Reduction of Radiation Exposure at Aged BWR Plants by Water Chemistry

1. Reduction of Radiation Exposure at Aged BWR Plants by Water Chemistry Nov.13 2008 Hidehiro Urata and Kenji Yamazaki Toshiba Corporation 1/23

2. Content • Background and Introduction • Water Chemistry Approach and Results • Conclusion

3. Roles of Water Chemistry at Operating BWRs Background and Introduction • Increase of radiation level due to ageing and up-rating of plant and higher burn-up fuel implementation is a concern regarding roles of water chemistry at operating nuclear power plants. • Main purposes of water chemistry • Radiation level reduction • Improvement of work environment • Reduction of radwaste • Improvement of safe and reliable operation • Structural material integrity • Fuel integrity

4. Radiation reduction Radiation dose reduction Radwaste reduction Optimum water chemistry Structural material integrity Fuel material integrity Secure and Reliable Operation Background and Introduction At aged BWR plants, radiation dose reduction for maintenance repair works, and mitigation of SCC are most important roles of water chemistry

5. Importance of radiation level reduction Radiation Exposure (Person Sv) Japanese BWR Oversea BWR （Good case) C: Possible effects of up-rating and higher burn-up From Y. Hayashida 2007 ISOE Asian Symposium, Seoul Korea Sep 12-14 2007 A: Reduction of works and Improvement of works B: Improvement of radiation environment A B Rationale Exposure Reduction by A+B Outage (days) Background and Introduction

6. Background and Introduction Mitigation of SCC Reduction of dose rate Objective Chemistry Remedy Hydrogen Water Chemistry Chemical Decontamination Recontamination Dose rate increase Influence on Dose Rate Dose rate Dose rate Water chemistry modification Decon. Operation Operation Increase of Co-60 Conc. Modification of oxide Factors Factors affecting radiation level buildup at aged BWR plants • Water chemistry for control of recontamination under low ECP

7. Background and Introduction Low DF Medium DF High DF (arbitrary unit) Re-contamination Rate DF ( Decontamination Factor) Dose before decon/after decon) Re-contamination after chemical decontamination depends on “ Decontamination Factor”. Medium Decontamination factor is preferable but not always obtained. Mechanism of the dependence should be evaluated.

8. Water Chemistry Approach and Result • Issues to be evaluated (1) Dependence of Recontamination on Decon. Factor (2) Water Chemistry for suppression of radiation level build up after water chemistry modification and chemical decontamination 2. Approach • Experimental approach by using BWR simulation loop • Parameters • Low ECP (ex. HWC) • TiO2, Zn (countermeasures)

9. Water Chemistry Approach and Results Reduction (Oxalic acid) Specimen preparation Pre-filming Oxidation (ozone) Chemical decontamination Reduction (Oxalic acid) Repetition (TiO2 treatment) HCl Co-60 deposition test Oxalic acid：2000ppm，95C Ozone：3ppm,80C Hydrochloric acid ：2mol/l,80C Measurement/Analysis Experimental (1) Procedure

10. Water Chemistry Approach and Results SUS316L 0.3mmt 10mm 40mm Experimental (2) Co-60 deposition test loop

11. Water Chemistry Approach and Results Results (1) Dependence on decon. factor 2.0 HWC FW DH>1.0ppm HWC 1.5 m-HWC FW DH<0.5ppm On Stainless steel specimen 1.0 m-HWC Co-60 0.5 W/O Zn W/O TiO2 0.0 Low DF Medium DF High DF • The V-shape dependency was reproduced. • Dependency is much strong under HWC.

12. Water Chemistry Approach and Results Results (1) Dependence on decon. factor

13. Water Chemistry Approach and Results ：60Co ：Fe2O3 ：NiFe2O4 ：Cr2O3 Results (1) Dependence on decon. factor Chemical decontamination with adequate control is important to suppress re-contamination.

14. Water Chemistry Approach and Results 2.0 HWC- W/O Zn HWC- With Zn 1.5 m-HWC-W/O Zn On stainless steel specimen) m-HWC- With Zn 1.0 Co-60 0.5 0.0 Low DF Medium DF High DF Results (2) Control by water chemistry - Zn - • Zinc is effective for suppression of recontamination under both HWC and m-HWC, even after inadequate decon.

15. Water Chemistry Approach and Results Results (2) Control by water chemistry - Zn -

16. Water Chemistry Approach and Results 2.0 Ref No treatment TiO2 deposition TiO2 deposition and Zn ＊ 1.5 On Stainless steel specimen 1.0 0.5 Co-60 0.0 Low DF Medium DF High DF Results (3) Control by water chemistry - TiO2 - • TiO2 can suppress Co-60 deposition to very low level and has synergy effect with Zn.

17. Water Chemistry Approach and Results Results (3) Control by water chemistry - TiO2 -

18. Water Chemistry Approach and Results ：60Co ：Fe2O3 ：NiFe2O4 ：ＺnCr2O4 ：Cr2O3 ：TiO2 Results (3) Control by water chemistry - TiO2 -

19. Water Chemistry Approach and Results TiO2 (Cathodic reaction) 2O2 + 8H+ +8e--> 4H2O O2 + e- -> O2- e- Heat Excitation (Anodic reaction) Fe ->Fe2+ + 2e- 3Fe2+ + 4H2O ->Fe3O4 + 8H+ + 8e- H2O + h+-> OH・+ H+ OH- + h+ -> OH・ h+ SS SS Possible explanation on role of TiO2 • Before ferrite (CoFe2O4) formation, Fe2+ can be oxidized to Fe3+ and Fe2O3 is formed. • Further discussion is needed to establish the mechanism for Co-60 deposition prevention.

20. TiO2 for SCC Mitigation TiO2 technology for BWR SCC mitigation is developed by Toshiba and TEPCO. ECP of stainless steel is decreased by TiO2 with photon, or without photon under low-HWC chemistry. Reduction of Oxygen Corrosion Potential (ECP) Oxidation of Steel Reduction of ECP Phto-electric Current Current K. Takamori, International Conference on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, Aug.14-18 2005 Snowbird Utah

21. Conclusion - 1 • Effects of Decontamination factor on re-contamination • The V-shape relationship between Co-60 deposition and DF could be clearly seen, especially in case of HWC condition. • In case of low DF, poorly protective residual oxides accelerate the re-contamination. • Under HWC condition, recontamination with inadequate DF is accelerated and thus control of decontamination is particularly important.

22. Conclusion - 2 • Water chemistry control • Zn injection can effectively suppress Co-60 deposition regardless of DF. • TiO2 can effectively suppress Co-60 deposition regardless of DF. • TiO2 technology is a promising candidate for simultaneous accomplishment of SCC mitigation and radiation exposure reduction. • Synergy effect of TiO2 and Zn can be seen.