Caustic precipitation of plutonium and uranium with gadolinium as a neutron poison

摘要

The caustic precipitation of plutonium and uranium from Pu- and U-containing waste solutions has been investigated to determine whether gadolinium could be used as a neutron poison for precipitation with greater than a fissile mass containing both Pu and enriched U. Precipitation experiments were performed using both process solution samples and simulant solutions with a range of 2.6 to 5.16 g/l U and 0 to 4.3:1 U:Pu. Analyses were performed on solutions at intermediate pH to determine the partitioning of elements for accident scenarios. When both Pu and U were present in the solution, precipitation began at pH 4.5 and by pH 7, 99% of Pu and U had precipitated. When complete neutralization was achieved at pH > 14 with 1.2 M excess OH-, greater than 99% of Pu, U, and Gd had precipitated. At pH > 14, the particle sizes were larger, and the distribution was a single mode. The ratio of hydrogen to fissile atoms in the precipitate was determined after both settling and centrifuging and indicates that sufficient water was associated with the precipitates to provide the needed neutron moderation for Gd to prevent a criticality in solutions containing up to 4.3:1 U:Pu and up to 5.16 g/l U.