Behaviour of defects under LOCA conditions

Following identification of flaws within a clad PWR pressure vessel, concerns were raised as to its safety in the event of a loss-of-coolant accident (LOCA)

The objective was to assess the integrity of the flawed vessel subject to a postulated LOCA.

A finite element model was developed for the pressure vessel with the observed defects.  Thermal analysis of the postulated LOCA was performed followed by stress analysis.  These results were input to an R5/R6 fracture mechanics assessment

The defects were shown to be stable under normal operating conditions, but the concerns were seen to be justified as flaws were predicted to grow rapidly under LOCA conditions.  However, the analysis demonstrated that, in its current condition, the vessel would survive one LOCA.

Accordingly an inspection plan was put into place to monitor the existing flaws, allowing the plant to continue in operation without the need of costly repairs or shutdown.  Safety was assured as long as the existing defects did not grow in operation.

Plant Integrity