The third major group of faults which need to be considered are those where the coolant flow fails or the feedwater flow to the boilers fails. In either case, there is a loss of the capability for removing the heat generated in the reactor core.

Firstly, consider gas circulator failure. If the sup­plies to gas circulators are lost, the motors run down at a rate depending on the inertia of the system. As soon as a mismatch occurs between the heat gen­erated in the core and the heat removal capability, channel gas outlet and fuel temperatures rise to a point where a reactor trip is initiated. However, the reactor is usually tripped on loss of volts to the gas circulator motor, but it is the trip on high tempera­ture which leads to the larger transient. Gas circula­tion is restored post-trip using alternative electrical supplies to subsidiary motors known as pony motors.

In the case of the failure of feedwater flow to the boiler, due either to a fault in boiler feed pumps or a breach of the main feed lines, the first effect is a rise in boiler gas outlet temperature followed by a rise in channel gas inlet temperature and, hence, in fuel can and channel gas outlet temperature leading finally to a reactor trip. It is, however, also of par­ticular importance in this case to study the structural temperatures about the diagrid and the core support at the bottom of the core. Excessive temperatures or temperature gradients in this region could cause dam­age which, although unlikely to cause an immediate hazard in terms of the release of radioactive material, could jeopardise the continued operation of the re­actor as a power-producing unit. Following the reactor trip emergency feed is restored using alternative pumps provided for the purpose.

There are a number of other faults which are con­sidered in the studies but they usually revert to one of those already described. They differ only in the parameter used to trip the reactor and the time from the start of the fault condition when the trip actually occurs. In this category are such items as loss of main grid supplies, tripping of the main turbine, and loss of less than all the main gas circulators.