Reactor safety

Design and operating standards

Essentially, preventive measures are the set of design and operating rules that are intended to make certain that a reactor is operated safely. The nuclear industry in the United States created a design philosophy referred to as “defense in depth” that numerous other countries have also adopted. In a nuclear power plant following the defense-in-depth model, all safety systems are required to be functionally independent, inherently redundant, and diverse in design.

Among the most well-known preventive measures are the reports and inspections for double-checking that a plant is properly constructed, rules of operation, and qualification tests for operating personnel to ensure that they know their jobs. Nuclear reactors must operate under a very high standard of quality assurance, requiring staff members to audit, evaluate, survey, and verify that all procedures and maintenance are being performed as they should be.

An important part of a safety system is strict adherence to design requirements: the reactor must have a negative power-reactivity coefficient; the safety rods must be injectable under all circumstances; and no single regulating rod should be able to add substantial reactivity rapidly. Another important design requirement is that the structural materials used in the reactor must retain acceptable physical properties over their expected service life. Finally, construction is to be covered by stringent quality-assurance rules, and both design and construction must be in accordance with standards set by major engineering societies and accepted by regulatory bodies.

Since no human activity can be shown to be absolutely safe, all these measures cannot reduce the risks to zero. However, it is the aim of the regulations and safety systems to minimize risks to the point where a reasonable individual would conclude that they are trivial. What this de minimis risk value is, and whether it has been achieved by the nuclear industry, is a subject of bitter controversy, but it is generally accepted that independent regulatory agencies—the United States’ Nuclear Regulatory Commission (NRC), the United Kingdom’s Office for Nuclear Regulation (ONR), the International Atomic Energy Agency (IAEA), and similar agencies around the world—are the proper judges of such matters.

The Windscale accident of 1957

Prior to the development of current preventive-design philosophies, the world’s first large-scale nuclear reactor accident took place in October 1957 at Windscale, Cumberland (now part of Cumbria), northwestern England. The Windscale plant was powered by a pair of identical reactors, known as Piles 1 and 2, that were an air-cooled, graphite-moderated design. Initially constructed between 1946 and 1950 to produce plutonium for nuclear weapons, they also provided energy for electricity production. However, with Britain fully engaged in the nuclear arms race, the reactors’ operations were subject to direct political influence, and Windscale engineers were driven to modify the fuel design in order to increase plutonium and tritium production. These modifications increased heat generation in the fuel and therefore temperature levels within the reactor cores.

After approximately seven years of successful operation, the increased production rate became too much for Pile 1 to handle. Possibly as a result of imperfections in one of the fuel elements, the core began to overheat, and fire broke out within the lithium-magnesium-clad uranium fuel slug. In an attempt to remove heat from the core, operators turned on all the reactor’s air-cooling fans. Unfortunately, this only fanned the flames and spread the fire through the core region. In addition, a significant amount of radioactive contamination was released through the ventilation stacks when the fans were turned on. Operators ultimately put out the fire by turning the fans off and forcing water through the core.

The Windscale event caused much less damage than the Three Mile Island, Chernobyl, and Fukushima accidents of later years (see below). Nevertheless, it provided an explicit demonstration that political agendas (in this case, participation in the nuclear arms race) must be separated from those of energy and safety.

The Reactor Safety Study of 1972–75

In 1972, as part of an effort to evaluate the risks from nuclear power plants, the U.S. Atomic Energy Commission (a predecessor of the NRC) authorized a major safety study. Conducted with major assistance from a number of laboratories, the AEC’s study involved the application of probabilistic risk assessment (PRA) techniques for the first time on a system as complex as a large nuclear power reactor. Also for the first time, the study compared the risk of a nuclear power plant accident with other events such as natural disasters and human-caused events. This work resulted in the publication in 1975 of a report titled Reactor Safety Study, also known as WASH-1400. The most useful aspect of the study was its delineation of components and accident sequences (scenarios) that were determined to be the most significant contributors to severe accidents.

The Reactor Safety Study concluded that the risks of an accident that would injure a large number of people were extremely low for the light-water reactor (LWR) systems being analyzed. This conclusion, however, was subject to very large quantitative uncertainties and was challenged. One basic problem with PRA techniques is that it cannot easily be confirmed by experience when the level of risk has been reduced to low values. That is to say, if PRA predicts that a reactor is subject to, say, one failure in 10,000 years, there is no way to prove that statement with only a few, or even with 10,000, years of experience. Thus, the results of the Reactor Safety Study as to risk levels were not confirmable. Nevertheless, updated versions of the report still provide the framework and reference for nuclear-related probability risk assessment.