25 PWRs have experienced IGA on both the primary and secondary sides of the Alloy 600 SG tubing, which has been a major contributing cause of the replacement of most of the SGs with mill-annealed tubing, not only in the United States but internationally Figure 25 illustrates the various corrosion processes found in different locations in a recirculating SG.26 PWR secondary system water chemistry has evolved through many changes over the years, largely in response to emerging technical issues associated with this degradation of structural materials in SGs In the early days of PWR operation, wastage became a problem in the secondary side of PWR SGs, resulting in a switch from the use of sodium phosphate inhibitor to all-volatile treatment (AVT) using ammonia, which in turn brought about the denting phenomenon Tighter control of impurities, oxidizing potential, and pH were necessary to mitigate the denting problem Despite continued chemistry improvements, many plants have had to replace SGs of earlier designs (e.g., those tubed with Alloy 600MA), as shown in Figure 26 Newer generation SGs are performing well, although there remain concerns about the adverse effects of lead impurity, causing Pb-assisted stress corrosion cracking (PbSCC), which is discussed later 37 Lead has been observed in various flow streams (final feedwater, heater drains, etc.) in the secondary systems of PWRs Lead is detected at some concentration in nearly all deposit analyses (SG and other locations) Lead is present in trace concentrations in secondary system materials of construction, as well as in chemical additives such as hydrazine.15 Figure 27 shows the worldwide causes of SG repairs through 2004 It can be seen that IGA is currently the most prevalent form of degradation Figure 28 compares the behavior of three types of SG tubing, Alloy 600MA (mill-annealed material used in early plants, Alloy 600TT (thermally treated material used in later plants), and Alloy 690TT (an improved alloy used in most replacement SGs) This diagram is taken from the 2008 PWR Secondary Water Chemistry Guidelines,27 which contains a much more detailed account of corrosion processes 600TT has reduced susceptibility under mildly oxidizing highalkaline conditions, that is, SCC is not observed until higher pH than for 600MA, and 600TT has approximately the same susceptibility as 600MA under acidic conditions 690TT is indicated as having a still smaller region of susceptibility in the high-alkaline region and as having no susceptibility in the acid region except under highly oxidizing conditions that are unlikely to occur in plants However, other work indicates that SCC can occur in 690TT at an acidic pH, especially if lead is present Also, SCC occurs in both 600MA and 600TT in the mid pH region if lead is present In the 1990s, improved pH control using amines became a regular practice, and fine-tuning, including using mixtures of different amines to control pH throughout the circuit and coordination with resin utilization, continues today Hydrazine is used to remove oxygen from the system Hydrazine levels have continually been reviewed and ‘optimized,’ with due regard to any impact on FAC in secondary systems, as FAC rates increase at very low oxygen concentrations Molar ratio control (MRC) describes a control strategy that adjusts the bulk water chemistry, generally sodium and chloride, such that the solution that is developed in the flow-occluded region is targeted to be near neutral MRC can involve the addition of chloride ions to ‘balance’ the cations that cannot be reduced via source term reduction programs MRC was widely practiced to minimize SCC concerns, but has not been actively employed at plants replacing to SGs tubed with Alloy 690TT With more plants replacing their SGs, less plants are adopting the MRC program Only ten plants were doing MRC in 2007,28 and they are all with original SGs with 38 Water Chemistry Control in LWRs U-bend cracks (PWSCC) Fatigue Free span ODSCC IGA ODSCC PWSCC Expansion transition PWSCC PWSCC or ODSCC ODSCC Denting Fretting, wear, corrosion, thinning Tubesheet Pitting IGA Expansion transition Tubesheet