This paper focuses on activation of oxygen isotopes in water and decay of this activated isotopes, i.e. 16N, 17N and 19O. An analysis of activation of water in pressurized water reactors and in fusion reactors was performed. Different evaluated nuclear data libraries were used in activation calculations (ENDF/B-VIII.0, FENDL-3.1b, JEFF-3.2 and TENDL-2015).
Progress in Nuclear Energy 117 (2019) 103042 Contents lists available at ScienceDirect Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene Review On the dose fields due to activated cooling water in nuclear facilities Andrej Žohar, Luka Snoj T ∗ Jožef Stefan Institute, Jamova cesta 39, SI-1000, Ljubljana, Slovenia ARTICLE INFO ABSTRACT Keywords: Activated cooling water PWR Fusion reactor MCNP Cooling pipes Steam generator Activated cooling water in nuclear facilities can present a significant radiation source around primary cooling system causing radiation damage to electrical components, increasing doses to personnel and in the case of fusion facilities additional heating to superconducting coils This paper focuses on activation of oxygen isotopes in water and decay of this activated isotopes, i.e 16N, 17N and 19O An analysis of activation of water in pressurized water reactors and in fusion reactors was performed Different evaluated nuclear data libraries were used in activation calculations (ENDF/B-VIII.0, FENDL-3.1b, JEFF-3.2 and TENDL-2015) The calculated activation rates with different nuclear data libraries agree well for the 16O(n,p)16N reaction and significantly differ for 17O (n,p)17N and 18O(n,γ)19O reactions In fusion reactor the specific activity of activated water isotopes is in the order of 1013 Bq/m3/MW, which is five orders of magnitude higher compared to specific activity in a typical fission pressurized water reactor, amounting to 109 Bq/m3/MW The results of specific activity of cooling water were used to perform parametric analysis of dose rates around pipes of cooling system and dose field around a steam generator in a pressurized water reactor as a representative of heat exchangers The analysis of dose rates around pipes include pipes featuring mm to cm thick walls and from 0.5 cm to 60 cm water radius Results can be used to estimate dose rates for all studied isotopes, provided the specific activity is known For heat exchangers the decay of 16N contributes majority to the dose rates in the air surrounding them while 17N and 19O decay contributes together less than 0.1% For a typical GW thermal power two loop pressurized water reactor the dose rates in air surrounding the stream generator are in the order of several mSv/h Introduction Water is cooling fluid in many nuclear facilities, such as fission nuclear reactors and some fusion reactors In fission reactors water is activated when passing through the reactor core, in fusion reactors however water is activated when cooling the blanket, or other components of the reactor such as diagnostic equipment Activation of water consist of activation of oxygen and hydrogen as primary constituents of the H2O molecule, activation of dissolved gasses, corrosion products and additions to water and fission products in fission reactor As all the latter are case specific, in this paper we will focus on activation of pure H2O only After being irradiated and activated the cooling water flows through the primary cooling circuit, commonly outside the primary biological shielding surrounding the reactor vessel There the activation products decay, emit radiation, which causes radiation damage to electrical components, increasing doses to personnel working around the cooling circuit and in the case of fusion facilities causes nuclear heating of various cold components such as superconducting coils cooled by liquid helium (Iida et al., 1997) Decay of activated cooling water can also be used to obtain ∗ important parameters of the heat producing component In nuclear power plants the decay of activated water is used to detect leakage of primary cooling system in the secondary cooling system (IAEA, 2000) Activated water can also be used to determine water flow and power of the reactor (Tsypin et al., 2003) In the case of fusion reactors the neutron yield of the reactor can be measured with the use of activated water (Nishitani et al., 2003) There are several papers on measurement of activation of cooling water in fission power plants and research reactors (Guo et al., 2018; Stepišnik et al., 2009) where the measurement are performed regularly for education of university students For fusion reactor only one experiment on the activation of water was performed at the JAERI FNS facility in Japan (Uno et al., 2001) Activated cooling water is also present in spallation source facilities (Santoro et al., 1999) The main contributors to the activity of clean cooling water are radioactive isotopes of oxygen and nitrogen produced by activation of oxygen isotopes in the cooling water, i.e 16N, 17N and 19O Majority of studies dealing with activation of cooling water and dose fields due to decay of activated water focuses on isotopes 16N and 17N (Blakeman et al., 2007; Santoro et al., 1997) while the majority neglects the effects of isotope 19O due to lower energies of gamma radiation emitted in Corresponding author E-mail addresses: andrej.zohar@ijs.si (A Žohar), luka.snoj@ijs.si (L Snoj) https://doi.org/10.1016/j.pnucene.2019.103042 Received 19 December 2018; Received in revised form 19 April 2019; Accepted 25 April 2019 Available online 16 May 2019 0149-1970/ © 2019 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/) Progress in Nuclear Energy 117 (2019) 103042 A Žohar and L Snoj decay and negligible activity compared to isotope 16N However in this paper activation and dose rates of all three isotopes of activated oxygen in cooling water are studied As hydrogen has two stable isotopes, 3H is also produced in cooling water from 2H activation However, as 3H halflife is in long compared to other activated isotopes and practically no gamma rays are emitted at decay, 3H dose rate contribution are neglected in this paper as they are in other papers The analysis presented in this paper focuses on calculating activation of oxygen isotopes and the dose fields around primary cooling pipes and heat exchangers of nuclear devices by using Monte Carlo particle transport code MCNP (Goorley et al., 2012) In the first part of the paper the neutron activation data, comparison of cross-sections for activation between different evaluated nuclear data libraries and results of activation calculations for fission and fusion reactors are presented The second part of the paper presents the parametric analysis of dose rates in the air surrounding pipes with different wall thickness