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Comprehensive nuclear materials 5 13 material performance in sodium , Comprehensive nuclear materials 5 13 material performance in sodium , Comprehensive nuclear materials 5 13 material performance in sodium , Comprehensive nuclear materials 5 13 material performance in sodium , Comprehensive nuclear materials 5 13 material performance in sodium , Comprehensive nuclear materials 5 13 material performance in sodium ,
5.13 Material Performance in Sodium T Furukawa and E Yoshida Japan Atomic Energy Agency, O-arai, Ibaraki, Japan ß 2012 Elsevier Ltd All rights reserved 5.13.1 Introduction 327 5.13.2 5.13.3 5.13.3.1 5.13.3.2 5.13.4 5.13.4.1 5.13.4.2 5.13.4.3 5.13.4.4 5.13.4.5 5.13.4.6 5.13.4.7 5.13.5 5.13.5.1 5.13.5.2 5.13.5.3 5.13.6 5.13.6.1 5.13.6.2 5.13.7 5.13.7.1 5.13.7.2 References Material Selection in the Consideration of Application in Sodium Corrosion Mechanism of Materials in Sodium Corrosion Produced by the Dissolution of Alloy Elements to Sodium Corrosion Produced Through Chemical Reaction with the Impurities in Sodium Corrosion Behavior and Factors Affecting Steel Immersion Time Temperature Dissolved Oxygen Sodium Velocity Alloy Elements Carburization and Decarburization Corrosion Estimation of FBR Materials Effect of Sodium on the Mechanical Strength of Steels Austenitic Stainless Steel Ferritic Steels Others (Ni Base Alloys, ODS) Damage to Steels with Sodium Compounds Sodium–Water Reaction Sodium Leak Tribology Self-Welding Frictional Wear 328 328 328 330 331 331 332 332 333 333 333 334 336 336 338 338 338 338 339 339 339 339 340 Abbreviations EBR-II FBRs FFTF FMS JAEA Monju ODS PFR PNC Experimental Breeder Reactor No (USA) Fast breeder reactors Fast Flux Test Facility (USA) Ferritic–martensitic stainless steel Japan Atomic Energy Agency Japanese Prototype Fast Breeder Reactor Oxide dispersion-strengthened steel Prototype Fast Reactor (UK) Power Reactor and Nuclear Fuel Development Corporation (present JAEA) 5.13.1 Introduction Sodium is one of the elements that exhibit the characteristics demanded of coolants for fast breeder reactors (FBRs) The physical properties of sodium are shown in Table and see Chapter 2.14, Properties of Liquid Metal Coolants Sodium is a solid at room temperature, and the melting and boiling temperatures are 97.82 and 881.4 C, respectively Therefore, sodium is in the liquid phase at FBR operating temperatures without pressurization For this reason, it is not necessary to adopt the proof-pressure design employed in light water reactors for sodium-cooled FBRs Moreover, the thermal conductivity and specific heat of sodium at 550 C are 0.648 W cmÀ1 CÀ1 and 1.256 J gÀ1 CÀ1, respectively, and sodium can transfer the heat of the reactor core to the power generation system efficiently Furthermore, its insignificant neutronmoderating capability is suitable for the coolant for the FBR, in which fast neutrons play a major role in the nuclear reaction On the other hand, the weakness of sodium as a coolant is its reactivity with oxygen and/or water 327 328 Table Material Performance in Sodium Physical properties of sodium Atomic number Atomic weight Melting point ( C/K) Boiling point ( C/K) Volume increase on melting (%) Density (g cmÀ3) Thermal conductivity (W cmÀ1 KÀ1) Specific heat (J molÀ1 KÀ1) 11 22.9898 97.8/371.0 881.5/1154.7 2.71 0.856 at 400 C 0.821 at 550 C 0.727 at 400 C 0.657 at 550 C 29.40 at 400 C 28.88 at 550 C Table elements1 Element Solubility equation Cu Ag Au Mg Zn When high-temperature sodium is leaked into the atmosphere, it reacts with the oxygen and moisture in the atmosphere, and the by-products of this reaction are known to cause structural damage to the reactor Moreover, in the event of a steam generator tube failure, high-temperature steam blows off into the sodium, and wastage of the adjoining tubes occurs 5.13.2 Material Selection in the Consideration of