Reliability analysis of passive emergency cooling system of WWER-440 type core

The passive emergency core cooling system of WWER-440 reactors consists of two independent identical subsystems, each comprising two similar presurized containers connected via discharge pipes to the reactor mixing chambers, one to the top chamber, the other to the bottom one. The system action starts autonomously upon pressure drop in the primary circuit below 6 MPa when the boric acid solution is forced out to the mixing chambers of the reactor by nitrogen overpressure. The system is activated upon, e.g., main circulating pipe rupture within roughly 10 seconds and its action is necessary as long as around 30 seconds following the accident. An analysis is made of possible system failures and their causes. The failure tree method was used in assessing the system reliability. It was shown that the following events mostly affected its reliability: a fault in container pressure measurement and display, a leak of the container safety valve, and the failure of the back valve in the discharge pipe to open. The analysis also showed that only under the condition when all four containers are operation worthy the reliability is satisfactory. In unit operation with one container out of operation for repair, the system reliability markedly decreased. The existing regulations that have so far permitted the shutdown of one container for three days should be amended to this effect. (Z.M.). 5 figs., 6 refs