Pressurised water reactor cold shutdown transient analysis

Abstract

The term “shutdown” as it applies to Nuclear Power Plants cannot be construed as meaning either dormant or inherently safe. Once a nuclear reactor is brought to the cold shutdown condition there are still decay process going on inside the reactor with the accompanying generation of heat in the megawatt range. There is a vast body of literature available investigating the occurrence of transients in reactors at varying levels of power and even hot standby, but comparatively little is available investigating transients in the cold shutdown condition. The objective of this investigation was to simulate the insertion of selected transients into a Westinghouse 3 loop pressurised water reactor – CPR 1000 - and carry out an analysis of said transient insertions in terms of the behaviour of the reactor. The transients that were analysed were: * Complete Residual Heat Removal failure without replenishment of Reactor Coolant System inventory * Loss of coolant accident (Hot and cold legs) * Inadvertent control rod withdrawal * Moderator dilution * Fuel clad failure The analysis revealed that by far the most severe repercussions – in fact meltdown – came about as a result of Residual Heat Removal failure without Reactor Coolant System replenishment. If steps are not taken to re-establish Residual Heat Removal in 5 hours and 10 minutes, the situation rapidly deteriorates to the point where meltdown occurs a further 2 hours and 29 minutes later. Large break Loss of Coolant Accidents were easily handled by the Residual Heat Removal System and held no real danger. Inadvertent rod withdrawal, moderator dilution and fuel clad failure either had to have key variables shifted so far away from the cold shutdown condition or the extent of the transient raised far beyond design based accident conditions, or both, yet did not hold any real threat as the available systems kept the reactor safe. It is recommended that selected combinations of transients are injected into the reactor and analysed and that Residual Heat Removal failure without replenishment is re-simulated with commercial code such as CORYs or MAPPS and analysed further.