A Composite Analytical Solution for Large Break LOCA

Abstract

The Canadian CANDU Industry is implementing a composite analytical solution to demonstrate, with high confidence, adequate safety margins for Large Break Loss of Coolant Accidents (LBLOCA) in existing CANDU reactors. The approach involves consolidating a number of individual approaches in a manner that alleviates reliance on any single analytical method or activity.


Using a multi-layered approach, the objective of this composite solution is to use a variety of reinforcing analytical approaches such that they complement one another to collectively form a robust solution. The composite approach involves: i) systematic reclassification of LBLOCA to beyond design basis events based on the frequency of


the limiting initiating events;


ii) more realistic modeling of break opening characteristics;


iii) application of Best Estimate and Uncertainty (BEAU) analysis methodology to provide a more realistic representation of the margins;


iv) continued application of Limit of Operating Envelope (LOE) methodology to demonstrate the adequacy of margins at the extremes of the operating envelope;


v) characterizing the coolant void reactivity, with associated uncertainties; and


vi) defining suitable acceptance criteria, accounting for the available experimental database and uncertainties.


The approach is expected to confirm the adequacy of existing design provisions and, as such, better characterize the overall safety significance of LBLOCA in CANDU reactors. This paper describes the composite analytical approach and its development, implementation and current status.