In the period 1970-1 a series of measurements at very high temperature (up to 1000°C) have been performed at the Battelle Northwest Laboratory (BNWL) in the High Temperature Lattice Test Reactor (HTLTR). The method of measurement in the HTLTR determines the excess neutron production in a test lattice from the ratio of the reactivity coefficients of a test lattice sample and a normalizing neutron absorber, usually copper.<15) The measurements have been repeated at several temperatures to obtain the temperature coefficient. Several correction factors, both analytical and experimental, must be applied to each measurement, and this complicates the analysis. With this technique only a small region of the exact lattice under study is required, and this fact made possible the study of lattices with high 233U or Pu concentrations.

Five different lattices have been measured with various 235U, 233U and Th loads. Lattice 4 was fuelled with Pu and Th, and lattice 5 was designed to simulate the Molten Salt Breeder Reactor (233U-Th). At high temperature the measured temperature coefficient is less negative than calculated. The measurements on Pu lattice showed a very good agreement with the calculations.