Time-dependent performance characteristics of fusion hybrid blankets

Energy multiplication and fissile fuel production in fusion fission hybrid blankets have been the subject of numerous studies during the last years. While most of the blanket neutronics investigations refer to stationary problems, we examine here the time dependent relationships among the various blanket parameters in order to reveal their immediate and long term effect to the blanket performance. Based on an appropriate parametrisation we present a convenient analytical method for solving the dynamic balance equations of (i) the free neutrons, (ii) the isotopes engendered and consumed in the several nuclear reactions, as well as (iii) the power production in a fission blanket driven by D-T neutrons. As results we describe burn-up and build-up of the important nuclear species in the blanket, including also fission products, and we discuss the time variation of fissile fuel content and its effect on the subcriticality of the blanket assembly; further, we illustrate the breeding ratio and fissile fuel yield as functions of fuel residence time and determine a maximum fissile enrichment achievable with a specific blanket design. Also we demonstrate the variation of inherent neutron multiplication as affected by the blanket exposure to fusion neutrons. Finally, a criterion for subcriticality is exhibited posing constraints to blanket design parameters which in turn restrict the blanket management to a proper range of loading, residence and extraction of fuel materials. (orig.)