In the same period of time a series of room-temperature critical experiments on the zero energy HITREX reactor have been performed at the CEGB Berkeley Nuclear Laboratories.

The HITREX-I core had a prismatic HTR lattice containing fuel of two types— teledial and annular—in a zone 2 m high of twenty-seven hexagonal blocks (399.5 mm across flats) surrounded by graphite reflectors. The measurements involved: reactivity, axial buckling, power distribution, fine structure effects, thermal spectrum distributions (Pu/U ratio), graphite damage (Rh reaction rate), control-rod worths. The theoretical evaluation has been performed with the WIMS-E modular scheme. Reaction rate distributions show a very good agreement between calculations and experiments, with a mean deviation of less than 1%. Also the prediction of the thermal flux within the central block is very well described by the analysis method. The major discrepancy between experiments and analysis is an underprediction of reactivity of about 1.5% Дk/k. It is likely that some 0.3% of this discrepancy is due to poor treatment of steel absorbers. The relative conversion ratio (RCR) in the annular fuel appears to be overestimated by 3.5%. This could result in an underestimate of up to 0.5% k/k in core reactivity/25"28’ An independent evaluation performed by the Dragon Project129’ based on 123 groups S4 cell calculations with the XSDRN code00’ overpredicts reactivity by about 1.5% Дk. An HITREX-2 series of experiments has been started, based on integral fuel blocks. It is also foreseen to heat a single block column up to 400°C.<3I)