Development and Application of BwrCrud to Long Term Evaluation of Activity Transport in BWRs

Transcription

1 Development and Application of BwrCrud to Long Term Evaluation of Activity Transport in BWRs K. Kobayashi, K. Lundgren 2, C. Bergström 2, K. Iida, Y. Iwasaki, K. Chikamoto 3, M. Nishi 3 Tokyo Electric Power Environmental Engineering Co., Inc, Tokyo,Japan 2 ALARA Engineering AB, Skultuna, Sweden 3 Japan NUS Co., Ltd, Tokyo, Japan INTRODUCTION Increase of radiation level in BWR primary system is caused by build-up of activated corrosion products on the primary system piping surface through the following main processes: corrosion products carried into the reactor by the feedwater deposit on the fuel surface and form fuel crud, the corrosion products deposited on the fuel surface are activated by neutroni and a small fraction is released to the reactor water again, the activated corrosion products are transported to the piping surface and subsequently deposit on the piping surface. Several measures have been applied to reduce the activity build-up such as reduction/removal of crud and control of Co-60 source which is a dominant contributor to the dose, with the materials of low Co content. Owing to these applications, the radiation level has been successfully lowered in recent years. On the other hand, the water chemistry conditions in BWRs in recent years show wide variety due to applications of new water chemistry control methods such as HWC operation, Zn injection and ultra low Fe operation and the application of chemical decontamination. These applications cause the corrosion/activated corrosion products behavior become more complicated than before. Under these circumstances, an improved model is required to achieve the control/reduction of activity build-up on BWR primary system. The improved model should be able to reflect the impact of the new water chemistry control methods on the corrosion/activated corrosion products behavior based on recent BWR operation experience and new findings. A new code, BwrCrud, has been developed to meet such a requirement. The models of the BwrCrud code have been developed based on a comprehensive review of measured data of many Japanese and European BWRs under a wide range of water chemistry condition. The development has been performed in cooperation with ALARA Engineering and needed two years to completion. BwrCrud allows user to evaluate the long-term impact on the reactor water activity concentration and the activity build-up on the primary system surface under HWC operation, Zn injection and so on. CONCEPT OF BWRCRUD CODE The BwrCrud code has been developed based on the comprehensive review of the measured data and considering new findings related to the effect of HWC, Zn injection, ultra low iron operation, tramp uranium on the core, and chemical decontamination 2-6. Furthermore, a fine tuning of the model parameters for transport processes has been performed by comparing the calculated values with measured water chemistry data of totally 8 Japanese and European BWRs. Design features and water chemistry condition (e.g. HWC and Zn injection) of the plants contained in the database are summarized in Table. A mass and activity flow scheme for the BwrCrud code is shown in Figure. Each box in the figure corresponds to the sub-model included in the BwrCrud code and the arrows show the transport processes of corrosion/activated corrosion products. The corrosion/activated corrosion products considered in the current version of BwrCrud are: Corrosion products - Fe, Ni, Co, Zn, Cr, Cu and Mn Activated corrosion products -Fe-59, Co-58, Co-60, Zn-65, Cr-5 and Mn-54 Activation of corrosion products is considered for fuel crud and in-core materials (e.g. Zircaloy surface of the fuel cladding, fuel spacer, control rod). For the in-core materials, BwrCrud accounts for their replacement history. Most of the transport processes take place in the reactor water and the reactor water condition is characterized by NWC/HWC and conductivity. Inflow of the corrosion products is partly by the feedwater and partly by the dissolution release due to the corrosion of the in-core and out-of-core reactor materials. The corrosion/activated corrosion products in the reactor water are transported and deposit on the primary system surface, while some part of those are removed by the RWCU system and carried

