High-level Waste Vitrification Research Articles

Corrosion testing of prospective chrome refractory materials in borosilicate glass melts

Justification is provided of significant practical relevance of the issue of corrosion and erosion resistance of refractory materials in furnaces with direct electric heating that are used for vitrification of high-level waste. Main promising materials are listed, and an algorithm is given for assessing their resistance to attack of borosilicate glass melts under operating conditions of electric furnaces. Corrosion testing of chrome refractory materials of joint Chinese and Belgian origin has been carried out under static and dynamic conditions in a low-melting borosilicate glass melt and in a borosilicate glass melt containing simulated liquid high-level waste (HLW). A search has been conducted among chrome refractory materials of Chinese and Belgian origin for ones demonstrating the highest glass melt attack resistance with regard to the conditions of HLW solidification. The results of these tests will be considered in the design of removable and small-scale melters when selecting the material for melter lining.

Issues Associated with Yellow Phase Formation and Other Disruptions in Japanese High-Level Waste Vitrification and Their Improvement

The Rokkasho Reprocessing Plant, located in northern Japan, includes facilities for reprocessing spent nuclear fuels and immobilizing high-level waste (HLW) into a stable-glass waste form based on borosilicate. The vitrification process consists of two large liquid-fed, joule-heated, ceramic-lined melters (LFCMs). During the active test campaigns at the Rokkasho Vitrification Facility, unstable melting performance and difficulty in glass operation were observed in the LFCM operation. The operating protocols that had been developed in the earlier full-scale mock-up testing with inactive waste simulants were found to be inadequate to manage a stable LFCM operation. We needed to understand correctly the fundamental causes for the operational difficulties experienced in the active test. First, we studied and summarized the troublesome element behaviors and disturbances in the HLW vitrification process, primarily on the LFCM. Second, we proposed an empirical model, including the transition of the phenomena that appeared in the active LFCM operation. Third, we analyzed the operating data and the trend of operation indexes to find out the fundamental causes for the unstable LFCM operation based on the results obtained in the described model. It was revealed that the presence of a significant amount of molybdenum and sulfur, together with the noble metals (NMs) Ru, Rh, and Pd in the Rokkasho HLW stream could lead to the formation of molybdate and sulfate salt phases [yellow phases (YPs)], and subsequently, the sedimentation of NMs at the bottom of the melter. The YPs may concentrate at the melter bottom and clog the pouring nozzle prior to the bottom accumulation of NM sludge, which can disrupt power deposition and cause difficulties during glass pouring operation. Based on these investigated results, we provide insights into short-term countermeasures, including numerical control of the heat balance in the melter, dilution of HLW feed with inactive waste simulants, and periodic rinsing of the bottom glass. We also provide an evaluation of their validities on the active operation through full-scale mock-up testing. However, the periodic rinsing and dilution operation produce additional glass-filled canisters and increase the total backend cost. Finally, we see a couple of important directions for future research and development. An advanced LFCM system has been developed for the long-term countermeasures to introduce recent advances in the LFCM design with YP and NM compatibilities. Furthermore, the modified or new glass formulations, which can incorporate more waste oxides into the glass matrix, known as high-waste loading glasses, also have been developed in parallel.

Reflective reviews on Japanese high-level waste (HLW) vitrification – Exploring the obstacles encountered in active tests at Rokkasho

Vitrification is currently the most widely used technology for the treatment of high-level waste (HLW) generated from spent nuclear fuel (SNF) reprocessing. Glasses for the solidification of HLW have been developed over half a century at different times in each country. Likewise in Japan, repeated system evaluations of borosilicate waste glass matrices and the liquid-fed joule-heated ceramic-lined melter (LFCM) process have been found to be applicable to a wide range of HLW compositions. The Rokkasho vitrification facility located in northern Japan encounters significant operational issues owing to the presence of sparsely soluble elements including noble metals (NMs) and separated phases, such as the yellow phases (YPs) constituents, in glass melts. These elements cause serious disruptions, jeopardizing the stable functioning of the LFCM. The active vitrification test was suspended because of the formation of YPs in the glass melts and subsequent sedimentation of NMs at the bottom of the melter. First, we analyzed the testing data to identify the fundamental causes behind the unstable LFCM operation. Our investigation revealed that the presence of significant amounts of Mo and S, along with NMs, specifically Ru, Rh, and Pd, in the Rokkasho HLW stream triggers the formation of molybdate and sulfate salt phases (YPs), which subsequently enabled the sedimentation of NMs at the bottom of the melter. Second, we selected eight major tasks related to difficulties of LFCM operation in the active tests and studied the findings in each task considering the preconditions of task success. Moreover, we strengthened our analysis by subjecting each task to 22 hypotheses tests in a systematic manner to minimize the risk of issues associated with generalizability. Third, we combined these findings and classified them into two categories based on the action system theory on management: variable system (Y) and actor’s system (X), assuming that the indirect or mechanism/environment factors of X should influence on the directly observed factors of Y. Our approach involved implementing a reflective actor’s system (X’) to improve the variable system for the controllable system (Y’), thereby mitigating unexpected events. We then summarized the main implications of this method to ensure predictable and reliable outcomes for the associated tasks. Finally, we concluded the potential effectiveness of strategic actions that could leverage organizational capabilities and processes to improve task performances in the context of a controllable system given the implications we identified.

