Nuclear criticality safety aspects of 238Pu

AbstractCalculations 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.