Application in Sodium In the process of material selection, it is necessary to take the environment into consideration by estimating the mechanical properties and thermal characteristics Environments specific to the FBR components include: (a) contact with the coolant (liquid metallic sodium), (b) high temperature at which the creep effects must be taken into account, and (c) neutron irradiation In this chapter, an outline of the compatibility of materials with sodium is provided The oxides formed on the surface of the material are easily reduced in high-temperature sodium, resulting in direct contact between the material and sodium Under this condition, the dissolution of elements contained in the material, such as iron, chromium, and nickel, in sodium and the reverse phenomenon (deposition) occur on the material surface due to the difference in chemical potential The behavior is fundamentally controlled by the solubility of the material elements in sodium and by the diffusion rate of the materials The solubility equation of the various elements in sodium is shown in Table Among the elements in austenitic stainless steel, the solubility of nickel is greatest Therefore, the phase transformation of austenite to ferrite through nickel dissolution is observed on the surface in longterm immersion in sodium The compatibility of various metals with sodium reported by Borgstedt and Mathews1 is shown in Solubility equation of metallic and amphoteric Cd Al Ga In U Pu Sn Pb Bi Cr Mo Mn Fe Co Ni log Swwpm ¼ 5.450 À 3055/T (K) log Swwpm ¼ 7.22 À 1479/T (K) Swt% ¼ À11 þ 0.52 Â (T(K) À 273.15) À Â 10À4 (T(K) 273.15)2 Swt% ẳ 0.1414 ỵ 2.08 106 (T(K) 273.15) ỵ 1.248 103 (T(K) 273.15)2 Swppm ẳ 1.4 ỵ 0.057 (T(K) À 273.15) log Swt% ¼ 3.67 À 1209/T (K) log Swwpm ẳ 1.4 ỵ 0.057/T (K) log Swt% ẳ 1.349 À 1010/T (K) log Swt% ¼ 4.48 À 1552/T (K) log Swwpm ¼ 4.36 À 6010.7/T (K) log Swwpm ¼ 8.398 À 10.950/T (K) log Swt% ¼ 5.113 À 2299/T (K) log Swt% ¼ 6.1097 À 2636/T (K) log Swt% ¼ 2.15 À 2103/T (K) log Swt% ¼ 5.67 À 4038/T (K) log Swwpm ¼ 9.35 À 9010/T (K) log Swwpm ¼ 2.738 À 2200/T (K) log Swwpm ¼ 3.640 À 2601/T (K) log Swwpm ¼ 4.720 À 4116/T (K) log Swwpm ¼ 0.010 À 1493/T (K) log Swwpm = 2.07 À 1570/T (K) Temperature range (K) 623–773 377–806 373–873 373–573 373–600 423–773 375–573 373–573 560–970 560–970 473–673 393–523 398–563 563–923 948–1198 500–720 550–811 658–973 673–973 673–973 Table The data can be used to create an index for the selection of the material used in sodium 5.13.3 Corrosion Mechanism of Materials in Sodium There are two known mechanisms of sodium corrosion One is corrosion produced by the dissolution of alloy elements to sodium, and the other is corrosion produced through chemical reaction with the impurities in sodium These two corrosion mechanisms are described in Sections 5.13.3.1 and 5.13.3.2 5.13.3.1 Corrosion Produced by the Dissolution of Alloy Elements to Sodium In this case, corrosion is dependent on the solubility in sodium of the elemental composition in the material, temperature, and the rate of solution The solution rate Rc is given by the following formula: Rc ¼ K ðCs À Ci Þ ½1 Material Performance in Sodium Table 329 Compatibility of materials with alkali metals1 Compatible with alkali metal up to ( C) Material Mg alloys Al alloys Cu alloys Ag and its alloys Au and its alloys Zn coatings Pb and its alloys Sn and its alloys Fe Low-alloy steels Ferritic steels High-Cr steels Austenitic steels Ni alloys Mo alloys W alloys Ti alloys Zr alloys V alloys Nb alloys Ta alloys Sintered A12O3 Stab ZrO2/CaO Stab ThO2/Y2O3 Glass UO2 UC Factors influencing compatibility Li Na K Rb and Cs n.c n.c 300 n.c n.c n.c n.c n.c 500 500 500 500 450 400 1000 1000 700 700 700 700 700 350 350 400 n.c n.c 350 400 n.c n.c n.c n.c n.c 700 700 700 700 750 600 1000 1000 700 700 700 700 700 500 350 550 250 750 750 300 400 400 n.c n.c n.c n.c n.c 700 700 700 700 750 600 1000 1000 700 700 