2 over by the main steam system. The primary system surface can be divide into several different areas of different materials and with/without influence of HWC. Table. Design features and water chemistry condition of the plants contained in the database Type Plant Fe Zn 2 HWC 3 A L H N A2 L M/L Y A3 L L Y A4 L L Y External recirculation loop plants Jet pump plants Internal pump plants A5 H/L L Y B H M/L Y B2 H M/L Y B3 M/L L Y B4 M/L L N B5 M/L L N B6 H/M L/H N C L L/M Y C2 L L/M Y C3 M/L L N C4 M/L L N C5 H/L L N C6 L L N C7 L L N. L : Feedwater Fe < 0.5ppb, M: Feedwater Fe 0.5ppb - ppb, H: Feedwater Fe > ppb 2. L: Reactor water Zn <0.5ppb, M: Reactor water Zn 0.5ppb - 2 ppb, H: Reactor water Zn >2ppb 3. Y: HWC, N: NWC Neutron irradiation Fuel exchange Steam lines Ni, Co, Zn Co, Zn, Ni Co, Zn, Ni Fuel crud In-core materials κ Reactor Water Zircaly Spacer Control rods Core structure HWC/NWC Feed water Fe Amorphous Fe Hematite Spinel RWCU System Ni, Cr Monoxide Corrosion products transport Activated corrosion products transport Figure. Simplified main flow scheme Figure 2. Schematic model for the transformation process of three different iron oxide form In BwrCrud, the corrosion/activated corrosion products behavior is described by an transformation process of three different iron oxide form - amorphous iron (e.g. FeOOH), hematite(fe 2 O 3 ), and ferrite (MeFe 2 O 4 ) - both on the fuel crud surface and on the primary system surface. The process that spinels are formed through the interaction of the amorphous iron and the hematite with divalent metal ions is schematically illustrated in Figure 2. Most iron inflow into the reactor is supposed to 2

3 be the amorphous iron form. The divalent metal ions (Ni, Co and Zn) are adsorbed both on the amorphous iron and on the hematite particles. The amorphous iron is supposed to be rapidly transformed to the hematite and subsequently the hematite reacts with the metal ions adsorbed on the particles and transformed to stable spinel form. Monoxide form is considered for Ni and Cr in addition to the iron oxides. The BwrCrud code has been developed utilizing recent computer technology. The code is programmed in C language and run from the console environment on Windows 95/NT. MODELS The models for fuel crud and activity build-up on the primary system surface have been developed based on the comprehensive review of the plant measured data, available laboratory data and investigation of recently proposed mechanisms. The models are described and discussed below. Fuel crud The model for fuel crud performance is schematically illustrated in Figure 3. The model includes the following areas of interest.. The reactor water with calculated concentrations of particles Reactor water and soluble species. The particles are either in amorphous iron, hematite, or spinel form 2. A fuel assembly with heated Zircaloy surface. The fuel assembly is characterized by a given power level and heated surface area. The given power level means a certain neutron activation rate of the fuel crud. 3. An inner layer of spinel form (i.e. MeFe 2 O 4, where Me = Steam Ni, Zn, Co) 4. An outer layer of hematite particles (i.e. Fe 2 O 3 ) and amorphous iron (e.g. FeOOH). The amorphous iron is Particles rather rapidly transformed to hematite. The amorphous iron is more effective in adsorbing in Ni, while the hematite is more effective in adsorbing Co and Zn. 5. An adsorbed layer of metals in form of surface complex. This adsorbed layer is formed on both the hematite and the amorphous iron particles. 6. A layer of precipitated monoxide (e.g. CrO 2 ) closed to the Zircaloy surface. 7. Trump uranium(tu) embedded in the fuel crud inner layer. Soluble species Spinel crystal Hematite particles 5a 3b 3 Amorphous ironc 3a Figure 3. Schematic model for fuel crud 2 TU 5c 7b 7a 7c 4 9a/9b 6 8 Monoxides Zircaloy 0 Ten transport processes between these areas are shown in Figure.3. The characteristics for these transport process are:. The boiling means in practice a transport of water to the fuel surface in parallel with the turbulence. 2. The deposition of particles is modeled as a process of mass transfer and adsorption in series. 3. The release of particles is modeled as a first order reaction with different release constants for hematite, amorphous iron, and spinels. 4. The amount of TU contributes to the release rate of particles through knockout processes. 5. The deposition of soluble species on hematite and amorphous iron is modeled in a way considering available sites in the iron oxides and different propertied for different soluble species(e.g. Co and Ni). 6. The transformation of soluble species adsorbed on the hematite to spinel form is modeled as a solid phase reaction. 7. The release of different species from the different oxide forms is modeled as first order reactions with an influence of the reactor water conductivity on the release rate. 8. The transformation of amorphous iron to hematite is modeled as a first order reaction. 3

