Project Summary
Degradation of Boraflex panels under gamma radiation can lead to loss of boron absorber in spent fuel storage pools. The US Nuclear Regulatory Commission (NRC) has required that US utilities produce a statement assessing the effects of Boraflex degradation on criticality safety margins. In response, the PECO Energy Company initiated a survey of panel conditions at the Peach Bottom Unit 2 Boiling Water Reactor, (BWR), ahead of the 1996 reactor reload.
The survey identified localised areas of degradation in the panels.
Serco Assurance secured a contract to evaluate the impact of observed and postulated levels of degradation on criticality safety. The evaluation included consideration of reactivity losses associated with the depletion of fissile content and the build up of fission products during fuel burnup (Burnup Credit)
Problem Solution
Reactor Physics and Criticality code methods were used to derive neutron multiplication factors in the storage pool for a range of postulated levels of Boraflex degradation, and for a range of fuel burnup.
Spent fuel compositions were calculated using the WIMS Reactor Physics code to model burnup in BWR fuel bundles, including depletion of fixed gadolinium poisons. Criticality calculations for the storage pool were made using MONK, a 3D Monte Carlo code.
On completion of the panel survey, further MONK calculations were made to assess criticality safety margins for existing pool conditions.
Outcome
By taking account of fuel reactivity loss due to burnup it was demonstrated that, even with global reductions in boron density down to 10% of design level, compliance with criticality safety criteria are maintained for the typical spent fuel assembly.
Calculations also showed that the small areas of localised degradation observed during the Unit 2 survey have no significant impact on criticality safety.
The Unit 2 fuel reload went ahead in the Autumn of 1996, and was completed in 19 days, a new record for a commercial BWR.
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