Comparative strength assessment of CANFLEX and 37-element fuel bundles

It is important that fuel bundles have adequate mechanical strength to withstand the loads applied to them during various operating conditions. This paper compares the mechanical strengths of CANFLEX and 37-element fuel bundles that have been subjected to hydraulic drag load. This paper examines the mechanical strengths of CANFLEX and 37-element fuel bundles under two conditions: normal operating conditions and refuelling conditions. These conditions cause axial compression in the fuel bundles, bending of the fuel elements, and bending of the end plates. The mechanical strength of the bundles was assessed using finite-element calculations. The results provide insights into the load-carrying mechanisms and capabilities of CANDU fuel bundles. During normal operating conditions, the last fuel bundle in a fuel channel is supported by the shield-plug; during refuelling conditions, the last fuel bundle in a fuel channel is supported by the side stops. Under these conditions, the total axial loads on the string of the fuel bundles are carried mainly by the fuel elements that contact the support. These conditions generate high stresses in both the end plates and in the fuel elements that contact the support. The CANFLEX fuel bundle contains 5 more radial ribs in each end-plate than the 37-element fuel bundle. Also, the radial ribs of the CANFLEX fuel bundle are wider than the radial ribs of the 37-element fuel bundle. These two features combine to strengthen the end plates of the CANFLEX bundle in a region that is highly stressed. On the other hand, the outer fuel elements of the CANFLEX fuel bundle have a smaller diameter than the fuel elements of the 37-element fuel bundle. This difference results in somewhat higher stresses in the outer elements of the CANFLEX fuel bundle design compared with the outer elements of the 37-element design. Overall, for the analysed conditions, the 37-element fuel bundle has the highest stress of the two fuel bundles. (author)