The design of British reactor systems incorporates the feature that the reactor operator is not part of the safety chain for reactor incidents. This philosophy places reliance on safety systems to detect unaccept­able conditions and to take corrective action within defined time limits.

Such safety mechanisms need to demonstrate their operation at defined intervals to give confidence in their reliability. Principally the safety mechanisms that receive the most attention are those associated with temperature protection. The following items come gen­erally under the heading of safety systems:

• Fuel temperature trips.

• Flux measurement trips (absolute and rate of change).

• Counter trip (absolute and rate of change).

• Barometric devices.

• Loss of reactor pressure — both rate of change and over/under pressure.

• Standpipe closures.

• Dome differential pressure (AGRs).

Many of these systems are tested on a frequent basis, usually monthly, and the results witnessed and as­sessed by an insurance inspector. The results of the tests are certified and the certificates signed by the Station Manager, Production Manager and Engineer­ing Manager.

The tests normally use injection methods to simu­late the source term. The final interruption of the output relay or guard line is prevented so as not to trip the reactor at each test. The remaining part of the chain is tested normally during the reactor’s biennial overhaul to prove its reliability.

Barometric devices are normally tested either in a rig or, in the case of boron balls, during the outages. Similarly, although not included in the above list, control rod drop tests are done during shut down. The test for control rods includes a measurement of its characteristic drop velocity over its range of insertion.

The list includes reactor standpipe pressure closures and although these cannot be strictly interpreted as safety devices, they are considered strategic enough to give a minor loss of coolant defect should they fail. They are therefore closely inspected following the repressurising of a reactor and after each refuelling operation to ensure a gas-tight seal and correct op­erating position.