700 700 700 500 350 550 250 300 450 400 n.c n.c n.c n.c n.c 700 700 700 700 750 600 1000 1000 700 700 700 700 700 500 350 550 250 Metal solubility, oxygen exchange Metal solubility Metal solubility High metal solubility High metal solubility High metal solubility Very high metal solubility Very high metal solubility Nonmetallic impurities Nonmetallic impurities Nonmetallic impurities Nonmetallic impurities Nonmetallic impurities Flow velocity Nonmetallic impurities Nonmetallic impurities Nonmetallic impurities Nonmetallic impurities Nonmetallic impurities Nonmetallic impurities Nonmetallic impurities Thermomechanical action Intergranular corrosion Intergranular corrosion Chemical reaction Excess of oxygen Nonmetallic impurities n.c ¼ not compatible 1.0E + 02 1.0E + 00 Ni Solubility (ppm) Mn 1.0E − 02 Fe C Mo 1.0E − 04 Si 1.0E − 06 Cr 1.0E − 08 1.2 1.4 1000/T (K) 1.6 Figure Solubility of alloy elements in sodium 1.8 where K is the solution rate constant, Cs is the solubility limit in sodium, and Ci is the actual concentration in sodium The solution rate constant K is controlled by diffusion The solubility of the alloy elements of the steel is shown in Figure 1.1–5 Included in Figure are the major elements of austenitic stainless steels (type 304SS and 316SS) and the Cr–Mo steels (2.25Cr–1Mo and Mod 9Cr–1Mo) which are used as the structural materials of FBRs The solubility of each of the elements in sodium at 550 C is less than a few parts per million This means that the compatibility of the steels with sodium is fundamentally excellent In the isothermal sodium condition, the corrosion of the steels stops when the dissolved elements reach saturation concentration at the temperature of sodium However, in the nonisothermal sodium condition, corrosion resulting from the difference in activity between sodium and the material surface occurs continually This corrosion behavior is called thermal gradient mass transfer In the cooling system, the elements in the materials in the high-temperature section 330 Material Performance in Sodium ΔW (mg cm-2) Temperature (ЊC) dissolve as a result of the temperature dependency of the solubility of the elements in sodium, and the dissolved elements are deposited on the steel surface in the low-temperature section by the same mechanism The results of thermal gradient mass transfer using a sodium loop made of SUS316, which is equivalent to AISI type 316SS, is shown in Figure The weight loss caused by the dissolution of the elements in the steel is measured in the high-temperature section, and the weight gain caused by the deposit of the dissolved elements in sodium is observed in the low-temperature section Figure shows the microstructure of the inner surface of the sodium pipe taken from the flowing sodium loop operated for 82 000 h Dissolution of the elements in the material is observed in the hightemperature section, and precipitation is observed in the low-temperature section located in the lower stream Generally, selective corrosion occurs at the initial stage as a result of the dissolution of the elements 600 Sodium flow→ 550 500 500 400 +1 390 600 520 470 410 –1 -1 Sodium velocity :3 m s Oxygen level :9 ppm Immersion time 5512 h 7141 h 10644 h Weight loss -2 -3 10 15 20 Distance from electro magnetic pump of sodium loop (m) 25 29 Figure Weight change in SUS316 after corrosion test in flowing sodium Reproduced from Maruyama, A.; Nomura, S.; Kawai, M.; Takani, S.; Ohta, Y.; Atsumo, H J Atomic Energy Soc Jpn 1984, 26, 59 in the steel, and then, the behavior moves to general corrosion with the progress of time In the hightemperature section, the dissolution of nickel, chromium, manganese, and silicon to sodium occurs easily, and molybdenum and iron remain in the material In the low-temperature section, chromium deposits easily with decreasing temperature Carbon transfer in the steel affects the mechanical properties of the FBR