Minimum critical mass of plutonium--polyethylene system found to be significantly lower than plutonium--water system

Theoretical homogeneous plutonium-polyethylene 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-polyethylene mixture in spherical geometry has been calculated to be approximately 360 g plutonium (+-5% for cross-section uncertainties) compared to approximately 530 g plutonium for a plutonium-water mixture under the same conditions. The 30 percent 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 (rho = 0.96 Mg/m3) than oxygen atom density in water