ODSCC Sludge Tubesheet PWSCC tube-end cracking Tubesheet Figure 25 Corrosion processes in recirculating steam generators, showing primary water stress corrosion cracking and outside diameter stress corrosion cracking on the secondary side Water Chemistry Control in LWRs 39 140 Operating plants Plants w/replacement SGs 120 134 84 88 81 134 134 134 72 64 67 59 52 45 51 37 22 29 17 10 7 7 78 134 134 132 131 132 134 131 131 132 132 133 134 12 132 134 12 133 133 130 121 63 68 55 46 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 22 20 31 41 40 11 93 99 60 108 115 127 80 79 Number of plants 100 Year Figure 26 Steam generator replacement status worldwide 100 90 80 Percent 70 60 50 40 30 20 10 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Year IGA Impingement Pitting Other Wear Thinning Fatigue Unknown SCC Preventive Figure 27 Worldwide causes of steam generator tube repair 600MA and 600TT tubing Currently, no plants with replaced SGs are believed to be using MRC Titanium-based inhibitors to minimize corrosion are also employed at some plants Boric acid treatment (BAT) involves the addition of boric acid to feedwater Such approaches are worthy of consideration, on the basis of plant-specific degradation mechanisms, operational considerations, and 40 Water Chemistry Control in LWRs 1.0 TT690 Potential (V vs Ec ) 0.8 Some tests indicate that 690 TT may be susceptible in the low pH region, especially if lead is 690 TT U-bend cracked present in near neutral AVT with lead and oxidizing sludge TT690 TT600 TT600 MA600 0.6 0.4 MA600 0.2 600 MA and 600 TT can be susceptible in mid pH range if lead or reduced sulfur is present pH 300 °C (572 °F) 10 11 12 Figure 28 Corrosion mode diagram for Alloys 600MA, 600TT and 690TT (based on Constant Extension Rate Tensile Tests at 300  C), showing regions where materials are susceptible to attack interactions The most recent developments are aimed at reducing deposit buildup in crevices, including the use of dispersants, such as polyacrylic acid (PAA), that is discussed in more detail later The historical trends in PWR secondary chemistry are shown in Figure 29 5.02.4.2 Chemistry Effects on Materials Degradation of SGs Corrosion of SG tubes has been the major issue affecting selection of secondary water chemistry parameters However, corrosion and FAC of SG internals and other secondary system components are also important concerns Corrosion of SG tube materials is mainly affected by the following water chemistry related factors, in addition to nonwater chemistry factors such as material susceptibility, temperature, and stress:  pH – Corrosion of several different types, including IGA/SCC and pitting, are strongly affected by the local pH High pH (caustic conditions) and low pH (acidic conditions) accelerate the rates of IGA/SCC  ECP – The ECP is a measure of the strength of the oxidizing or reducing conditions present at the metal surface The rate of corrosion processes are strongly affected by the ECP Secondary side SCC in tube alloys tends to be accelerated by increases in ECP, that is, by the presence of oxidizing conditions  Specific species – Some impurity species accelerate corrosion of tubing alloys as a result of their effects on pH and ECP In addition, lead and reduced sulfur species (e.g., sulfides) appear to interfere with formation of protective oxide films on the tube metal surfaces, and thereby increase risks of IGA/SCC, independent of influences on pH or potential Similarly, chlorides tend to increase the probability of pitting These factors have been most thoroughly explored for mill-annealed Alloy 600 (600MA) As discussed in Chapter 5.04, Corrosion and Stress Corrosion Cracking of Ni-Base Alloys, tests indicate that the other tubing alloys, that is, stress-relieved Alloy 600 (600SR), thermally treated Alloy 600 (600TT), nuclear grade Alloy 800 (800NG), and thermally treated Alloy 690 (690TT), exhibit similar tendencies, but have increased resistance to corrosive attack, in the order listed, with 690TT having the highest resistance Laboratory tests and plant experience indicate that 690TT has very high resistance to IGA/SCC on the