and water diameter and equivalent biological dose rates in the air surrounding a heat exchanger are presented For the model of a heat exchanger a vertical steam generator in a typical GW thermal power pressurized water reactor was used Dose rates presented in the paper are the H*(10) ambient dose equivalent for biological dose rates and dose rates in silicon for electronic components Fig Cross-section energy dependence for activation of oxygen nuclide taken from the JEFF-3.2 data library (OECD/NEA Data Bank, 2014) As there are many different evaluated cross-sections for the above mentioned water activation reactions in different evaluated nuclear data libraries, the cross-sections can be significantly different In this paper cross-sections from four different libraries were used: ENDF/BVIII.0 (Brown et al., 2018), JEFF-3.2 (OECD/NEA Data Bank, 2014), FENDL-3.1b (Koning and Trkov, 2016) and TENDL-2015 (Koning et al., 2015) Cross-section for activation of 16O (Fig 2) in all studied libraries is the same as it is derived from the same experimental data (Nelson and Michaudon, 1999) For 17O activation however the cross-sections differ between libraries as presented in Fig The cross-section in JEFF-3.2 library is taken from TENDL-2012 and cross-section in FENDL-3.1b is taken from TENDL-2010, which are predecessors of TENDL-2015 library Evaluated cross-sections in TENDL libraries are based on computations by software for simulation of nuclear reactions TALYS (Koning and Rochman, 2012) The TENDL-2010 library is based on TALYS 1.20 version, TENDL-2012 is based on TALYS 1.50 version and TENDL-2015 is based on the TALYS 1.74 version for computation of cross-sections Due to this the cross-sections between this three libraries are similar unlike the cross-section from ENDF/B-VIII.0 library, which is taken from ENDF/B-V library which was released in 1978 and is based on computations by MODNEW (Uhl, 1972) and measurements performed by Menlove (Menlove et al., 1970) In Fig the cross-sections for reaction 18O(n,γ)19O from all studied evaluated nuclear data libraries are presented With the release of the ENDF/B-VIII.0 evaluated nuclear data library the cross-section for activation of 18O was added The cross-section is based on the Neutron activation of water Oxygen and nitrogen activated isotopes in cooling water are produced from activation of oxygen isotopes via the 16O(n,p)16N, 17O (n,p)17N and 18O(n,γ)19O reactions Activated isotopes in cooling water decay by emitting various decay products with different energies Summarized data is presented in Table (Chadwick et al., 2011) The marked energies in table present the dominant energies of decay pro16 ducts emitted at decay As 16N decays via decay path 16N O + γ, high energy gamma rays are emitted (E = 6.13 MeV) with half-life of 16 O + n + γ with half-life of 7.13 s 17N decays via decay path 17N 4.14 s Emitted neutrons can activate components outside the primary circuit and produce neutron induced gamma-rays Activated isotope 19 19 O has a half-life of 26.9 s and decays via decay path 19O F + γ 16 16 17 17 Reactions O(n,p) N and O(n,p) N are threshold reactions with energy threshold at 10 MeV and MeV respectively Reaction 18O (n,γ)19O already takes place at thermal energies as presented in Fig Due to threshold reactions the activation of water is expected to be higher in fusion reactors like ITER compared to fission reactors due to higher neutron energies (14 MeV neutrons from deuterium-tritium fusion) Table Summarized data of activated isotopes of cooling water obtained from ENDF/B-VII.1 data library (Chadwick et al., 2011) The marked energies present the dominant energies of decay products Progress in Nuclear Energy 117 (2019) 103042 A Žohar and L Snoj TALYS 1.64 version and TENDL-2015 is based on the TALYS 1.74 version for computation of cross-sections As in the case for activation of 17O the cross-section for activation 18O taken from the ENDF/B-VIII.0 library differs significantly compared to cross-sections in other studied nuclear data libraries The difference in cross-section between other studied libraries is in the epithermal region at around 0.08 MeV In this region the cross-section for 18O(n,γ)19O from FENDL-3.1b and TENDL2015 libraries exhibit a resonance peak, while the cross-section from JEFF-3.2 library has no peak Due to this the calculated reaction rates are expected to be higher with the use of FENDL-3.1b and TENDL-2015 library Activation of cooling water It is difficult to measure the absolute value of activity of activated isotopes in cooling water in nuclear facilities due to short decay times of isotopes and high energy radiation which can cause high dose rates Another difficulty is the placement of large detectors (e.g High Purity Germanium detector) close to the primary cooling system for accurate measurements as the systems are normally shielded The easiest way to determine the absolute value of isotope activity is by calculations from parameters of nuclear facility The change of specific activity of a studied isotope in the cooling water a with time is described by: Fig Cross-sections for reaction 16O(n,p)16N taken from ENDF/B-VIII.0, JEFF3.2, FENDL-3.1b, TENDL-2015 libraries and experimental results from EXFOR database a (t ) = F (1 (1) t ), e −1 where λ is a decay constant [s ] and F is an average reaction rate in region of interest over irradiation time which is described by: F= t V (r, E , t ) i (E ) n (r , t )dE dV dt , (2) where the (r, E , t ) is the neutron flux at position r , i (E ) is the microscopic cross-section for studied reaction and n (r, t ) the number density of target atoms at position r In nuclear facilities cooling water circulates in the primary cooling system and is exposed to neutron flux for a short time Hence the change in specific activity of studied isotope is described using a system of equations: Fig Cross-sections for reaction 17O(n,p)17N taken from ENDF/B-VIII.0, JEFF3.2, FENDL-3.1b, TENDL-2015 libraries and experimental results from EXFOR database ao = e ti = ao e T, + F (1 e ti) (3) where ao is the specific activation of coolant on the outlet of heat producing component, is the specific activation of coolant on the inlet of heat producing components, ti is the exposure time and T is circulation time the coolant needs from the outlet to the inlet of heat producing component From the eq (3) the equilibrium value of specific activity at the outlet of the heat producing component can be obtained: ao = F 1 ti e (ti + T ) e (4) In eq (4) the F is the average reaction rate over whole heat producing component The intensity of neutron fluxes as well the energy spectrum can significantly change through the heat producing component This changes can be taken into account by