Leaving aside the possibly harmful environmental effects of burning fossil fuel, major accidents, such at that at Aberfan in South Wales involving a coal waste heap, have occurred with many fatalities amongst the public. Chemical plants have also affected the public directly through release of toxic chemicals, such as at Bhopal in India. In the nuclear context significant release of radioactivity from a nuclear power station would not be confined to the site of the power station. Depending on weather conditions, the effects could be measurable at a considerable distance from the site. Nuclear power therefore imposes an involuntary risk on a large population.
It is a fundamental requirement of the Nuclear Safety Principles that the risks posed by a nuclear plant are demonstrably “As Low As Reasonably Practicable” (ALARP), taking into account economic and safety requirements. Any safety case must show that a robust process has been adopted to both identify and assess potential options to enhance safety against ALARP philosophy and methodology. The Nuclear Safety Principles set out limits on the doses and risks that should be achieved by the AGR stations. These limits, the Basic Safety Limits (BSL) and Basic Safety Objectives (BSO), define the boundaries between which the ALARP principle is invoked.
When deciding whether this risk is acceptable, it is necessary to compare it with other sources of risk experienced in everyday life. The Health and Safety Executive (HSE) have produced guidance on how to determine whether a risk is acceptable. This guidance is contained in the document “The Tolerability of Risk from Nuclear Power Stations”, (1992). Figure 93 illustrates the basic concepts, with respect to the annual risk of death to an individual.
There is a level of risk which is considered unacceptable, save in extraordinary circumstances. No nuclear power station should be operated if the risk it presents exceeds this upper level. There is also a level of risk which is so small that it can be considered broadly acceptable. A nuclear power station which presented this level of risk would be considered to have met the requirements for reducing risk to the lowest practicable level.
Between these two extremes, the decision as to whether to reduce this risk still further is based on a comparison between the reduction in risk and the cost of achieving it. This region is known as the ALARP region. There is an obligation on all operators of nuclear power stations in the UK to reduce the risk from the power station to a value which is As Low As Reasonably Practicable. The closer the risk presented by the plant is to the upper level of acceptability the more likely it would be that practicable safety improvements could be made and would be required. Conversely, the lower the risk the more difficult it would be to improve safety further. The comparison of benefits and disbenefits to establish what is “reasonably practicable” can be carried out on a quantitative or qualitative basis. A quantitative assessment allows the calculation of a ‘cost-benefit ratio’ which helps with the test of “disproportionality” described above. Engineering judgement and expert opinion are considered to be valuable and key to qualitative assessments is to ensure they are structured in such a way as to encourage a systematic review.
The Health and Safety Executive have specified numerical values for the levels of risk which are considered unacceptable and broadly acceptable. A risk of death of one in 10,000 per year to a member of the public is the maximum that could be tolerated from any large industrial plant. This figure is derived by comparison with risks associated with activities which are generally regarded as acceptable.
Nuclear power stations are expected to achieve a risk target 10 times lower than this, which is one in 100,000 per year maximum risk to any member of the public. In addition a broadly acceptable risk is one of 1 in one million per year. Risk levels up to 10 times higher are acceptable for those working at the plant. It should be noted that the risk covers the effects of normal operation and those of potential failures. The first kind relates to planned and authorised discharges of radioactive material, and the second to plant faults leading to an unplanned release of radioactivity.
The discussion above may be summarised as follows:
For any activity the level of risk may be so great that the activity cannot be allowed to continue. This upper limit defines the boundary between risks which are just tolerable and those which are intolerable.
Even when the risk is tolerable, it must be reduced to a level which is as low as reasonably practicable.
A point is reached at which the risk is, or has been made, so small that no further precaution is necessary.
