Experimental investigation of moderator density variation effects in an SCWR-like fuel lattice

Reaction rate distribution measurements are reported from experiments in an SCWR-like test lattice performed with separated moderator and coolant regions. This work was done at the zero-power research reactor PROTEUS at the Paul Scherrer Institute in Switzerland. Four water-tanks were inserted into the four central moderator channels of the lattice. The derived reaction rates, captures in U-238 (C-8) and total fissions (F-tot), as well as their ratio (C-8/F-tot), were mapped on the lattice and compared to MCNPX whole-reactor model calculations. There were two measurement campaigns: one in which the moderator density of the water in the tank was effectively increased (by changing the mixture of H2O and D2O, so as to alter the hydrogen density) and one in which this density was decreased. Excellent agreement, viz, within the la uncertainty of 0.4% and 0.8% for F-tot and C-8, respectively, has been obtained between experiment and calculation for both measurement campaigns. These experiments thus offer a generic validation of Monte Carlo simulations of moderator density variation effects in SCWR lattices, which are characterized by considerably different water densities in the separate moderator and coolant regions.