Critical experiments on single-unit spherical plutonium geometries reflected and moderated by oil

Abstract

DEDICATION ABSTRACTExperimental critical configurations are reported for several dozen spherical and hemispherical singleunit assemblies of plutonium metal. Most were solid but many were hollow-centered, thick, shell-like geometries.All were constructed of nested plutonium (mostly ?Pu) metal hemispherical shells. Tbree kinds of critical mnfigmdons are reported. Two required interpolation andor extrapolation of data to obtain the critical mass because reflector conditions were essentially infinite. The first hds the plutonium essentially fully reflected by a hydrogen-rich oil; the second is essentially unreflected. The third kind reports the critical oil reflector height above a large plutonium metal assembly of accurately known mass (no interpolation required) when that mass was too great to permit fid oil reflection.Some configurations had thicknesses of mild steel just outside the plutonium metal, separating it from the oil. When used, the thickest of these was 50 mm. Steel laminations were used on both spherical and hemispherical assemblies; but hemispheres did not have steel against the plane h. O i l -r e f l d experiments were taken to or very close to delayed criticality; but those without oil were necessarily extrapolated to criticality for safety reasons. Still, extrapolations are believed to predict criticality quite accuratey because of the strong lrneanty in the h c t i o d relationship between radius and r e c i p r d mukiplidon.These experiments were performed at the Rocky Flats Critical Mass Laboratory in the late 1960s. 7'hey have not been published in a form suitable for benchmarkquality comparisons against state-of-the-art computational techniques until this paper. The age of the data and other factors lead to some diEculty in reconstructing aspects of the program and may, in turn, decrease confidence in certain details. Whenever this is true, the point is acknowledged.The plutonium metal was alpha-phase =%I containing 5.9 wt-% %. All assemblies were formed by nesting 1.667-m-thick (nominal) bare plutonium metal hemispherical shells, also called hemishells, until the desired mnfiguration was achieved. Very smaU tolerance gaps machined into radial dimensions redud the ef&ctive density a small mount in all cases, Steel components were also nested hemispherical shells; but these were nominally 3.333-mm thick. Oil was used as the reflector because of its chemical cornpalib* with plutonium 4.