Swiss Solutions for Providing Electrical Power in Cases of Long-Term Black-Out of the Grid

A better understanding of nuclear power plant electrical system robustness and defence-in-depth may be derived from comparing design and operating practices in member countries. In pursuing this goal, the current paper will focus on Switzerland. It will present in general the protective measures implemented in the Swiss nuclear power plants to ensure power supply, which comply with the 'Defence-in-depth' principle by means of several layers of protection. In particular it will present the measures taken in case of a total station blackout. The different layers supplying electricity may be summed up as follows. The first layer consists of the external main grid, which the plant generators feed into. The second layer is the auxiliary power supply when the power plant is in island mode in case of a failure of the main grid. A third layer is provided by the external reserve grid in case of both a failure of the external main grid and of the auxiliary power supply in island mode. As a fourth layer there exists an emergency electrical power supply. This is supplied either from an emergency diesel generator or a direct feed from a hydroelectric power plant. In the fifth layer, the special emergency electrical power supply from bunkered emergency diesel generators power the special emergency safety system and is activated upon the loss of all external feeds. A sixth layer consists of accident management equipment. Since the Fukushima event, the sixth layer has been reinforced and a seventh layer with off-site accident management equipment has been newly added. The Swiss nuclear safety regulator has analysed the accident. It reviewed the Swiss plants' protection against earthquakes as well as flooding and demanded increased precautionary measures from the Swiss operators in the hypothetical case of a total station blackout, when all the first five layers of supply would fail. In the immediate, a centralized storage with severe accident management equipment was jointly set up by the operators. This equipment would be transported to the plant site by land or air. In a second step, each operator installed additional severe accident management diesel generators in each plant and prepared the necessary cabling and switch gear. Particular attention was dedicated to establish procedures so that the hooking and operation of the accident management equipment could be directly performed by shift personnel. The presentation shall show both current practices and recent design changes of safety-related electrical systems in nuclear power plants in Switzerland. (authors)