UO₂ Core Design of a Direct-Cycle Fast Converter Reactor Cooled by Supercritical Water

The core design of a fast converter reactor adopting enriched UO₂ fuel is studied for maximizing the power rating of the direct-cycle, supercritical water-cooled fast reactor with the same reactor pressure vessel as the breeder and mixed-oxide (MOX) fueled converter. The coolant void reactivity is kept negative by placing thin zirconium-hydride layers in the blanket fuel assemblies facing the driver fuels, as in our fast breeder reactor design. Compared with the fast converter adopting MOX fuel, the electric power output is increased 11%, from 1444 to 1625 MW(electric). It is attained by the reduced blanket fuel fraction for keeping negative reactivity at coolant voiding. The positive reactivity at flooding the core is much larger than that of the MOX core, but it can be managed by the control rod system. The conversion ratio, the surviving ratio, is 0.85, reduced 0.1 from that of the MOX converter. The enrichment of UO₂fuel reaches 16.9%. The specific fissile inventory is the highest, compared with the MOX-fueled converter and breeder due to the lower fission cross sections of ²³⁵U. The cores of the supercritical water-cooled reactors are radially heterogeneous. The decoupling problem is, however, much smaller than that of the liquid-metal fast breeder reactor due to the smaller core diameter. The hydrogen loss from the zirconium hydrides at steady state and accidental conditions does not impose a problem.