Bowing and corresponding thermal stress phenomena in fuel pins during transients

Although the thermal mechanical behaviour of fuel pins has been studied in detail for steady-state conditions, much less attention has been paid to these problems during transients. The present paper deals with bowing and thermal stress phenomena in fuel pins during blow-down transients. By means of FEM the transient circumferential temperature distribution is analysed in a ziracloy cladded UO2 fuel pin, for a representative spectrum of parameters. The transient heat transfer coefficient at the outer-cladding wall are obtained from out-of-pile flow-down experiments. Solutions for both temperature dependent and independent material properties are given. The results show that the circumferential temperature variations in the canning are more pronounced during transients as compared to steady-state conditions. The influence of the gap resistance remains small. Thermal stresses and related fuel pin bowing phenomena remain within acceptable limits. (Auth.)