A study on the effect of rise in calandria vault cooling water and concrete temperature during system degraded cooling and station black out conditions in 540 MWe Indian PHWRs

Full text of publication follows: The calandria vault houses the calandria vessel which contains heavy water moderator and natural UO2 in number of coolant tubes. Demineralized water is circulated through calandria vault housing in Indian PHWRs to remove part of nuclear heat produced in calandria and to cool the concrete structure. It also provides shielding against neutron and gamma radiation. The calandria vault concrete structure acts as biological shield. The calandria vault cooling system consists of 3 pumps (normally 2 working and 1 standby) and 2 plate type heat exchangers to remove the heat generation due to attenuation of gamma rays. The calandria vault wall is also subjected to heat load, the contribution being mostly from attenuation of gammas. It is of interest to estimate the temperature distribution of the wall to evaluate whether concrete peak temperature as well as differential temperature are within acceptable limits. An analysis was done to estimate the temperature rise of the calandria vault water and calandria vault concrete during degraded cooling (1 pump and 1 heat exchanger)of the system and Station Black Out (Off site and On site power failure). During Station Black Out forced circulation in calandria vault cooling system is not available, so the heat loss is to the Reactor Building atmosphere through calandria vault wall concrete. For estimation of the concrete temperature, two-dimensional finite difference explicit scheme was used with time dependent and spatial variation in heat generation and convective boundary conditions were considered. Calandria vault concrete is in contact with the calandria vault water through stainless steel liner and 3 mm air gap. Calandria vault water temperature distribution in the flow direction was predicted in the above mentioned events by considering the heat gain from the neighbouring systems and heat generation in water. Outer surface of calandria vault wall is in contact with Reactor building atmosphere. Temperature rise in that region during the above events are also considered. From these above analyses it was concluded that concrete peak temperature and differential temperatures are within the acceptable limits. (authors)