SAFETY STANDARDS

There may be differences in the regulations and standards for future reactor licensing. In countries favourable to nuclear power, the licensing frameworks are likely to evolve or be extensions of existing frameworks for currently operating plant. However, there are a number of countries that are not favourable towards nuclear power where the approach will be dependent on the perception of relative risks to benefits. There are also a number of countries where no new plants are planned or where moratoria are already in place in which case there is no issue. Within Europe for example, five out of the eight EU member

Table 8.6. Standards for future reactors (in countries where there are not moratoria)

Current regulations and standards are likely to be appropriate for evolutionary type plant Current regulatory regimes may need to be extended for more revolutionary type plant Future reactors are expected to include greater protection against severe accidents. Increased level of PSA

Increased use of BE methods to demonstrate more realistic safety margins Increased use of Risk Informed methods following USNRC lead

EUR 20055 EN (2001).

states with nuclear power are in this category. These include Belgium, Germany, Netherlands, Spain and Sweden.

The standards for future reactors will depend on internationally accepted standards, e. g. as shown in Table 8.6. Some countries believe that their current regulations and standards are already appropriate for future evolutionary LWRs. There are, however, certain requirements that are likely to be imposed which assume greater significance for future reactors.

It seems likely that future reactors will have to include greater provision against severe accidents. This may be required to be demonstrated by both PSA and deterministic means.

PSA methodology is used currently on existing plant to identify weaknesses and therefore enable modifications to be implemented. PSA practices have improved significantly over recent years; these methods provide an accepted means of assessing the safety of a plant. PSAs provide a means of verifying the design basis of a plant supported by deterministic analysis making conservative assumptions. They also can be used to assess whether there are any ‘cliff-edge’ concerns about safety with the design, e. g. just beyond the design basis. This has led to modern LWR designs, which restrict the source term for radioactive release for beyond design basis, including severe accidents.

There is certainly a trend in some countries to extend the design basis for new plants to cover severe accident challenges. However, this could clearly have a major impact on competitiveness of the plant, through the cost of including specific or additional components.

From a severe accident perspective in LWRs, the strength of the containment is crucial in limiting radioactive release. However, at present, the containment is only built to withstand DBAs where safety systems are assured, and assumed to respond subject to a single failure criterion. Present analyses demonstrate a margin between the design pressure and the actual failure pressure and this is useful in evaluating the implication of certain severe accidents. However, if all severe accidents were included within the design basis envelope, then the containment would need to be strengthened to withstand higher loads.

There are differences worldwide in the approach to containment and its function in severe accidents. For example, within the IAEA member states, some countries have already made significant improvements, others have plans for improvement that have not yet been implemented, others prefer to adopt a different approach to severe accident management.

The extension of the design to include severe accidents has been proposed in Germany. However, at the time of writing, there is a consensus to terminate the use of nuclear energy over the next 30 years and therefore standards for future plants are no longer under consideration. Other countries, e. g. France are taking a similar position. If such proposals are adopted, there could clearly be major differences in approach to licensing across the nuclear operating countries.

In many cases, the evolutionary designs contain more advanced safety features, some of which already mitigate against severe accident vulnerabilities. The EPR design, for example, has a debris retention component. Some VVER-1000 reactors’ future designs will adopt a similar approach. Thus, the addition of core catchers to prevent melt attack of the containment base-mat is one feature that has already been introduced into the design to cover severe accidents.

It would, however, be very difficult to extend the design basis to cover all potential severe accident scenarios. Steam explosion vulnerabilities are still uncertain and it would be difficult to demonstrate by deterministic means that a containment is sufficiently strong to withstand all possible loadings, taking account of the uncertainties.

Other approaches on design have been to take advantage of more inherent mechanisms, passive injection, gravity driven flow, e. g. as in AP-600.

A chapter is devoted to passive plants later in the book.

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