The initial state of NPP is the operation at rated power. As a result of an initial event, loss of all sources of alternating current electrical power, all MCPs (main coolant pumps) are tripped, stop valves of a turbine generator are closed, the primary circuit makeup-blowdown system is disconnected, the power supply to pressurizer is disconnected, BRU-Ks (steam dump valve to turbine) are disconnected, and the main and auxiliary feedwater systems of secondary circuit are stopped. Besides as a result of diesel generators failure to start all active safety systems do not function.

After scram (because of three or more MCPs switching-off) the reactor power reduces down to a residual heat level. After the ending of MCPs coastdown the natural circulation of the primary coolant is established. Firstly the heat removal from the primary circuit is carried out due to BRU-A (steam dump valve to the atmosphere) operation, and then via steam generator pulse safety device. As a result of SPOT operation the part of heat from primary circuit is removed in an environment, and other heat is removed through BRU-A (loss of the boiler water from steam generator proceeds). After appropriate decreasing of residual heat, dumping devices of the second circuit are closed, and loss of boiler water from steam generator stops. The heat removal from the primary circuit is carried out due to steam generator PHRS operation under the closed circuit, steam is condensed in heat exchanger modules, and the condensate returns back in steam generator. The reliable cooling of the core is provided. Thus, results of calculation show that steam generator PHRS operation prevents any core damage in considered BDBA.

XIII-9. Conclusions

A number of relatively innovative passive safety means are used in the new Russian plant designs with V-392 reactors to fulfil the fundamental safety functions; such as reactivity control, fuel cooling, and radioactivity confinement. For example, the estimated core melt frequency for V-392 is three orders of magnitude less than the V-320 reactor. The engineering solutions using natural circulation in V-392 design increases the safety level compared to operating WWER-1000 plants.

REFERENCES TO ANNEX XIII

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

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