The knock-limited performance of several fuels blended with S-2 reference fuel
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Ruthenium Behavior at Phase Separation of Borosilicate Glass-12259
The Rokkasho reprocessing plant (RRP) located in Aomori, Japan, vitrifies high level waste (HLW) into a borosilicate glass. The HLW is generated from the reprocessing of spent fuel and contains ruthenium (Ru) and other platinum group metals (PGMs). Based on the recent consequences after a huge earthquake that occurred in Japan, a hypothetical blackout was postulated for the RRP to address additional safety analysis requirements. During a prolonged blackout, the borosilicate glass could phase separate due to cooling of the glass in the melter. The Ru present in the glass matrix could migrate into separate phases and impact the durability of the borosilicate glass. The durability of the glass is important for quality assurance and performance assessment of the vitrified HLW. A fundamental study was performed at an independent university to understand the impact of a prolonged blackout. Simulated HLW glasses were prepared for the RRP, and the Ru behavior in phase separated glasses was studied. The simulated HLW glasses contained nonradioactive elements and PGMs. The glass compositions were then altered to enhance the formation of the phase-separated glasses when subjected to thermal treatment at 700 deg. C for 24 hours. The synthesized simulated glasses contained 1.1 % Ru by weight as ruthenium dioxide (RuO{sub 2}). A portion of the RuO{sub 2} formed needle-shaped crystals in the glass specimens. After the thermal treatment, the glass specimen had separated into two phases. One of the two phases was a B{sub 2}O{sub 3} rich phase, and the other phase was a SiO{sub 2} rich phase. The majority of the chemical species in the B{sub 2}O{sub 3} rich phase was leached away with the Material Characterization Center-3 (MCC-3) protocol standardized by the Pacific Northwest National Laboratory using an aqueous low-concentrated nitric acid solution, but the leaching of the Ru fraction was very limited; less than 1% of the original Ru content. The Ru leaching was much less than those of the other elements, and the needle-shaped crystals of RuO{sub 2} were observed in the B{sub 2}O{sub 3} rich phase in the specimen after the leaching test. Another experiment was performed using another glass specimen which had been prepared with the same frits, but used reagent RuO{sub 2} of granular shape at lower content (0.0073% by weight as RuO{sub 2}). The leached fractions of elements for the latter specimen increased to almost the same fraction (more than 10% of the original Ru content) as observed for boron and sodium, when the phase separated glass was leached using the MMC-3 protocol with non-acidic de-ionized water. Based on the results of this study, it was concluded that needle-shaped RuO{sub 2} crystals are contained in the B{sub 2}O{sub 3}-rich phase after phase separation of the borosilicate glass after a hypothetical blackout. The leaching fraction for the needle-shaped RuO{sub 2} present in the phase separated glass is much lower than those for boron or sodium. Ruthenium behavior has been studied for a hypothetical loss of cooling in the liquid fed ceramic melter for high level waste by taking into account the phase separation of borosilicate glass. The needle-shaped crystal of ruthenium dioxide after bi-nodal-type phase separation of the borosilicate glass at 700 deg. C migrated into the B{sub 2}O{sub 3} rich phase, but remained without dissolution by an acidic aqueous solution. Additionally, granular ruthenium dioxide can be a morphological form of ruthenium after bimodal-type phase separation of the vitrified high level waste with borosilicate glass media. After the phase separation of the borosilicate glass, the crystal shape of the ruthenium dioxide is either needle-shaped or granular, and the leachable fraction of ruthenium is relatively much lower than those of major components (boron and sodium) in the vitrified borosilicate glass. The fraction of leached ruthenium increased to almost the same fraction as observed for boron and sodium when the phase-separated glass was leached with ultrapure water. (authors)
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Boron effects on fission product behavior under severe accident conditions
This paper addresses basic studies for the evaluation of boron (B) release from a degraded BWR control rod and its effect on cesium (Cs) and iodine (I) chemistry during a severe accident. Possible increase of gaseous iodine release induced by released B was predicted under various atmospheres by chemical equilibrium calculations. Results showed that the change in atmosphere affects B release fraction from the control rod and also the generation of volatile iodine species with the formation of CsBO{sub 2}. Under a steam-starvation atmosphere, B release fraction could be reduced by the formation of stable iron (Fe)-B alloys in the molten control rod materials and, therefore, the generation of gaseous iodine as well as CsBO{sub 2} are considered to be suppressed. The calculations results show that the B release kinetics as well as the atmosphere play an important role for the Cs and I vapor species. For the validation of these results, basic experiments for the evaluation of B release kinetics, thermochemical properties and phase states of Cs-B-O complex compounds, and reproduction tests focused on FP release and transport are underway. (authors)
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The Effect of Glass Chemistry on Caesium Volatility in Iron Phosphate Glass Systems - 19344
