Skip Navigation LinksSafety Case

 

1. Objectives of the Safety Case

Safety cases are produced in accordance with common standards and principles. This section describes the major features of these standards and the arrangements for review of the safety case.

The need for all nuclear power stations to be operated to a common set of safety standards has long been recognised. The International Atomic Energy Agency (IAEA), which has member countries throughout the world, has drawn up a series of Safety Guides and via its International Nuclear Safety Advisory Group, has also produced a series of safety documents. These cover all aspects of nuclear power station design and operation and provide a framework within which an individual country can develop its own safety standards.

The responsibility for the safety of a nuclear power station rests with those who manage and operate the plant. The national government has the responsibility to establish a legal framework for the operation, backed by an independent regulator to confirm that the plant is operated safely and within the legal framework. International co-operation helps to ensure that all nuclear power plants meet accepted international standards.

In the area of radiation protection, the International Commission on Radiological Protection (ICRP) has provided an internationally accepted set of dose limits. All nuclear power stations in the United Kingdom are operated within ICRP limits. In the area of reactor safety the approach taken in the United Kingdom follows the advice given in the Safety Guides and Codes issued by the IAEA.

The safety case for an Advanced Gas- Cooled Reactor demonstrates that it presents an acceptably low level of risk to the public and to the operating personnel. This takes into account both normal operation and credible fault conditions.

2. Compliance with the Safety Case

It is a principle of the operation of AGR power stations that, for every routine or abnormal operation involving nuclear safety, a safety case must be prepared, and must be subjected to an independent assessment by the Company s internal safety division.

The safety case identifies the limits and conditions necessary in the interests of safety. Those relating to normal operation are derived mainly from fault studies, and are formalised as Operating Rules. Each AGR has a set of Operating Rules which have been approved by the Health and Safety Executive. All operations carried out must comply with the Operating Rules. These Rules specify the fundamental safety limits for the plant, for example:

-        pressure and temperature limits for the primary gas circuit;

-        limiting values for the settings of trip levels for reactor protection;

-        availability requirements for essential plant, and

-        conditions under which a reactor may be refuelled.

Changes to the Operating Rules require the endorsement of the Nuclear Safety Committee and the approval of the Health and Safety Executive before they may be put into effect.

3. Modifications to Plant or Procedures

Changes to the plant or to the operations carried out at a nuclear power station are referred to as modifications. Any proposals for modifications must be submitted in the form of a paper setting out all relevant information, such as the details of the design and intended manufacturing standards, restrictions to be observed during installation, commissioning requirements, training requirements, quality assurance arrangements and effects on approved drawings and station documentation. This paper is then submitted for approval, by a procedure which varies in accordance with the extent of the safety significance of the modification. Any abnormal or one-off operation, or proposal for a significant test in a reactor, would be submitted by the same procedure.

The most significant modifications, referred to as Category 1, are defined as those which, if inadequately conceived or executed could cause a serious increase in the risk of a radiological hazard. These require endorsement by the Nuclear Safety Committee and the agreement of the Nuclear Installations Inspectorate before they may be put into effect. As well as being submitted to the Nuclear Safety Committee, Category 1 proposals are subjected to an independent assessment by the Company’s internal safety division.

Modifications with a lesser, but still significant risk are referred to as Category 2. They are subjected to an independent assessment by the Company’s internal safety division before implementation, and their clearance is reported to the Nuclear ADVANCED GAS-COOLED REACTOR SAFETY STATEMENT 91 AGR SS 4/97 Safety Committee.

Category 3 modifications, involving no significant risk may be approved by the Station Director, and are reported to the Company’s internal safety division. These arrangements ensure that any proposed change to the plant or the processes at a nuclear power station receives a level of scrutiny appropriate to its safety significance. In a similar way, if a plant fault occurs which may affect the ability of the plant to carry out its intended safety function, a safety case is required for continued reactor operation. It is the responsibility of the Station Director in the first instance to decide whether the plant should be shut down, or subjected to other restrictions, while the safety case is developed.

4. Reference Safety Case

The safety case may change in detail, but the underlying principles remain unaltered. If a new operation is intended (for example, refuelling at a power level not previously covered in the safety case) then a new safety case would be prepared. This safety case would identify any new Operating Rules or changes to existing Rules that were required. Similarly, if new plant is installed at a power station, a safety case is required for that new plant before it may be installed or operated. Specific safety cases are also required for abnormal or one-off operations.

Operational nuclear power stations can therefore be said to have a “living safety case”. This safety case provides the justification for all the safety related activities carried out on the site, incorporating all the changes which have occurred since initial construction. This all-inclusive documentation is referred to as the Reference Safety Statement or Reference Safety Case for the power station and it is defined within a Schedule listing all the documents which describe the safety case.

5. Periodic Safety Review of the AGR Safety Case

Advanced Gas-Cooled Reactors were designed and constructed according to the best currently available advice in the form of national and international guidelines, current British Standards and other design codes. They are however expected to operate for several decades, during which time guidelines and design standards will change, and a significant proportion of the plant originally installed will be replaced. Each replacement represents a small change in the safety case, and its documentation supplants a small part of the previous safety case. The Reference Safety Case helps to keep the recording of the safety case in order, but it may become difficult after a period of years to discern the precise current case. It is therefore appropriate to undertake from time to time a comprehensive review of the safety case against current guidelines.

Such an exercise is called a Periodic Safety Review (PSR), and is a requirement of the Nuclear Site Licence. It is undertaken at 10-yearly intervals, or as directed by the NII, and is submitted to NII. Because the guidelines change, PSRs reveal discrepancies between the plant and current standards. These are rectified where it is reasonably practicable to do so. The operational history of the plant is analysed during the review to identify any trends which need addressing, even if there is no immediate concern.

A major part of the periodic review is to assess the effects of ageing. Plant lifetime assessment is a significant part of the overall safety strategy for the AGRs. The Periodic Safety Review is carried out in five broad subject areas. These are shown by the table below:

Within each of these subject areas a number of topic reports are produced covering all aspects of the safety case. On conclusion of the review a summary report is presented to the Nuclear Safety Committee and is furnished to the Nuclear Installations Inspectorate. Any additional work required is agreed with the NII.

The Periodic Safety Review ensures that the safety cases for all the AGR power stations are assessed and maintained to common standards. These standards are in line with the best practices found in other countries and applied to other reactor types.