outside diameter on tubing (OD IGA/SCC) in Water Chemistry Control in LWRs 41 Pb remediation Dispersants Titanium Molar ratio control MPA, DMA ETA chemistry Morpholine chemistry Boric acid addition EPRI water chemistry guidelines Ammonia chemistry Phosphate 1975 1980 1985 1990 1995 2000 2005 Figure 29 Evolution of water chemistry for pressurized water reactor secondary systems normally expected crevice conditions, but OD IGA/ SCC could possibly occur as a result of upsets or as a result of long-term fouling and accumulation of aggressive species in deposit-formed crevices Alloy 800NG also has high resistance to OD IGA/SCC, but laboratory tests indicate that it is about twice as susceptible as Alloy 690TT, and it has experienced limited amounts of IGA/SCC in plants, while no operation-related corrosion of 690TT has been reported Laboratory tests and some plant experience indicate that 600TT is significantly more resistant than 600MA but less resistant than 800NG and 690TT Water chemistry selected to protect SG tubes appears to be satisfactory for most balance-of-plant (BOP) components such as turbines The main corrosion concerns in the BOP that affect secondary system water chemistry are FAC of carbon steel piping, tubing, and heat exchanger internals and shells, and ‘ammonia’ attack of copper and copper alloy tubes In addition, FAC has also affected some recirculating SG internal components (e.g., feedrings, swirl vanes) FAC is mainly influenced by the at-temperature pH and oxygen content around the secondary system ‘Ammonia’ attack of copper alloys is mainly influenced by the concentrations of ammonia and oxygen at the copper alloy locations, but is also accelerated by increases in concentrations and pH associated with other amines, although not as strongly as by increases in ammonia Once-through steam generators (OTSGs) have different thermal hydraulics and (in original SGs) tube materials than recirculating steam generators (RSGs) These differences have led to OTSGs having somewhat different tube corrosion experience than RSGs of the same vintage For the most part, OTSGs have experienced somewhat lower rates of tube degradation However, significant IGA/IGSCC has been detected in the upper bundle free spans of several units, especially at scratches, and SG replacement has been performed or is planned at all units The locations in SGs that are most affected by IGA/IGSCC are those where free circulation of secondary water is impeded by the local geometry, for example, in crevices formed by tube support plates or by sludge piles that can accumulate on the tube sheet Impurities in the secondary water can concentrate in these locations by boiling and evaporation in a process called ‘hideout.’ The key issue influencing water chemistry regimes in PWR secondary system is to minimize SG degradation by controlling sludge buildup, reducing (and balancing, e.g., MRC) the concentration of impurities (i.e., sodium, chloride and sulfate) in deposits at the tube-tubesheet and tube-tube support plate interfaces The use of advanced amines to control pH has increased significantly in the past few years, as discussed in a following section Figure 30 shows the main approaches used in typical chemistry control strategies Impurities are removed from SGs by blowdown of the coolant Over the past 20 years or so, average 42 Water Chemistry Control in LWRs Key issue: Mitigating IGA/IGSCC in concentrating regions Approach: Control local chemistry Molar ratio control Reduce Na increase Cl Reduce iron Redox potential Amines dispersants Reduce Cu increase N2H4 Inhibitors Boric acid TiO2 Figure 30 Pressurized water reactor secondary chemistry control strategies blowdown impurity concentrations in US SGs have been reduced from several ppb to the sub-ppb range Many PWRs today have SG blowdown concentrations near or below the analytical detection limit Minimization of impurities is recommended but has been insufficient to prevent or completely mitigate IGA/IGSCC at most plants with susceptible tube material and design, as it can result in sodium-rich feedwater Cations such as sodium can be more effectively retained by