Investigations of LWR design modifications for improved fuel utilization in once-through cycle

Neutronics aspects of a reactor core throughout its cycle are investigated in a search for increasing in-core utilization of the residual fissile isotopes content in the cycle discharged disposal. Design considerations are explored in order to account for power peaking due to water gaps and for convenient neutron leakage leading to a utilization of burnup dependent effective bucklings and cross sections. At convenient burnup levels, changes in H2O/UO2 volume ratio are introduced allowing an intense depletion of the residual fissile isotopes existing in assemblies with high exposure levels. The effects of design changes are evaluated from an interplay of different nuclear parameters for pin cell, assembly and core calculations. A systematic analysis of industrial methodology and available data guided the development of an analytical model that reproduced several nuclear parameters with an accuracy similar to those from more complete models and with a reduced computational effort. The effects due to design modifications introduced at burnup levels near the end-of-cycle, in an equilibrium cycle condition, have indicated the possibility of a better in-core utilization of the residual fissile isotopes existing in the cycle-discharged disposal. The potential benefits are significant to warrant an examination of the mechanical and thermal hydraulics considerations involved in implementing the design modifications under study