Preventive measures against power failure

Nuclear power plants depend on electric energy supply ("power"), because they have numerous electrically driven systems. These systems include those that are vital for safety. Therefore, preventive measures are taken in the event of the external power supply of a nuclear power plant failing.

Power supply in a nuclear power plant

A nuclear power plant produces electricity and feeds it into the public grid. At the same time it consumes electricity since it needs power for its electrically driven components (such as pumps, valves, ventilation, etc.). Generally, nuclear power plants in Germany can get power via three different paths:

• Own production

  In normal operation, the power plants provide themselves with the power required for their own use. The remaining power produced is fed into the public grid.

• Via main off-site power connection from the public grid

  When the power plant is not operated or has been shut down (for example during an inspection or after an incident), the nuclear power plant is supplied with power for its own use by the public grid via the main off-site power connection.

• Via spare off-site power connection from the public grid

  If the main off-site power connection fails, one switches to a spare off-site power connection that is connected with another voltage level of the public grid and is independent of the main off-site power connection.

Failure of external power supply

If the connection to the external power supply fails completely, a nuclear power plant can no longer release its generated power into the grid. In that case, the nuclear power plant reduces its capacity drastically to a level that corresponds with its own use of electrical power. Thus, the nuclear power plant can generate its own electricity in isolated operation separated from the public grid. This process is called "load rejection to auxiliary power".

If the load rejection to auxiliary power fails, the nuclear power plant is automatically shut down and covers its demand of electric power gradually via

• Emergency diesel power

  German nuclear power plants have several emergency diesel power aggregates which supply power to all systems required for safety (e.g. cooling water pumps, instrumentation and control). Their number and performance varies depending on the respective plant. Generally, the plants have fuel reserves available for several days. If required, more fuel can be added.

• Neighbouring power plant

  A neighbouring power plant (e.g. gas turbine or hydroelectric power plant) or a neighbouring nuclear power plant unit (in the case of several power plant units present on the plant premises) can provide power. Some plants have a direct connection.

Station blackout: Additional failure of emergency diesel power

If the external diesel power has failed and if the emergency diesel power aggregates fail on top of that, this is referred to as a “station blackout”. In that case battery-supported power supply is available for all areas of instrumentation and control as well as selected, smaller electrical devices. This battery supply can provide the aforementioned areas with power for a limited period of time, even in the event of a failure of the emergency diesel power. However, the battery supply cannot provide larger electrical devices with power, e.g. cooling water pumps.

Furthermore, each nuclear power plant has mobile, protected diesel aggregates available that can be used in the event of a station blackout and reload the batteries and provide selected systems with power.