boiling in a crevice than chloride Hence, excess cations over anions or anions over cations result in specific corrosion issues because of concentration processes in local environments The original all-volatile treatment used ammonia to control pH, but a less-volatile chemical than ammonia would improve pH control throughout the circuit Early work employed morpholine, but now several other amines are used Since the initial application of advanced amine chemistry about 15 years ago, there has been tremendous success in reducing the transport of corrosion products to the SGs by improving the attemperature pH around the BOP, especially in the two-phase regions This has resulted in mitigation of FAC and thus reduced generation of corrosion products that ultimately get transported to the SGs Ethanolamine (ETA) remains the most used amine at US plants, with $75% of the US plants using ETA or ETA with other amines, such as dimethylamine (DMA) or 3-methoxypropylamine (MPA), to control secondary cycle pH, as shown in Figure 31.17 Several plants now use a mixture of amines to achieve the optimum pH throughout the secondary system, with $25% of the US plants using MPA or MPA with other amines while 12% of the plants use morpholine or morpholine with other amines 0% 4% 5% 4% 16% 56% 2% 7% 6% ETA MPA ETA/DMA ETA/MPA MPA/DMA MPA/Morph Morph/DMA ETA/Morph Morph Figure 31 Amines used in the secondary systems of US pressurized water reactors The proper control of oxygen in pressurized water reactor (PWR) secondary feedwater, using an oxygen scavenger such as hydrazine and/or carbohydrazide, has been an enduring issue The requirements for oxygen concentration necessitate that some optimization take place Maintaining reducing conditions – that is, low electrochemical potential – in the SG is essential to minimize SCC On the other hand, some oxygen in the feedwater counteracts corrosion of Water Chemistry Control in LWRs 1985 < 1992 1993 1994 1995 1996 1998 43 2005 % of US PWRs using advanced water chemistries 120 99 100 80 60 76 62 73 40 58 40 68 44 44 20 23 Using advanced amines 57 30 23 30 30 50 41 31 33 28 On molar ratio control Using >100 ppb FW N2H4 35 19 37 41 17 Using boric acid treatment Figure 32 Pressurized water reactor secondary chemistry trends carbon steel surfaces and the transport of corrosion products to the SG Recent work has investigated the effect of hydrazine and oxygen on the ECP of SG tubing materials (Alloys 600 and 690) as well as stainless steel (304 and 316) and carbon steel during PWR startup conditions These laboratory studies have shown that changes in the concentration of hydrazine, used to ensure a reducing potential in the SG, within the typical range allowed and employed (e.g., 20–150 ppb) have no discernable effect on FAC at feedwater temperatures (e.g., 180–235  C) Figure 32 shows the trends in using advanced water chemistry regimes in the secondary systems at US plants 5.02.4.3 Control of Sludge Fouling of SGs Corrosion products in the secondary side of PWR SGs primarily deposit on the SG tubes These deposits can inhibit heat transfer, lead to thermal–hydraulic instabilities through blockage of tube supports, and create occluded regions where corrosive species can concentrate along tubes and within tube-to-tube support plate crevices The performance of the SGs can be compromised not only through formation of an insulating scale, but also through the removal of tubes from service due to corrosion Although the application of various amines to control the at-temperature pH (pHT) in specific locations of PWR secondary systems has been successful in reducing the corrosion of BOP metals and thus reducing corrosion product transport to SGs, a complementary strategy now exists for significantly reducing SG fouling through online application of dispersant, which inhibits deposition By inhibiting deposition of the corrosion products, the dispersant facilitates more effective removal from the SGs via blowdown This strategy has been employed at fossil boilers for many decades However, due to the use of inorganic polymerization initiators (containing sulfur and other impurities), polymeric