Plutonium Silicate Alteration Phases Produced by Aqueous Corrosion of Borosilicate Glass
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silicate) [6]. Silicon-based colloids have been implicated in the migration of spent weapons plutonium from the Nevada Test Site [7], although Kersting et al. attributed the plutonium incorporation to sorption rather than co-precipitation. EXPERIMENT
Monolithic samples of LaBS glass loaded with 10 weight % plutonium were reacted with water vapor at 100% relative humidity and 200'C for periods of 14 to 56 days. The test procedure is described in detail elsewhere [1, 81. Parallel corrosion tests, whereby crushed LaBS glass was immersed in deionized water at 90 'C (a modified ASTMC-1285 Product Consistency Test or PCT), were conducted for extended periods of time (> 98 days). The surfaces of the glass samples, along with alteration phases, were examined with a transmission electron microscope (TEM) and scanning electron microscope (SEM) to determine the characteristic alteration products. Vapor alteration of the LaBS glass for 14 days produced macroscopic crystallites of a plutoniumlanthanide silicate material, which is, to our knowledge, the first plutonium-based alteration phase observed in glass-water reaction. An extensive alteration layer was found on the glass surface containing amorphous aluminosilicate layered with bands of a cryptocrystalline plutoniumlanthanide silicate that was similar to the surface crystals but relatively depleted in lanthanides. Of particular interest was evidence of size selection among the lanthanide elements lanthanum, neodymium, and gadolinium, as well as separation of the lighter lanthanides from plutonium. There is little evidence that gadolinium, an important neutron absorber, is separated from the plutonium. 739 Mat. Res. Soc. Symp. Proc. Vol. 608 ©2000 Materials Research Society
After vapor hydration at 200'C and 100% relative humidity for 14 days, the surface of the LaBS glass monolith was speckled with minute (- 5 gim) crystallites that appeared white against the dark glass. The surface of the glass itself had a slight crust, or alteration film, which appeared less advanced than what would typically be observed from a similar test on a HLW glass. Samples of the white surface crystallites were embedded in epoxy resin and thin sectioned with a Riechert ultramicrotome. The thin-sectioned material was examined using a JEOL 2000FX II transmission electron microscope operating at 200 keV and equipped with x-ray (EDS) detectors and an electron energy loss spectrometer (EELS). Cross sections of the altered surface film on the glass were taken by scoring the specimen with a diamond scribe, and embedding fractured material for thin sectioning in a manner similar to the surface particles. Intact glass surfaces were also examined with an SEM and light microscopy. Samples of LaBS glass from crushed glass immersion tests at 90'C were also examined. The LaBS glass reacted very slowly under these conditions, and little evidence of an alteration layer was observed until at least 98 days of testing. Longer-term tests have not been examined in detail owing to a shift in program focus to the t
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