structural materials Decarburization occurring in the high-temperature section, in particular, has the potential of degrading creep strength The effect of sodium on the mechanical strength is described in Section 5.13.5 5.13.3.2 Corrosion Produced Through Chemical Reaction with the Impurities in Sodium The most important element in the impurities in sodium is oxygen Sodium is the reducing agent, and its affinity to oxygen is very strong The temperature dependence of the oxygen saturated in sodium is shown in Figure The solubility of oxygen in sodium is significantly higher than in water However, the control of impurities in sodium can be achieved by using the cold trap technique6 based on the theory of the deposit of dissolved impurities in sodium The introduction of oxygen into sodium may occur during nuclear plant construction, refueling, the supplementing of the reactor cover gas, the opening of the coolant boundary for maintenance operations, etc These are the paths for contamination through oxides adhering to the components and the impurities in the gas The relationship between the standard free energy of the formation of iron oxides and the temperature is shown in Figure The thermodynamically stable oxide in sodium is sodium oxide, Na2O Sodium flow (a) 525 ЊC, 82 000 h (BD-1) (b) 575 ЊC, 82 000 h (BD-2) (c) (d) 625 ЊC, 82 000 h (BD-3) 420 ЊC, 82 000 h (BD-4) 10 μm Figure Corrosion of the inner surface of sodium loops made of type 304SS operated for 82 000 h Reproduced from Yoshida, E.; Kato, S.; Wada, Y Liquid Metal Systems; Plenum press: New York, 1995 Material Performance in Sodium Dissolved oxygen (ppm) 1.0E + 04 oxidize iron thermodynamically (i.e., iron oxide cannot be formed) The iron is oxidized by the formation of complex Na–Fe oxides.7 In addition to oxygen, impurities in sodium include elements in the steel, hydrogen, and nitrogen Hydrogen and nitrogen induce changes in the microstructure that lead to the potential degradation of mechanical properties 1.0E + 03 1.0E + 02 1.0E + 01 5.13.4 Corrosion Behavior and Factors Affecting Steel 1.0E + 00 1.0E - 01 1.5 2.5 1000/T (K) Figure Temperature dependence of saturated oxygen concentration in sodium -300 O2 Na 2/3 ΔG0f (KJ mol-1 O2) -400 Fe 2O O 2Fe The basic mechanism of the steel corrosion in sodium is described in Section 5.13.3 In this section, the corrosion behavior and its effects are described The major data are obtained for austenitic stainless steels The important factors which influence the corrosion of the steels in sodium are: (1) immersion time, (2) temperature, (3) dissolved oxygen, (4) sodium velocity, (5) alloy elements, and (6) carburization and decarburization The data on each factor are described in the following sections 5.13.4.1 -500 1/2 e 3O a 2O F 2N O3 2/3 -600 O4 1/2 Fe Na O3 e a 4F N 2/3 e a 5F N -700 O2 Fe Na -800 200 331 400 600 800 Temperature (K) 1000 1200 Figure Relationship between ÁG0f and temperature in oxides consisting of iron and sodium Iron, the main element in the steel, is corroded by the following reaction: Feẵs;l ỵ 3Na2 Oẵs;l ! Na4 FeO3 ẵs ỵ 2Naẵs;l 0 ẳ ỵ43:8 KJ mol1 ; Gr298 ẳ ỵ26:1KJ mol1 Hr298 where [g], [l], and [s] stand for gas, liquid, and solid This is an endothermic reaction However, Gibbs energy of reaction (ÁGr0 ) becomes negative at 380 C, and the reaction progresses spontaneously above that temperature In addition, Na2O cannot directly Immersion Time As mentioned in Section 5.13.3.1, nickel, chromium, and manganese dissolve easily in sodium Therefore, when austenitic stainless steel is immersed in sodium, the selective corrosion caused by the dissolution of these elements progresses in the initial stage This process is based on the solid diffusion of the elements in the steel, and is described by Fick’s second law Diffusion in sodium is given by the following two formulas: dM dc ẳ D dt dx ẵ2 p Mt ị ẳ p Dt C0 Cs ị p ẵ3 where M is the diffusion amount, D