Implementation of bleed and feed procedures in Siemens PWRs

Abstract

In the last few years work in the field of emergency operating procedures has concentrated on the development, validation and implementation of bleed and feed procedures. This is part of a national strategy in Germany aiming at a further reduction of the residual risk. Designing bleed and feed procedures for all relevant event sequences would have the potential to reduce the core damage frequency by at least a factor of 4 from 3.1 × 10 −6 to 7.5 × 10 −7 per year for the Konvoi plants. A further reduction to 4.4 × 10 −7 per year seems to be achievable if loss of scram in the case of small-break loss-of-coolant accidents and SGTR turns out to be of minor importance. At present procedures are being developed to cope with the complete loss of steam generator feed either by mechanical system failures or by loss of a.c. power supply. Preference is given to the secondary-side bleed and feed procedure and it has been shown that this procedure has the potential to prevent or delay the necessity of primary-side bleed initiation even if it is only partially successful. The failure probability of the two bleed and feed procedures has been estimated to be of the order of 10 −2. The result is dominated by operator errors. Failures of mechanical system functions are negligible. This shows that there is still a potential for improving the procedures with regard to the man-machine interface. Since some retrofitting is required it will still take 1–2 years before the procedures become fully operational in all Siemens pressurized-water reactors.