Thermal bowing of reactor fuel elements

Abstract

Every fuel element will be bowed because of initial inaccuracies in manufacture and inevitable temperature asymmetries around its circumference which develop in operation. This initial bow will cause changes in the heat transfer conditions around the circumference, inducing temperature changes and hence changing the bow. A general theoretical expression has been derived for the amplification of the initial bow in terms of the physical properties of the coolant, fuel and can, the dimensions of the system and the power rating. It is shown that some combinations of these parameters will lead to complete instability, i.e. the element will continue to bow until it touches the channel wall or another element, leading to serious local overheating of the can. The maximum bow and corresponding excess can temperature must be limited to an acceptable value. Usually the only convenient way to do this is to reduce the unsupported axial length of the element. The determination of the maximum ‘safe’ value is discussed. Experiments are reported which confirm the predicted variation of induced bowing with the more important parameters over the range of general interest.