The immobilization of technetium-99 ((99)Tc) in a suitable host matrix has proven to be a challenging task for researchers in the nuclear waste community around the world. In this context, the present work reports on the solubility and retention of rhenium, a nonradioactive surrogate for (99)Tc, in a sodium borosilicate glass. Glasses containing target Re concentrations from 0 to 10,000 ppm [by mass, added as KReO(4) (Re(7+))] were synthesized in vacuum-sealed quartz ampules to minimize the loss of Re from volatilization during melting at 1000 °C. The rhenium was found as Re(7+) in all of the glasses as observed by X-ray absorption near-edge structure. The solubility of Re in borosilicate glasses was determined to be ~3000 ppm (by mass) using inductively coupled plasma optical emission spectroscopy. At higher rhenium concentrations, additional rhenium was retained in the glasses as crystalline inclusions of alkali perrhenates detected with X-ray diffraction. Since (99)Tc concentrations in a glass waste form are predicted to be <10 ppm (by mass), these Re results implied that the solubility should not be a limiting factor in processing radioactive wastes, assuming Tc as Tc(7+) and similarities between Re(7+) and Tc(7+) behavior in this glass system.

This study presents the characterization of salt phases that formed on simulated low‐activity waste glass melts during a rhenium solubility study. This study with rhenium salts is also applicable to real applications involving radioactive technetium salts. In this synthesis method, oxide glass powder is mixed with the volatile species, vacuum‐sealed in a fused quartz ampoule, and then heated in a furnace. This technique restricts the volatile species to the headspace above the melt but still within the sealed ampoule, thus maximizing the concentration of these species that are in contact with the glass. Above the previously determined solubility of Re7+ in this glass, a molten salt phase segregated to the top of the melt and crystallized into a solid layer. This salt was analyzed with X‐ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, as well as wavelength dispersive spectroscopy and was found to be composed of alkali perrhenates (NaReO4, KReO4) and alkali sulfates. Similar crystalline inclusions were found in the bulk of some glasses as well.

It has long been accepted that the precision of isotope ratio measurements by inductively coupled plasma mass spectrometry (ICP-MS) is poorer than consideration of counting statistics would suggest is possible. This has been attributed to the coupling of a scanning mass analyser to a noisy ICP ion source, the sample introduction system and the ion extraction method. Thus, the ion output from the source varies as the analyser scans between successive masses so that their ratios vary. A variety of noise sources have been identified, mainly from noise power frequency spectra, but the sensitivity of these spectra as diagnostic tools is limited by the statistics of the signals fed to the analysing software, and it becomes necessary to extend the investigation by using isotopic ratio precision as the ultimate criterion. Even when the effect of (previously identified) major sources are greatly reduced, poor precision is still obtained. In this study the effect of poor ion distribution in the plasma and ICP-MS interface is identified as the principal cause and by avoiding this, precision values are found which agree with statistically expected values.

No abstract

No abstract