The principles established by the Reactor Safety Guide were given an unexpected test in 1979 when Three Mile Island Unit 2 near Harrisburg, Pennsylvania, suffered a severe accident. Through the failure of an important valve to operate correctly, cooling water to the core was lost, parts of the core were melted and the rest of it destroyed, and a large quantity of fission products was released from the primary reactor system to the interior of the containment structure. The equipment failure was exacerbated by reactor operator error, as the emergency core cooling system was deactivated by operator action because of a misinterpretation of the type of accident that was occurring. Fortunately, the containment vessel of the reactor building fulfilled its function, and only a small amount of radioactivity was released, demonstrating the wisdom of incorporating this structure into a rigorous design. All the same, a severe accident had occurred.

Many investigations of the Three Mile Island accident followed. Recommendations differed among them, but a common thread was that the human element was a much more important factor and higher risk to the safe operations of a nuclear power plant than had been recognized. The human element pertained not only to the operating staff but also to the management of nuclear plants and even the NRC itself. As a result, following the accident many changes in operator training and in technical and inspectorate staffing were implemented, just as a number of hardware enhancements were introduced. It is generally believed that these changes have been effective in reducing the likelihood of the occurrence of accidents as severe as that at Three Mile Island. As a side issue to this, however, the operating costs of nuclear power plants have escalated sharply as more and more highly trained people have been added to the operating staffs.

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The significance of the human element, particularly as it relates to plant management and rigorous high-level regulatory decision making, was borne out again by the Chernobyl disaster of 1986. One of the four reactors in a nuclear power station approximately 100 km (60 miles) north of Kiev, Ukraine (then part of the Soviet Union), exploded and caught fire as the result of an ill-conceived experiment (a test to quantify how long the steam turbines would run while coasting to a stop if the reactor was abruptly shut down). Before the event was brought under control, an estimated 25 percent of the radioactive contents of the reactor had been released in a high cloud plume. Approximately116,000 people had to be evacuated, and a large area surrounding the plant received fallout so great that it could not be farmed or pastured as a result of this accident. Significant levels of radiation were detected as far north as Scandinavia and as far west as Switzerland.

In September 2005 the Chernobyl Forum, comprising seven United Nations organizations and programs, the World Bank, and the governments of Belarus, Russia, and Ukraine, published a three-volume, 600-page report assessing the impact of the accident on public health. Approximately 50 emergency workers had died of acute radiation sickness shortly after the accident, and 9 children had died from thyroid cancer because of radiation exposure. From among the 200,000 emergency workers who were present at the site in the first year following the accident, the people who were evacuated, and the 270,000 residents of the most heavily contaminated areas, an additional 3,940 people were likely to die from cancer during a prolonged period after the accident.

Investigation of the Chernobyl accident placed the largest blame, as with the Three Mile Island mishap, on poor management both at the plant and within the government bureaucracy. Because these accidents primarily resulted from human failings rather than from some intrinsic factor, most experts have continued to believe that nuclear energy can be a safe source of power. There is, however, a condition on the conclusion that nuclear power is by and large a safe form of power. The facilities for generating this power must be designed, built, and operated to high standards by knowledgeable, well-trained professionals, and a regulatory mechanism capable of enforcing these standards must be in place.

Systems and structures

Mitigating measures, also referred to as safety systems, are systems and structures that prevent accidents from proceeding to a catastrophic outcome in the event they do occur. Two of the principal mitigating measures, described in the section Reactor design and components, are (1) the safety rod systems that quickly put the reactor into a subcritical state and prevent a supercritical accident and (2) the containment structure that prevents radioactive materials from being released into the atmosphere. Other significant mitigating measures include the emergency core-cooling system, whose purpose is to provide sufficient cooling of the core and fuel region within the vessel upon a loss of reactor coolant, and the emergency power system, which is designed to supply electrical power to support systems in the event that the normal supply is disrupted. Emergency power systems are necessary so that detectors, circulating pumps, valves, and other critical components continue to operate as necessary to remove decay heat. An extreme mitigating measure is the evacuation of personnel who might otherwise be heavily exposed in a reactor installation.

According to PRA studies, three categories of events are primarily responsible for the risks associated with LWRs—namely, station blackout, so-called transient without scram, and loss of cooling. In station blackout, a failure in the power line to which the station is connected is postulated. The proposed emergency defense is a secondary electrical system, typically a combination of diesel generators big enough to drive the pumps and a battery supply sufficient to run the instruments. In transient without scram, the assumed event is an insertion of positive reactivity—for example, through an undesired withdrawal of the shim rods. The protective safety system response in this case is the rapid and automatic insertion of the safety rods. In loss of coolant, the event is assumed to be caused by a mechanical failure of the normal cooling system such that a certain amount of the coolant is lost. The emergency response is activation of an emergency core-cooling system. In all such measures, proper operator action and proper functioning of the appropriate backup system are paramount aspects of emergency response.

Other reactor designs pose different types of risk. For example, neither the pool-type liquid-metal reactor (LMR) nor the high-temperature gas-cooled reactor (HTGR) is at major risk with regard to loss of coolant flow and perhaps not with regard to station blackout. However, the LMR and perhaps the HTGR are at some risk from events that might cause air or water to enter the coolant system. The hazard is that reactor materials, sodium or graphite, could chemically react with air and water, causing what is known as an exothermic reaction that releases large amounts of heat in addition to the decay heat already existing within the core region. The hazard is greater with sodium in the LMR than it is with graphite in the HTGR.