Criticality of actinoid elements

New calculations and experimental data on critical parameters of actinoid elements are given. Nuclides having an even number of neutrons in the nucleus may form a critical system only if they are in the form of metals, oxides, or other compounds with a small number of elements that retard neutrons. In nuclides with an odd number of neutrons criticality may be reached at almost any content of the neutron retardant. Critical mixtures of fissioning nuclides and means for ensuring nuclear safety in processing and storing 238Pu are considered. The assumption that metallic 238Pu has a finite critical mass was confirmed at the Los Alamos laboratory. Comparison of 10 g of metallic 238Pu with 10 g of 239Pu by a substitution method showed that the reactivities of these isotopes are equivalent. Since metallic 239Pu in spherical form without reflector has a critical mass of around 15 kg, it is considered that metallic 238Pu also may form a similar critical system. Based on experimental data from storage of 238Pu, it was assumed that in systems with a heat spectrum, if the 238Pu content in the radioisotope mixture is no less than 76%, the danger of criticality does not exist. Calculations were confirmed experimentally for 238Pu and 244Cm. The reliability of data for the other nuclides is determined by the precision of the nuclear-physics constants. (H.E.)