is the diffusion coefficient, C0 is the initial concentration, and Cs is the surface concentration at time (t) Formula [2] is Fick’s first law and its integration is given as the total diffusion Formula [3] is the function given as the approximate formula From these formulas, it is understood that the corrosion behavior at the initial stage observes the parabolic law proportional to the root of time At the initial process of the corrosion, the parabolic type behavior caused by the selective corrosion is dominant The corrosion behavior is called start-up corrosion 332 Material Performance in Sodium 100 400 Weeks et al.9 Steady-state corrosion Metal loss Start-up Temperature (ЊC) 600 500 700 Immersed time, t Selective loss (Ls) ∝ t1/2 Bulk loss (Lb) ∝ t Total loss (Ls + Lb) Corrosion rate (µm year–1) Bagnall and Jocobs12 10 Menken30 Zebroski10 Kolster11 Thorley and Tyzack8 JAEA Figure Schematic representation of corrosion data for austenitic stainless steel The corrosion behavior of iron, which is the main element of stainless steel, is general corrosion and the corrosion progresses as a linear function of time Therefore, the corrosion of the dominant factor changes from start-up corrosion to general corrosion with the progress of time (Figure 6) The corrosion behavior that dominates general corrosion is called steady-state corrosion The time required for a change from start-up corrosion to steady-state corrosion is 2000–5000 h, although it is dependent on conditions such as sodium volume, temperature, velocity, and dissolved oxygen 5.13.4.2 Temperature Temperature dependence of the corrosion rate in the sodium of austenitic stainless steels is shown in Figure The corrosion rate CR is described by the following Arrhenius function: E ½4 CR / exp À RT where R is the gas coefficient (8.3145 J molÀ1 KÀ1), E is apparent activation energy, and T is temperature The apparent activation energy E is a function of temperature The values are reported as 100–120 kJ molÀ1 by numerous researchers (Table 4) The energy is lower than that of the activation energy of the diffusion of iron, chromium, and nickel, in Austenitic stainless steel Oxygen content: 10 ppm Na velocity: 3.8 m s–1 0.1 0.9 1.1 1.3 1.5 1000/T (K) Figure Corrosion rate of austenitic stainless steels in flowing sodium stainless steel ($250–300 kJ molÀ1), and it agrees with the activation energy of solubility in sodium (Figure 1, Fe: 82.5 kJ molÀ1 and Cr: 104 kJ molÀ1) In fact, it is understood that the corrosion of stainless steel is dominated by the dissolution process of the major elements (iron, chromium, and nickel) of the steel The effect of dissolved oxygen on sodium, which influences the dissolution reaction, is described in the following section 5.13.4.3 Dissolved Oxygen The effect of dissolved oxygen on the corrosion rate is described by the following formula because the corrosion process is dominated by the reaction process of oxides CR / ½O2 n ½5 where CR is the corrosion rate, [O2] is the oxygen concentration, and n is a constant The constant n, reported by the researchers, is listed in Table This result suggests the possibility Material Performance in Sodium Table Comparison of apparent activation energy on corrosion rate Sodium temperature ( C) Activation energy (kJ molÀ1) Reference Thorley Weeks Zebroski Kolster Bagnall Maruyama 450–725 538–705 500–700 650–700 593–723 500–650 73.5 108.8 110.5 114.2 167.4 92–109 [8] [9] [10] [11] [12] [13] Table Comparison of oxidation coefficient on corrosion rate Bibliography O2 content (ppm) Coefficient (n) Reference Thorley Zebroski Roy Kolster 5–100 12, 50 5–30 1–8 8–40 2.5–9 1.5 1, 1.56 1.2 0.91 >1 0.8 [8] [10] [14] [11] Maruyama Sodium Velocity It has been observed that the corrosion rate in sodium increases as sodium velocity increases However, it is known that the increase ends when the velocity reaches a certain limit It is believed that the limit is a