CANDU Fuel Compression Tests at Elevated Temperatures

Abstract

An inlet header large break loss of coolant accident (LOCA) in CANDU reactors with fuelling against flow can cause the fuel to shift in the channels with a consequent reactivity insertion. This results in an increased fuel power transient, and a potential increase in the analyzed consequences for such events. As the reactors age and the channel axial gaps increase, the magnitude of the predicted power transient increases. A design solution to reduce the power transient is to limit the amount of fuel movement by reducing the channel axial gap. This solution was implemented into Ontario Hydro's Bruce B and Darlington reactors. A consequence of a reduced channel axial gap is the potential for the fuel column axial expansion to become constrained by the channel end components in large break LOCAs. This experimental program investigated the effects of pellet cracking and elevated sheath temperatures on the ability of the fuel elements, of the 37element bundle design, to sustain axial loads. The unirradiated fuel elements tested were either in the as-received condition or with the U02 fuel pellets cracked in a mechanical process to simulate the effect of irradiation. The load deformation characteristics demonstrated that, for a given amount of axial compression, the loads sustainable by the elements at elevated sheath temperatures were low. As a result, excess axial expansion would be easily accommodated without further challenge to pressure tube integrity.