Evaluation of Neutron Spectral Effects in a Plutonium-Fueled Lattice by Foil Activation

The investigation of the neutron spectral effects in actual cell geometry in a plutonium-fueled lattice is the final step of a program undertaken at LFCR of CNEN on the investigation of the physics problems for the plutonium recycle in light water reactors. Spectral indices have been measured by foil activation techniques, in the fuel pins and in the moderator, both in a uniform lattice and in the presence of a cross-shaped water gap; the presence of the water gap reduces the value of the index ²³⁹Pu-fission/²³⁵U fission by about 8%. The measured values have been compared with the results of the design calculational methods as well as those of a more detailed method and agreement is good. In particular, it has been found that the use of two thermal-energy groups (in a five-group scheme) is necessary to describe properly the effect of water gaps on the plutonium effective cross sections. The ²³⁹Pu absorption rate has been evaluated from the measured spectral indices by applying a previously tested correlation method; the presence of the water gap also reduces the ²³⁹Pu absorption/²³⁵ U-fission-ratio by about 10%.