The UK currently has a stockpile of 99,000 m{sup 3} of intermediate level waste (ILW), with this figure expected to rise to 191,000 m{sup 3} by 2125. Current immobilisation options include cementation and thermal treatment, with cementation being the preferred candidate due to the simplicity of processing. However, thermal processing such as vitrification could offer significant volume reductions, up to 75 % for some waste-streams. In comparison, cementation is generally associated with a volume increase, with a typical waste package seeing a volume increase of 200 - 400 %. For vitrification to be accepted as the preferred treatment option of ILW in the UK, the issue of volatility during the vitrification process must be addressed. Volatile components of ILW such as Cs-137 and Sr-90 account for almost 50 % of the current total radioactivity produced by the UK's ILW stockpile. Therefore, the efficient retention of these elements into the glass immobilisation medium must be considered. Various approaches can be employed when trying to increase volatile retention, however this project has focussed on the influence of glass chemistry on volatility. A series of iron phosphate glasses containing 5 mol % of B{sub 2}O{sub 3}, MnO and ZnO have been produced, where additives have been substituted in place of Fe{sub 2}O{sub 3} in a standard 40Fe{sub 2}O{sub 3}-60P{sub 2}O{sub 5} system. All glass samples were characterised using Raman spectroscopy, SEM-EDX, thermal analysis, XRD and XRF. The glasses were then doped with caesium carbonate and re-melted in the system described below. A simple off-gas system has been developed and commissioned to capture and characterise volatiles released from the glass during melting. The off-gas system has been refined several times, with the most recent design comprising of a sealed stainless steel system, connected to two gas washing bottles containing 150 ml of 50 % nitric acid. Nitrogen gas is passed through the system for the duration of the experiment at a flow rate of ∼ 0.9 L/min to ensure efficient carryover of the volatiles released from the glass melt into the solutions in the gas washing bottles. The total retention of the volatiles can be measuring by analysing the contents of the gas washing bottles using ICP-MS and XRF of the final glass product. (authors)
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The Effect of Glass Chemistry on Caesium Volatility
The UK currently has 99,000 m3 of intermediate level waste (ILW). The current preferred treatment route is cementation. However, thermal treatment could offer a better long term option due to higher waste loading and lower storage costs. This project will focus on volatile retention in the vitrification process to make thermal treatment a more viable option. A series of iron phosphate base glasses were prepared with the nominal molar composition xM{sub y}O{sub z}(40 - x)Fe{sub 2}O{sub 3}-P{sub 2}O{sub 5} (M{sub y}O{sub z} = B{sub 2}O{sub 3}, MnO, ZnO with x = 5%). Samples are given names of the form IPG-1-X5 where X = B, Mn or Zn. For all batches, reagents were weighed and mechanically mixed to produce 150 g of glass. melted at 1150 deg. C for 3 h, Samples were immediately transferred to an annealing furnace and annealed at 450 deg. C for 1 h before being cooled to room temperature at 1 deg. C min{sup -1}. 75 g batches was produced with a target of 2 wt.% caesium oxide. Each batch was placed into a mullite crucible and placed in the sealed system seen below. Each canister was placed into a top-loading resistance furnace and ramped to 950 deg. C at a rate of 3 deg. C min{sup -1} under a nitrogen atmosphere with a flowrate of ∼0.9 L min{sup -1}. The samples were held at this temperature for 3 h before power to the furnace was removed and the system allowed to cool naturally. Once cool, the steel canister was opened and the entire apparatus (steel canister, mullite crucible and tubing) was washed through with ∼50 ml of 0.5 M HNO{sub 3} to allow for a complete mass balance of the caesium species. A series of Iron Phosphate glasses have been produced and characterised to investigate the influence of glass chemistry on the volatility of caesium. All glasses were doped with ∼2 wt.% Cs{sub 2}O and re-melted at 950 deg. C for 3 hours in a specially designed furnace set-up. Results of these experiments showed that the addition of 5 mol. % ZnO greatly increased the retention of Cs{sub 2}O whilst B{sub 2}O{sub 3} was detrimental to the Cs{sub 2}O retention. A series of iron phosphate based glasses have been produced with 5 mol.% additions of B{sub 2}O{sub 3}, MnO and ZnO, substituted in place of Fe{sub 2}O{sub 3}. All samples were analysed before and after being doped and re-melted with a 2 wt.% addition of Cs{sub 2}O. The effect of each additive on the base glass has been reported showing that both B{sub 2}O{sub 3} and ZnO increase the glass transition temperature, whilst MnO additions decrease the thermal stability of the glass. The Q-speciation of the glasses was investigated, and the results showed that the additives did not significantly influence the Q-speciation in comparison to the base glass. It has been shown that the addition of ZnO dramatically increases the retention of Cs during re-melting by almost a factor of 10 in comparison to the base glass, whilst the addition of MnO addition increased the caesium retention but only by a factor of ∼3. The addition of B{sub 2}O{sub 3} had a detrimental effect on the volatility in comparison to the 40Fe{sub 2}O{sub 3}-60P{sub 2}O{sub 5} base glass.
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