dividing the heat producing components in smaller sections with similar neutron fluxes and energy spectrum in which the reaction rates are calculated In general there are n equations for n regions in the heat producing component plus one equation for the region outside the heat producing component The general form of the system of equations is: Fig Cross-sections for reaction 18O(n,γ)19O taken from ENDF/B-VIII.0, JEFF3.2, FENDL-3.1b, TENDL-2015 libraries and experimental results from EXFOR database The cross-sections from FENDL-3.1b and TENDL-2015 library are similar except in the high energy region (above 30 MeV) and are due to this overlapped in the above graph Moghabghab resonance parameters below MeV (Mughabghab, 2006) while above MeV the cross section is based on J Kopecky and D Nierop evaluation for EAF-3 library The cross-section in the JEFF-3.2 library is taken from the TENDL-2012 while the cross-section in the FENDL-3.1b library is taken from the TENDL-2014 library The TENDL2012 library is based on TALYS 1.50 version, TENDL-2014 is based on a1 = an + e t i1 + F1 (1 e t i1) an = an e t in + Fn (1 e t in) an + = an e T (5) From eq (5) the equilibrium of activity of isotope at the outlet of heat producing component (an ) can be calculated Progress in Nuclear Energy 117 (2019) 103042 A Žohar and L Snoj As there are many different nuclear facilities where cooling water is activated a general analysis of activation of water in each of them is difficult to perform In this paper an analysis of activation of cooling water in a typical PWR and fusion reactor is presented PWR reactor was chosen as a representative of fission reactors as they are the most common type of fission power reactors neutron flux is highest Two orders of magnitude lower reaction rates are in the downcomer while in lower and upper plenum the reaction rates are five orders of magnitude lower as in the core due to support structures in lower plenum and control rods in upper plenum The uncertainties in the calculation results are due to systematic error of Monte Carlo calculation, uncertainty in the model and uncertainty of nuclear data However, only the Monte Carlo statistical uncertainties are presented in Table For lower and upper plenum the greatest contribution to uncertainty is the Monte Carlo statistical uncertainty To improve the statistic of simulation the calculation times would need to be extended However, the contribution to the total reaction rate from this two regions is negligible compared to the contribution of the core and additional calculation time would not change the final result significantly For calculation of specific activity the total value of reaction rates are needed but the spectral analysis of reaction rates were also performed The results of reaction rate spectra in core for some libraries are presented in Fig For activation of 18O results of reaction rate spectra in core from two different libraries are present to show the effect of additional resonance peak in cross-section in TENDL-2015 library From calculated reaction rates and exposure times the equilibrium specific activity at the outlet of the reactor vessel was calculated using eq (5) and the obtained values for all studied nuclear data libraries are presented in Fig The most activated isotope of cooling water is 16N due to high natural abundance and higher cross-section for reaction The equilibrium specific activity is four orders of magnitude higher than equilibrium specific activity of 17N and two orders of magnitude higher than equilibrium specific activity of 19O Due to differences in cross-section for activation of 17O the equilibrium specific activity of 17 N obtained with the ENDF/B-VIII.0 library is by a factor of three higher compared to results obtained with other libraries The equilibrium specific activities of 19O obtained with libraries TENDL-2015 and FENDL-3.1b are by a factor of three higher than equilibrium specific activity obtained with the use of JEFF-3.2 library due to resonance peak in the cross-section at epithermal energy Despite significant differences in cross-section for activation of 18O in ENDF/B-VIII.0 library the equilibrium specific activity is comparable to results obtained with the use of TENDL-2015 and FENDL-3.1b This is due to the resonance peaks at fast neutron energies 3.1 Activation of cooling water in PWR To obtain the absolute value of specific activity of activated isotopes of cooling water at the outlet of heat producing component, which in the case of PWR is the reactor vessel, from eq (4) some parameters are needed The parameters are the exposure times of cooling water to neutron flux, circulation time and the reaction rates The exposure time and circulation time can be determined from volume flow rate of cooling water and volume of it in different regions In the studied case the circulation time of cooling water was calculated to be around 8.1 s, while the exposure time was calculated to be around 4.2 s of this 1.4 s to high neutron flux in reactor core The reaction rates were calculated by the Monte Carlo particle transport using the MCNP code A geometrical model of a typical GW two loop PWR reactor vessel was constructed in MCNP The model is presented in Fig and neutron spectra for all studied regions is presented in Fig The reaction rates in the reactor vessel were calculated as follows The reactor vessel was divided in four sections: downcomer, lower plenum, core and upper plenum and then the section averaged reaction rates were calculated by multiplying the neutron flux calculated by the track length estimator (F4 tally in MCNP) with the corresponding cross-section The reaction rates were calculated using all studied nuclear data libraries and density of water at 600 K Results of average reaction rates in regions for some libraries are presented in Table The highest reaction rates are in the reactor core where the 3.1.1 Time dependence of specific activity Results of specific activity presented in Fig present the equilibrium value However, during start-up, shutdown and power changes of reactor the value of specific activity changes The behaviour in specific activity can be simulated using the calculated reaction rates in specific areas This is described with a set of eq (6) (Žohar and Snoj, 2016): A o (t ) = F4 (1 e t ), F3 (1 e t )e t i3 (1 e t i 4), N i=0 ae iT , t < ti + F4 ti t t < ti3 ti + ti3 + ti2 + ti1 and N T t (6) where a is the saturated value of specific activity after one cycle at the outlet of reactor vessel The first equation in eq (6) presents the specific activity produced in region (upper plenum) The second equation presents the saturated specific activity produced in region plus specific activity produced in region (reactor