Qualification of HTR fuel and graphite in European material testing reactors

The Juelich Nuclear Research Centre (KFA) has been involved in development and irradiation testing of fuel and structural materials for high-temperature reactors (HTR) for 25 years. During this time several HTR reactor concepts have been designed and developed employing a variety of fuel element concepts with various fuel operating conditions. Coated particles and fuel element matrix components, as well as the integral fuel elements, have undergone extensive irradiation testing in various material testing reactors (MTR). Since 1962, about 150 irradiation experiments have been performed in the reactors R2-Studsvik, BR2-Mol, HFR-Petten, Siloe-Grenoble and FRJ2-Juelich. Rapid feedback of the results from irradiation testing as an essential part of successful fuel element and material development requires above all short irradiation time and therefore high thermal neutron flux. The tests are carried out in instrumented capsules purged with He/Ne gas simulating HTR nominal and extreme operating conditions, including power and temperature transients. The irradiation temperature can be varied by changing the neutron flux and by changing the purge gas composition and is measured by thermocouples in the specimen or in the surrounding structure. The fission gases produced during irradiation are swept out and analysed for their activity. The amount of the fission gases and the composition indicate the fuel quality. Isothermal irradiations and experiments for the determination of neutron-induced creep have been carried out for graphite as structural material for core and fuel elements. Specimen dimensions and physical properties were measured out-of-pile between the irradiation cycles. The tests were carried out at temperatures between 400 and 1200 deg. C up to a fast neutron fluence of 2x1026m-2, the typical lifetime conditions of the graphite side reflector in pebble bed cores. (author). 4 refs, 9 figs