Qualification and interpretation of MR test reactor irradiation data on WWER-440 type fuel rods for fuel thermal model validation

Conducted jointly by the Kurchatov Institute of Atomic Energy of the USSR and the Finnish state owned power utility Imatran Voima Oy, an irradiation program, SOFIT, including a series of characterised and instrumented test rod bundles is in progress to examine the thermal and mechanical behaviour of WWER type fuel rods. The main purposes of the work are to promote the flexibility and the economy of the fuel utilisation, and to verify, for the WWER annular fuel and Zr1%Nb cladding, the fuel behaviour computer programs used in the Soviet Union and in Finland. The more specific objectives are to obtain information on fuel temperature, internal gas pressure and cladding elongation during irradiation for different fabrication parameters. Qualification and interpretation of in-pile results from the first irradiation, completed at maximum local exposure of about 16 MWd/kgU, are presented. Quantitative evaluation of affecting phenomena could be deduced from the restored thermocouple readings. The measured temperatures show dependency on the as-fabricated gap size and weakness on the helium fill pressure. The results of the calculations, obtained by the existing steady state fuel behaviour computer codes are compared with the experimental temperatures. The importance of careful qualification of the data is emphasised. The related uncertainties are discussed. Preliminary fission gas data of some of the rods have become available, which show low or moderate gas release. (author). 5 refs, 7 figs, 1 tab