The reactor-pumped laser: Kinetics effects on the SPR III reactor

The Sandia Pulsed Reactor III (SPR III) is being used as the pumping source for evaluation of energy deposition and laser physics in gas mixtures for reactor-pumped lasers. The test apparatus includes moderating and reflecting materials both near and in the glory hole, and occasionally, small quantities of uranium deposited on ceramic tubes in the glory hole. These materials significantly affect reactor behavior. The most prominent change that occurs with moderating material in the glory hole is an increase in the effective neutron lifetime of the reactor, and a factor of 2 to 3 has been observed in these tests. This affects several aspects of the reactor's behavior: initial reactor period, pulse width, and yield versus initial reactivity insertion. The most significant effect occurs when small quantities of uranium (ranging from 0.4 grams to 6 grams U3O8) are included in the package. There is an apparent increase in lifetime up to a factor of 7 over the normal value, and a large change in the pulse symmetry. Investigation suggests that the moderating material and uranium behave as a pseudo delayed-neutron precursor with a delay time of about 1.6 milliseconds, and an apparent yield of 0.00017. This paper provides a detailed discussion of these observations