Steam generator - tube ruptures: reduction of the core melt frequency by primary bleed and RPV-water-level controlled HPI

Loss of coolant accidents (LOCA) because of Steam Generator Tube Ruptures (SGRT) have been investigated in the German Risk Study (Phase B) for the NPP Biblis B, a 1300 MWe PWR. The risk study is performed on behalf of the Federal Minister of Research and Technology. In addition to the risk study the Federal Minister for Environment, Nature Conservation and Nuclear Safety is sponsoring an accident management (AM)-project for reducing the core melt frequencies by hardware changes or by using plant system functions, which e.g. are locked by the reactor protection system during an emergency core cooling (ECC)-situation. The contribution of a severe accident sequence with SGTR as an initial event to risk of uncontrolled radioactivity releases to the environment is important in case of containment bypass caused by a leak at the secondary side. The main steam line components of this NPP are neither designed for a pressure as high as the zero head of the high pressure injection (HPI)-pumps nor for static and dynamic waterloads. Therefore automatic system functions are planned in order to avoid the initiation of HPI in case of steam generator tube failures up to a double ended break of one U-tube. System failures, operator errors or breaksizes > 6 cm2 can cause the actuation of the ECC-criterion and the automatic initiation of the HPI. Operator initiated system functions are planned in order to enable the manual switch off of the HPI. Alternative measures, which will improve the reliability of the shut off of the HPI, and AM-measures, which will be appropriate in case of a HPI shut off failure, have been developed