In order to ensure that a nuclear power station is operated within the limits and constraints assumed in the safety case, it is necessary to provide the op­erator with clear concise definitions of these limits and constraints. It is essential to impress upon the operator the overriding importance of compliance with any instructions which embody these limits and constraints.

Notwithstanding any instructions the licensee may give to his employees as a responsible operator of a nuclear power plant, the HSE requires as a condi­tion of the site licence that such instructions are drawn up. The licence condition also indicates the statu­tory requirements for making any changes to these instructions.

There are three separate but closely related docu­ments or sets of documents specified in the licences for the latest stations, whilst for the earlier stations there were only two.

The first document contains the Operating Rules. These specify the actual limits in terms of tempera­ture, gas pressure and power to which the reactor may be operated and the principles covering the degree of essential plant availability required. The Operating

Rules can only be changed with the explicit approval of the HSE after the licensee has followed his own procedures for agreeing such changes internally. It is emphasised that the licensee has no authority for making any change to the Operating Rules, which would lead to a relaxation of the limits or constraints, under any circumstances. Clearly if as a result of some investigation or operating experience a limit was found to be insufficiently rigorous, the licensee would be expected to act accordingly.

In order to change or suspend an operating rule, the station manager is required to prepare a submis­sion for the Nuclear Safety Committee. This submis­sion explains the need for the change, provides the necessary safety arguments supporting the change and, if appropriate, details additional precautions which may be necessary to ensure compliance with the safety principles. If agreed by the Nuclear Safety Committee, the submission is sent to the HSE by the Director of Health and Safety for their agreement with a request for the necessary formal consent.

If the requirement for change is urgent, agreement may be given by the Director of Health and Safety on behalf of the Nuclear Safety Committee. It is still necessary however to obtain the consent of the Health and Safety Executive.

The second document contains the Identified Op­erating Instructions (IOIs) and details the interpre­tation of the limits contained in the Operating Rules in terms of the actual measurement of parameters from the plant. It also gives details of the constraints on plant availability amplifying the principles em­bodied in the Operating Rules. The IOIs are a recent innovation brought about by the increase in complexi­ty of the AGR plant and consequently of the safety case, compared with the magnox reactors. Because the alteration of the Operating Rules is a lengthy process involving finally the consent of the HSE, it was agreed that the detailed interpretation of the Operating Rules could be relegated to a document requiring a less formal method of alteration. Indeed it is these details which, because of operating experience or of minor plant faults, are most likely to require amendment. Such amendment, of course, may only be carried out if the limits and principles in the Operating Rules are not violated. The amendment of the IOIs requires only the agreement of the head­quarters departments and divisions of the CEGB. The HSE is informed of the change within 14 days of its implementation; their explicit consent is not required. It is likely on the latest stations that in future the HSE will require to be informed of changes to the IOIs immediately before implementation.

The Plant Operating Instructions (POIs) form the final level of documentation directly related to the Operating Rules. They are the detailed instructions on the operation of the station as a whole. They give the step by step instructions on, for example, starting up and shutting down the unit, refuelling, operation

under fault conditions, etc. It is required that the POIs should be consistent with the Operating Rules and identified operating instructions and for this rea­son all changes are furnished to. the HSE within 14 days of their implementation. The changes themselves are the subject of a procedure involving only the station staff and the agreement of other organisations is not required.

The format of Operating Rules has remained very similar for all stations. The detail varies to take ac­count of a particular plant design and its mode of operation. There are sections covering:

• Fuel elements — clad temperature limits.

• Pressure vessel cooling system — setting of safety relief valves.

• Circuit activity — burst can detection.

• Fuel handling.

• Essential supplies — availability of plant and of consumable supplies such as CO2 or diesel fuel.

• Control and safety circuits — trip settings.

• Carriage of nuclear matter on site.

• Moderator.

• Impurities in the gas circuit.

• Structural temperature.

Each rule or set of rules within a section usually has associated with it a section in the identified operating instructions. It is a general principle that only those limits and constraints which are under the direct control of the operator are included in the Operating Rules. If operation outside a particular limit or with a prohibited plant state is prevented, either by the inherent design of the station or by the use of safety interlocks, an operating rule is unnecessary.