2.1 Design objectives

Following a trip or planned shutdown, a reactor still produces heat due to fission product decay. This is about 7% initially falling rapidly during the first hour to around 1.4%. It then falls more gradually to about 0.6% after 1 day and 0.3% after 1 week. Even after several weeks it is still significant. Normally this heat is readily removed.

However, if due to a fault in the normal cooling system the heat is not removed, the temperature of the reactor and coolant gas could slowly increase even­tually causing fuel failure and release of the active fission products to the gas coolant. The gas tem­perature increase would also cause the pressure to rise with the eventual opening of the safety relief valves. This would allow a path to atmosphere for the fission products, although there are filters on the gas side safety relief valves to remove particulate activity and limit any release.

There is therefore a safety requirement to cool the reactor fuel in the short and long term post-trip. The

approach adopted in design is to provide adequate cooling systems so that whatever the circumstances, normal or fault, the fuel and plant are maintained below prescribed safety limits.

In addition to safety limits, there will be economic constraints. If these were to be exceeded, the plant or fuel might sustain damage which would delay return to service or require some fuel to be discharged. The post-trip cooling systems must therefore also be de­signed for compliance with economic limits.

The same basic constraints must, of course, also be met when the reactor trips in the event of a fault. The reactor post-trip cooling systems protect against a wide range of faults including loss of primary or secondary circulation where the primary circuit inte­grity is maintained and for other faults where it may be breached.

The effect of such faults on the design of post-trip cooling systems is three fold. First, the nature of the fault may enhance the demand on the system; for example, where reactor pressure is lost. Second, the fault may arise from a plant fault which prevents part of the post-trip cooling system from acting. Third, plant provided for post-trip cooling may be damaged as a consequence of the fault. Each of these aspects must be taken into account in developing the design.

The overriding design objective is to remove post trip decay heat from the reactor in a reliable manner under all credible operational and fault conditions so that safety limits are not exceeded. The need to comply with economic constraints is less crucial but still im­portant in safeguarding the capital investment in the plant.