Modeling Xenon Transport in Molten Salt Fueled Reactors

Modeling Xenon Transport in Molten Salt Fueled Reactors

Valmor F. de Almeida¹, Benjamin S. Collins¹, Robert K. Salko¹, and Robert Z. Taylor²

¹Oak Ridge National Laboratory, Oak Ridge, TN 37831-6181

²University of Tennessee, Knoxville, TN

Abstract

There exists a renewed interest in molten salt nuclear reactor (MSR) technology in view of its favorable safety attributes, namely, 1) the fuel is already in the liquid state (hence no fuel melt down concerns), 2) operation near ambient pressure, 3) no fire or explosion hazard, 4) with loss of pumping power, the fuel is drained by gravity from the reactor’s core into storage tanks retaining the fission products in the frozen salt. There are also economic attributes that justify the revival of the molten salt reactor development, and there are disadvantages and concerns that need to be resolved before this reactor type can be adopted. Nevertheless, our focus is in one aspect of any MSR design that must be implemented, that is, the removal of volatile fission products during the reactor’s operation. This is typically accomplished by contacting the molten salt with an inert gas, e.g., He, either as cover gas in the pump assembly or as entrained bubbles into the salt (or both). Noble gases are typically removed from the salt in this operation mode, in particular ¹³⁵Xe which is the most notable thermal neutron absorber of the volatile fission products. Ability to continuously remove volatile fission products is a key enabling feature of high-burnup MSR’s and their economic advantage over existing commercial reactors.

The addition of a gas to a molten salt flow significantly complicates the modeling and simulation of the coupled mass, heat, momentum, and neutron transport in the presence of fission, transmutation and decay. We are developing modeling and simulation capability for coupled transport in MSR’s and this talk describes current efforts to develop an integrated simulation tool. Our point of the departure as far as data and model are concerned is the Oak Ridge National Laboratory Molten Salt Reactor Experiment which operated in 1965-1969. We are specifically developing a model for the removal of ¹³⁵Xe by means of He bubbles in the LiF-BeF₂ salt mixture with soluble UF₄ fuel under reactor operation conditions. The distinct difficulties faced at this point are: the development of a non-isothermal, two-phase, volume averaged fluid flow model with volatile mass transport from the liquid phase into the gas phase, diffusion/dispersion, and generation of the fission products in the liquid phase, and coupling of speciation to neutron transport. The computational platform used is leveraging ORNL codes from the Consortium for Advanced Simulation of Light Water Reactors (CASL), a DOE Energy Innovation Hub.

This talk will also provide an outlook of the additional modeling of redox reactive fluid flows needed to account for corrosion of vessel components and deposition of metallic fission products as interfacial transport phenomena.