Immobilization by vitrification is one potential disposition option for aportion of the United States’ excess plutonium inventory. Research has beenperformed to determine the glass forming region of a frit, plutonium andrare earth system. The frit contains mainly oxides of aluminum, silicon andboron; small amounts of ZrO2 and SrO are also included. The rareearth elements provide a flux to the glass during processing. The rareearths are also added as neutron absorbers (to prohibit criticality) in thefinal vitreous product.

This report will show the compositional region, using Th as a Pu surrogate,that should be targeted in future studies for maximum Pu solubility in thelanthanide borosilicate glass system. Durability data and process variableswill also be provided.

The Savannah River Site has the majority of the United States' supply ofneptunium currently stored in an acid solution in one of their canyonfacilities. A program is being developed that could be utilized to ship thismaterial, as glass, to Oak Ridge National Laboratory where the Np could beleached from the glass, purified by ion exchange and made into targetmaterial for the production of Pu-238. Ion exchange purification dictates nomaterial be in the leachate making the isolation of the Np difficult. Wehave developed a process using thorium as a surrogate for Np that couldimmobilize the Np into a soda borosilicate glass for shipment. To achieverecovery of the Np, the glass can be phase separated prior to leaching withnitric acid. Phase separation would produces a Np-rich sodium-borate phaseand a Si-rich phase similar to a Vycor® glass. The nitric acid selectivelyattacks the sodium-borate phase allowing high Np recovery in a solution thatcontains only sodium and boron. These can be easily separated from Np by ionexchange. Essentially all of the silicon which would interfere with ionexchange by precipitation is retained in the Vycor®-type phase. Thistechnology may also be applied to other actinides stored in relatively puresolutions.

This paper will report the optimization of variables for maximizing Th (a Npsurrogate) recovery while minimizing Si release. Th solubility in glass,heat treatment conditions and leaching parameters will be discussed.Transmission Electron Microscopy (TEM) with energy dispersive spectroscopy(EDS) data will be included to show phase separation after heattreatment.

Immobilization by vitrification is one potential disposition option for aportion of the United States' excess plutonium inventory. Research has beenperformed at the Savannah River Site (SRS) to determine the optimumcomposition of a lanthanide borosilicate frit for the vitrification ofplutonium using a Plackett-Burman design and simplex algorithm as astatistical tool. This technique uses various response variables to rank andoptimize a composition. The variables used in this study correspond tohomogeneity, durability, actinide solubility and devitrification afterheat-treatment.

The optimized frit composition was determined using a constant ThO2 loading of 20 wt%. No noticeable trends were followedwith respect to the individual components which may indicate a relativelyrobust system able to accommodate variations in the feed.

Batches containing various loadings of ThO2 were melted todetermine if actinide solubility was improved in the optimized compositioncompared to that of a similar lanthanide borosilicate glass. No noticeableimprovement in ThO2 solubility was realized as a result of usingthis optimization technique.

This paper presents results of an investigation of the microstructures and durabilities of glasses for immobilization of excess Pu, Am, and Cm at the Savannah River Site (SRS). The glasses investigated had compositions based on commercial lanthanide glasses developed in the 1930's. All the glasses were prepared remotely in shielded cells and the analyses performed using instruments in radio hoods or benches. Durabilities were measured using the ASTM C-1285 standard leach test (PCT). Results for three glasses are presented. Two glasses contained 15 and 7 wt.% Pu, respectively. The third glass contained Am and Cm. The 15 wt% Pu glass was not completely amorphous and contained crystals of undissolved PuO2 and as well as dissolved PuO2. The 7 wt% Pu glass was completely amorphous. The Am/Cm glass contained <1 wt% actinides and was homogenous. The PCT tests for the Pu glasses indicated that B and Ba were leached congruently. Sm and Pu were leached at slower rates. In some cases release rates for specific Ba, Sm, and Pu isotopes were measured by analyzing the leachates by mass spectroscopy. For the Am-Cm glass release rates for B and Ba were equal indicating congruent dissolution. All three glasses were 25 to 50 times more durable than the borosilicate glasses developed at SRS for immobilization of the fission product HLW resulting from reprocessing operations at SRS.