Minimum Critical Mass of Plutonium-Polyethylene System Found to Be Significantly Lower Than Plutonium-Water System

Theoretical homogeneous plutonium-poly ethylene mixtures have significantly lower critical masses than theoretical homogeneous plutonium-water mixtures of comparable H/Pu ratio for H/Pu ratios close to optimum. The minimum critical mass for an optimally moderated, full water-reflected, homogeneous plutonium-poly ethylene mixture in spherical geometry has been calculated to be ∼360 g plutonium (±5% for cross-section uncertainties) compared to ∼530 g plutonium for a plutonium-water mixture under the same conditions. The 30% drop in critical mass is attributable to increased hydrogen atom density and the superior moderating characteristics of carbon over oxygen combined with a higher carbon atom density in polyethylene (ρ = 0.96 Mg/m³) than oxygen atom density in water (ρ = 1.0 Mg/m³).