Mathematical relations are presented for estimating the release fractions of gaseous fission products from coated fuel particles and fuel elements containing them. The relations are based on simplified models of the release process, with particular emphasis on the following mechanisms: recoil, diffusion from fuel, diffusion through particle coating, and diffusion through fuel element matrix. The characteristics of fission-gas release by these mechanisms, acting singly and in combination, are considered, and the application of the theoretical relations to experiment planning and interpretation is discussed. Special attention is given to methods for analysis of data from continuous, in-pile (sweep capsule) release experiments and neutron-activation release experiments. (auth).

Three levels of fission product diffusional release models are solved exactly. First, the Booth model for a homogeneous uncoated spherical fuel particle is presented and an improved implementation is suggested. Second, the release from a fuel particle with a single barrier layer is derived as a simple alternative to account for a coating layer. Third, the general case of release from a multicoated fuel particle is derived and applied to a TRISO-coated fuel. Previous approaches required approximate numerical solutions for the case of an arbitrary number of coatings with arbitrary diffusivities and arbitrary coating interface conditions.

The US and FRG have developed methodologies to determine the performance of and fission product release from TRISO-coated fuel particles under postulated accident conditions. The paper presents a qualitative and quantitative comparison of US and FRG models. The models are those used by General Atomics (GA) and by the German Nuclear Research Center at Juelich (KFA/ISF). A benchmark calculation was performed for fuel temperatures predicted for the US Department of Energy sponsored Modular High Temperature Gas Cooled Reactor (MHTGR). Good agreement in the benchmark calculations supports the on-going efforts to verify and validate the independently developed codes of GA and KFA/ISF. This work was performed under the US/FRG Umbrella Agreement for Cooperation on Gas Cooled Reactor Development. 6 refs., 3 figs., 3 tabs.