An application of the new way to prevent core melting in pressure tube reactors (CANDU type)

Abstract

The pressure tube reactors, especially CANDU type, have a calandria low pressure vessel (near to atmospheric pressure) immersed into a concrete vault filled with water. The accident analysis done by ELFIN-HTCELL code for the channel heat up and by fluid flow PHOENICS code as applied for moderator cooling system efficacy, showed that even the moderator cooling system operates, in some transients sequences where the normal heat sinks are lost, and the top core pressure tubes can reach burst conditions, which means that the fission product secondary retaining barrier gets destroyed, and yet the core can be cooled by water admission through the ruptured tubes from the emergency core cooling system (ECC), if it is available. Otherwise, if in many accident sequences the moderator cooling system remains the ultimate heat sink for the core fuel, and it is not available even from the accident start, a core melt appears. Taking into account the “natural” advantage offered by the presence of both pools in calandria and in the vault, separated by the calandria vessel, the introduction of density locks between them could be a safety passive design solution. When the temperature of moderator water gets higher the density lock cold-hot interface loss stability and thus the density locks get “open” fully permitting the admission of the cool water from the vault pool in calandria. Therefore, by natural circulation the decay heat is transferred via an air-cooling tower, and no mechanical moving parts are needed to open this circuit. Also, if the vault water is borated, it can be used to stop the nuclear reaction when the normal shutdown systems are not available and a positive reactivity coefficient appears, e.g. large loss of coolant accident (LOCA).