function of oxygen concentration in sodium and/ or the structure of the sodium loop According to Thorley and Tyzack,8 Roy,14 and Kolster,11 the limit is 3.8, 6–7, and m sÀ1, respectively It is believed that the effect of such a sodium velocity is based on the thickness of the boundary layer between the material surface and the flowing sodium In fact, the thickness of the boundary layer decreases as sodium velocity increases, and the diffusion of alloy elements via the boundary layer increases 5.13.4.5 O2 cont.: ppm Velocity: 1.48 m s–1 Immersion time: 2930–5254 h 650 ЊC 1.0 600 ЊC 0.1 20 30 10 Nickel concentration in steel (mass %) 40 Figure Effect of nickel content in stainless steel on the corrosion rate [13] that the control of dissolved oxygen may significantly influence the corrosion behavior 5.13.4.4 10 Corrosion rate (µm year–1) Bibliography 333 Alloy Elements The effect of the alloy elements on corrosion is examined because long-term corrosion occurs as a result of thermal gradient mass transfer The effect of the chromium and nickel concentration on the steels is particularly significant Figure shows the effect of the nickel content of stainless steel on the corrosion rate On the other hand, the dependence of the corrosion rate on elements in the steels is hardly observed in austenitic stainless steels, such as types 304SS, 316SS, and 321SS, because of the slight difference in chemical composition (Figure 9) 5.13.4.6 Carburization and Decarburization In monometallic sodium loops, the difference of the carbon activity, which is the driving force of the carbon transfer of the material, increases as temperature increases Therefore, decarburization occurs in the high-temperature section and carburization occurs in the low-temperature section On the other hand, in bimetallic sodium loops which consist of austenitic stainless steel and ferritic steel, it is easy for decarburization to occur in the ferritic steel, which has a high carbon activity due to the difference in carbon activity between different materials, whereas carburization in austenitic stainless steel, which has low carbon activity, easily occurs at elevated temperature Carbon is an important element in maintaining the superior mechanical strength of steel Therefore, the carburization/decarburization behavior of the steels via sodium is important from the perspective of mechanical properties 334 Material Performance in Sodium 750 650 Temperature (ЊC) 550 450 304SS 316SS Corrosion rate (μm year–1) 10 Fuel cladding tube (FCT) 321SS ~9 ppm O2 ~2.5 ppm O2 ~9 ppm O2 ~2.5 ppm O2 ~9 ppm O2 ~2.5 ppm O2 20 ppm O2 10 ppm O2 ppm O2 Sodium velocity 2–4 m s-1 (FCT: 2–4.8 ms-1) 0.1 0.9 2.5 ppm O2 Immersion time 1000–7200 h 1.1 ppm O2 1.3 1000/T (K) 1.5 Figure Comparison of corrosion rates of austenitic stainless steels Carbon concentration in sodium (ppm) 100 10–1 Decarburization Type316 (0.06 wt% C) FFTF (566 ЊC) FFTF (474 ЊC) Type304 L (0.025 wt% C) Data band of the carbon concentration in EBR-II primary cooling system EBR-II (470 ЊC) 10–2 10–3 400 Carburization 500 600 700 Temperature (ЊC) Figure 10 shows the boundary between carburization and decarburization in monometallic sodium loops consisting of austenitic stainless steel (single alloy).15 At a carbon concentration of 0.2 ppm in sodium, the temperature boundary is $650 C, with decarburization occurring over that temperature and carburization occurring below that temperature Although the boundary is influenced by the carbon concentration in sodium (carbon activity), it is necessary to take decarburization and carburization into consideration to apply austenitic stainless steel when the temperature is above 550 C, such as fuel cladding tubes On the other hand, in bimetallic sodium loops that consist of ferritic steel and austenitic stainless steel (two alloys), decarburization and carburization also occur in the temperature range of the structural materials