NEUTRON TEMPERATURE-MEASUREMENTS IN A CRYOGENIC HYDROGENOUS MODERATOR

Benchmarkings of neutronic calculations are most successful when there is a direct correlation between a measurement and an analytic result. In the thermal neutron energy region, the fluence rate as a function of moderator temperature and position within the moderator is an area of potential correlation. The measurement can be done by activating na tural lutetium. The two isotopes of the element lutetium have widely d iffering cross sections and permit the discrimination of flux shape an d energy distributions at different reactor conditions. The Lu-175 has 1/v dependence in the thermal energy region, and Lu-176 has a resonan ce structure that approximates a constant cross section in the same re gion. The saturation activation of the two isotopes has been measured in an insulated moderator container at the center of a thermal heterog eneous reactor designed for space nuclear propulsion. The measurements were made in a hydrogenous (polyethylene) moderator at three temperat ures (83, 184, and 297 K) and five locations within the moderator. Sim ultaneously, the reactivity effect of the change in the moderator temp erature was determined to be positive with an increase in temperature. The plot of activation shows the variation in neutron fluence rate an d current with temperature and explains the positive reactivity coeffi cient. A neutron temperature can be inferred from a postulated Maxwell -Boltzmann distribution and compared with Monte Carlo or other calcula tions.