Long Term Variation in Composition and Neutron Yield in Pile Plutonium

The neutron economy of a thermal reactor or system of reactors using Pu239 as fuel for long time periods is examined. In the first case treated, only the change in neutron absorption and production due to plutonium isotope growth is considered. In the second, and more complete case, the effect of neutron absorption by fission products is included. Both analyses are developed in terms of an idealized system in which: (1) a uniform and constant flux of 3 × 1014 neutrons/cm2/sec is assumed; (2) the Pu239 level is kept constant by internal regeneration or from external sources; (3) neutron escape and neutron capture by structure, moderator, and coolant are neglected; (4) excess neutrons beyond those needed to propagate the chain are absorbed in the fertile material, U238, to regenerate Pu239; and (5) contributions to the neutron economy from U238 and U235 fission are not included. In the first case (omitting fission product absorption), the system is found to be approximately regenerative, i.e., at equilibrium, about as much Pu239 is formed as is destroyed. In the second case (including fission products), the regenerative properties become relatively poor unless fission products are removed periodically. A particular processing cycle is examined, in which chemical separations occur at 0.2-year intervals (nvt — 2 × 1021 neutrons/cm2) and is found to yield an almost regenerative system, so that relatively little Pu239 has to be supplied from external sources.