![]() However, as experience shows, it cannot be totally excluded that events occur that challenge the integrity of the barriers beyond what they are designed to cope with (their design basis), resulting in failure of one or more of the barriers. ![]() Thus, the prevention of radioactive releases to the environment is based on what is known as the principle of defense-in-depth. Various safety systems, such as emergency cooling systems, are in place to protect the integrity of these barriers also in case of very unlikely events. These are typically the tubes of zirconium alloy into which the uranium oxide fuel pellets are sealed to make up the reactor core, the steel pressure boundary of the reactor core coolant system, and the reactor containment, typically a solid concrete structure enclosing the main parts of the reactor. In normal operation and in a number of events and disturbances taken into account in the design of the nuclear power plant, these radionuclides are prevented from escaping to the environment by several physical barriers. Energy is released in the process and ends up as heat in the reactor core. New neutrons are also produced so that a chain reaction can be maintained. The fission process: A uranium nucleus is split in two parts (fission products) when hit by a neutron. All three severe accidents discussed in this paper had their root causes in system deficiencies indicative of poor safety management and poor safety culture in both the nuclear industry and government authorities. ![]() To achieve these objectives the importance of maintaining high global standards of safety management and safety culture cannot be emphasized enough. It is concluded that essential objectives for reactor safety work must be: (1) to prevent accidents from developing into severe core damage, even if they are initiated by very unlikely natural or man-made events, and, recognizing that accidents with severe core damage may nevertheless occur (2) to prevent large-scale and long-lived ground contamination by limiting releases of radioactive nuclides such as cesium to less than about 100 TBq. Impacts include health effects, evacuation of contaminated areas as well as cost estimates and impacts on energy policies and nuclear safety work in various countries. The root causes and impacts of three severe accidents at large civilian nuclear power plants are reviewed: the Three Mile Island accident in 1979, the Chernobyl accident in 1986, and the Fukushima Daiichi accident in 2011.
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