Thermal-Mechanical Analysis of a Low-Enriched Uranium Foil Based Annular Target for the Production of Molybdenum-99

Abstract

One of the US Department of Energy’s Global Threat Reduction Initiative (GTRI) goals is the elimination of the use of high enriched uranium (HEU) for the production of the radioisotope molybdenum-99 (Mo-99). One strategy to achieve this goal is to use an irradiation target that utilizes a low-enriched uranium (LEU) foil. This paper considers an annular target geometry, where an LEU foil of open cross section is sandwiched between two concentric aluminum tubes. A recess is cut on the inner tube to hold the LEU foil and facilitate assembly. The target must contain the fission products until it can be opened and the LEU foil removed for further processing. The thermal contact resistance between the LEU foil and the aluminum tube cladding needs to be low enough to ensure that the LEU temperature does not exceed the operating temperature specified by the reactor safety case. Numerical models using the commercial finite element code Abaqus FEA are used to explore the potential for gaps opening between the LEU-foil and the aluminum tubes, and the stresses developed in the aluminum tubes during heat generation in the target during irradiation. Parametric studies are conducted using constructed analytic models to explore the impact of the ratio of heat transfer coefficients between the inner and the outer tubes and the inner-to-outer tube thickness ratio. It is concluded that the current annular target design is safe at high LEU heat generation rates and the use of these targets will likely not compromise the reactor safety.