Hydrogen Distribution Analysis for CANDU 6 Containment Using the GOTHIC Containment Analysis Code

Abstract

Hydrogen may be generated in the reactor core by the zirc-steam reaction for a postulated loss of coolant accident (LOCA) scenario with loss of emergency core cooling (ECC). It is important to predict hydrogen distribution within containment in order to determine if flammable mixtures exist. This information is required to determine the best locations in containment for the placement of mitigation devices such as igniters and recombiners. For large break loss coolant accidents, hydrogen is released after the break flow has subsided. Following this period of high discharge the flow in the containment building undergoes transition from forced flow to a buoyancy driven flow (particularly when local air coolers (LACS) are not credited). One-dimensional computer codes (lumped parameter) are applicable during the initial period when a high degree of mixing occurs due to the forced flow generated by the break. During this period the homogeneous assumption of lumped codes is applicable. However, during the post-blowdown phase (when the break flow has subsided) the assumption of homogeneity (particularly in large volumes) becomes less accurate and it is necessary to employ three-dimensional codes to capture local effects. This is particularly important for purely buoyant flows which may exhibit stratification effects. In the present analysis a three-dimensional model of CANDU 6 containment was constructed with the GOTHIC computer code using a relatively coarse mesh adequate enough to capture the salient features of the flow during the blowdown and hydrogen release periods. A 3D grid representation was employed for that portion of containment in which the primary flow (LOCA and post-LOCA) was deemed to occur. The remainder of containment was represented by lumped nodes. The results of the analysis indicate that flammable concentrations exist for several minutes in the vicinity of the break and in the steam generator enclosure. This is due to the fact that the hydrogen released from the break is primarily directed upwards into the steam generator enclosure due to buoyancy effects. Once hydrogen production ends, the concentrations in these areas become non-flammable again. In the boiler room (volume above the reactivity mechanisms deck) flammable mixtures are not predicted due to the large volume available for mixing. In other areas of containment, particularly below the break, flammable mixtures are not predicted as there is insufficient forced flow to propagate hydrogen downwards.