Use of natural circulation mechanism core cooling of high temperature helium-cooled reactors as a means of safety enhancement

Attempts were made to establish operating limits of the natural circulation mechanism as applied to nuclear power plants with gas-cooled thermal and fast reactors where helium is used as coolant. For this purpose parametric analysis is carried out for a closed loop with the reactor core as source of coolant heating and cooler (shutdown heat exchanger, steam generator or high-temperature heat exchanger) placed above the level of the upper end of the core. The gas moves due to its various density in the circuit riser and downcomer. The analysis made it possible to conclude that among numerous measures envisaged in design of gas-cooled nuclear reactors to provide safe operation, the natural circulation mechanism can be considered as one of the reliable and simple means of reactor core cooling except for depressurization case