Measurement and calculation of neutron densities in BWR high burn-up fuels

The computer codes PANAMA and FRESCO developed at the Research Center Julich have been used for the prediction of fuel performance and fission product release behavior during the normal operation of the Japanese High-Temperature Engineering Test Reactor, HTTR. Basis for the calculations was the so-called ʹStandard HTTR Operation Planʹ with a nominal operation time of 660 efpd including a 110 efpd period with enhanced fuel temperatures. Fuel performance model calculations with the PANAMA code have shown that for the temperature distribution given, only a small additional failure fraction is expected. The diffusive release of metallic fission products from the fuel occurs mainly from the central core layers with the maximum temperatures whereas there is little contribution from the upper layer. Silver most easily escapes the fuel. The release data for strontium and cesium also reveal a significant fraction to originate from still intact particles. The comparison with the calculations obtained with the JAERI models has shown a good agreement for the release from the coated particles.