SAFETY THROUGH DESIGN

Already, a considerable degree of harmonisation has been achieved within the international community, on the principles of safety for commercially operating reactors. The implementation of these principles may be achieved at different levels across the countries operating nuclear plant but considerable progress has been made. Further, international safety standards will become increasingly stringent. This means that future reactor designs are likely to have to demonstrated even higher standards of safety than at present, to meet more demanding national regulatory requirements and international safety standards.

In order to do this, design principles will need to be considered for future plant (Carnino, 1999), which build on the principles already established for present generation plant. These are discussed below.

There needs to be assurance that all technical safety needs are complied with in design. The following safety design principles are now accepted in most countries, operating nuclear plant (Table 7.4). Many of these have been put forward by the IAEA and are included in the IAEA list of 25 safety principles, listed in the next chapter.

The design must be such that plant operation is reliable, stable and manageable. Prevention of accidents is the prime goal. For many new evolutionary designs, the goal has been extended to provide better protection against severe accidents (Table 7.5).

Table 7.4. Safety fundamentals in design

Design must ensure the nuclear installation is suited for reliable, stable and easily manageable operation Design must include appropriate defence-in-depth principle Technology must be proven or qualified by experience or testing or both Man-machine interface and human factors must be included in the design and in the development of operational requirements Radiation exposures to site personnel and releases to the environment must meet ALARA principles

A comprehensive safety assessment and independent verification must confirm that the design meets the safety objectives before the operator completes his submission to the licensing authority

Table 7.5. Evolutionary plants: safety features

Objective

Achieved by:

Increased margins and grace periods

Larger components and water volumes Lower power densities

Improved safety system reliability

Simpler redundant and diverse safety systems, greater physical separation, utilisation of high reliability components

Preclusion of high pressure core melt ejection

Reliable depressurisation systems

Increased inherent safety

Passive cooling and condensation systems

Corium confinement and cooling

Introduction of core catchers

Robust defence-in-depth

Strong containments to withstand internal and external challenges

Hydrogen management and control

Hydrogen recombiners

Juhn (1999).

The ‘defence-in-depth’ principle that a number of levels of protection and multiple barriers are included to prevent radioactive release is well accepted. This ensures that the combinations of failures that could occur that could lead to a significant release are of very low probability. In advanced designs, the tendency is to increase the robustness of this principle by appropriate design.

An important requirement is to ensure that the design technology is proven. Advantage should be taken of experience, if relevant, if not by further testing or possibly a combination of both.

Man-machine interfaces and human factors must be considered in the design and must be incorporated into the development of operational requirements. A key objective of newer designs is to reduce human errors.

The ALARA principle should be adopted in the design in respect of staff exposure on site and in the releases of radioactive materials to the environment. A reduction of exposures is the goal in newer designs.

Confirmation of the design via a comprehensive safety assessment and independent verification should be carried out to ensure that safety requirements are met prior to submission of the case to the regulating body.

The case must show that the risk to workers and the public is continually decreasing and demonstrate that operation is environmentally friendly.

This can be achieved by a suitable containment, which is designed to reduce the frequency of large releases to very low levels. This needs to be demonstrated via appro­priate analysis (probably via deterministic and probabilistic means in addition to improved defence-in-depth).

In general, the protection of the workers and the public impacts must be demonstrated in the design, operational procedures and environmental assessments.

Development of a transparent and stable process for the licensing of plant.

A well-established and stable generating framework is an important requirement with good interfacing between the licensee and the regulatory body. The process can be enhanced via a rigorous self-assessment process coupled with independent assessment.

Need to gain public acceptance on the benefits of the proposed new design.

Harmonisation of regulatory approach, which may be more possible for new designs, is a good means of increasing public understanding and acceptance of nuclear safety.

An extremely important requirement is that there should be no serious accidents on current plants and that the nuclear industry is seen to act with integrity.

Safety requirements can be met while still maintaining costs at a level for nuclear plant to remain competitive with other generators.

The economics of nuclear power generation is improved by longer fuel cycles and by longer life (including life extension on current plants). This will clearly remain true for new designs as well.

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