Aged nuclear explosive melt glass: Radiography and scanning electron microscope analyses documenting radionuclide distribution and glass alteration

Assessment of the long-term performance of nuclear melt glass under saturat ed conditions provides insight into factors controlling radionuclide releas e into groundwater. Melt glass samples were collected from an underground n uclear detonation cavity at the Nevada Test Site that was in contact with g roundwater for more than 10 years. The samples were made into thin sections and the distribution of alpha-activity mapped using CR-39 plastic detector s. The melt glass is visually heterogeneous and the results of the alpha-tr ack radiography indicate that the highest alpha-activity is associated with areas of dark colored glass. Analyses of the thin sections by alpha-spectr ometry show the prominent actinide species to be Pu-238, Pu-239,Pu-240 and Am-241. Scanning electron microprobe analysis of the bulk glass shows consp icuous alteration layers lining internal Vesicle surfaces in the glass. X-r ay diffraction patterns for the alteration phases are consistent with clay mineral compositions. Glass dissolution models indicate these layers are to o thick to have formed at ambient temperatures over the IO year period in w hich they remained in a saturated environment. This implies the alteration layers likely formed at temperatures higher than ambient during cooling of the cavity following the underground detonation. Mobilization of this clay alteration layer as colloidal particles in groundwater represents a potenti al source of actinide release into the environment.