4 9. The precipitation and release of species as monoxides are modeled as first order reactions with specie-dependent constants. 0. The neutron activation of corrosion products. Activity build-up on primary system surface The model for Activity build-up on primary system surface is schematically illustrated in Figure 4. The basic concept of the model is similar to the fuel crud model. Seven different transport processes are shown in Figure 4. The main characteristics for these process are:. A corrosion process according to Ref.7 is applied in the model. Corrosion is described as both a metal release and a formation of oxide film through operation time dependent equations. NWC and HWC conditions are treated separately. 2. The deposition of particles is modeled as a first order reaction. 3. The release of particles is modeled as a second order reaction with respect to the amount of deposed materials. 4. The deposition of soluble species on the hematite and the amorphous iron is modeled in a similar way as the corresponding deposition on fuel crud. Reactor water Amorphous iron Hematite Particles Oxide film Stainless steel Figure 4. Schematic model for activity build-up on primary system surface 5. The release of different species from different oxides forms is modeled in a similar way as the corresponding release from the fuel crud. 6. The transformation of ion species adsorbed on the hematite to spinel form is modeled as a solid phase reaction. 7. The transformation of amorphous iron to hematite is modeled as a first order reaction. CALCULATION RESULTS Validation (Comparison) calculation The reactor Kashiwazaki Kariwa 5(KK5) was selected as an example of BwrCrud calculation. KK 5 started operation in 990 and is under 8th cycle operation as of August, 999. The plant data including design data, operation records, and measured water chemistry data were used for the input of the calculation. Note that monthly-averaged data has been used as the water chemistry data. The calculated trends up to March, 999 of the reactor water activity concentrations of Co-60 and Co-58 and the activity build-up on the surface of recirculation piping are presented and compared to measured data in Figure 5 and Figure 6, respectively. The calculated reactor water activities show some deviation during the three first cycles, but show a good agreement in later cycles. The calculated Co-60 build-up on the surface of recirculation piping is somewhat lower than the measured data. 2 a b 5c 5a Soluble species 4d/5d 4a Adsorbed layer 4c Monoxide 6 4

5 3.0E0 2.5E0 2.0E0 Co-60.2E02.0E02 8.0E0 Co-58 [Bq/cc].5E0.0E0 [Bq/cc] 6.0E0 4.0E0 5.0E00 2.0E0 0.0E E Figure 5. Comparison of measured data with calculated concentrations of Co-60 and Co-58 in reactor water in KK5.20E00 Co E-0 Co-58.00E E-0 Dose rate [msv/hr] 8.00E E E E-0 Dose rate [msv/hr] 2.00E-0.50E-0.00E E E E Figure 6. Comparison of measured data with calculated Co-60 and Co-58 contact dose rates on recirculation piping in KK5 Prediction calculation Based on the results obtained from the validation calculation, prediction calculations have also been carried out to estimate the variation of the contact dose rate on recirculation piping during a 0 years operation period for KK5 under different water chemistry conditions. The prediction period is from August 999 to 2009 (8th -7th refueling outage). Figure7 shows the comparison of the predicted dose rates on recirculation piping for the cases with Zn injection resulting in 2ppb and 5ppb in the reactor water, respectively and the case without Zn injection acting as a reference. The predicted results show that the dose rate is reduced by Zn injection while is changed little by keeping the latest water chemistry condition of the plant. Figure 8 shows a comparison of predicted dose rates on recirculation piping for the reference case with keeping the latest water chemistry conditions of the plant, the case with full system decontamination during the 8th refueling outage, and the case with Zn injection after the full system decontamination. The predicted dose rate of the decontamination case shows increase by recontamination after the full system decontamination but the dose rate remains lower than that of the reference case. On the other hand, the predicted result of the case with Zn injection after the full system decontamination shows lower dose rate than that of the case without Zn injection by suppression of recontamination. The predicted results show totally reasonable behavior reflecting the latest knowledge. 5

