Chemistry and Materials Challenges in Generation IV Supercritical Water Reactors

1. UNRESTRICTED / ILLIMITÉ Chemistry and Materials Challenges in Generation IV Supercritical Water Reactors D. Guzonas Presented at the workshop of the Canadian National Committee, International Association for the Properties of Water And Steam 2009 May 11

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3. UNRESTRICTED / ILLIMITÉ 3 Temperature-pressure phase diagram of water

4. UNRESTRICTED / ILLIMITÉ 4 Dependence of Density on Temperature

5. UNRESTRICTED / ILLIMITÉ 5 The Generation IV International Forum In 2001, nine countries, including Canada, initiated the Generation IV International Forum (GIF) by signing a charter to collaboratively develop the next generation of nuclear energy systems Goal to develop next generation systems that can be licensed, constructed and operated in a manner that will provide a competitively priced and reliable supply of energy, while addressing nuclear safety, waste, proliferation and public perception concerns by the 2030 timeframe Canada and four other nations, signed the Framework Agreement for International Collaboration on Research and Development of Generation IV Nuclear Energy Systems on February 28, 2005 Today, eight countries plus Euratom are actively participating through the Framework Agreement

6. UNRESTRICTED / ILLIMITÉ 6 Challenges The GIF SCWR Materials and Chemistry Provisional Project Management Board (PPMB) has identified two major challenges that must be overcome to ensure the safe and reliable performance of an SCWR: Insufficient data are available for any single alloy to unequivocally ensure its performance in an SCWR, especially for alloys to be used for in-core components Current understanding of SCW chemistry is inadequate to specify a chemistry control strategy, as the result of the large changes in physical and chemical properties of water through the critical point, coupled with the as yet poorly understood effects of water radiolysis

7. UNRESTRICTED / ILLIMITÉ 7 SCWR Materials

8. UNRESTRICTED / ILLIMITÉ 8 Pressure Tube vs Pressure Vessel Pressure vessel and pressure tube designs share common issues with respect to materials for out-of-core components and fuel cladding strong synergies between materials R&D needs of the two designs

9. UNRESTRICTED / ILLIMITÉ 9 Alloys Studied under SCWR Conditions

10. UNRESTRICTED / ILLIMITÉ 10 Key Experimental Variables Key experimental variables identified to date: Temperature Water density (pressure) Dissolved oxygen concentration Water conductivity Concentrations of additives Surface finish

11. UNRESTRICTED / ILLIMITÉ 11 Mechanical Properties Also needed: Information on fracture toughness, tensile strength, creep resistance Understanding of irradiation-induced changes to cladding, structural materials due to: Growth Swelling He-bubble formation Dislocation microstructure Precipitate microstructure Irradiation-induced composition changes Focused modeling can improve understanding of materials-environment interactions within a shorter time frame key degradation processes (e.g., general corrosion, pitting, SCC initiation and growth, irradiation and thermal creep) being modeled using the latest computational techniques

12. UNRESTRICTED / ILLIMITÉ 12 Summary of Damage Types Relevant to the SCWR Candidate Material Classes

13. UNRESTRICTED / ILLIMITÉ 13 Surface Modification Ideal material for critical SCWR applications possesses: good resistance to corrosion at the surface good resistance to SCC, creep and radiation damage in the bulk No alloy has yet been identified that possesses all these attributes Potential solution ? modify the surface of a material possessing the required bulk properties to impart the desired corrosion resistance Coatings (metals, ceramics) Grain boundary engineering

14. UNRESTRICTED / ILLIMITÉ 14 SCWR Chemistry

15. UNRESTRICTED / ILLIMITÉ 15 Water Chemistry – Introduction Compared to the large body of work on materials testing, little work on SCWR water chemistry has yet been carried out Long-term goal is to specify a suitable water chemistry for the SCWR design Candidate water chemistry regimes and specifications for key chemistry parameters: pH dissolved oxygen and hydrogen concentrations concentrations of any other additives allowable concentrations of impurities must be identified prior to any long-term materials testing