dispersants had not been utilized in the nuclear industry Only recently has a PAA dispersant been available that meets the criteria for nuclear application, and progress has been made in reducing SG fouling by application of an online dispersant to substantially improve the efficiency of blowdown iron removal Dispersant application is proving to be a highly promising technology for markedly decreasing SG fouling, delaying (or possibly eliminating) the need for expensive chemical cleaning and effectively reducing the frequency of sludge lancing for SG maintenance Online application of PAA to the feedwater system has been successfully demonstrated to greatly increase the efficiency of the blowdown system in eliminating feedwater corrosion products from fouling the SGs The first application occurred at Arkansas Nuclear One Unit for a three-month trial in early 2000, which demonstrated a significant improvement in the blowdown iron removal efficiency from $2% to $60% with 4–6 ppb PAA in 44 Water Chemistry Control in LWRs With dispersant Iron removal efficiency 100 10 Without dispersant 0.5 1.5 Dispersant concentration (ppb) Figure 33 Iron removal efficiency during dispersant test at McGuire pressurized water reactor the feedwater The second application, in 2005–2006, at McGuire Unit for a 6–9 month trial in their replacement SGs tubed with Alloy 690TT showed a similar significant improvement., as shown in Figure 33.15 The following conclusions are evident from the McGuire demonstration described in the above reference:  PAA is an effective dispersant A feedwater PAA concentration approximately equal to the feedwater iron concentration (2 ppb in this case) appears to effectively remove approximately 50% of the influent feedwater iron under steady-state operating conditions  Although blowdown copper spikes with initial PAA application (albeit to a much lesser extent than iron), it quickly returns to normal levels and remains there  Filter element consumption is manageable  Blowdown cation conductivity and ammonia behavior changed during the trial, but these changes are believed to be mainly due to changes in plant configuration and not PAA  The SG thermal performance level has improved slightly with dispersant application, most likely due to slight beneficial changes in the tube deposit thermal properties The long-term trial at McGuire demonstrated the significant improvement in blowdown iron removal efficiency with application of PAA dispersant (a follow up to the successful short-term trial at ANO-2 in 2000) Based on the success of the McGuire long-term trial, evaluations are in progress with SG vendors looking toward technical concurrence for long-term use in their fleet of recirculating SGs 5.02.4.4 Lead Chemistry PbSCC is a serious concern that can affect all SG tubing materials currently employed A better understanding of lead behavior is needed at SG and feedwater temperatures before possible mitigation techniques can be successfully developed It is well known that soluble lead at very low concentrations can contribute to SCC of nickel alloys Likewise, it is well known that some locations on the secondary side of PWR SGs will accumulate lead in the solid state (i.e., deposit) with local concentrations considerably in excess of those observed to accelerate cracking in laboratory testing Recent investigations using analytical transmission electron microscopy15 have identified lead in the cracks of many tubes pulled from PWRs However, the absence of extensive operating SG tube failures at rates comparable to what might be predicted based on laboratory studies of PbSCC indicates that some mitigating phenomenon could be present.15 EPRI has published a sourcebook on lead29 that summarizes the state-of-knowledge regarding PbSCC and its effects on PWRs It incorporates PbSCC laboratory testing, the current understanding of lead transport and other physical chemistry aspects of lead, and the accumulated industry experience regarding PbSCC and its mitigation Three clearly understood and accepted facts regarding lead in PWR secondary water systems became clear as this sourcebook was being put