The behaviour of SGHWR fuel elements under accident conditions

A loss-of-coolant accident (LOCA) may affect the fuel element in two ways. Because the coolant is discharging from the reactor circuit (blowdown) the hydraulic forces acting on the element change, and may become larger than in normal service. In addition, the varying flow alters the heat transfer from the elements so that it may reach higher temperatures than in normal service. Two extreme cases are being studied. In the first, a breach in the reactor circuit is postulated adjacent to a fuel element, i.e. in a pressure tube or immediately above or below it. The fuel element is thus in the path of the discharging coolant. The SGHWR fuel element is more readily deformed by bending and by compression than it is by tension, and the more severe conditions for the fuel element are therefore those where the coolant discharges from a breach in the pressure tube at or immediately above the fuel element, so that abnormal vertical or horizontal forces act on it. The other extreme case is that of a fuel element in a channel where the coolant flow ceases because of the change in flow conditions following the breach. The element is assumed to dry out and to remain in steam which is stagnant and at a reducing pressure. The stored heat and the residual heating cause a transient rise in temperature which may in turn result in some mechanical deformation of the element's structure before it is cooled by quenching. These two conditions have been studied experimentally. First results are described from a simple slowdown rig ('Bluster'). The spray-quench rig which has been used to test half-length elements is also described, with a discussion of some results. It is concluded that the integrity of the element and the spray-cooling mechanism is likely to remain unimpaired