Investigations of Pressure and Temperature Evolutions in the Reactor Containment Building of a Fast Reactor After a Core Disruptive Accident

ABSTRACT

Prototype Fast Breeder Reactor is a 1250 MWt, 500 MWe pool type liquid sodium cooled nuclear reactor presently under construction at Kalpakkam. During a Core Disruptive Accident, about 350 kg of sodium along with the fission gases are expelled into an enclosed space (hereinafter referred to as ‘the enclosure’) above the top shield, inside the reactor containment building (RCB). In order to protect the public and environment against radioactivity release, the RCB is to be isolated. For this purpose area gamma monitors and ventilation duct monitors are envisaged. In order to improve the reliability of isolation, it is planned to measure the pressure and temperature rise in the enclosure during the sodium fire and use them as additional signals to isolate RCB.

When the sodium burns in the enclosure, it exchanges heat with the complementary shielding materials by natural convection and thermal radiation. The enclosure is not a leak-tight compartment. Hence, natural draft of RCB air takes place through the openings in the enclosure. The burning rate of sodium depends strongly on the concentration of oxygen which varies with time due to its consumption during sodium burning. Detailed zero dimensional transient thermal hydraulic models have been developed to predict the evolutions of temperature of air and internal structures as well as air pressure. The governing equations are solved by Runge- Kutta method of order 4. The code has been validated against the ideal case of zero porosity in the enclosure walls. Based on the parametric studies, the optimum porosity of the enclosure walls to have a successful pressure signal has been identified. The values of pressure and temperature settings for isolation of RCB have been finalized.

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