core) The equations follow this order until the time the cooling water goes through the whole reactor vessel After that the last equation describes the specific activity behaviour The first analysis performed was the change of specific activity of all activated isotopes of cooling water during the start of the studied reactor with no activated cooling water Special attention was given to Fig MCNP model of reactor vessel in a typical PWR with marked regions for reaction rates calculation and marked direction of water flow in reactor vessel Progress in Nuclear Energy 117 (2019) 103042 A Žohar and L Snoj Fig Lethargy neutron flux spectra in all four studied regions of a GW thermal pressurized water reactor the time and number of recirculation cycles it takes for the specific activity of a radionuclide to reach the saturated value and the results are presented in Fig The specific activity of all products is asymptotically increasing in steps due to recirculation cycles The results show that it takes 4.3 (21 cycles) for specific activity of 16N to reach saturated value, 2.9 (14 cycles) for specific activity of 17N to reach saturated value and 14.3 (70 cycles) for specific activity of 19O to reach saturated value The specific activity changes in the last cycles are too small to be visible in the graph The specific activity behaviour during power changes and after shutdown was also analysed for the studied reactor To simulate this the Table Reaction rates in studied regions of reactor vessel in a GW thermal power PWR Region 16 O(n,p)16N ENDF/ B-VIII.0 [cm−3s−1] 17 O(n,p)17N TENDL-2015 [cm−3s−1] 18 Downcomer Lower plenum Reactor core Upper plenum 1.39·105± 6.71·102± 1.11·107± 1.53·102± 9.85± 0.79 0.037± 0.009 780± 62 0.022± 0.007 5.05·103± 4.54·102 3.60± 0.38 1.99·105± 2.11·104 1.92± 0.21 6.98·103 1.61·102 5.56·105 4.15·101 O(n,γ)19O FENDL-3.1b [cm−3s−1] Fig Reaction rate per energy bin in the core of a GW thermal power PWR for activation of all isotopes of cooling water obtained for some nuclear libraries For activation of 18O results from two different libraries are taken to present the effect of additional resonance peak in the cross-section Progress in Nuclear Energy 117 (2019) 103042 A Žohar and L Snoj Fig Equilibrium specific activities of activated isotopes in cooling water for all studied nuclear data libraries for a typical GW thermal power PWR Fig 11 Comparison of neutron flux energy spectrum in cooling water for fission and fusion reactors comparison of both neutron spectra in cooling water is presented in Fig 11 The absorption lines in the fast neutron region of fission spectra are due to elastic scattering of neutrons on 16O As already mentioned in the beginning of the paper due to higher energies of neutrons in fusion reactors and threshold reactions for activation of 16O and 17O the activity of cooling water is expected to be higher in fusion reactors Due to this calculations of activation of cooling water in fusion reactor using MCNP were performed The methodology for calculation was same as for calculation in fission reactor Reaction rates in water pipes cm from first wall of reactor were calculated using the MCNP for all studied nuclear data libraries The neutron spectrum used was from a D-T plasma and the exposure time was estimated to be s It was also assumed that the cooling water was not activated before the cooling of reactor as the circulation time of system is large enough for all activated isotopes to decay before new activation The results of activated cooling water for an ITER like reactor with 500 MW thermal power is presented in Fig 12 As predicted for ITER like fusion reactors the specific activity of cooling water for all isotopes is higher compared to fission reactors For isotope 16N and 17N the specific activity is four orders of magnitude higher while for isotope 19 O the specific activity is one orders of magnitude higher Due to lower thermal neutron flux in fusion reactors compared to fission reactors the specific activity of 17N is higher than specific activity of 19O despite lower natural abundance The specific activity for 19O obtained with the ENDF/B-VIII.0 library is higher compared to results obtained with TENDL-2015 and FENDL-3.1b library due to higher cross-section in the energy region of fast fusion neutrons Fig Time dependence of specific activity from the start of a reactor with no activated cooling water to saturated value Steps in the graphs corresponds to individual cycles of cooling water Fig 10 Time dependence of specific activity during power changes and after shutdown power level in simulation was changed from full power to 50% power and kept constant till the specific activity of all isotopes reached new saturated value Then the power was changed back to full power and after the specific activity of all isotopes reached saturated value the reactor was shut down The results of the simulation are presented in Fig 10 The specific activity behaviour during power changes is similar to the behaviour during the reactor start-up The times and numbers of cycles needed to reach new saturated values are the same The last part of the simulation presents specific activity behaviour after a rapid reactor shutdown 15 after reactor shutdown the specific activity of all isotopes falls below 0.001 Bq/m3 3.2 Activation of cooling water in fusion reactors In fission reactors the energies of neutrons at birth are distributed according to Maxwell spectrum with peak energy below MeV and average energy of neutrons at around MeV On the other side in fusion reactors fusing deuterium and tritium (D-T) the energies of neutrons at birth are around 14.1 MeV, an order of magnitude higher Due to this the neutron spectrum between fission and fusion reactors differ The Fig 12 Equilibrium specific activities of activated isotopes in cooling water for all studied nuclear data libraries for an ITER like fusion reactor of 500 MW thermal power Progress in Nuclear Energy 117 (2019) 103042 A Žohar and L Snoj exchangers wary significantly between different types of nuclear facilities and a general analysis of dose field surrounding them is difficult to perform Due to this an analysis of biological dose field in the air surrounding a vertical steam generator in a typical PWR was performed The results and analysis of both studies are presented in following chapters Table Comparison of specific activity of activated isotopes of cooling water in fission and fusion reactors normalized to MW thermal power Only the Monte Carlo statistical uncertainties are presented in table Activated isotopes Fusion reactor specific activity [Bq/m3/MW] Fission reactor specific activity [Bq/m3/MW] 16 5.92 1013 (1 ± 0.0069) 9.60 10 (1 ± 0.0069) 17 19 N N O 9.46 109 (1 ± 