16. UNRESTRICTED / ILLIMITÉ 16 Water Chemistry – Key Issues Four key issues identified: Radiolysis of SCW Understanding Corrosion Product Transport and Deposition Specification of Water Chemistry for Detailed Testing Identification of Methods for Chemistry Monitoring and Control

17. UNRESTRICTED / ILLIMITÉ 17 Radiolysis Radiolytic production of oxidizing species (e.g., ·OH, H2O2, O2, HO2·/O2-·) can increase corrosion of reactor components as well as affect corrosion product transport and deposition Current PWRs and PHWRs limit formation of oxidizing species by ensuring the presence of excess hydrogen at concentrations sufficient to chemically lower the net production of oxidizing species by radiolysis insufficient data exist to determine whether this strategy would be effective in an SCWR Coolant could be very oxidizing immediately downstream of the core Work is on-going to develop an improved understanding of SCW radiolysis through a combination of experiment and modeling

18. UNRESTRICTED / ILLIMITÉ 18 Corrosion Product Transport Release and transport of corrosion products from surfaces of system components a serious concern for all water-cooled nuclear power plants High levels of corrosion product transport can result in: increased deposition on fuel cladding surfaces, leading to reduced heat transfer and the possibility of fuel failures increased production of radioactive species by neutron activation, ultimately increasing out-of-core radiation fields and worker dose In addition, nuclear and thermal power stations experience deposition of copper and silica species (which are volatile in steam) on turbines at levels that can cause turbine failure Supercritical thermal stations experience suggests corrosion product deposition could be significant in an SCWR

19. UNRESTRICTED / ILLIMITÉ 19 Distribution of deposits in a fossil-fired SCW boiler

20. UNRESTRICTED / ILLIMITÉ 20 Water Chemistries Most experimental work on SCWR materials has been carried out using a limited range of water chemistries pure water pure water with added oxygen (50 - 8000 ppb) hydrogen water chemistry (H2 concentration ~ 30 cm3/kg water). Thinking ‘outside the box’ may be helpful in devising novel water chemistries (e.g., LiOH addition)

21. UNRESTRICTED / ILLIMITÉ 21 Corrosion Product Release in Different Water Chemistries

22. UNRESTRICTED / ILLIMITÉ 22 Chemistry Monitoring Relevant chemistry parameters (e.g., conductivity, pH, ECP, concentrations of dissolved H2 and O2) must be monitored and controlled in an SCWR and in in-reactor test loops Existing methods of chemistry monitoring are predominantly: ex-situ (cooled and de-pressurized) off-line (batch laboratory analysis of grab samples) These will be inadequate in an SCWR, as a result of the large changes in water chemistry around the critical point Reliable monitoring of key chemistry parameters will likely require development of in-situ or on-line probes need for more work on this topic

23. UNRESTRICTED / ILLIMITÉ 23 Summary Many unresolved issues remain wrt materials selection for an SCWR: significant progress made in acquiring data on materials properties needed to choose a short list of candidate alloys for longer term testing Data on materials properties currently available for about 90 alloys A number of out-reactor test facilities are now operating Some testing of irradiated materials has also been performed Round robin testing is planned and databases are under development will facilitate comparison of data from different laboratories and enable correlations to be developed (e.g., effect of Cr content of alloys) While the pace has not been as rapid, some progress in understanding water chemistry issues such as radiolysis and corrosion product transport in SCW has been made

24. UNRESTRICTED / ILLIMITÉ 24 Acknowledgements The author would like to acknowledge many valuable discussions with: P. Tremaine (U of Guelph), J.-P. Jay-Gerin (U. of Sherbrooke), W. Zheng (MTL) SCWR PPMB members and alternates, past and present, including L. Heikenheimo, H. Matsui, P. Arnoux, J. Jang, J. Kaneda, S. Kasahara, G. Was, S.S. Huang T. Allen (U. Wisconsin) and J. Kysela (Rez) NRCan and AECL for funding various parts of this work

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