HIERARCHICAL APPROACH TO SAFETY GOALS

For establishing a hierarchical structure of safety goals the following general aspects are expected to be considered:

 The hierarchy is to be applicable to all types of nuclear installations (examples are however often provided for water-cooled NPPs, specifically LWRs).

 The hierarchy is to be applicable to all relevant lifetime stages.

 The hierarchy is to cover the applicable states of the installation, e.g. operational states and accident conditions.

 The hierarchy is to complement and be in agreement with the structure of the IAEA Fundamental Safety Principles and Safety Standards.

 The hierarchy is to be consistent with the structure and intents of defence-in-depth and support its implementation.

 The Top Level safety goals express overall requirements on society level, while lower levels will successively detail the top level goals.

 Safety goals on different levels are to be consistent and traceable, allowing to derive lower level goals from higher level ones.

 Higher level safety goals are as far as possible to be technology neutral, while lower level goals are expected to be increasingly technology specific.

 The hierarchy is to include qualitative as well as quantitative safety goals.

 The structure is to be clearly and unambiguously defined, making it easy to understand, implement and communicate.

It has to be noted that some further considerations may need to be taken into account during the process of applying the actual safety goals forming the hierarchy, which are discussed in Section 4.

2.3.1. Proposed hierarchy of safety goals

The proposed hierarchy of safety goals is shown in Figure 2. The picture also illustrates how the levels of safety goals relate to different aspects of safety (society/site/facility) and to technology (technology independent/specific), and the potential overlaps. In Table 1, each of the levels of safety goals is briefly characterised. A more detailed discussion about each level is provided in the following sub-sections. In addition, in Section 3, the derivation of the hierarchy of safety goals is discussed in more detail and examples are given of specific safety goals on the various levels.

FIG. 2. Proposed hierarchy of safety goals.

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TABLE 1. HIERARCHICAL LEVELS OF SAFETY GOALS

4 Safety goals mentioned in this table and throughout this section are expected to be seen as examples and do not constitute a complete list of safety goals.

Level Overall Objective Description

Explanations on the Nature of Safety Goals and Examples⁴ qualitatively and may presuppose, e.g.

the prevention of unreasonable harm to the public and the environment. These

Upper Level safety goals are high-level and used as a bridge to support the development of Intermediate and Low Level safety goals from the Top Level.

In some countries, this is done by relating to levels of risks from other involuntary sources of risk, using quantitative or semi-quantitative expressions of relation between risks from nuclear installations and risks from other involuntary sources of risk, e.g. considerations related to independence and protection of barriers, redundancy and independence, doses for normal operation, amounts of radioactive waste generated, etc.

This level also includes the definition of some high-level quantitative safety goals, e.g. overall large early release frequency (LERF) for the site.

TABLE 1. HIERARCHICAL LEVELS OF SAFETY GOALS (cont.)

2.3.2. Top level safety goals

Top Level safety goals are the highest level safety goals as defined in national legislation or regulations. In many countries, nuclear safety is ultimately governed by qualitative safety goals at the society level, which are often defined in nuclear legislation but may also be issued by regulatory authorities. These safety goals differ in wording between countries, but generally presuppose the prevention of unreasonable harm to the workers, public and the environment from ionizing radiation, in line with the IAEA Fundamental Safety Objective [1].

Top Level safety goals are important as high-level statements, but may not be sufficient themselves to be used as a basis for defining detailed safety goals.

2.3.3. Upper level safety goals

The Upper Level safety goals provide an interpretation of the Top Level safety goals in terms of overarching requirements to ensure adequate protection and limiting risks of undue harm to people and the environment in operational states and accident conditions. Upper Level safety goals are expressed in more detail than the Top Level safety goals, providing a bridge to the more detailed technical safety goals at the Intermediate and Low Levels. The Upper Level safety goals may also reflect different aspects of risk, such as effects on individuals and society at large.

Upper Level safety goals are typically technology neutral and have a site-wide scope thus providing a basis for Intermediate and Low Level safety goals, which may require an interpretation in numerical terms of what constitutes an unreasonable risk to an individual or to the society.

Upper Level safety goals typically define harmful effects in a way that allows detailed interpretation on the underlying levels. In some countries this is done by comparison with other involuntary sources of risk, e.g., with fatality risks from other sources of energy production or cancer fatality risks from other human-made causes, to which an individual is exposed. In addition, Upper Level safety goals may also be qualitative to draw attention to particular nuclear safety aspects, such as minimisation of radioactive waste, effective leadership and management, etc. Upper Level safety goals may also consider tolerable levels of disruption to the population in the site vicinity, e.g. evacuation, relocation-areas and time scales, and agricultural restrictions.

Level Overall Objective Description

Explanations on the Nature of Safety Goals and Examples

A large number of specific deterministic safety goals are in use, e.g. related to maximum fuel cladding temperature in an LWR.

This level may also include quantitative probabilistic safety goals, e.g. for a specific plant unit, the frequency of large release, core or fuel damage, barrier strength, or SSC reliability.

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2.3.4. Intermediate level safety goals

Intermediate Level safety goals are normally to a large extent technology neutral but can include the highest level safety goals for application to specific technologies. This is also the appropriate level to distinguish between facility and site-wide safety goals. Thus, the scope is basically site-wide, but may also include aspects related to specific facilities.

Intermediate Level safety goals are aimed to cover crucial general safety principles and provisions such as defence-in-depth, safety margins, physical barriers (including considerations related to independence and protection of barriers), and redundancy and independence.

Safety goals on Intermediate Level also include site level requirements as appropriate, e.g.

related to potential releases (total frequency for the site for large (early) release) or site level capability to handle external hazards with a certain frequency and magnitude.

2.3.5. Low level safety goals

The Low Level safety goals are technical and aim at assuring the nuclear installation meets the higher level safety goals, by addressing siting, design and operational aspects of a nuclear installation. Technical safety goals are also more directly useful as means to evaluate the adequacy of existing or proposed designs of safety related SSCs.

At this level, various aspects of the design and operation of a specific nuclear installation are assessed against safety goals. There is a wide range of safety goals on this level to address both operational states and accident conditions.

Some Low Level safety goals are qualitative and relate to whether a risk, or a condition that may result in a risk, is acceptable. Quantitative deterministic safety goals may relate to maximum or minimum values of crucial parameters, such as fuel temperature, pressure or water levels. Quantitative probabilistic safety goals are expressed as frequencies or probabilities of unacceptable states or consequences. Low level safety goals can constitute requirements or acceptance criteria for design and operation.

3. DERIVATION OF SAFETY GOALS