When the short term transient is terminated with flow restoration after a short period of stagnation, it is
necessary to ensure that the reactor remains in a sale state. Heat is generated post-trip by fission product decay at a rate which reduces with time. When the clad temperatures are at their peak, a considerable amount of heat is lost to the graphite by radiation. The introduction of a small amount of coolant How is sufficient to bring these clad temperatures down somewhat. Graphite temperatures however continue to rise. Since the reactor pressure has reduced from 10-15 bar and since, as a result of the incident, a number of gas circulators and boilers may not be available immediately, less heat may be removed from the core as a whole than is being generated. When the clad temperatures approach the rising graphite temperatures, they too may begin to rise again. Fur­ther, if the breach cannot be sealed, air mav enter the vessel. The heat of oxidation of the graphite is trivial compared with fission product heat at 400- 500°C, but above this it becomes increasingly sig­nificant. In addition, air is a less satisfactory heat

MAGNOX IGNITION TEMP. 64D°C

Fie. 4,5 A topical short term transient

transfer medium than CO: at any given pressure because it has a much lower density.

A computer code is used capable of modelling all these characteristics as they affect a single channel in the reactor. The study is usually carried out to deter­mine the highest initial graphite temperature con­sistent with the graphite transient ultimately being controlled. Each of the probable initial steady states in terms of numbers of gas circuits available, essential plant available and reactor power is examined and a graphite temperature limit derived. In addition, the effect of possible operator action is investigated and where this action is acceptably reliable an alleviation of the graphite temperature limit may be allowed. The actions which an operator may take to control the long term transient are; to inject CO2 reducing the oxygen/graphite reaction, reduce gas inlet tem­perature by over feeding the boilers thus increasing heat removal capability, seal the breach in the vessel, bring into service extra gas circulators or increase the speed of those already running.