The initial state of the NPP is the operation at rated power. As a result of break of main coolant pipeline, the discharge of significant mass of coolant of the primary circuit takes place. When the pressure of the primary circuit becomes less than 13.7 MPa, the scram is initiated. Stop valves are closed after trip of the reactor. In this condition, the main coolant pump is switched off and coast down when the difference between the temperature of coolant in a hot leg and saturation temperature under actual pressure becomes bellow 10 °C. The steam generator PHRS starts up by the fact of decrease of the difference between primary and secondary side pressure. The boric acid solution is supplied to the reactor from traditional hydroaccumulators with initial pressure of 4.0 MPa by corresponding primary pressure decrease, without any signal actuation. The containment PHRS will provide condensation of the steam in the containment.

Thus, in the first stage of the accident, primary pressure is decreased due to loss of coolant and operation of the passive heat removal system. Further cooling down and pressure decrease are realized via steam generator PHRS and containment PHRS. When the pressure difference between primary circuit and containment is decreased to 0.6MPa, passive valves of the emergency depressurization system (primary circuit untightening subsystem) open to connect reactor inlet and outlet with the fuel pond space. When the reactor and containment pressure difference is decreased below 0.3 MPa, the ECCS tanks (elevated hydroaccumulators open to the containment) begin to flood the reactor. This sequence results in creating of so called emergency pool where the reactor coolant system is submerged to and in connection of this emergency pool with the spent fuel pond. The natural circulation along the flow path shown in Figure XII-3 (reactor inlet plenum — core — reactor outlet plenum — ‘hot’ depressurization pipe — fuel pond — ‘cold’ depressurization pipe — reactor inlet plenum) provides the long term heat removal from the core in case of the LOCA combined with complete loss of power supply. The water in the emergency pool and spent fuel pool reaches the saturation point in about 10 hours. The steam generated will condense on the internal surface of the steel inner containment wall, and condensate flows back into the emergency pool. This configuration ensures also the heat removal from reactor vessel bottom to keep the corium inside the reactor in case of postulated core melt event.

XII-6. Conclusions

Several passive safety systems based on natural circulation phenomena are used in WWER-640/407 reactors to fulfill the fundamental safety functions of reactivity control, fuel cooling, and radioactivity confinement. Implementation of these systems made it possible to significantly increase power plant safety in terms of the expected severe core damage and excessive radioactivity release frequencies.

REFERENCES TO ANNEX XII

[1] INTERNATIONAL ATOMIC ENERGY AGENCY, Natural circulation data and methods for advanced water cooled nuclear power plant designs, IAEA-TECDOC-1281 (IAEA, Vienna, 2000).

[2] INTERNATIONAL ATOMIC ENERGY AGENCY, Status of advanced light water reactor designs 2004, IAEA-TECDOC-1391 (IAEA, Vienna, 2004).