Assessment of Fuel Cooling under Natural Circulation Conditions in Intact Loop for LOCA Scenarios in Gentilly-2

Abstract

Thermosyphoning experiments in the multiple-channel RD-14M facility have shown that, under certain two- phase conditions in a multiple-channel facility, the coolant in some of the channels may flow opposite or reverse to the nominal forward direction. Subsequently, void from the inlet headers may enter the inlet feeders of the channels where the flow is in the forward direction or void from the outlet headers may enter the outlet feeders of the channel where the flow is in the reverse direction. (In this paper, these two phenomena are referred to respectively as inlet feeder draining and outlet feeder voiding.) This void would reduce the feeder hydrostatic head and, therefore, the channel flow. The resulting channel two-phase flow would eventually stratify exposing the upper fuel elements andpart of the pressure tube to steam. These fuel elements andpart of the pressure tube would heat up. Based on these results, Hydro Quebec proposed design modifications to Gentilly 2 to avoid high loop void conditions in postulated accident scenarios with loss of forced coolant circulation. SOPHT simulations of LOCA scenarios indicated that these design changes reduce significantly the intact loop void in most of the scenarios. For a few scenarios of low probability (i.e., in the multiple failure category), the intact loop void could still be sufficiently high for a short period of time to raise concern for fuel cooling. For these few LOCA scenarios, an assessment of fuel and fuel channel cooling under natural circulation conditions in the intact loop -was made. This paper presents the models that were developed and used for this assessment and the results of this assessment.