0.0070) 1.10 109 (1 ± 0.0135) 4.1 Parametric analysis of pipes 6.75 10 (1 ± 0.0074) 1.33 107 (1 ± 0.0147) Pipes are connectors between major components in the cooling loop and guide tubes for instruments measuring parameters of coolant Due to this the diameter of cooling water and thickness of walls changes throughout nuclear facilities A parametric study of the dose filed in the air surrounding the pipes due to decay of activated cooling water was performed to include all possible sizes of pipes The aim of this parametric study is to provide guidelines on expected dose rates around different pipes containing activated water The results of dose rates were calculated by using the MCNP with the ENDF/B-VIII.0 library at 50 cm distance from surface of pipe The simulated model was a two meters long pipe with reflecting surfaces (boundary conditions) at the end and surrounded by air The material used for the pipe wall was stainless steel (SS 304) as it is used as the main material in primary cooling circuit of PWR Despite the simulated pipes being a part of primary cooling system the thermal isolation was not modelled in the analysis The source for Monte Carlo calculations was a uniform and isotropic emission of decay particles in the whole water volume in pipes The parametric analysis of dose rates included pipes with wall thickness from 0.1 to 8.1 cm and water radius from 0.5 to 60.5 cm This limits were chosen to include all possible pipe sizes in the primary cooling loops of nuclear facilities On the graphs of results the dimensions for pipes according to ANSI B36.19 schedules 160 for sizes 1.27 cm (1/2 inch) through 30.5 cm (12 inches) and according to ASME Boiler and Pressure Vessel Code, Section III, Class components for larger pipes are given as red dots Neutrons emitted in decay of isotope 17N can activate components around primary cooling system Two types of gamma rays are produced at activation: prompt and delayed In this paper only the study of prompt gamma dose rates due 17N decay was performed as the activation analysis of the pipes was not performed The results of activity were also normalized to MW thermal power of reactor for comparison of results between fission and fusion reactors and are presented in Table For isotope 16N and 17N the specific activity is five orders of magnitude higher while for isotope 19O the specific activity is two orders of magnitude higher Dose field in nuclear facilities Gamma rays and neutrons emitted at decay of isotopes of activated cooling water cause radiation damage to electrical and structural components in vicinity of primary cooling system and increased dose rates to personnel performing tasks close to the cooling system Due to this the determination of dose field is important for designing of shielding for components and personnel As already stated in the paper measurements of dose field due to decay of activated cooling water in nuclear facilities is in most cases difficult if not impossible As a result of that the dose field needs to be obtained using computational methods like Monte Carlo or deterministic methods For complex nuclear facilities the Monte Carlo method is the preferred method In this paper the dose fields were calculated using the Monte Carlo code MCNP In Monte Carlo calculations the dose rate at a specific location in studied facility is calculated using the particle flux at studied location and flux-to-dose conversion factors For effects on biological tissue (the H*(10) ambient dose equivalent) the flux-to-dose factors from standard ICRP-21 (ICRP, 1973) are used for gamma rays while for neutrons the flux-to-dose factors from standard ICRP-74 (ICRP, 1996) are used which have been independently validated by several different institutions (Traub, 2010) For dose rates on the silicon components the flux-to-dose factors from standard ASTM E722-14 (ASTM International, 2014) for neutrons are used while for gamma rays the dose rates are calculated using energy deposited in studied material All flux-to-dose factors used in this paper are presented in Fig 13 All nuclear facilities have some common components in cooling systems like pipes As there are many different sizes of pipes through the cooling system of a facility a parametric analysis is needed to cover all possible pipe sizes for general study On the other hand heat 4.1.1 Biological parametric analysis Results of dose rates were normalized to one source particle to study the diffusion of prompt gamma rays due to decay of 17N and results are presented in Fig 14 They show that there is a region where the dose rates are at highest due to greater absorption of neutrons compared to thinner pipes and lower absorption of prompt gamma rays compared to Fig 14 Parametric results of biological gamma dose rates due to decay of 17N normalized to one source particle The red dots present the pipe parameters according to ANSI B36.19 schedules 160 and ASME BPVC section III Fig 13 Flux to dose conversion factor using different standards, for both neutrons and rays in terms of Sv/h and silicon equivalent Gy/h per particle flux Progress in Nuclear Energy 117 (2019) 103042 A Žohar and L Snoj Fig 15 Parametric results of biological dose rates normalized to water volume The red dots present the pipe parameters according to ANSI B36.19 schedules 160 and ASME BPVC section III thicker pipes The maximum is a around 10 cm of water radius and cm of wall thickness From the results it is also visible that pipes according to ANSI B36.19 standard 160 are in the maximum of dose rates For other isotopes of activated cooling water the highest doses are in region with small water radius (few cm) and thin walls (few mm) The results of parametric analysis (d) obtained by MCNP were renormalized such a way that they can be multiplied with specific activity of water (a) and divided by volume flow rate of activated water to obtain the dose rates in Sv/h at distance 50 cm from pipe surface: D=d a V 4.1.2 Silicon parametric analysis Dose rates for electronic components were calculated in cm thick silicon dummy model Neutron dose rates were calculated using ASTM standard For gamma rays the dose rates were obtained using tally multiplier which consisted of number density of silicon, total crosssections for gamma interaction in silicon and gamma heating number of silicon As in the case for biological dose rates, the results were normalized to one source particle to study diffusion of prompt gamma rays due to decay of 17N and are presented in Fig 16 The highest dose rates are around 10 cm of water radius and cm of wall thickness due to absorption of neutrons and low absorption of prompt gamma rays in (7) Such approach allows easy estimation of dose rates around pipes by using pre-calculated