Thanks to multiple safety systems and technological solutions, modern plants have a statistical likelihood of a serious incident occurring less than once every million years.
As the chance of a disruption or an accident can never be fully excluded, plant design will include features to prevent the progress of any disrupting events or to limit them.
Safety systems are dimensioned to meet the requirements of normal needs many times over: if one device were to fail, another can take care of the task. The design of a nuclear power plant is based on defence-in-depth principle: if one safety system should fail, another system, based on a different operating principle, can take over. Should the second system fail, yet another system of a different type will assume the task.
In order for a disruption to advance to an accident, it would require simultaneous failure of several systems, which is highly unlikely.
Defence-in-depth principle is also visible in the structures of the power plant. Nuclear fuel is located within several protective shells, each of which is able to contain all radioactive materials by itself.
There are several inherent safety features in light water reactors. If for example the cooling of the nuclear fuel in the reactor would fail, the reactor will turn itself off so that the chain reaction in the fuel can no longer continue. Residual heat generated in the reactor has to be removed even when the reactor has been shut down.
If the supply for cooling water stopped for some reason, cooling water can be obtained from the plant’s cleanwater tanks for a considerable length of time. Electricity supply to the plant is also secured with diesel generators, with its own emergency power supply plant as well as by having several connections to the national grid.