Lattice parameter measurements on cluster-type fuel for advanced thermal reactor

The behavior of neutrons in a highly heterogeneous unit cell consisting of D2O moderator, H2O coolant and a 28-pin fuel cluster contained in a pressure tube has been studied through lattice parameter measurements covering three different 235U enrichments, four coolant void fractions and two lattice pitches. A single-region core configuration was adopted, with which measurements were made to determine - in relation to coolant void fraction - the critical D2O level, as well as various lattice parameters. A strong dependence on coolant void was observed for the critical level and the lattice parameters, in the case of the smaller 22.5cm pitch lattice, due to the positive effect on core reactivity exerted by the slowing-down faculty of H2O in the epithermal energy region. With the larger 25.0cm pitch, however, no meaningful dependence on void fraction was shown by any of the measured values, and this was ascribed to a compensating negative effect due to enhanced thermal neutron self-shielding in the fuel region produced by the H2O coolant. The results of cell calculations obtained by means of the METHUSELAH-II code showed generally good agreement with experimentally determined data, for both critical D2O levels and lattice parameters, in the case of coolant-filled lattices (0, 30 and 70% void fractions). For lattices devoid of coolant (100% void fraction), however, discrepancies in lattice parameters - particularly in rho28 - produced corresponding deviations in core reactivity amounting to 1% in excess of those incurred with other void fractions. (auth.)