values in this paper Results for all activated isotopes are presented in Fig 15 For easier readout of physical quantities from the figures the data for certain pipe diameters are provided in tabular format in appendix A Unlike results normalized per source particle the highest dose rates are for pipes with big water radius (over 50 cm) and thin wall (few mm) For prompt gamma rays due to decay of 17 N the region with highest dose rates is not present due to small water volumes for normalization If the specific activity of all three isotopes of activated water would be the same value the highest contribution to biological dose rates would be due to neutrons from 17N and the lowest contributor are gamma rays from 19O decay However, as already presented in the paper the specific activities of isotopes are different due to differences in activation The majority contribution to the total dose rate is thus from 16N decay while the dose rates from 17N and 19O contribute the same order of magnitude Fig 16 Parametric results of gamma dose rates in silicon due to decay of 17N normalized to one source particle The red dots present the pipe parameters according to ANSI B36.19 schedules 160 and ASME BPVC section III Progress in Nuclear Energy 117 (2019) 103042 A Žohar and L Snoj Fig 17 Parametric results of dose rates in silicon normalized to water volume The red dots present the pipe parameters according to ANSI B36.19 schedules 160 and ASME BPVC section III Fig 18 Comparison between the CAD model and the constructed MCNP model of the steam generator Figures of the steam generator are mirrored for better comparison Progress in Nuclear Energy 117 (2019) 103042 A Žohar and L Snoj pipe walls as in the case of biological dose rates The parametric results of dose rates for silicon were renormalized in a way they can be multiplied with specific activity of water and divided by volume flow rate of activated water to obtain dose rates in Gy/h at distance 50 cm from pipe surface Results for all activated isotopes are presented in Fig 17 For easier readout of physical quantities from the figures the data for certain pipe diameters are provided in tabular format in appendix B The highest dose rates are for pipes with big water radius and thin wall similar to biological dose rates However, unlike biological dose rates, the lowest contribution to the total dose rate at same specific activity for all activated water isotopes is due to neutrons from 17N decay while the highest contribution is due to gamma rays from 16N decay 4.2 Dose field around heat exchanger Important components in cooling loops in nuclear facilities are hear exchangers The decay of activated cooling water presents one of the main radiation sources around heat exchangers as they are normally positioned away from primary radiation source Steam generator is the heat exchanger in nuclear power plants and is located together with the primary coolant pump in an area separated from reactor core For efficiency of heat exchange the water is majority of flowing time in the heat exchanger In the case of steam generators in PWR the water is more than 70% of circulation time in steam generators Thus the majority of the radioactive isotopes of water decay in the heat exchangers As the primary cooling pumps can be located next to the heat exchangers the decay causes increasing doses to workers performing emergency repairs on the pumps during operation Fig 19 MCNP source (marked with red dots) used to obtain dose field due to decay of activated cooling water energies the sources were made separately for each studied isotope 4.2.1 Steam generator model A detailed geometrical computational model of the steam generator and the radiation source was constructed in MCNP The computational model of the steam generator was based on the steam generators in a two loop PWR and the resulting geometrical model is presented in Fig 18 Material used in the model are low alloy steel SA 508 Cl 3a, stainless steel SS 304, Inconel 690 TT, borated water for the primary side of the steam generator with 1400 ppm boron concentration and pure water for secondary part of the steam generator The model of the steam generator was surrounded by air and concrete structure similar to structures in the power plant By the construction of the computational model it was taken care that the mass of the model was preserved The final mass of the model deviated from the real mass by 1.38%, i.e 4.7 tons To construct the MCNP model of the steam generator the U-tubes (over 5000 U-tubes) have been defined using universes and repeated structures in hexagonal lattices MCNP has several limitations for definition of particle source especially if the source description depends on defined cells in universes and lattices To use the axial function along tubes a cylinder needs to be defined within the source definition To properly define a cylinder in the source term two parameters are needed Due to this it is not possible to define an axial function for each U-tube as there are too many U-tubes If a larger cylinder would be used to encompass all U-tubes the number of cell for the primary water inside U-tubes needs to be given In such a case MCNP distributes points inside such cylinder for source locations thus creating point source Due to this limitations the particle source was modelled as a set of discs In the area of U-tubes the centres of the discs were placed in the middle of the tube with the radius of the tube In the height the source discs were placed in layers with 50 cm distance between each layer At the bottom of the steam generator the source was defined as layers of discs on a cm×2 cm grid with 30 cm distance between layers The source location in the steam generator is presented in Fig 19 The probability for selection of a disc as the source was defined using the exponential decay of the radioactive isotopes of activated cooling water As each radioactive isotope has its own decay time and decay products with different 4.2.2 Dose field results As the model of steam generator was taken from a GW thermal power PWR the results of dose fields were normalized to activity of activated water in such PWR which were presented earlier in the paper Fig 20 The gamma dose field distribution outside the steam generator due to decay of isotope 16N The arrow presents the direction of primary cooling water flow 10 Progress in Nuclear Energy 117 (2019) 103042 A Žohar and L Snoj Fig 21 Results of prompt gamma and neutron due to decay of isotope Time the cooling water is in the steam generator was estimated to be around s Calculated gamma dose field due to decay of 16N is presented in Fig 20 The highest dose rates are at the bottom of the steam generator, where the hot primary water enters