Y. V. PUZANOV, SUE SIA ‘Radon’, Russia DOI: 10.1533/9780857097446.1.327

Abstract: The axiomatic basics of quantitative safety/risk assessments are discussed. Deterministic and probabilistic analysis methods are then introduced. As an illustrative example, safety assessment for the environment is given in terms of the probability of radionuclide escape from a near-surface disposal facility. Emergency accident levels are correlated with the probabilities of those accidents occurring.

Key words: safety, risk, equipment failure, probability of failure, emergent event tree, failure trees.

9.1 Introduction

Increased attention is being given to issues of safety in the nuclear field as evidenced by the large numbers of publications on this topic. These can be divided into two types according to how the term ‘safety’ is interpreted. The first, the subjective type, concerns the safety of the environment, population or personnel in the proximity of radioactive materials and ionizing radiation sources. The second, the objective type, discusses the safety of nuclear power plants (NPP) or nuclear hazardous objects, although the influence of these objects on the environment or people is also discussed. In the first case, safety is treated in terms of the ability to protect from the effects of ionizing radiation. In the second case, safety is treated as a property of an object, i. e. the property of not rendering an action hazardous, not resulting in contamination or not resulting in the spread of radioactivity. This ambi­guity can lead to misunderstandings in safety assessments. For example, with respect to the ability to protect, it is necessary to take into account the availability/absence of radiation detectors and means of personal protec­tion. If, on the other hand, safety is the property of an object and the need is to avoid hazardous action, taking, for example, the NPP, then other factors must be included associated with the object’s composition, for example the physical barriers designed into the system.

Scientific safety assessment requires that the true character of both sub­jective and objective safety problems is considered quantitatively in math­ematical terms. In this chapter safety assessment is considered using the following system of definitions and axioms:

Definition 1. The safety of an object indicates the state of its immunity from the harmful influence of other objects or factors dangerous to it.

Immunity assumes the ability to resist harmful influence. This ability can be realized through the presence of systems and the elements which prevent penetration of the dangerous object and characteristics of the protected object, i. e. through the presence of a specified protective shield separating the dangerous and protected objects. In other words, it is possible to say that the object is protected if it is supplied with a protective shield (Fig. 9.1).

Definition 2. The protective shield is the means of reducing or eliminating harmful influence on the protected object.

Objects may be divided into the protected or dangerous by analyzing their interaction. The object which has harmful influence is considered as danger­ous. The object, whose safety should be ensured, is protected. For the theory to be developed we must formulate:

Axiom 1. There are protected objects and dangerous objects or factors.

This axiom is needed to clearly establish the safety of the object in question, as it is protected, and dangerous objects may change their roles. For example, if we consider the harmful affects of radioactive wastes (RAW) on the environment, the latter is protected and the wastes are a dangerous object. If we are talking about external influences on disposal wastes in a facility, then all the potential, unauthorized entrants such as terrorists, meteorites, infiltrating moisture, insects and rodents, that is, in fact, the environment, become a dangerous object, and the wastes are what must be protected. This shows that to distinguish the protected objects from the dangerous it is necessary to consider the postulate of their interaction:

Axiom 2. A dangerous object has a harmful influence on the protected.

From Definition 2 and the axioms, the obvious conclusion can be formulated:

9.1 Protective shield.

Safety principle: For the safety of a protected object it should be separated
from the dangerous object or factors by means of a protective shield.

These definitions, axioms and the safety principle are the axiomatic basis of the safety theory as a scientific area. The safety principle enables us to understand that quantitative safety assessments can be made, characterizing the state of the protective shield, its performance and reliability. Therefore, the subject of study of the safety theory is the protective shield. The theory involves the use of mathematical modeling of the system’s functioning as the protective shield and the components that make up the protective shield, as well as numerical methods and natural, physical experiments to determine the characteristics and parameters of these systems.