Criticality of Plutonium Compounds in the Undermoderated Range, H:Pu ≤ 20

Experimental criticality data do not exist for most plutonium compounds. To obtain guidelines for nuclear criticality safety use, a survey utilizing transport-theory calculations was made to determine the critical masses of bare and water-reflected spheres as a function of density and H:Pu ratio for 12 of these compounds in the undermoderated range (H:Pu ≤ 20). The compounds considered were: PuH₂, PuH₃, PuN, PuC, Pu₂C₃, PuO₂, Pu₂O₃, PuF₃, PuF₄, PuCl₃, Pu(NO₃)₄, Pu(C₂O₄)₂. Also derived were core density exponents which permit critical masses to be predicted for compounds with densities ranging down to one-fifth of their theoretical values. The validity of the calculations was examined by comparing results with the limited criticality data on homogeneous PuO₂ systems in the undermoderated range. Comparisons were also made for Pu metal systems and for three heterogeneous Pu-fueled assemblies.