Contents
Safety Assessment Principles (SAP)
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The NII assesses the safety cases submitted to them by licensees of nuclear power stations. In order to provide a framework for this assessment the Health and Safety Executive have produced the document “Safety Assessment Principles for Nuclear Plants” (1992). These ADVANCED GAS-COOLED REACTOR SAFETY STATEMENT 13 AGR SS 4/97 principles were aimed primarily at the safety assessment of new power plants.
Since the last AGR power station to be built was commissioned in 1988 the principles were not strictly designed for AGR power stations. They are, however, useful in assessing proposals for changes to the safety case for existing stations and for safety case reviews.
The Safety Assessment Principles (SAPs) describe the standards which will be followed by the NII in their assessment of safety cases submitted to them. Within these principles licensees of nuclear power stations are at liberty to construct their own safety cases. The SAPs are therefore not prescriptive.
Design solutions to particular problems are devised by the licensee and assessed by the NII. The SAPs require the licensee to study the risk presented to the public during normal operation and fault conditions. They also specify standards that should be met by the installed plant and safety systems. At the beginning of the document five fundamental principles are stated. These principles were derived from the recommendations of the International Commission on Radiological Protection. The safety case for an AGR power station is required to demonstrate compliance with these principles, which are as follows:
No person shall receive doses of ionising radiation in excess of statutory dose limits as a result of normal operation.
The exposure of any person to radiation shall be kept as low as is reasonably practicable.
The collective effective dose to operators and to the general public as a result of operation of the nuclear installation shall be kept as low as is reasonably practicable.
All reasonably practicable steps shall be taken to prevent accidents.
All reasonably practicable steps shall be taken to minimise the radiological consequences of any accident.
These five principles relate to the three main aspects of safety at a nuclear power station. These are dose limitation, accident prevention and accident mitigation. The SAPs provide further interpretation of acceptability of risk by defining a Basic Safety Limit and a Basic Safety Objective. The Basic Safety Limit represents the upper limit of tolerability of risk, while the Basic Safety Objective, which is set at a risk one tenth of this, is the risk level below which detailed assessment by the regulator of the safety case is not required.
