Development of Iron Phosphate Waste Forms for Fission Products Waste Streams
- MO-SCI Corporation, Rolla, MO (United States)
- Missouri Univ. of Science and Technology, Rolla, MO (United States)
The current baseline spent nuclear fuel reprocessing procedures generate secondary waste streams that generally include large amounts of MoO3, ZrO2, rare earth and noble metals that are poorly soluble in borosilicate glasses and thereby limit the waste loading. The main goal of this SBIR project was to develop suitable iron phosphate-based compositions for vitrifying the MoO3-rich waste (Collins-CLT: the most likely reprocessed waste composition) with greater waste loadings than can presently be achieved with borosilicate glass, while retaining acceptable chemical durability. An iron phosphate waste form containing 40 wt% of the high MoO3 Collins-CLT waste simulant, designated 40wt%-5, was developed and tested. Monazite-type (Rare Earth)PO4 and ZrP2O7 crystals are present in the as-cast 40wt%-5 sample, and are present in greater relative sizes and concentrations in the CCC (canister centerline cooling)-treated sample, along with crystalline Fe2P2O7. The precipitation of these phases affects the composition of the residual glass matrix. The chemical durability of the as-cast and CCC-treated 40wt%-5 waste forms were evaluated using the product consistency test (PCT) and the vapor hydration test (VHT). From the PCT, the normalized elemental release rates from both the as-cast and CCC-treated 40wt%-5 samples were lower than the DOE high level waste requirement, even though elemental releases from the CCC-treated sample is three times greater than from the as-cast sample. The increased dissolution rates of the CCC-treated 40wt%-5 sample is attributed to a more reactive residual glassy phase, one that recedes faster in water than does the residual glass in the as-cast sample. The Raman and high-pressure liquid chromatography (HPLC) analyses show that the residual glass in the CCC-treated sample consists of longer P-anions, associated with a lower O/P ratio, and such glasses are generally more reactive. The residual glass in the CCC-treated sample also has a lower Fe/P ratio, due to the precipitation of Fe(II)2P2O7 crystals, and greater Cs- and Mo-contents, and these compositional factors also contribute to the faster dissolution rates of the CCC-treated waste form. To avoid the precipitation of Fe(II)2P2O7, which strongly relates to the increased dissolution rates of the CCC-treated 40wt%-5, H3PO4 was used for the phosphorus source instead of NH4H2PO4, for producing 40wt%-5 waste form. Fe(III) ions were dominant in 40wt%-5 samples prepared with H3PO4 and Fe(II)2P2O7 was not observed in either the as-cast or the CCC-treated 40wt%-5 samples. As a result, the PCT chemical durability of the 40wt%-5(H3PO4) samples is excellent and is not affected by cooling rates of the waste form. The 40wt%-5 waste form was successfully melted in a commercial-scale cold crucible induction melter (CCIM) and the chemical and structural properties of these large-scale melted samples were similar to those of the smaller-scale waste forms melted by conventional means. The properties and structures of glasses from both the Na2O-MoO3- Fe2O3- P2O5 system and Cs2O-MoO3- Fe2O3- P2O5 system were studied to understand how Mo is incorporated into the iron phosphate base glass and how that incorporation affects glass properties and characteristics. Static dissolution test results for these glasses were fit to a two-stage model, based on an initial parabolic behavior followed by a linear behavior. In general, the respective dissolution rates decrease with increasing O/P ratio, decreasing Mo/(Mo+Fe) ratio, and decreasing alkali content. The distributions of various Mo coordination sites and phosphate sites were acquired by Raman band deconvolution and HPLC analysis. Changes in phosphate network connectivity mostly account for the compositional dependence of the properties of these alkali-iron-molybdenum-phosphate glasses.
- Research Organization:
- MO-SCI Corporation, Rolla, MO (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- SC0011906
- OSTI ID:
- 1419255
- Type / Phase:
- SBIR (Phase II)
- Report Number(s):
- DOE-MoSci-0011906
- Country of Publication:
- United States
- Language:
- English
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