Mechanical analysis of cylindrical part of canisters for spent nuclear fuel

This report describes mechanical analyses of cylindrical part of the VVER 440-, BWR- and EPR-type canisters for spent nuclear fuel. The task was first to evaluate the stresses at maximum design pressure and further by increasing pressure load to determine the limit collapse load and corresponding safety factor. Maximum design pressure 44 MPa is a sum of the hydrostatic pressure 30 MPa caused by 3 km ice layer, 7 MPa caused by ground water pressure at the deepest disposal depth of 700 m and 7 MPa from bentonite swelling pressure.

The analysis presented in this report concern the middle area of the canisters, where the cast iron insert is considered to be more critical than in the ends of the canister. For the model a piece from the middle area of the canister was separated by two planes perpendicular to the axis of the canister. This piece was studied first by two-dimensional plane strain model, where the planes are constrained and no elongation of the canister takes place. In the second model one of the planes was constrained and the other plane was allowed to displace in axial direction, which remains as a plane during deformation and to which axial pressure force is directed. This analysis, which corresponds better the real condition in the canister, was performed as three-dimensional. The analyses gave however practically equal results due to plastic deformation. Thus the analysis can be done by two-dimensional plane strain model leading to same accuracy with less computation effort. Analyses were performed as large displacement and large strain analyses by the PASULA computing package, which has been developed at VTT for a variety of structural analysis and for heat conduction calculations. A special routine was developed for automatic mesh generation. Before the analysis of the VVER 440-, BWR- and EPR-type canisters the calculation methodology was validated with test results, which were received from pressure tests performed with a short BWR canister in Germany.

The geometry of the cross section of the canister is always eccentric in practice due to manufacturing tolerances and material properties may be slightly non-homogenous. This was studied by making structural eccentricity to the models. The behavior of copper is actually more complicated than in the models applied in this study. For instance creep of copper was not modeled, since very long-term creeping data is lacking. On the other hand, the analyses showed that the cylindrical parts of the canisters could retain their integrity even without copper layer. Thus the copper layer can give extra margin against canister failure. Without copper layer and with 5 mm eccentricity the safety margins with design load of 44 MPa against failure for the VVER 440, BWR and EPR inserts are 2.6, 2.1 and 3.5, respectively.