The Nuclear Criticality Safety Aspects of Plutonium-238

Calculations show that a 10% isotopic abundance of ²³⁸ Pu in a bare spherical ²³⁹ Pu-H₂O system increases the minimum critical mass by ∼48%, thereafter the increase continues exponentially. These results, in addition to several subcritical neutron multiplication experiments with ²³⁸ Pu in both solution and dioxide form, confirm that ²³⁸ Pu behaves as a neutron poison in a well-moderated medium. Neutron multiplication experiments were conducted with four SNAP-19B (IRHS) heat sources with ∼1 kg ²³⁸ Pu per source. The results indicated that a neutron multiplication <1.2 existed when the heat sources were assembled in different array configurations. The criticality unit value for ²³⁸ Pu in either solution or dioxide form has been conservatively established at 2.0 kg. Thermocouples installed on process vessels have proven to be a new criticality control technique for determining gradual ²³⁸ Pu sludge buildup. Heterogeneous water-filled storage vaults provide the safest method for storing large quantities of PuO₂ containing at least 76% ²³⁸Pu.