Modeling of thermal properties of a TeO2 target for radioiodine production

Abstract

Three radionuclides of iodine (I-121, I-123 and I-124) are of great interest for modern nuclear medical diagnostics. They can be all produced by the (p,n) nuclear reaction using isotopically enriched solid TeO2, targets. The produced radioiodine can be rapidly separated from the target material by dry-distillation from the melted target after the irradiation. Since TeO2 has low thermal conductivity, the most critical issue in the design of a production target is the provision of its effective cooling in order to avoid melting of the oxide layer during the irradiation. A compact solid target irradiation system (COSTIS) has been designed for the irradiation of TeO2 targets, suitable for routine production of radioiodine. The target is a circular Pt-disk that carries the TeO2 melted into a circular grove in the center of the disk. The target coin is manually inserted into COSTIS, fixed pneumatically in the irradiation position, released remotely after irradiation and falls down driven by gravity into a transport container. The engineering design of the cavity for helium cooling of the front face of TeO2 and the impinging water jet cooling the back face of the target disk was done based on a simulation of the thermal behavior of the target during the irradiation. A straightforward numerical method for the prediction of the thermal properties of the solid target has been developed. The approach is based on calculations without using the common practice of Prandtl and Nusselt empirical correlation. The fluid flow description in the boundary layer was refined in such a way, that the heat flux, exchanged between the solid and fluid, is obtained directly from Fourier law. The governing equations are based on the local thermodynamic equilibrium and conservation equation of mass, momentum and energy. In order to solve the set of governing equations, the finite-volume method is used. This procedure gives rapid answers whether the proposed geometry satisfies the design criteria. (C) 2003 Elsevier B.V. All rights reserved.