Award Date

1-1-2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Committee Member

Mohamed B. Trabia

Number of Pages

292

Abstract

This dissertation presents a study of the spent nuclear fuel canisters for maximizing the compressive stress depth through the closure-weld region wall thickness. Induction coil heating technique can be used to relieve the residual stresses from the closure weld and induce a state of compression through the wall thickness. This technique involves localized heating of the material by the surrounding coils. The material is then cooled to the room temperature by quenching; A three-dimensional finite element model was developed for the canister using the sequential method. This method consisted of a sequential thermal-stress analysis where nodal temperatures from the thermal analysis were applied as body force loads in the subsequent stress analysis. This model, which was computationally intensive, has been used to verify the results of the model developed in two-dimensions and ensure its accuracy; The effects of induction coil heating and subsequent quenching were also determined by using a two-dimensional axisymmetric finite element model of the canister. This model made use of the direct method. This method included only one type of analysis that uses coupled-field element type containing all necessary degrees of freedom for the heat transfer and the stress analyses. Direct coupling is advantageous when the coupled-field interaction is highly nonlinear and is best solved in a single solution using a coupled formulation. The results of the two-dimensional axisymmetric model were almost identical to the results of the three-dimensional model. Therefore, the computationally efficient two-dimensional axisymmetric model was used for the subsequent optimization problem; The finite element results were validated using the results obtained from an experimental test. A canister mockup which consists of an outer shell and a support ring was manufactured. The mockup was subject to solution annealing process. At the end of the process, a compressive stress state developed on the shell outer surface. The stresses on the canister outer surface were obtained based on the readings of the strain gages that were attached to several points on the mockup. The results of the experimental test were consistent with the finite element solution; The parameters of most promising designs were tuned to further maximize the depth of compressive stress through the wall thickness. This was handled as an optimization problem that was subject to geometrical and stress constraints. Two different solution methods were implemented for this purpose. First, ANSYS optimization subroutine was used to obtain an optimum solution. These results were subsequently improved using a successive heuristic quadratic approximation. This routine provided the dimensions of the best design that result in the maximum compressive stress in the canister closure-weld region.

Keywords

Annealing; Canisters; Closure; Closure-weld; Fuel; Induction; Induction Annealing; Minimization; Nuclear; Nuclear Fuel Canisters; Process; Region; Residual; Residual Stresses; Spent; Stress Corrosion; Stresses; Weld

Controlled Subject

Mechanical engineering

File Format

pdf

File Size

5754.88 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

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