the steam generator The values are on the order of 10 mSv/h At the extent of the U-tubes in the steam generator the values for gamma dose rates are on the order of a few mSv/h (up to mSv/h) and at the top part of the steam generator the gamma dose rates are below mSv/h At the bottom and at the length of the U-tubes the asymmetry of the gamma dose field is visible due to radioactive decay during flow through the steam generator On the side of the steam generator, where the water is flowing up, the gamma dose rates are around mSv/h, while on the side the water is flowing down the gamma dose rates are around mSv/h Neutrons emitted in the 17N decay can penetrate the steam generator and cause radiation in the air The neutron dose field is presented in Fig 21a The intensity of neutron dose field is an order of magnitude higher than prompt gamma dose field due to 17N decay The highest neutron dose rates are at the bottom of the steam generator, where the primary cooling water enters the steam generator, while at the length of U-tubes the dose rates are lower due to neutron capture in steam generator components At decay of 17N neutrons are emitted and some of them activate components in the steam generator thus inducing prompt gamma ray emission The gamma dose field due to this prompt gamma rays is presented in Fig 21b The intensity of the gamma dose field from 17N decay is on the order of μSv/h which is three orders of magnitude lower than gamma dose field due to 16N decay At the bottom and at the height of U-tubes the dose rates are the same order of magnitude This is due to the absorption of neutrons from 17N decay It is more likely the neutrons are going to activate isotopes of metals that compose the steam generator than activate isotope 18O in water Due to the design of the steam generator, the neutrons are going to activate more atoms at the length of U-tubes and less at the bottom of the steam generator despite higher activity in the bottom part From the analysis of prompt 17 N and gamma dose field due to decay of isotope 19 O Fig 22 Calculated spectrum of prompt gamma rays due to decay of 17N in the air surrounding the steam generator The area under spectrum was normalized to gamma spectrum outside the steam generator the lines from deuterium, isotopes of iron, nickel and niobium are visible as presented in Fig 22 The last studied isotope of activated cooling water is 19O The gamma dose field is presented in Fig 21c Compared to the gamma dose field due to 16N (Fig 20) the intensity of the field is several order of magnitude lower The gamma dose field at the bottom of the steam generator and at the length of the U-tubes is in order of several μSv/h The total dose field due to activated cooling water is order of mSv/ h The majority contribution to the total dose field is due to decay of Table The contributions of each studied isotope of activated cooling water to total dose field at the length of U-tubes Activated isotope Percentage of total activity [%] Percentage of total dose rate [%] 16 98.63 0.01 1.36 99.981 0.008 0.011 N N 19 O 17 11 Progress in Nuclear Energy 117 (2019) 103042 A Žohar and L Snoj isotope 16N while decay of 17N and 19O contribute less than 1% to the total dose field The contributions of each studied isotope of activated cooling water to total dose field are presented in Table enough to absorb neutrons but at the same time thin enough for prompt gamma rays to penetrate the wall and cause radiation in the air surrounding pipes Heat exchangers are one of the bigger components in a cooling loop and the cooling water is majority of circulation time in them As they are normally shielded form radiation of the source, decay of activated cooling water can present the main source of radiation In the case of steam generator in a GW thermal power PWR, the decay of activated cooling water causes biological dose rates in order of several mSv/h in the air surrounding the steam generator while in a GW thermal power fusion power plant the biological dose rates can be on the order of 100 Sv/h due to five orders of magnitude higher activity of activated water isotopes, especially higher activity of isotope 16N, indicating that using water as coolant in fusion reactors might not be the best choice from radiation point of view In both type of reactors the majority contribution to the dose rate is from decay of isotope 16N while the decay of isotopes 17N and 19O contribute combined less than 0.1% In fusion reactor higher activity of cooling water will cause not only higher biological radiation and activation of structural and electrical components but also additional nuclear heating to important component like superconducting coil windings, which can significantly affect the cooling power needed to cool the superconducting coils at K Due to this decay of activated cooling water needs to be taken into account when designing shielding and cooling systems in fusion reactors Conclusion Activated cooling water in nuclear facilities can present important source of radiation next to the radiation source in the nuclear facility itself thus causing radiation damage to electrical components and increasing doses to personnel working near primary cooling system As measurements of activity and dose fields surrounding the cooling systems are difficult if not impossible a methodology for calculating the results using Monte Carlo method was presented in the paper A study of results obtained with the use of the four most commonly used nuclear data libraries was presented Several differences between data libraries were observed, especially for activation of 17O and 18O, while the crosssection for activation of 16O is same in all studied libraries Specific activity for all three studied isotopes of cooling water were calculated for model of PWR reactor and fusion reactor From the results it was observed that the specific activity of water in fusion reactor is higher at the same thermal power The value for fusion reactors was calculated to be in the order of 1013 Bq/m3/MW for isotope 16N, which is five orders of magnitude higher compared to fission reactor In the air surrounding cooling pipes the biological and electronic dose rates are in the same order of magnitude for all isotopes except for neutron dose rates due to decay of 17N Neutron dose rates are lower for electronics compared to biological dose rates for several orders of magnitude For prompt gamma rays produced at absorption of neutrons the highest dose rates are for pipes with water radius around 10 cm and pipe thickness around cm For this parameters the pipe wall is thick Acknowledge The authors acknowledge the financial support from the Slovenian Research Agency (research core funding No.P2-0073) Appendix