Makai Mihály - Reactor Core Monitoring: Background, Theory and Practical Applications

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Part I
Safety First

Part I is a concise summary of the most important safety considerations taken into account during the design and operation of nuclear reactors, in particular for nuclear power plants.

Springer International Publishing AG 2017

Mihly Makai and Jnos Vgh Reactor Core Monitoring Lecture Notes in Energy 10.1007/978-3-319-54576-9_1

1. Reactor Safety Goals

Mihly Makai 1 and Jnos Vgh 2

(1)

Budapest, Hungary

(2)

Alkmaar, The Netherlands

Mihly Makai

Email:

Abstract

This chapter describes the safety goals generally applied for the design, licensing, operation and decommissioning of nuclear power plants (NPPs).

1.1 Safety Goals

The present section gives a general description of the safety goals applied at NPPs, discussing also the differences between regulatory goals and designers goals . Nuclear regulatory authorities usually require the fulfilment of technology neutral or technology independent safety goals, while designers must obviously apply design-oriented, technology-specific safety goals, as well.

1.1.1 Fundamental Safety Principles

Principles and basic definitions are best outlined in the IAEA Safety Standards Series with []):

1. Principle: Responsibility for safety An effective legal and governmental framework for safety, including an independent regulatory body, must be established and sustained.
2. Principle: Role of government An effective legal and governmental framework for safety, including an independent regulatory body, must be established and sustained.
3. Principle: Leadership and management for safety Effective leadership and management for safety must be established and sustained in organizations concerned with, and facilities and activities that give rise to, radiation risks.
4. Principle: Justification of facilities and activities Facilities and activities that give rise to radiation risks must yield an overall benefit.
5. Principle: Optimization of protection Protection must be optimized to provide the highest level of safety that can reasonably be achieved.
6. Principle 6: Limitation of risks to individuals Measures for controlling radiation risks must ensure that no individual bears an unacceptable risk of harm.
7. Principle: Protection of present and future generations People and the environment, present and future, must be protected against radiation risks.
8. Principle: Prevention of accidents All practical efforts must be made to prevent and mitigate nuclear or radiation accidents.
9. Principle: Emergency preparedness and response Arrangements must be made for emergency preparedness and response for nuclear or radiation incidents.
10. Principle: Protective actions to reduce existing or unregulated radiation risks Protective actions to reduce existing or unregulated radiation risks must be justified and optimized.

The above ten fundamental safety principles form the general basis on which IAEA safety requirements for protection against exposure to ionizing radiation are formulated. One can see that the above high level safety principles are very general and technology-neutral; thus there is room for various interpretations when defining specific safety goals for design, operation and decommissioning.

1.1.1.1 Safety Goals

A safety goal is a set of quantitative and/or qualitative requirements to be fulfilled in order to ensure that the desired level of safety is achieved. Consistent and internationally harmonized and acknowledged safety goals might represent solid technical basis for carrying out safety assessments to determine whether a nuclear facility meets safety expectations, or not. However, probably their most important role is to support/justify specific design solutions and facility operation modes.

In the last decade concerted efforts were made to establish an internationally acknowledged hierarchical system of safety goals, see [] for details).

• Within the hierarchy, high level, technology-neutral safety goals are properly linked to low level, technology-specific goals;
• The framework provides practical assistance to designers, vendors, operators and regulators to achieve uniform and comparable levels of safety when dealing with various nuclear facilities using various technologies at various sites;
• It ensures the public unambiguously, that the necessary and sufficient protection is provided in all cases.

Obviously, an appropriate hierarchy of safety goals should be applicable for all possible nuclear installation types, during their entire lifetime and in all possible operational states, including accidents. Reference [.

Fig. 1.1

Basic types of safety goals []

Fig. 1.2

Framework of safety goals proposed by the IAEA []

The proposed safety goal pyramid consists of four levels. The top level of the hierarchy corresponds to the fundamental safety objective (see Sect. ). The 3rd upper level is basically concerned with the whole site and is still technology-neutral. The 2nd intermediate level also provides generic safety principles related e.g. to defence-in- depth and physical barriers . If quantitative safety goals are included here, then they are basically technology-neutral and site independent. The first low level contains technology-specific safety goals for all facilities located at the specific site. The quantitative goals given here are technology-specific, e.g. maximum fuel cladding temperature, LRF and CDF target values, etc.

The MDEP (Multinational Design Evaluation Programme) is an international initiative launched by the nuclear safety regulators of 15 countries involved in the safety assessment of Generation III reactors ( EPR , AP1000 , AES-2006 , ABWR and APR1400 ). The basic aim of MDEP is the harmonization of safety requirements and to share country-specific knowledge accumulated in relation with the various Gen III designs. During the course of its activities the MDEP encountered the problem of heterogeneous and country-specific safety goals; therefore it decided to elaborate a different approach in order to facilitate a broader harmonization of regulatory requirements. The MDEP proposed a top-down approach consisting of three hierarchical levels (see Fig. ).

Fig. 1.3

Structure of safety goals and targets as proposed by the MDEP []

First the top-level safety goals are established, then the structure of the lower levels is defined, together with the method applicable to derive lower level safety goals. Its main novelty is that the proposed hierarchical structure of safety goals is based on the defence-in- depth (DiD) concept and it puts forward a method how to develop lower level safety goals by using higher level safety goals. The concept is technology-neutral and applicable for water-cooled and non-water cooled reactors alike. Note that the MDEP is not proposing an ultimate system of safety goals, rather it proposes a method for deriving the system of safety goals for any type of reactors. The concept is based on the recognition that although the top level safety goals are by definition technology-neutral, the lower levels must inevitably contain technology-specific goals and targets, in order to supply usable and appropriate guidance for the design and operation of a specific facility.

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