Variant calculations of WWER 440 loss-of-coolant accidents

The computer programme LENKA is described intended for the thermal and hydraulic analysis of a loss-of-coolant accident in the PWR primary coolant circuit. The actual reactor is substituted with a system of zones and connecting pipes. The programme considers three types of zones, viz., a zone filled with a homogeneous single-phase coolant or a two-phase water-water vapour mixture; a zone in which the separation is expected of steam and liquid after two-phase mixture formation; a zone filled with a subcooled liquid and a compressed two-phase gas above the surface thereof. Five intact loops were substituted with one equivalent loop, a damaged loop was modelled separately. It was found that the process of the accident following the rupture of the cool circulation piping branch primarily depended on the crack size and circulating pump operation. The effect of other factors, such as pressure losses of the shut-down pump, the technique of calculating friction losses for two-phase flow, heat supply to the core and heat removal in steam generators was found to be less significant. When the cooling configuration was observed no practical effect was found of the time of the pipe rupture on the core cooling. (J.P.)