Assessment of Fuel Fitness for Service Following Standing Start Process During Gentilly-2 Annual Outage

Abstract

During a planned annual outage at Gentilly 2, primary and back-up heat sinks are provided for removing decay heat from fuel. The primary heat sink is provided by either the steam generators or the shutdown cooling system heat exchangers. Forced coolant flow transports the decay heat to these heat sinks. The backup heat sink is provided by coolant subcooling, end-shield and moderator coolants, piping metal mass. and the steam generators. Intermittent buoyancy-induced flow (IBIF) of the coolant or standing-start process transports the decay heat to these heat sinks. The analysis in this paper assesses fuel and fuel channel fitness-for-service after repeated standing start cycles when only the back-up heat sink is assumed available. This analysis defines a set of suitable conditions for return to full-power operation, without any inspection or analysis after the fie1 and fuel channels have been subjected to repeated 1BIF cycles. Two temperature limits are used in the analysis: the fuel sheath temperature is limited to 450 degrees C, and the pressure tube temperature is limited to 400 degrees C in IBIF cycles. The analysis uses the computer programs THERMOSS-111 for calculating the number of IBIF cycles, the channel-coolant temperature and heatup times in each of the cycles, HOTSPOT for calculating the transient sheath-to-coolant heat-transfer coefficient and the pressure tube temperature transient, ELESTRES for generating initial conditions of a fuel element, and ELOCA.Mk6 for assessing the transient thermal-mechanical behaviour of the fuel element. A set of stringent criteria for fuel bundles and pressure tubes to return to service are defined, justified, and examined. Whenever possible, fuel sheath and pressure tube degradation mechanisms are quantified using the temperature limits and the results calculated by the computer codes. A 450 degrees C fuel sheath temperature map is constructed. This map defines the operation envelope for fuel sheath remaining below 450 degrees C should a loss of forced coolant circulation occur in a shutdown state. If the fuel sheath temperature limit is not exceeded, the pressure tube temperature will be below 400 degrees C during IBIF cycles. With these limits, the results of the analysis show that the fuel and the fuel channel are suitable for returning to full power after repeated IBIF cycles. Presented at the 19Ih Annual CNS Conference, Toronto, October 18-21, 1998.