Analysis of Reverse Flow Consequences During Gentilly-2 Channel Refuelling

Abstract

The use of a dynamic model coupled to a thermal-hydraulic code developed for the prediction of fuel bundle velocities is presented. The fuel bundle motion can be induced by severe reverse flow conditions following an hypothesized inlet pipe break accident in a CANDU reactor channel. The model is validated using the reverse flow experiment results obtained at STERN Laboratories. The test series was aimed at studying the fuel string motions during a channel flow reversal. The experiments have led to an understanding of the driving mechanism for bundle motion under a reverse flow in a closed channel. Based on a SOPHT model, an approach is developed to establish the safety consequences of a LOCA during refuelling at Gentilly-2. The fuel string displacement model is extended to explain the slowing down of the fuel string when the reactor channel is coupled to the fuelling machines during refuelling. Due to the damping of the nearly stagnant water, a hydraulic drag opposes the motion when the fuel string leaves the channel core region. This leads to a deceleration phase as long as the fuel movement remains free of any obstacle. Simulation results are presented for large and small LOCA's affecting a single Gentilly-2 channel during the various refuelling stages. The model can thus be used, together with other mechanical tests and analyses, to assess fuel velocity at impact and to analyses fuel integrity, channel integrity and, fuelling machine integrity with their corresponding time scales, probabilities of occurrence and possible safety consequences.