While the US programme was based on the Th cycle, the British programme (UKAEA and CEGB) has been based almost exclusively on low enriched uranium. The UKAEA programme was started in 1969 in Winfrith with lattice parameter measure­ments at room temperature in an undercritical system using the NESTOR reactor as a neutron source.

The central part of the HECTOR reactor was later used for measurements up to 427°C.<16) Various reaction rates have been measured and calculated for the NESTOR experiments with tubular pins and teledial fuel. The discrepancies in relative conver­sion ratio (RCR), 238U/235U fission ratio (FR) and Pu/U fission ratio range from 0.5 to 3% for WIMS calculations, slightly higher for simplified Dragon methods.<17_21) The HECTOR experiments were rather difficult to analyse because of the marked heterogeneity in composition and temperature.

There appeared to be a tendency to overestimate the magnitude of the negative temperature coefficient.1221 In a phase II of the UKAEA programme cross-pin and cross-block power and damage distributions and thermal spectrum characteristics have been measured in hexagonal block geometry in the ZENITH-I reactor. The experi­ments have been repeated at 400°C to check the effect of temperature on the measured distributions and control-rod worths. These experiments were terminated in June 1971. Like in the HECTOR case, there was a tendency to overestimate the negative temperature coefficient and the spectral changes at the core-reflector boundary were rather poorly calculated.

In a phase III the cold ZENITH-II reactor has been used for measurements on block to block and cross core distributions, three-dimensional effects on a large core, effect of absorbers, burnable poisons, non-uniform enrichment, Pu02/U02 fuelled blocks, control-rod worths and interactions, local criticality situations/23’24’