There are several thousand fuel channels in the mag — nox reactor through which the coolant passes.

Ideally the maximum fuel surface and gas outlet temperature of each channel should be checked, but in practice only a proportion of the channels are monitored in magnox reactors.

The chromel/alumel thermocouple with stainless steel sheath is used for metallurgical and nuclear reasons in power reactors up to about 850°C (Volume F, Chapter 4). Above 850°C, the thermocouple may be affected by deterioration of the protective oxide film.

On magnox reactors the fuel element surface tem­perature is measured by attaching the thermocouple to the fuel element can. The finning of the can is drilled at a selected point to take the thermocouple hot junction. Both time response and the accuracy are affected by the integrity of the thermocouple fit. In some cases continuous ‘trailing lead’ thermocouples are used, but because of the complication in fuel­handling operations, channel multiple plug and sockets are widely used. Great care must be taken to ensure that the materials and the design of the plugs and sockets will maintain a high insulation value in the reactor environment.

The 3 mm diameter thermocouple, used on magnox fuel elements, has a resistance of about 2 П/m and the fairly high lead resistance together with the com­pensating cable results in a total resistance per ther­mocouple of the order of 600 fi. The output is about 40 per °С so that 16 mV may be delivered around normal power conditions in a magnox reactor. Some thermocouples are used for excess temperature pro­tection and others for measurement only.

For maximum reactor power, the fuel elements are operated at as high a temperature as allowed by fault considerations which take account of a margin above

operating temperature. When this margin becomes zero, a reactor shutdown is initiated via temperature trip amplifiers and the reactor safety system.

5.3.2 Other temperature measurements

Temperature measurements on magnox reactors are discussed in Chapter 3 of this volume.