together:  In laboratory testing, the presence of lead accelerates SCC of mill-annealed 600MA, stress-relieved 600SR, and thermally treated 600TT stainless steels as well as thermally treated Alloy 690 (690TT)  In operating PWRs, lead is present in the secondary system  In two cases, a large ingress of lead to the secondary system has occurred as a result of lead blankets having been left behind in SGs; the tubes in the affected SGs cracked sooner and faster than in the other SGs at the same units Set against these known facts are the following four points:  The mechanisms by which lead is transported from its ultimate source to the SG tube and into a crack are not well understood, and a comprehensive evaluation of possible mechanisms has not been performed Water Chemistry Control in LWRs  The threshold concentration at which lead will accelerate SCC in SGs is not well defined  No definitive indicator of PbSCC is available  There is no well-characterized mechanism by which lead accelerates SCC Recent work has shown that adsorption/desorption of Pb on corrosion products and SG tubing surfaces could potentially be a major sink/source, respectively, for Pb microscopy.15 There is no direct evidence of adsorption in SGs; however, there is sufficient potential for this mechanism that direct high-temperature measurements under SG conditions have been performed As a result of ongoing laboratory studies, microscopy15 speculates that formation of a lead layer slows repassivation, after a passive film at the crack tip is disrupted, potentially to an extent to which a crack can initiate and propagate 5.02.5 Chemistry Control for FAC in BWRs and PWRs FAC causes wall thinning of carbon steel piping, vessels, and components, as discussed in Chapter 5.06, Corrosion and Environmentally-Assisted Cracking of Carbon and Low-Alloy Steels The wall thinning is caused by an increased rate of dissolution of the normally protective oxide layer, for example, magnetite, that forms on the surface of carbon and low-alloy steels when exposed to highvelocity water or wet steam The oxide layer reforms and the process continues If the thinning is not detected in time, the reduced wall cannot withstand the internal pressure and other applied loads The result can be either a leak or a complete rupture The rate of wall loss (wear rate) of a given component is affected by temperature, fluid bulk velocity, the effect of component geometry on local hydrodynamics, the at-temperature pH, the liquid phase dissolved oxygen concentration, and the alloy composition The addition of chromium to steels decreases the rate of FAC Materials used to replace piping damaged by FAC include low-alloy steels containing chromium and molybdenum (P11, 1.25% Cr–0.5% Mo and P22, 2.25% Cr–1% Mo) and carbon or lowalloy steels clad with stainless steel Corrosion models are used to estimate wall thinning and determine where monitoring is required An example of the approach commonly used in the United States is described by Chexal and Horowitz.30 45 The main chemistry factors that affect the rate of FAC are pH and dissolved oxygen concentration FAC is not an issue for PWR primary systems As indicated earlier, laboratory studies have shown that changes in the concentration of hydrazine in the PWR secondary system feedwater, used to ensure a reducing potential in the SG, have no discernable effect on FAC at feedwater temperatures, within the typical range allowed and employed The chemistry parameter that a BWR plant has some degree of control over is dissolved oxygen Oxygen affects the form and solubility of the oxide layer, the dissolution of which is inherent in FAC Several plants inject oxygen into the system, as the rate of FAC increases dramatically if the oxygen concentration is less than about 25 ppb Plant data are shown in Figure 34 Use of HWC in a BWR can significantly reduce the amount of oxygen in the main steam, extraction steam, and heater drain systems, thus potentially increasing the FAC rates in these areas of the plant The effect of NMCA on the corrosion behavior of carbon steel in 550  F (288  C) water containing various amounts of oxygen