Comparison of probabalistic fracture mechanics and service based LOCA estimates for NPP with VVER

The deterministic definition of Design Basis Accident (DBA) is based on circumferential double ended guillotine break (DEGB) of the largest pipe in the primary coolant system-large break loss of coolant accident (LB LOCA). The emphasis is on the use of LB LOCA as tool for analyzing containment performance and ECCS dimensioning. After implementation of Leak Before Break (LBB) methodology the Safety Evaluation issued in 1984 found that probability of a full double-ended rupture of tough piping in a typical PWR primary coolant system is vanishingly small. Potential for failure is low enough so that full DEGBs need not be postulated as a DBA for defining of structural loads. At present exist greater knowledge and understanding to this problem base on - over 40 years of regulatory experience and several thousand years of collective operating experience; - results of extensive research on piping behaviour of related deterministic and probabilistic fracture mechanic analyses. Due to this circumstances US NRC recognize the need to redefine the LB LOCA. One key element of this approach is the utilizing the LOCA initiating event frequencies to evaluate the technical basis for potential related rule changes. In the paper are discussed two existing approaches and their application on VVER piping. (orig.)