1.1 INTRODUCTION

1.1.1 Safety and Design

The designer of a fast reactor, just like the designer of any other engineering enterprise, has to take into account what might happen if something goes wrong. He or she has to make sure that whatever happens the risk of injury — either to the operating staff or the general public — or of damage to property, is very slight.

There are basically two ways of making a reactor safe. First the overall design concept is chosen so that it is inherently safe. That is to say that for a number of possible accidents the design is such that the reactor behaves safely and damage does not spread even if no protective action, automatic or deliberate, is taken. But it is not possible to guard against all accidents in this way, however well the overall design is chosen. The second way to make the reactor safe is to incorporate protective systems. These are devices designed specifically to prevent the damaging consequences of accidents. A protective system can be active, such as an automatic shutdown system, or passive, such as a containment barrier.

The design aim is to make sure that the risk to the public is suf­ficiently small to meet the criteria of acceptability. To ascertain that the aim has been met the designer has to determine the response of

the reactor, with its protective systems, to a range of accidents. To test the systems thoroughly it is often necessary to assume that certain accidents happen, even though no way is known by which they could actually take place. These are known as “hypothetical accidents”. The final step is to analyse the accidents, whether hypothetical or not, and to ensure that the risks meet the criteria imposed by the authorit­ies that regulate nuclear activities. These criteria vary of course from country to country.

The main concern in reactor safety is to make sure that the radio­active materials — fuel, fission products and activation products — are contained adequately and do not escape to the environment. This is the main subject of this chapter, which is confined to consideration of the safety of the reactor alone. Questions of the safety of fuel man­ufacture and transport and of waste disposal are not addressed here, nor are other risks associated with the steam and electrical plant, for example.

The safety of nuclear reactors in general is discussed by Lewis (1977) and Farmer (1977). Detailed accounts of some of the subjects touched on briefly here are given by Graham (1971) and Waltar and Reynolds (1981).