and hydrogen has been studied and the data confirm that there is no adverse effect of NMCA on FAC.7 The carbon steel segments of the BWR vessel bottom-head drain line have been identified as being FAC susceptible because of the flow conditions and the potential for low dissolved oxygen concentrations However, a significant number of inspections have been performed recently at US plants and little thinning has been observed The 2008 edition of the BWR Water Chemistry Guidelines7 recommends that feedwater oxygen should be maintained above 30 ppb to minimize FAC of carbon and low-alloy steels 5.02.6 Water Chemistry Control Strategies Sometimes, step changes in chemistry strategy are unavoidable, as with the move to reducing chemistry in BWRs In these cases, the operators must be prepared to deal with adverse effects Some BWRs adopting reducing conditions experienced a large jump in out-of-core radiation fields, which may be avoided with prior zinc injection Addition of new chemicals requires extensive qualification For example, the successful demonstrations of BWR online noble Water Chemistry Control in LWRs Relative FAC wear rate (expressed as percentage of the average wear rate of components at 10 pbb oxygen) 46 160 140 Plant A 120 Plant A 100 Plant B Plant C 80 Plant D 60 Average 40 20 0 10 20 30 40 50 60 70 80 Dissolved oxygen (ppb) Figure 34 Plant data showing the relationship between flow-assisted corrosion and dissolved oxygen (Oxygen values are localized, calculated by the CHECWORKS codes from measured values at condensate or feedwater locations.) chemistry and PAA dispersants in PWR SGs resulted from detailed monitoring and evaluation during the first injections If possible, changes in chemistry should be made in baby steps, with monitoring at each step, before further changes are implemented Examples of this strategy are the gradual increases in lithium/pH and dissolved hydrogen in PWR primary systems These incremental changes minimize adverse side-effects and allow a planned approach to the optimum plant-specific chemistry control program The US nuclear power industry produces guidance documents to assist plant personnel in determining a plant-specific chemistry control strategy The early versions of these documents, developed in the 1980s, listed water chemistry specifications and actions to be taken if the limits were exceeded As more chemistry options became available, the guidelines evolved into providing guidance on selecting the most appropriate chemistry for a specific plant Thus, the 2008 BWR Water Chemistry Guidelines7 offers recommendations on controlling ECP, zinc injection, and feedwater iron control Likewise, the 2007 PWR Primary Water Chemistry Guidelines12 provides guidance on pH control and zinc injection, and the 2008 PWR Secondary Water Chemistry Guidelines27 discusses impurity control, amines, and dispersants Theses documents are used by all US nuclear power plants and provide the technical basis for similar guidelines used in many other countries Development of a strategic water chemistry plan, as discussed in these documents, is seen as crucial to controlling material degradation in the future References 10 11 12 Swan, T.; Wood, C J In Developments in Nuclear Power Plant Water Chemistry, VIIIth International Conference on Water Chemistry of Nuclear Reactor Systems, Oct 23–26, 2000; BNES: Bournemouth, UK, 2000 Fruzzetti, K.; Wood, C J In Developments in Nuclear Power Plant Water Chemistry International Conference on Water Chemistry of Nuclear Reactor System, Jeju Island, Korea, Oct 23–26, 2006 Cohen, P Water Coolant Technology of Power Reactors; Gordon and Breach: New York, 1969 Jones, R L In International Water Chemistry Conference, San Francisco, Oct 11–15, 2004; EPRI: Palo Alto, CA, 2004 Garcia, S.; Wood, C Recent advances in BWR water chemistry In VGB NPC’08 Water Chemistry Conference, Berlin, Sept 14–18, 2008 Cowan, R.; Hussey, D Radiation field trends as related to chemistry in United States BWRs In 2006 International Conference on Water Chemistry of Nuclear Reactor Systems, Jeju Island, Korea, Oct 23–26, 2006 EPRI Boiling Water Reactor Water Chemistry