Pre-calculated factors for biological dose rates Table Tabulated pre-calculated factors for biological dose rates due to gamma rays from decay of Water radius [cm] Pipe thickness [cm] 1.067 0.478 1.334 1.670 2.108 2.413 3.017 3.652 4.445 5.715 0.556 0.635 0.635 0.714 0.874 0.935 1.113 1.349 Pre-calculated factor 1.06 10 18 5.21 10 18 1.95 10 17 8.43 10 17 2.33 10 18 1.24 10 17 4.25 10 17 1.65 10 16 3.93 10 16 Sv m6 h Bq s (1 ± 0.02%) 16 N at 50 cm distance from pipe surface presented in Fig 15 Water radius [cm] Pipe thickness [cm] 7.065 1.588 8.414 (1 ± 0.02%) (1 ± 0.02%) 10.954 1.067 0.478 1.334 1.670 2.108 2.413 3.017 0.556 0.635 0.635 0.714 0.874 3.652 0.935 5.715 1.349 4.445 1.113 2.69 10 20 4.09 10 20 2.94 10 19 3.64 10 18 4.31 10 17 1.36 10 20 1.30 10 19 1.22 10 18 1.15 10 17 3.332 34.950 (1 ± 0.02%) (1 ± 0.02%) 5.6 36.850 5.9 39.350 (1 ± 0.02%) (1 ± 0.02%) Pre-calculated factor 2.885 16.193 Tabulated pre-calculated factors for biological dose rates due to prompt gamma rays from decay of Pipe thickness [cm] 2.301 13.653 (1 ± 0.02%) (1 ± 0.02%) Table Water radius [cm] 1.826 Sv m6 h Bq s (1 ± 0.19%) 7.065 1.588 13.653 (1 ± 0.17%) (1 ± 0.15%) 16.193 34.950 (1 ± 0.13%) (1 ± 0.10%) 36.850 39.350 (1 ± 0.08%) (1 ± 0.06%) 12 16 3.29 10 15 1.02 10 14 8.40 10 13 1.41 10 6.26 10 15 15 8.29 10 14 8.50 10 13 (1 ± 0.03%) (1 ± 0.03%) (1 ± 0.03%) (1 ± 0.04%) (1 ± 0.04%) (1 ± 0.07%) (1 ± 0.07%) (1 ± 0.07%) N at 50 cm distance from pipe surface presented in Fig 15 Pipe thickness [cm] 10.954 8.01 10 Sv m6 h Bq s 17 Water radius [cm] 8.414 (1 ± 0.17%) (1 ± 0.17%) 6.3 Pre-calculated factor 1.826 2.301 2.885 3.332 5.6 5.9 6.3 Pre-calculated factor 1.16 10 16 6.50 10 16 2.14 10 15 2.40 10 16 1.32 10 15 1.21 10 14 1.30 10 14 1.26 10 14 Sv m6 h Bq s (1 ± 0.06%) (1 ± 0.05%) (1 ± 0.05%) (1 ± 0.05%) (1 ± 0.06%) (1 ± 0.09%) (1 ± 0.10%) (1 ± 0.11%) Progress in Nuclear Energy 117 (2019) 103042 A Žohar and L Snoj Table Tabulated pre-calculated factors for biological dose rates due to neutrons from decay of Water radius [cm] Pipe thickness [cm] 1.067 0.478 1.334 1.670 2.108 2.413 3.017 3.652 4.445 5.715 0.556 0.635 0.635 0.714 0.874 0.935 1.113 1.349 Pre-calculated factor 1.97 10 17 9.67 10 17 4.43 10 17 2.17 10 16 7.06 10 16 2.45 10 15 3.36 10 16 1.32 10 15 5.36 10 15 Sv m6 h Bq s (1 ± 0.01%) 17 N at 50 cm distance from pipe surface presented in Fig 15 Water radius [cm] Pipe thickness [cm] 7.065 1.588 8.414 (1 ± 0.01%) (1 ± 0.01%) 10.954 1.067 0.478 1.334 1.670 2.108 2.413 3.017 3.652 4.445 5.715 0.556 0.635 0.635 0.714 0.874 0.935 1.113 1.349 1.45 10 19 6.93 10 19 2.52 10 18 1.03 10 17 4.32 10 17 3.22 10 19 1.63 10 18 5.30 10 18 1.92 10 17 3.332 34.950 (1 ± 0.02%) (1 ± 0.02%) 5.6 36.850 5.9 39.350 (1 ± 0.02%) (1 ± 0.02%) Pre-calculated factor 2.885 16.193 Tabulated pre-calculated factors for biological dose rates due to gamma rays from decay of Pipe thickness [cm] 2.301 13.653 (1 ± 0.01%) (1 ± 0.01%) Table Water radius [cm] 1.826 Sv m6 h Bq s (1 ± 0.02%) 6.3 19 Water radius [cm] Pipe thickness [cm] 7.065 1.588 10.954 13.653 (1 ± 0.03%) (1 ± 0.03%) 16.193 34.950 (1 ± 0.05%) (1 ± 0.05%) 36.850 39.350 (1 ± 0.05%) (1 ± 0.06%) 1.01 10 14 3.48 10 14 9.71 10 14 2.27 10 13 1.66 10 14 6.24 10 14 2.04 10 13 2.31 10 13 Sv m6 h Bq s (1 ± 0.02%) (1 ± 0.03%) (1 ± 0.03%) (1 ± 0.04%) (1 ± 0.04%) (1 ± 0.13%) (1 ± 0.14%) (1 ± 0.15%) O at 50 cm distance from pipe surface presented in Fig 15 8.414 (1 ± 0.02%) (1 ± 0.03%) Pre-calculated factor 1.826 2.301 2.885 3.332 5.6 5.9 6.3 Pre-calculated factor 8.32 10 17 1.39 10 16 4.91 10 16 3.33 10 15 3.30 10 15 2.90 10 16 7.24 10 16 3.33 10 15 Sv m6 h Bq s (1 ± 0.07%) (1 ± 0.07%) (1 ± 0.08%) (1 ± 0.09%) (1 ± 0.10%) (1 ± 0.27%) (1 ± 0.27%) (1 ± 0.27%) Appendix Pre-calculated factors for dose rates in silicon Table Tabulated pre-calculated factors for dose rates in silicon components due to gamma rays from decay of 16N at 50 cm distance from pipe surface presented in Fig 17 Water radius [cm] Pipe thickness [cm] 1.067 0.478 1.334 1.670 2.108 2.413 3.017 3.652 4.445 5.715 0.556 0.635 0.635 0.714 0.874 0.935 1.113 1.349 Pre-calculated factor 1.88 10 22 9.65 10 22 3.75 10 21 1.68 10 20 8.02 10 20 4.22 10 22 2.34 10 21 8.33 10 21 3.33 10 20 Sv m6 h Bq s (1 ± 0.02%) (1 ± 0.02%) Water radius [cm] Pipe thickness [cm] 7.065 1.588 8.414 10.954 (1 ± 0.02%) (1 ± 0.02%) 2.885 34.950 5.6 36.850 (1 ± 0.02%) (1 ± 0.02%) 2.301 13.653 16.193 (1 ± 0.02%) (1 ± 0.02%) 1.826 39.350 3.332 5.9 6.3 (1 ± 0.02%) Pre-calculated factor 1.66 10 19 6.94 10 19 2.21 10 18 2.39 10 17 2.98 10 19 1.34 10 18 2.17 10 17 2.70 10 17 Sv m6 h Bq s (1 ± 0.03%) (1 ± 0.03%) (1 ± 0.03%) (1 ± 0.04%) (1 ± 0.04%) (1 ± 0.06%) (1 ± 0.07%) (1 ± 0.07%) Table 10 Tabulated pre-calculated factors for dose rates in silicon components due to prompt gamma rays from decay of 17N at 50 cm distance from pipe surface presented in Fig 17 Water radius [cm] Pipe thickness [cm] 1.067 0.478 1.334 1.670 2.108 2.413 3.017 3.652 4.445 5.715 0.556 0.635 0.635 0.714 0.874 0.935 1.113 1.349 Pre-calculated factor 6.11 10 25 6.55 10 24 5.51 10 23 7.48 10 22 9.23 10 21 1.98 10 24 2.28 10 23 2.43 10 22 2.42 10 21 Sv m6 h Bq s (1 ± 0.23%) Water radius [cm] Pipe thickness [cm] 7.065 1.588 8.414 (1 ± 0.21%) (1 ± 0.21%) 10.954 13.653 (1 ± 0.19%) (1 ± 0.16%) 16.193 34.950 (1 ± 0.13%) (1 ± 0.10%) 36.850 39.350 (1 ± 0.08%) (1 ± 0.07%) 13 1.826 2.301 2.885 3.332 5.6 5.9 6.3 Pre-calculated factor 2.51 10 20 1.44 10 19 4.89 10 19 3.25 10 18 5.27 10 20 2.96 10 19 2.99 10 18 3.50 10 18 Sv m6 h Bq s (1 ± 0.06%) (1 ± 0.06%) (1 ± 0.05%) (1 ± 0.06%) (1 ± 0.06%) (1 ± 0.09%) (1 ± 0.09%) (1 ± 0.10%) Progress in Nuclear Energy 117 (2019) 103042 A Žohar and L Snoj Table 11 Tabulated pre-calculated factors for dose rates in silicon components due to neutrons from decay of Water radius [cm] Pipe thickness [cm] 1.067 0.478 1.334 1.670 2.108 2.413 3.017 3.652 4.445 5.715 0.556 0.635 0.635 0.714 0.874 0.935 1.113 1.349 Pre-calculated factor 2.26 10 24 1.17 10 23 4.28 10 23 1.75 10 22 7.35 10 22 5.23 10 24 2.71 10 23 7.46 10 23 3.30 10 22 Sv m6 h Bq s (1 ± 0.01%) 16 Water radius [cm] Pipe thickness [cm] 7.065 1.588 8.414 (1 ± 0.01%) (1 ± 0.01%) 2.301 16.193 3.332 34.950 (1 ± 0.02%) (1 ± 0.02%) 36.850 39.350 (1 ± 0.02%) (1 ± 0.02%) 1.826 10.954 13.653 (1 ± 0.02%) (1 ± 0.02%) N at 50 cm distance from pipe surface presented in Fig 17 2.885 5.6 5.9 6.3 Pre-calculated factor 1.40 10 21 5.00 10 21 1.44 10 20 4.09 10 20 2.34 10 21 9.12 10 21 3.60 10 20 4.38 10 20 Sv m6 h Bq s (1 ± 0.03%) (1 ± 0.03%) (1 ± 0.03%) (1 ± 0.04%) (1 ± 0.04%) (1 ± 0.12%) (1 ± 0.13%) (1 ± 0.14%) Table 12 Tabulated pre-calculated factors for dose rates in silicon components due 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fission spectra are due to elastic scattering of neutrons on 16O As