An Investigation of an In-Vessel Corium Retention Strategy for the Wolsong Pressurized Heavy Water Reactor Plants

An external reactor vessel cooling as a means for an in-vessel retention has been selected as one of the tentative severe accident management strategies for the Wolsong plants, which are typical CANDU 6 reactors. The strategy takes advantage of the plant-specific features: (a) the power density is low, (b) the calandria vessel and the calandria vault have large water volumes, (c) the calandria is always submerged in the water of the calandria vault during a normal operation, (d) the stainless steel layer of the molten corium is negligible even though the unoxidized Zircaloy could form a metal layer, (e) no insulation structure is designed around the calandria vessel, (f) the bottom area of the calandria is large enough to transfer a sufficient amount of the corium decay heat into the calandria vault water, and (g) the water supply from the backup water sources into the calandria vault is available for a long-term external cooling of the calandria. The above design features cause a severe accident progression to be considerably delayed, and they minimize the in-vessel retention issues applied to a certain pressurized light water reactor. Furthermore, the thermal analysis demonstrates that the molten corium on the bottom of the calandria is externally coolable in terms of the critical heat flux, although phenomenological uncertainties still exist. This paper shows the feasibility and the evaluation results of the in-vessel retention strategy via an external vessel cooling for the CANDU 6-type plants, which have not been addressed as yet.