Guidelines – 2008 Revision; EPRI: Palo Alto, CA, 2008 Edsinger, K In Nuclear News; Tompkins, B., Ed.; 2008; pp 34–36 Fruzzetti, K.; Perkins, D PWR chemistry: EPRI perspective on technical issues and industry research In VGB NPC’08 Water Chemistry Conference, Berlin, Sept 14–18, 2008 Andresen, P.; Ahluwalia, A.; Hickling, J.; Wilson, J Effects of PWR primary water chemistry on PWSCC of Ni alloys In 13th International Conference on Environmental Degradation of Materials in Nuclear Power Systems, Whistler, Canada, Aug 19–23, 2007 Andresen, P.; Ahluwalia, A.; Wilson, J.; Hickling, J Effects of dissolved H2 and Zn on PWSCC of Ni alloys In VGB NPC’08 Water Chemistry Conference, Berlin, Sept 14–18, 2008 EPRI Pressurized Water Reactor Primary Water Chemistry Guidelines: Revision 6; EPRI: Palo Alto, CA, 2007 Water Chemistry Control in LWRs 13 Pathania, R.; Yagnik, S.; Gold, R E.; Dove, M.; Kolstad, E Evaluation of zinc addition to PWR primary coolant In 7th International Symposium on Environmental Degradation of Materials in Nuclear Power Systems, Breckenridge, CO, NACE: Houston, TX, 1995; pp 163176 14 Molander, A.; Jenssen, A.; Norring, K.; Koănig, M.; Andersson, P.-O Comparison of PWSCC initiation and crack growth data for Alloy 600 In VGB NPC’08 Water Chemistry Conference, Berlin, Sept 14–18, 2008 15 Fruzzetti, K.; Rochester, D.; Wilson, L.; Kreider, M.; Miller, A Dispersant application for mitigation of steam generator fouling: Final results from the McGuire long-term trial and an industry update and EPRI perspective for long-term use In VGB NPC’08 Water Chemistry Conference, Berlin, Sept 14–18, 2008 16 Haas, C.; Ahluwalia, A.; Kucuk, A.; Perkins, D PWR operation with elevated hydrogen In VGB NPC’08 Water Chemistry Conference, Berlin, Sept 14–18, 2008 17 Hussey, D.; Perkins, D.; Choi, S Benchmarking radioactivity transport and deposition in PWRs In VGB NPC’08 Water Chemistry Conference, Berlin, Sept 14–18, 2008 18 Stevens, J.; Bosma, J Elevated RCS pH program at Comanche peak In VGB NPC’08 Water Chemistry Conference, Berlin, Sept 14–18, 2008 19 Nordmann, F Worldwide chemistry objectives and solutions for NPP In VGB NPC’08 Water Chemistry Conference, Berlin, Sept 14–18, 2008 20 21 22 23 24 25 26 27 28 29 30 47 Tigeras, A.; Stellwag, B.; Engler, N.; Bretelle, J.; Rocher, A Understanding the zinc behavior in PWR primary coolant: A comparison between French and German experience In VGB NPC’08 Water Chemistry Conference, Berlin, Sept 14–18, 2008 Frattini, P L.; Blok, J.; Chauffriat, S.; Sawicki, J.; Riddle, J In VIIIth International Conference on Water Chemistry of Nuclear Reactor Systems, Oct 23–26, 2000; BNES: Bournemouth, UK, 2000 Riess, R Personal communication, 2008 Byers, W.; Wang, G.; Deshon, J Limits of zinc addition in high duty PWRs In VGB NPC’08 Water Chemistry Conference, Berlin, Sept 14–18, 2008 Perkins, D.; Ahluwalia, A.; Deshon, J.; Haas, C An EPRI perspective and overview of PWR zinc injection In VGB NPC’08 Water Chemistry Conference, Berlin, Sept 14–18, 2008 Millett, P J; Hundley, F Nucl Energ 1997; 36, pp 251–258 EPRI Personal communication from K Fruzzetti, 2009 EPRI Pressurized Water Reactor Secondary Water Chemistry Guidelines – Revision 6; EPRI: Palo Alto, CA, 2008 Choi, S Personal communication, 2009 EPRI Pressurized Water Reactor Lead Sourcebook; EPRI: Palo Alto, CA, 2006 Chexal, V.; Horowitz, J Chexal–Horowitz flowaccelerated corrosion model – Parameters and influences In ASME PVP-Vol B, Current Perspectives of International Pressure Vessels and Piping Codes and Standards, Book No H0976B, 1995 ... bursts: Zinc required 20 Water Chemistry Control in LWRs Increasing concerns about core internals cracking led to the need to increase hydrogen injection rates, which in turn resulted in the introduction... percentage of PWR injecting zinc 50 45 Percent of PWRs injecting Number of units injecting 40 35 30 25 20 15 10 1993 1994 19 95 1996 1997 1998 1999 2000 2001 2 002 2003 2004 20 05 2006 2007 Year... depleted zinc show greater benefit than those using natural zinc, as shown in the figure Water Chemistry Control in LWRs 35 Percentage of units within range 70 3 .5 ppm 60 50 40