Release of Rare Fission Gases from Spherical Elements with Coated Fuel Particles

The release of the rare fission gases, krypton and xenon, from a high-temperature reactor pebble-bed core is predominantly determined by the heavy-metal contamination of the matrix material during manufacture. In the case of the Thorium High-Temperature Reactor prototype fuel, particles with failed coatings contribute <10% to the total core release of the xenon and krypton isotopes with the exception of long-lived ⁸⁵Kr. In a series of irradiation experiments with spherical fuel elements, a linear relation between the gas release and the contamination of the matrix material was established. At mean fuel temperatures of 700°C (973 K), only ∼1% of the ^85mKr and ¹³³Xe produced by fuel contamination is released. The experimental data for the steady-state release of 13 krypton and xenon isotopes can be explained by describing the graphitic matrix material as a two-component. system. Component 1 is attributed to the graphitic grains of the raw material, and component 2 to the material between the grains, such as the amorphous, nongraphitized binder coke. The total contamination-induced release from the fuel elements is given by the retention characteristics of the two components working in parallel, followed in series by the gas-phase transport through the interconnected porosity of the fuel element structure. As a consequence of this model, the apparent activation energy for the steady-state release depends on the half-lives of the isotopes of the same species yielding, e.g., 5 kcal/mole (21 kJ/mole) for ¹⁴⁰Xe and 9 kcal/mole (38 kJ/mole) for ¹³⁸Xe.