A major design intent of all nuclear power stations is that the operator should not be faced with the need to carry out urgent actions or to make rapid decisions in order to prevent a nuclear incident. The term operator in this context may be the station staff cor­porately, a station department or an individual opera­tor who may or may not be in the central control room.

On the latest stations, the safety case makes no demand on the operator beyond the monitoring of the state of plant for 30 minutes following the onset of any fault. He may, however, within this time, need to take some simple actions to prevent economic damage to plant. On the earliest stations, operator action of a straightforward nature is very occasion­ally called for within 15 to 20 minutes, to prevent a hazardous situation arising. For slowly-developing faults, operator action may be claimed as protection against the possibility of a release on a time scale of many hours.

It is essential that during normal operation and under fault conditions the operator is kept aware of any difficulties or deficiencies in the plant and to this end a comprehensive alarm system is provided. The alarms are divided into two groups, urgent, which need immediate attention and, non-urgent, which may be accepted and the fault dealt with when convenient. The AGR stations have equipment which automati­cally takes care of all operations required immediate­ly post-trip to ensure that the reactor is adequately cooled even if the grid supply fails or there is dam­age to plant. A display is provided which enables the operator to follow the post-trip sequence of plant initiation and operation, and he is only required to intervene in the very unlikely event that adequate cooling is not established. Although this intervention is not claimed in the safety case the option exists.

The operator can initiate a reactor trip if the nor­mal protection equipment fails, or if he perceives a situation developing which might lead to a hazardous condition. This operator action is not claimed in the safety case except for very slowly developing faults but is a prudent option.

Operator action is required in the long term fol­lowing an accident to alleviate the consequences and to restore the situation to as near normal as possible. In the event of a release of activity, he is required to seek the advice and assistance of organisations outside the station and to monitor the extent of any release. He is expected to take appropriate action as best he may to deal with any on-site emergency, such as a major fire, until outside help arrives.

The reliance on the automatic protection equip­ment is only valid if the station is operated within the limits assumed by the safety case. These limits which also include the trip settings of the safety cir­cuits and the availability of essential plant required post-trip, are clearly defined in the Operating Rules and Identified Operating Instructions. It is mandatory upon the operator to ensure that these rules and instructions are followed at all times.