It is not possible to instrument every fuel element in a reactor because of the very high numbers, so an alternative method has to be adopted to assess the highest fuel element temperature in the reactor. It is probable that not more than 2.5ro of the fuel ele­ments are instrumented. Such a small proportion makes it statistically difficult to derive an assessment ot temperature. To give a confident result other rele­vant temperatures have to be taken into account.

The assessed temperature arrived at by means of a statistical analysis using fuel element temperature, channel gas outlet temperatures, bulk core inlet and outlet gas temperatures.

The result of the sum gives a maximum temperature of any fuel element in the reactor and it is this tem­perature which determines the operating condition tor driving the reactor.

Core restraint structure

The determination of the maximum operating tempera­ture and temperature differentials between adjoining components is necessary to avoid mechanical failure of the structure due to differential expansion. Once at full power and with the components thermally ‘soaked’ problems should not occur, but with changes of power and temperature strict observation of temperature of the restraint and support core structures is essential.