Passive Safety Characteristics of Fuel for Modular HTR and Fuel Performance Modeling under Accident Conditions

The high fission product retention potential of coated particle fuel has been utilized for small-sized modular High Temperature Reactors in such a way that the inherently passive limitation in the maximum accident temperature guarantees an almost complete fission product retention and enables the reoperation of the reactor. An extensive program has produced the experimental results which constitute the basis of the claim to inherent safety under this aspect. Models and codes have been developed (i) to predict a realistic, at least a conservative overall release from the primary circuit, (ii) to reduce the large number of experimental results to a small set of characteristic coefficients, (in) to predict release beyond experimental conditions. Conservative predictions of release from the core have been achieved using a traditional pressure vessel model for release from particles and simplified diffusion models for fission product transport. This approach relies completely on the fact that nearly all accident conditions have been covered by experimental work and is limited by the finite number of experiments. The second purpose of data reduction has been achieved with two different types of modeling approaches whereby a new combined model would be equally relevant to all volatile fission products. This development goes into the direction of a true mechanistic prediction which is an ongoing effort. The safety design of the 200 MW_th HTR-Modul is based on KFA experimental results from fuel accident condition performance testing and the modeling effort has been applied to the safety review. (author)