United States Nuclear Regulatory Commission - Protecting People and the Environment

Phenomenon Identification and Ranking Tables (PIRTs) for Power Oscillations Without Scram in Boiling Water Reactors Containing High Burnup Fuel (NUREG/CR-6743)

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Publication Information

Manuscript Completed: August 2001
Date Published: September 2001

Prepared by:
B.E. Boyack, A.T. Motta, K.L. Peddicord, J.G.M. Andersen,
C.A. Alexander, B.M. Dunn, T. Fuketa, L.E. Hochreiter,
R.O. Montgomery, F.J. Moody, G. Potts, D.W. Pruitt, J. Rashid,
R.J. Rohrer, J.S. Tulenko, K. Valtonen, 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

Availability Notice


In the United States, two types of regulatory criteria have been used in safety analyses to address reactivity accidents. One criterion is a limit of 280-cal/g fuel on peak fuel-rod enthalpy. The other criterion consists of several threshold values that are used to indicate cladding failure. In the 1970s, high burnup was considered to be around 40 GWd/t (average for the peak rod). Data out to that burnup had been included in databases for criteria, 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 burnups and that continued extrapolation of transient data from the low-burnup database was not appropriate. The US Nuclear Regulatory Commission (NRC) is addressing these issues. It 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. The NRC is also preparing to develop a new criterion to replace the current 280-cal/g coolability limit. 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 instability power oscillations arising during an anticipated transient without scram in boiling water reactors containing high burnup fuel. The initiating event is a trip of both recirculation line pumps.

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