PRELIMINARY SURVEY OF REQUIREMENTS FOR HEAVY WATER MODERATED REACTOR LATTICE EXPERIMENTS IN THE PROPOSED CRITICAL FACILITY.

Calculations have been carried out to assess the fuel and control requirements for lattice measurements in heavy water moderated reactor cells, using a 'split-table' type machine with a U235/graphite driver region. The METHUSELAH code was used to calculate cell-averaged cross-section data for the heavy-water moderated region, and GYMEA for the U235/graphite driver region and the graphite reflector. CRAM calculations in four groups were then made for critical configurations in a range of driver region compositions and assembly geometries. It was established that for the type of natural uranium steam-generating heavy water reactor lattice considered, an adequately close approximation to the equilibrium neutron spectrum for the critical SGHWR could be established over a 30cm radius at the centre of the assembly, using approximately one tonne of SGHWR type fuel, 9 kg U235 and 13 tonnes of graphite. The latter two quantities would be somewhat increased if practical packing densities for the machine were taken into account. Adequate reactivity control for operation is available by use of a practicably small number (about 5) of conveniently sized plates 'black' to thermal neutrons.