8.1 Control and instrumentation — extent and differences from magnox reactors

The basic requirements of control and instrumentation on AGRs are similar to those for magnox reactors, though the higher operating temperatures, coolant pressure and greater power density place great empha­sis on temperature constraints in the reactor structure including the boilers, and on the detailed monitoring of reactor coolant composition and the feedwater steam circuit.

Because of the smaller margins and complex inter­relations, the number of measurement-, is larger and there is considerable emphasis on reliability since mam measurements are involved in safetv-related operations and the Operating Rules.

The C and 1 provisions on Hevsham 2 are tvpical of other AGRs and are based on the centralised con­trol and monitoring philosophy. Under normal op­eration each unit can be operated by a single unit operator in the Central Control Room (CCR) with a minimum of local-to-plant staff.

The extent of instrumentation on Heysham 2, com­pared with previous AGRs, has been increased con­siderably mainly because of an increase in the amount of auxiliary plant associated with post-trip heat re­moval. Because of the segregated train policy and diverse cooling requirements, this has further increased the amount of C and I equipment. Extensive use is made of computer-based control, display and monitor­ing systems.

Modulating control is provided to allow the unit performance to be optimised and to keep plant para­meters within working limits without requiring continu­ous operator supervision. Sequence control is provided to assist the operator in the start-up and shutdown of some plant systems. Both of these are provided by computer-based systems.

Automatic protection systems are provided to ter­minate all fault sequences for which prompt action is required with high reliability. Automatic post-trip sequence equipment is described in Section 8 of this chapter. These systems are independent of the main computer systems.