An engineer's view of safety regulation

Nuclear safety is primarily a matter for engineers, and that is just as well. A reactor operates at a crossroads between many disciplines, including chemistry, mechanical engineering, physics, automation, computing, neutronics and thermodynamics. To design and safely operate these technological monsters requires theoretical and practical knowledge of which engineers have a better grasp than managers or administrators. An accident may occur when a chain reaction goes out of control or a break appears in the cooling system. So the parameters which must be monitored are the fuel and moderator temperature coefficients, excess radioactivity, the neutron life cycle, capture cross-sections for neutrons of varying energies, calorific capacity and thermal conductivity, and radiation resistance of cladding. In short, a series of values with which accountants and financiers are wholly unfamiliar.

If you open a book on nuclear power written by an engineer, you will find the key principles and full details of how a reactor works, with a section on safety. This describes two possible approaches: a deterministic approach based on the defence-in-depth concept; or a probabilistic approach based on risk calculation. The first one dates from the 1950s, the second took shape in the late 1960s. For many years they were presented as rivals, but the quarrel of the ancients and moderns is now largely behind us. The probabilistic approach, discussed in Chapter 4, is based on identifying fault sequences which have the potential to initiate a major accident. The deterministic approach considers a whole list of plausible incidents and accidents, but without assigning them a probability. It frames rules, either to avoid such events purely and simply, or to mitigate them, but without trying to calculate the scale of the reduction. For example it is a basic rule that a failure on a single device must not be able to cause an accident on its own. Defence in depth is closely linked to the deterministic approach, because it involves raising a succession of barriers to prevent the release of radioactive materials into the environment. Furthermore, the dimensions of each barrier include a safety margin.