Moderator temperature effect on reactivity in light water moderated experimental reactors

The moderator temperature coefficient of reactivity has been measured in a U-233 fuelled plate type light water moderated reactor (KAMINI). Using neutron cross sections based on WIMS library, lattice homogenization code SMAXY and 3D core calculation code COMESH the moderator temperature coefficient of reactivity is predicted very well (within 0.6 pcm per °C). For U-235 and Pu-239 fuelled KRITZ experiments also, moderator temperature coefficients of reactivity have been predicted and compared with Studsvik results using lattice code MURLI and 2D core calculations. Our predictions of the moderator temperature coefficient of reactivity is fairly good for these cores. For U-233 fuelled KAMINI, the close agreement on moderator temperature coefficient of reactivity was not found using more recent U-233 cross-sections derived from ENDF/B-IV (WIMKAL). By comparing the α (capture to fission ratio) of U-233 at low energies between WIMS and ENDF/B-IV and VI, the better comparison with WIMS is attributed to the flatter α behavior with energy. There appears to be a need for measurement of α of U-233 below 0.1 eV.