Mitigation measures for severe accidents in pressurized water reactors of the Federal Republic of Germany

Aware of the safety reserves existing in conservatively designed systems on the one hand and the public discussion about the residual risk in view of Chernobyl on the other, owners of nuclear power plants in the Federal Republic of Germany felt obliged to take measures for a further improvement of NPP safety standards. With these measures the management of beyond design basis events will also be possible. The steps comprise instructions for safety objective oriented procedures, accident management measures and the backfitting of equipment for preventing severe core damage or for mitigating the consequences. They contribute to a further reduction of the residual risk. Accident management measures are possible with the help of existing engineered systems or by components or systems to be backfitted. Preventive measures are preferred to mitigation procedures. As a part of on-site accident management, backfitting measures for primary pressure relief, for the filtered venting of the containment, for air filtration for the control room and other measures were promised to the Federal Secretary of Environment by the power generation industry in the Federal Republic of Germany. Because the measures make possible the controlled pressure relief of the reactor coolant system, a core meltdown in the high pressure path can be prevented. Thus, various possibilities for core cooling in the low pressure range are possible. To prevent containment over-pressurization failure in the hypothetical case of core meltdown, the Federal German PWR containments are presently equipped with a system for venting via aerosol filters. By this means the pressure in the containment is kept below the failure limit and uncontrollable radioactive release is prevented. 6 figs, 1 tab