Severe degradation of BWR fuel failures: Coolant activity analysis

The Electric Power Research Institute (EPRI) has been investigating an apparent recent increase in the frequency of severely degraded failed fuel rods in BWRs. These incidents involve abnormal levels of coolant radioactivity from the failed rod, and are followed by a steep increase in the activity from tramp uranium, most of which is deposited on fuel rod surfaces and therefore tends to carry over to subsequent operating cycles. Analyses of coolant activity from several BWRs experiencing degradation of failed fuel rods were carried out with extensive use of the CHIRON code, developed for EPRI by S. Levy Incorporated. CHIRON is based on steady-state conditions, but its trending plot capability was extremely useful for studying the time variation of radiochemical parameters. It is observed that the failure degradation process usually starts three to eight months after a primary failure occurrence. It is possible from measurements of Np239 coolant activity to reach quite accurate estimates of the amount of fuel released from the failed rod(s) over time. The neptunium calculations can also be used to estimate the size/length of the cladding rupture. An equation has been developed to predict offgas at the start of the next cycle from offgas at end of cycle. It is concluded that a failed BWR fuel rod can deteriorate severely within a few months, and therefore all failure occurrences should be acted upon with the serious consequences of severe damage in mind. A set of recommended guidelines is established for monitoring and interpreting activity signals, and for acting upon failure observations