Each station has active maintenance facilities where plus units and other reactor assemblies (such as con­trol rod assemblies) handled by the fuelling machine can be serviced. Glove boxes with appropriate shield­ing are used at the higher levels, with remote handling tools at the lowrer levels such as at the neutron scatter plug where activity is high. Some sub-assemblies such as closure units or control rod actuators may be re­moved for detail maintenance in special workshops for contaminated components.

The other facilities include a test and training faci­lity which embodies a full scale replication of the reactor charge path for use with the fuelling machine. Fuelling machine maintenance facilities include pro­vision for nose unit, grab and hoist servicing. Pro­vision is made for operations such as fuel element bottle inspection and test before re-use. Recovery equipment for visual inspection of the reactor charge path and removal of damaged components is pro­vided either as part of the fuelling machine or as a separate unit.

8 AGR post-trip heat removal systems and essential electrical supplies

Comprehensive protection systems are provided to shutdown the reactor under fault conditions (see Sec­tion 9 of this chapter), but these actions alone are insufficient to ensure that the reactor remains in a safe long term shutdown state, since heat continues to be generated in the core from the radioactive decay of fission products. This shutdown decay heat is an inevitable consequence of the nuclear fission process und it is entirely outside the control of the operators, wen after a normal controlled shutdown. In order to prevent overheating of the fuel, and the consequential damage to the reactor structure and primary pressure circuit components which could result in a release of radioactivity, it is necessary to maintain a reactor ‘-ooling function following shutdown of the reactor.

The post-trip cooling function is achieved in prin­ciple in a similar manner to that employed to cool the reactor core vvhen at power, by:

• Circulating sufficient coolant gas to transfer the fission decay heat from the fuel to the boilers.

• Providing sufficient feedwater to the boiler systems to remove this heat from the primary circuit and reject it to the environment.

Whilst cooling the reactor core, the plant must be operated in such a manner that the maximum compo­nent temperatures and stresses are maintained within design limits to avoid unacceptable plant damage. This cooling function is effected through the combined operation of a number of plant systems. Some of these are normally operational during power operation of the reactor, whilst others are provided specifically for post-trip heat removal duties, and are shutdown during normal power operation. Automatic sequencing equipment is provided to carry out the large number of operations required to terminate the normal power operational cooling system and to establish the post­trip heat removal systems in service.

The same principles of post-trip heat removal are applicable to all AGR power stations. However, the plant systems differ considerably in detail between stations and Heysham 2 is described in depth, being the latest station and thus most representative of current CEGB practice.