Thermo-mechanical FE-analysis of butt-welding of a Cu-Fe canister for spent nuclear fuel.

摘要

In the Swedish nuclear waste program it has been proposed that spent nuclear fuel shall be placed in composite copper-steel canisters and then placed in holes in tunnels located some 500 m underground in rocks. The canisters consists of two cylinders of 4850 mm length, one inner cylinder made of steel and one outer cylinder made of copper. Outer diameter of canister is 880 mm and wall thickness for each cylinder is 50 mm. The steel cylinder, which contains the spent nuclear fuel, is placed inside the copper cylinder. A copper end is butt welded to the copper cylinder using electron beam welding. To obtain penetration through the thickness with this weld method, a backing ring is placed at the inside of the copper cylinder. In this paper, the temperature, strain and stress fields present during welding and after cooling after welding are calculated numerically using the FE-code NIKE-2D. Aim is to use the present calculations to estimate risk for creep fracture during subsequent design life. A large strain formulation is employed for calculation of transient and residual stresses in the canister based on calculated history of temperature field present in the canister during the welding process. The contact algorithm available in NIKE-2D is used to detect possible contact between the steel and copper cylinders during the welding. To simplify the calculations, rotational symmetry is assumed. For large gap distances between the steel and copper cylinders the residual stress field is calculated to have a shape similar to that observed in butt welded pipes with maximum axial stress values at the yield stress level. For small gap distances the backing ring will come in contact with the steel cylinder which leads to large residual stresses in the backing ring. The maximum accumulated plastic strain in the weld metal and HAZ was calculated to about 5 % for both gap distances. (Atomindex citation 24:037517)