Objective
To study mechanisms and properties determining fuel and fission product behaviour during both, base and off-normal conditions. This activity involves unirradiated and irradiated "classical" and "improved" fuel samples of various composition and over a wide range of temperatures, up to very high burn-up, and makes use of appropriate computer models. The final aim of the activity is the improvement of the safety of fuel operation in a reactor.
Progress to end 1990
The Laboratory continued its cooperation with the International Fission Gas Release Project Riso III (Dk) by incorporating the extensive experimental data resulting from the programme into the OFT data bank and evaluating them with existing TU fuel performance codes. - The OECD-coordinated activity for analysing fuel and fuel debris of the Three Mile Island (TMI) damaged reactor has been concluded. An apparatus for thermal diffusivity measurements on active specimens with the laser flash technique has been constructed. - Nitride fuels with a "tailored" structure and heterogeneous fuels (U,Pu)O2 and UN) were fabricated for short-term irradiatons in the HFR-reactor. Irradiations of fuels for future reactors to test their behaviour at the beginning of life (BOL) and at the end of life (EOL), NILOC (HFR) and NIMPHE (PHENIX), respectively, have been continued. Out-of-pile tests were performed to study changes in structure and composition of mixed nitride fuel pins in an axial temperature gradient. - Measurements of the heat capacity of UO2 up to 8000K were concluded and the results are being analysed. Radiative properties of oxides (thoria, urania, zirconia) were measured in the solid and the liquid range. A model for the total emissivity of urania was developed. - The code MITRA has been adapted to perform source term calculations. A computer code for the calculation of the thermochemical equilibrium of fission products was written and a database for fission product compounds has been implemented with interface to the SOLGASMIX/MITRA codes. A shielded Knudsen cell for irradiated UO2 fuel has been assembled. - Work in 1990 on the safety of nuclear fuels has resulted in 33 (status september '90) contributions to conferences, articles in scientific journals, reports and chapters in books; two patents were granted
Detailed description of work foreseen in 1991 (expected results)
Riso III results will undergo final evaluation and fuel work will concentrate on MOX fuel and on the structural and chemical changes at local burn-ups of up to 15 %. Laboratory work will principally deal with SIMFUEL with 6 and 8 % burn-up. Modelling work will continue. Annealing tests will be performed under oxidizing and reducing atmosphere on U02 samples irradiated up to 55 GWd/t, in order to determine fission gas release as a function of O/M . A remotely controlled thermal diffusivity apparatus will be mounted in a hot cell. BOL and EOL irradiations NILOC and NIMPHE, respectively, will be continued and a high burn-up irradiaton POMPEI will be prepared. A study of new fuels for future reactors under off-normal conditions will be launched. Measurement of the thermal conductivity of molten UO2 in the presence of W will be attempted. Vapour pressures and the dissociation temperatures of advanced fuels will be measured.
Short description of evolution of work in 1992
Oxide fuel transient studies will be extended to high-burn up MOX fuel, both experimentally and in modelling. Work on the RIM-effect and its implications for fuel performance and spent fuel storage will be brought to an end. - Annealing tests with irradiated fuels and Knudsen cell measurements of fission product release will continue. - NIMPHE irradiations in Phenix will be extended to 12 at % b.u. Six KNK (carbide) pins (with 7 at % b.u.) will undergo PIE. - Source term work will continue and the thermal conductivity of high burn-up fuel will be studied.
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76125 Karlsruhe
Germany