AbstractCalculations show that a 10% isotopic abundance of238Pu in a bare spherical239Pu-H2O system increases the minimum critical mass by ~48%, thereafter the increase continues exponentially. These results, in addition to several subcritical neutron multiplication experiments with238Pu in both solution and dioxide form, confirm that238Pu behaves as a neutron poison in a well-moderated medium. Neutron multiplication experiments were conducted with four SNAP-19B (IRHS) heat sources with ~1 kg238Pu 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 for238Pu 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 gradual238Pu sludge buildup. Heterogeneous water-filled storage vaults provide the safest method for storing large quantities of PuO2containing at least 76%238Pu.