Publications

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This study is focused on describing the desorbed off gases due to heating of the AgIMordenite (MOR) produced at ORNL for iodine (I₂) gas capture from nuclear fuel aqueous reprocessing. In particular, the interest is for the incorporation of the AgI-MOR into a waste form, which might be the Sandia developed, low temperature sintering, Bi-Si oxide based, Glass Composite Material (GCM). The GCM has been developed as a waste form for the incorporation any oxide based getter material. In the case where iodine may be released during the sintering process of the GCM, additional Ag flake is added as further insurance in total iodine capture and retention. This has been the case for the incorporated ORNL developed AgIMOR. Thermal analysis studies were carried out to determine off gasing processes of ORNL AgIMOR. Independent of sample size, ~7wt% of total water is desorbed by 225°C. This includes both bulk surface and occluded water, and are monitored as H2O and OH. Of that total, ~5.5wt% is surface water which is removed by 125°C, and 1.5wt% is occluded (in zeolite pore) water. Less than ~1 wt% total water continues to desorb, but is completely removed by 500°C. Above 300°C, the detectable remaining desorbing species observed are iodine containing compounds, including I and I₂.

This study encompasses initial scoping tests on the incorporation of a novel iodine loaded getter material into the Sandia developed low temperature sintering glass ceramic material (GCM) waste form. In particular, we studied the PNNL Ag-I-Aerogel. Optical microscopy indicates inhomogenous samples based on particle sizes and variations in color (AgI vs Ag/AgO on silica). TGA/MS data when heated in air indicates loss of iodine and organics (CO2) between 250-450°C a total of ~15wt% loss, with additional / small iodine loss when during 550°C hold for 1 hr. TGA/MS data when heated in N2 indicates less organic and slightly less iodine loss below 550°C, with no loss of iodine in 550°C 1 hour hold. Furthermore, a substantial mass loss of sulfur containing compounds is observed (m/e of 34 and 36) between 150 – 550°C in both air and N2 sintering atmospheres. In an effort to capture iodine lost to volatilization during heating (at temps below glass sintering temperature of 550°C), we added 5 wt% Ag flake to the AgIaerogel. Resulting data indicates the iodine is retained with the addition of the Ag flake, resulting in only a small iodine loss (< 1wt%) at ~350°C. No method of curtailing loss of sulfur containing compounds due to heating was successful in this scoping study.

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Two large size Glass Composite Material (GCM) waste forms containing AgI-MOR were fabricated. One contained methyl iodide-loaded AgI-MOR that was received from Idaho National Laboratory (INL, Test 5, Beds 1 – 3) and the other contained iodine vapor loaded AgIMOR that was received from Oak Ridge National Laboratory (ORNL, SHB 2/9/15 ). The composition for each GCM was 20 wt% AgI-MOR and 80 wt% Ferro EG2922 low sintering temperature glass along with enough added silver flake to prevent any I2 loss during the firing process. The silver flake amounts were 1.2 wt% for the GCM with the INL AgI-MOR and 3 wt% for the GCM contained the ORNL AgI-MOR. The GCMs, nominally 100 g, were first uniaxially pressed to 6.35 cm (2.5 inch) diameter disks then cold isostatically pressed, before firing in air to 550°C for 1hr. They were cooled slowly (1°C/min) from the firing temperature to avoid any cracking due to temperature gradients. The final GCMs were ~5 cm in diameter (~2 inches) and non-porous with densities of ~4.2 g/cm³. X-ray diffraction indicated that they consisted of the amorphous glass phase with small amounts of mordenite and AgI. Furthermore, the presence of the AgI was confirmed by X-ray fluorescence. Methodology for the scaled up production of GCMs to 6 inch diameter or larger is also presented.

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Herein, we study the durability of the Sandia Bi-Si oxide Glass Composite Material (GCM) waste form when formulated with different weight percent levels of AgI-MOR. The post-iodine exposure AgI-MOR material was provided to SNL by ORNL. Durability results for the GCM fabricated with 22 and 25% AgI-MOR indicate releases of Ag and I at the same low rates as 15% AgI-MOR GCM, and by the same mechanism. Iodine and Ag release is controlled by the low solubility of an amorphous, hydrated silver iodide, not by the surface-controlled dissolution of I₂- loaded Ag-Mordenite. Based on this data, we postulate that much higher loading levels of AgIMOR are probable in this GCM waste form, and limits will govern by retention of mechanical integrity of the GCM versus the solubility of silver iodide.

Radionuclide 137Cs is one of the major fission products that dominate heat generation in spent fuels over the first 300 years. A durable waste form for 137Cs that decays to 137Ba is needed to minimize its environmental impact. Aluminosilicate pollucite CsAlSi 2O6 is selected as a model waste form to study the decay-induced structural effects. Whereas Ba-containing precipitates are not present in charge-balanced Cs0.9Ba0.05AlSi 2O6, they are found in Cs0.9Ba 0.1AlSi2O6 and identified as monoclinic Ba 2Si3O8. Pollucite is susceptible to electron-irradiation-induced amorphization. The threshold density of electronic energy deposition for amorphization was determined to be ∼235 keV/nm 3. Pollucite can be readily amorphized under F+ ion irradiation at 673 K. A significant amount of Cs diffusion and release from the amorphized pollucite occurs during the irradiation. However, cesium is immobile in the crystalline structure under He+ ion irradiation at room temperature. The critical temperature for amorphization is not higher than 873 K under F+ ion irradiation. If kept at or above 873 K all the time, the pollucite structure is unlikely to be amorphized; Cs diffusion and release are improbable. A general discussion regarding pollucite as a potential waste form is provided in this report. © 2014 American Chemical Society.

The minimum amount of silver flake required to prevent loss of I{sub 2} during sintering in air for a SNL Glass Composite Material (GCM) Waste Form containing AgI-MOR (ORNL, 8.7 wt%) was determined to be 1.1 wt% Ag. The final GCM composition prior to sintering was 20 wt% AgI-MOR, 1.1 wt% Ag, and 80 wt% Bi-Si oxide glass. The amount of silver flake needed to suppress iodine loss was determined using thermo gravimetric analysis with mass spectroscopic off-gas analysis. These studies found that the ratio of silver to AgI-MOR required is lower in the presence of the glass than without it. Therefore an additional benefit of the GCM is that it serves to inhibit some iodine loss during processing. Alternatively, heating the AgI-MOR in inert atmosphere instead of air allowed for densified GCM formation without I{sub 2} loss, and no necessity for the addition of Ag. The cause of this behavior is found to be related to the oxidation of the metallic Ag to Ag{sup +} when heated to above ~300{degrees}C in air. Heating rate, iodine loading levels and atmosphere are the important variables that determine AgI migration and results suggest that AgI may be completely incorporated into the mordenite structure by the 550{degrees}C sintering temperature.

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