6 Dose rate[msv/hr] Zn injection (5ppb) Zn injection (2ppb) Ref Figure 7. Prediction results of dose rate on the surface of recirculation piping - Bar chart focuses on the effect of Zn injection.6.4 Dose rate [msv/hr] Zn injection after full system deconatmination Full system decontamination Ref Figure 8. Prediction results of dose rate on the surface of recirculation piping - Bar chart focuses on the effect of chemical decontamination CONCLUSION AND FURTHER WORK The BwrCrud code which can take account of the impact of the new water chemistry control methods such as ultra low Fe operation, HWC operation, and Zn injection has been developed. The models of BwrCrud have been founded on the basis of the comprehensive review of measured BWR data under wide range of water chemistry condition and the recently proposed mechanisms. The validation calculation results for KK5 show reasonably good agreement with measured data. Some prediction calculations for 6 TEPCO plants including KK-5 have been carried out. These extensive calculations show that the code works in the expected way and the calculation results show the reasonable trends specific to different water chemistry conditions while further improvement is needed to obtain better results. Particularly, the actual 6

7 corrosion product behavior in BWRs operated with HWC and Zn injection is complicated and hence it is necessary to continue the effort to improve the model based on new findings. We will perform further studies on the effects of ph, of Zn injection and of in-core corrosion environment under HWC and the results of those studies will be used to further improve t the BwrCrud model. REFERENCES. K. Lundgren, C. Bergström, K. Haraguchi, H., Nishimura, K, Kobayashi, K. Chikamoto and G. Granath, BwrCrud Development and Validation of a New Code for Improved Simulation of Activity Transport in BWR Primary Systems. Proc. 998 JAIF Int Conf on Water Chemistry in Nuclear Power Plants, Kashiwazaki City (998). 2. T. Kelén and H. P. Hermansson, The KEMOX-2000 Project: Minimizing Radiation Doses by Optimizing Oxide Conditions. Proc. Water Chemistry of Nuclear Power Systems 7. BNES, Bournemonth (996). 3. M. Ullberg and M. Tanse Larsson, Iron to Nickel Ratio of BWR Fuel Crud Effects and Interpretation. Proc. Water Chemistry of Nuclear Power Systems 7. BNES, Bournemonth (996). 4. K. Yamazaki, A New Water Chemistry Control Method for Radiation Reduction of BWR. 997 Int. ALARA Symp, Orland, Florida (997). 5. Y. Nishino, T. Sawa, K. Ohsumi and H. Itoh, Reaction Rates of Amorphous Iron Hydroxide with Nickel and Cobalt Ions in High Temperature Water. J. Nucl. Sci. Technol. 26(2), 2-29(989). 6. Y. Asakura, M. Kikuchi, S. Uchida and H. Yusa, Deposition of Nickel and Cobalt Ions on Heated Surface under Nucleate Boiling Condition. J. Nucl. Sci. Technol. 26(2), 2-20(989). 7. Y. Hemmi, Y. Uemura and N. Ichikawa, General Corrosion of Materials under Simulated BWR Primary Water Conditions. J. Nucl. Sci. Technol. 3(5), (994). 7