Author: Masaharu Tsubokura
Editors: Akihiko Ozaki M.D., Yuki Senoo
129. Nuclear power accidents are evaluated based on their impacts
The atomic bombs on Hiroshima and Nagasaki during the Second World War caused enormous damage to the residents. Since then, human-made radiation exposure has occurred several times, including nuclear weapon testing associated with the development of atomic bombs and hydrogen bombs and accidents at nuclear processing facilities and nuclear power plants. The Chernobyl and Three Mile Island nuclear power plant accidents are well-known, in addition to the Fukushima Daiichi nuclear disaster.
The former Soviet Union succeeded in developing the world’s first commercial nuclear power generation. In Chernobyl, on April 26, 1986, a stress test of the power plant was conducted under the condition of failure in the transmission of the external power supply. However, an accumulation of several factors occurred in this experiment that resulted in a massive steam explosion, a fire lasting more than a week, and the release of radioactive substances (called positive scrum).
First of all, the operator forcibly continued the experiment even though the output was lower than planned and there was a structural defect in the system (control rod) that was supposed to control the chemical reaction within the reactor. Due to these circumstances, the status of reactor became even more out of control when the emergency stop button was activated.
Such nuclear accidents are evaluated in eight stages from “Level 0” to “Level 7” depending on their impacts (International Nuclear Event Scale). The disasters at Chernobyl and Fukushima were recorded as level 7, which indicates they were “severe accidents.”
130. Misoperation and nuclear power plant damage ?
Other than the atomic bombs on Hiroshima and Nagasaki during the Second World War, human-made radiation exposure has occurred several times, including nuclear weapon testing and accidents at nuclear processing facilities and nuclear power plants. The Chernobyl and Three Mile Island nuclear power plant accidents are well-known accidents besides the Fukushima Daiichi nuclear disaster.
On March 28, 1979, radioactive substances leaked into the environment from a nuclear power plant located in the U.S. state of Pennsylvania (Three Mile Island), just three months after starting commercial operation. This was the first nuclear power plant accident involving the evacuation of 100,000 people.
Due to human error, the water supply pump stopped working. This caused the reactor pressure and temperature to rise, the pressure relief valve to open, and the reactor to shut down eventually.
Because the pressure relief valve broke and remained open, the reactor coolant was lost. In addition, the operator failed to recognize that the coolant had reduced. As a result, the nuclear reactor core was damaged and radioactive substances leaked into the environment.
Fortunately, area residents’ levels of radiation exposure were low. However, this accident was marked as level 5, according to the International Nuclear Event Scale, which indicates an accident involving risk outside the facility.
The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, on 2nd and 9th July 2017 was reproduced for MRIC Global under the author's permission.