United States Nuclear Regulatory Commission - Protecting People and the Environment

Phenomenon Identification and Ranking Tables (PIRTs) for Loss-of-Coolant Accidents in Pressurized and Boiling Water Reactors Containing High Burnup Fuel (NUREG/CR-6744)

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Publication Information

Manuscript Completed: August 2001
Date Published: December 2001

Prepared by:
B.E. Boyack, A.T. Motta, K.L. Peddicord, C.A. Alexander, J.G.M. Andersen,
J.A. Blaisdell, B.M. Dunn, D. Ebeling-Koning, T. Fuketa, G. Hache, L.E. Hochreiter,
S.E. Jensen, S. Langenbuch, F.J. Moody, M.E. Nissley, K. Ohkawa, G. Potts, J. Rashid,
R.J. Rohrer, J.S. Tulenko, K. Valtonen, N. Waeckel, W. Wiesenack

Los Alamos National Laboratory
Los Alamos, New Mexico 87545

H. Scott, NRC Project Manager

Prepared for:
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code W6245

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Abstract

In the United States, cladding embrittlement criteria and related evaluation models are used to address loss-of-coolant accidents. The embrittlement criteria are a peak cladding temperature of 1204 °C (2200 °F) and an equivalent oxidation of 17% of the cladding wall thickness calculated with the Baker-Just correlation. Evaluation models address ballooning, rupture, flow blockage, and oxidation kinetics. In the 1970s, high burnup was considered to be around 40 GWd/t (average for the peak rod). Data out to that bumup had been included in databases for codes, and regulatory decisions. It was believed that some extrapolation in burnup could be made and fuel burnups in licensed reactors up to 62 GWd/t (average for the peak rod) were permitted. By the mid 1980s, however, unique changes in pellet microstructure had been observed from vendor and international data at higher burnups along with increases in the rate of cladding corrosion. It thus became clear that other phenomena were occurring at high bumups and that continued extrapolation of transient data from the low-bumup database was not appropriate. The US Nuclear Regulatory Commission (NRC) is addressing these issues. The NRC is performing research with respect to high burnup fuel to acquire and develop the requisite understanding of the performance of high burnup fuel under accident conditions. It is also conducting research to determine if current embrittlement criteria and evaluation models are adequate for highburnup fuel or if modifications are needed. To support these efforts, The NRC has commissioned the formation of a Phenomena Identification and Ranking Table (PIRT) panel to identify and rank the phenomena occurring during selected transient and accident scenarios in both pressurized water reactors and boiling water reactors containing high burnup fuel. Because the PIRT identifies and ranks phenomena for importance, currently existing experimental data, planned experiments, computational tools (codes), and code-calculated results can be screened to determine applicability and adequacy using the PIRT results. This PIRT identifies and ranks phenomena for loss-of-coolant accidents in both pressurized and boiling water reactors. A spectrum of break sizes has been considered in preparing the PIRT.

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