What are the main groups of causes of accidents at radiation hazardous facilities. What zones of radioactive contamination (contamination) arise during accidents at nuclear power plants? What are the phases of accidents at nuclear power plants and describe them.
The main groups of causes of accidents at radiation hazardous facilities:
• equipment failure due to imperfection of the design of the installation, violation of the technology of its manufacture, installation or operation;
• erroneous actions of personnel or intentional violation of operating rules;
• external events (the fall of aircraft and missiles, natural disasters, the impact of various types of weapons, sabotage acts).
In accidents involving the release of radioactive substances at nuclear power plants, areas of radioactive contamination (contamination) of the area are formed in the form of a circle (in the accident area) and an elongated ellipse (in the wake of a radioactive cloud); the ellipse may have the correct shape under normal topographic and meteorological conditions and incorrect under difficult topographic and meteorological conditions (rough terrain, changes in wind direction and speed, etc.).
For the correct and effective organization and conduct of protective measures, the areas of radiation pollution of the area are divided into zones:
• external exposure (moderate, strong, dangerous and extremely dangerous);
• internal exposure (hazardous and extremely hazardous).
In accidents involving destruction of the reactor, all irradiation zones are formed. The greatest danger in this case is external exposure. In accidents without destruction of the reactor, the most dangerous is the internal irradiation of the human thyroid gland.
Accidents at radiation hazardous facilities have four phases of development: initial, early, middle and late.
The initial phase of the accident is the period of time preceding the start of the release (discharge) of radiation into the environment, or the period of detection of the possibility of exposure to the population outside the sanitary protection zone of the enterprise. In some cases, this phase is not fixed due to its transience.
The early phase of an accident is the period of direct release (discharge) of radioactive substances into the environment, places of residence or distribution of the population. The duration of this period can be from several minutes or hours in the case of a single release (discharge) to several days in the case of a prolonged release (discharge).
The average phase of an accident includes a period during which there is no additional release of radioactivity from a source of release (discharge) into the environment. This phase can last from a few days to a year after the accident.
The late phase of the accident (recovery phase) is the period when the population returns to normal living conditions, that is, until the need to take protective measures ceases. It can last from several weeks to several years or decades depending on the power and radionuclide composition of the release, the characteristics and size of the contaminated area, the effectiveness of radiation protection measures.