Verification of impact of pressure change in hydro accumulators on safety of VVER-440/213 reactors in the case of LB LOCA

Operational safety of nuclear units and its demonstration is one of the most closely followed issues. In this connection and with relation to the established safety limits, an emphasis is on the verification of transients and accidents. Tools employed for such verification are system thermal hydraulic computer codes which calculate T-H parameters of the basic primary and secondary components. Since 1990 in the Czech Republic for these analyses are used such western system computer codes as RELAP5, ATHLET and CATHARE. While RELAP5 has been used most within the initially stage, ATHLET and CATHARE were used as independent codes. One of most important operational problems of NPPs with WER-440/213 is maintaining the nitrogen pressure in hydro-accumulators, with the consequential problem whether it would be possible to reduce this pressure without affecting safety. In 1995 - 1997 we have performed a number of the relevant analyses using RELAP5 and ATHLET for initiating events LB and SB LOCA. In 1997 ATHLET 1.1 C, 1.2A and 1.2B(using NPA ATLAS) were applied. The paper gives a brief description of the models used for the primary and secondary circuits, selection of the boundary and initial conditions and also defines the LB LOCA acceptance criteria to be used for WER reactors. The results are discussed in relation to the safety limits (fuel conditions) at different pressure in hydro-accumulators, different number of hydro-accumulators in operation and ECCS influence. (author)