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March 24, 1999 Dr. William D. Travers Dear Dr. Travers: SUBJECT: HIGH BURNUP FUEL PHENOMENA IDENTIFICATION AND RANKING During the 460th meeting of the Advisory Committee on Reactor Safeguards, March 10-13, 1999, we reviewed the status of the NRC confirmatory research program on high burnup fuel. During this review, we had the benefit of discussions with representatives of the Offices of Nuclear Regulatory Research (RES) and Nuclear Reactor Regulation (NRR), the industry, and of the documents referenced. CONCLUSIONS AND RECOMMENDATIONS
DISCUSSION In our report dated June 15, 1998, we discussed the NRC research to confirm the regulatory decision to limit the extent of fuel burnup. In that report, we suggested that the staff develop an understanding of what data and analyses would be required of licensees to support applications for extending fuel burnup beyond the current limit of 62 GWd/t. Development of such an understanding is a challenge since data on high burnup fuel behavior under accident conditions are sparse and scattered. The RES staff will undertake an expert opinion elicitation to identify
the physical and chemical phenomena that will affect fuel behavior, establish
the state-of-knowledge concerning these phenomena, and rank them in terms
of their importance to safety. The phenomena identification and ranking
elicitation is to be done for accident scenarios found by RES to be risk
important. These are loss-of-coolant accidents for both pressurized water
reactors (PWRs) and boiling water reactors (BWRs), control rod ejection
accidents for PWRs, and anticipated transients without scram (ATWS) for
BWRs. RES plans to use this expert opinion elicitation to refine its We are enthusiastic about the use of a disciplined, scrutable expert opinion elicitation to plan and refine the NRC research. We believe the elicitation to be an essential addition to the planning for extended fuel burnup being done by the Nuclear Energy Institute (NEI) and NRR staff. We encourage that both NEI and NRR participate in the effort being undertaken by RES. Much remains to be done to complete the planning for the phenomena identification and ranking. RES will need to develop the formalism for conducting and documenting the expert opinion elicitation. RES can adapt one of several formalisms developed by the NRC in other efforts. RES should expand the original scope of its phenomena identification and ranking elicitation to include the issues of accident source term. The revised accident source term (NUREG-1465) approved by the Commission was developed from analyses of fuel taken to burnups that are moderate in comparison to burnups being achieved in current plants. We are concerned that the accident source term will have to be modified to account for the effects of extended fuel burnup. Chemical forms and volatilities of radionuclides may be affected by burnup because of higher oxygen potentials in the fuel and the fuel-cladding gap. Releases of radionuclides from the fuel may be increased because of higher concentrations of interstitial oxygen, more extensive connection of intergrannular porosity, and smaller grain sizes in the so-called "rim" region. RES has not established a technically defensible position on modifications of the accident source term to account for fuel burnup. Adequate data have not been marshaled. Analytical tools used to date do not appear to include adequate descriptions of pertinent phenomena and processes. Superior analytical tools may be available in the U.S. and in other countries. Additional data may be available from research done abroad. RES will gain substantial benefit for developing a position on modifying the accident source term by including elicitation of expert opinion on the effects of burnup on radionuclide behavior in fuel. In October 1998, we met with representatives of France, Germany, and Japan to discuss technical issues of mutual interest and as a result formed a Quadripartite Working Group on High Burnup Fuel. We believe this group could contribute to the planned expert opinion elicitation for phenomena identification and ranking. Dr. William J. Shack did not participate in the Committee's deliberations regarding this matter. Sincerely, /s/ Dana A. Powers References:
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