Nuclear safety of a nuclear power plant is conditioned by failure free and reliable operation of all its facilities and systems that prevents occurrence of an accident or removing consequences of failures, retaining radioactive materials in defined areas, and preventing their spreading into the environment in case of their release. The present level of safety of nuclear reactors ensures that all systems are able to cope with failures separately without endangering the population and the environment.
Basic principle of safety of nuclear power plants is based on a defence in depth principle. The defence in depth consists of 4 physical barriers and 5 levels of protection. The defence in depth principle is designed for compensation of potential human errors and equipment failures. It concentrates on several protection barriers, including those preventing release of radioactive substances into the environment. The principle covers also protection of the barriers themselves and other measures for protection of the population and the environment against damage if these barriers are not fully effective.
Reliability of each of these barriers is very high and probability of simultaneous damage of all three barriers and release of radioactive material into the environment is very low.
- The first barrier consists of crystal fuel matrix in form of ceramic tablets.
- The second barrier is fuel cladding – hermetically sealed tube made out of zirconium alloys.
- The third barrier is a hermetically sealed primary circuit pressure system with the reactor pressure vessel preventing release of coolant with radioactive substances into the environment at all anticipated temperatures and pressures.
- The fourth barrier is the containment – 1,5 m thick steel-concrete envelope, retaining radioactive substances that could be released through potential damage of the first three barriers.
- level: conservative design, quality assurance, inspection activities, safety culture;
- level: conduct of operation including response to abnormal operation conditions ensuring integrity of the first two physical barriers;
- level: safety and protection systems for fulfilling three basic safety functions;
- level: measures to maintain integrity of the containment;
- level: measures of on-site and off-site emergency plans to mitigate effects of RA substance released into the environment.