Temperature-Dependent k_∞ for a ThO₂-PuO₂ HTGR Lattice

The temperature dependence of k_∞ for a graphite-moderated ThO₂-PuO₂-fueled lattice has been measured in the high temperature lattice test reactor. Values of measured at equilibrium temperatures from 20 to 1000°C serve as a benchmark for evaluating computational methods and cross sections used in the design of a plutonium-fueled high temperature gas-cooled reactor (HTGR). The calculated and measured changes of k_∞ with temperature are in agreement. However, the magnitudes of calculated values are ∼0.8% higher than the measured values for k_∞. When plutonium is used as the initial fissile fuel, the temperature coefficient of reactivity is changed favorably from that of an HTGR lattice fueled initially with ²³⁵ U. The reactivity decrease associated with elevating this plutonium-thorium fuel to operating temperature is ∼30% less than that obtained with ²³⁵U-Th fuel, so the excess reactivity for which the control system must compensate is reduced with plutonium fueling. Above the operating temperature range, plutonium tends to make the temperature coefficient of the HTGR more negative due to increased neutron capture in the 1.06-eV resonance of ²⁴⁰Pu. This effect will improve the self-limiting characteristics of the reactor during a power excursion.