Evaluation of the Performance of Advanced Refractory Materials Intended for the Roof of an Electric Furnace for High-Level Waste Vitrification

Evaluation of the Performance of Advanced Refractory Materials Intended for the Roof of an Electric Furnace for High-Level Waste Vitrification

Vitrification of simulated high-level liquid waste by laser

Vitrification of simulated high-level liquid waste by laser

MoO3 Solubility and Chemical Durability of V2O5-Bearing Borosilicate Glass

In the vitrification of high-level radioactive liquid waste (HLW), the separation of sodium-molybdate melts is a problem because it reduces the chemical durability of the vitrified waste. A glass with both high MoO3 solubility and chemical durability is required for the safe disposal of radioactive waste. In this study, we investigate the effects of vanadium oxide on the phase separation of the molybdenum-rich phase and the water resistance of the resulting glass by phase equilibrium experiments and chemical durability test. Phase equilibrium experiments were performed on SiO2-B2O3-Al2O3-ZnO-CaO-Na2O-LiO2-MoO3 system glasses and on glasses with V2O5 added. The results showed that MoO3 solubility increased when V2O5 was added. The increase in MoO3 solubility in borosilicate melts may be associated with the viscosity-lowering effect of V2O5. Chemical durability tests were performed on borosilicate glass compositions obtained from phase equilibrium experiments. The normalized leaching rates of V2O5-bearing glasses were higher than those of other glasses. This is due to the higher network modifier/network former ratio of the glass tested. The normalized elemental mass loss of glass containing waste components increases with increasing leaching duration. This suggests that the waste component prevents the formation of a gel layer at the reaction front.

Corrosion testing of domestically produced baddeleyitecorundum and chromium-containing materials in borosilicate glass melts

Practical importance of corrosion and erosion resistance of refractories was justified for the design of furnaces with direct electric heating that are used for vitrification of high-level waste (HLW). The main promising materials are listed, and the algorithm of assessment of their resistance to the effects of borosilicate melts is given for the operating conditions of electric furnaces. Corrosion testing of refractories was carried out under static and dynamic conditions in a low-melting borosilicate glass melt and in a borosilicate glass melt containing simulated liquid HLW. Refractory materials were selected that have the highest resistance in a glass melt under conditions of HLW immobilization. The testing results will be used for choosing lining material in the design of removable and small-scale melters. Ill. 5. Ref. 11. Tab. 3.

Influence of Radioactive Sludge Content on Vitrification of High-Level Liquid Waste

The radioactive sludges formed at the bottom of high-level liquid waste (HLW) storage tanks pose challenges when the HLWs are vitrified. This study aims to determine the influence of the sludge content (enriched in Na2O, Al2O3, NiO, Fe2O3, and BaSO4) on the structure and properties of waste glasses in order to find the optimal ratio of sludges to HLW during vitrification. In the experiments, the simulated sludge and simulated HLW were mixed at different ratios from 0:8 to 4:4, with an overall waste content of 16 wt %, in a borosilicate glass wasteform. It is found that the glass density, molar volume, sulfur retention, and glass transition temperature changed little when increasing the sludge content of the glasses, while the viscosity, chemical durability, and crystallization features of the glasses varied notably. The crystals formed in the glasses during the thermal treatment were exclusively Fe-substituted diopside (Ca, Mg, Fe)2Si2O6. An increase in the Al2O3 and NiO content of the glasses may have been responsible for the increased crystallinity at high temperatures. The leaching rate of Si, B, Na, and Cs from the glasses declined with the increasing addition of sludge to the glasses. Although all the glasses fulfilled the requirements for vitrification processing and glass-product performance, it is recommended that the sludge content of the whole waste should not exceed 25 wt %. This study guides further research on the immobilization of high-level sludges.

Alternative reductants for foam control during vitrification of high-iron High Level Waste (HLW) feeds

Foaming during vitrification of radioactive waste in Joule-Heated Ceramic Melters (JHCM) is exacerbated by trapping of evolving gases, such as CO2, NOx and O2, beneath a viscous reaction layer. Foaming restricts heat transfer during melting. Sucrose is employed as the baseline additive at the Hanford site in Washington State, USA to reduce foaming. Alternative carbon-based reductant additives were explored in simulated, inactive Hanford high-iron HLW-NG-Fe2 feeds, for both their effect on foaming and to give insight to the behaviour of multivalent species in glass melts under different redox conditions. Graphite, coke (93% C), formic acid and HEDTA additives were compared with sucrose, and a feed with no additive. Graphite and coke additions proved most effective in reducing the maximum foam volume by 51 ± 3% and 54 ± 2%, respectively, compared with 24 ± 5% for sucrose. Lower foaming could result in more efficient vitrification in JHCMs. Reductants also affected redox ratios in the multivalent species present in the feed. The order of reduction, Mn3+/Mn2+ > Cr6+/Cr3+ > Ce3+/Ce4+ > Fe3+/Fe2+ was as predicted on the basis of their redox potentials. There is less reduction overall, particularly in the Fe3+ → Fe2+, than predicted by the calculations, attributed to the oxygenated atmosphere of the experiments.

Numerical Studies of Processes in an Induction Furnace with a Cold Crucible for Vitrification of High-Level Waste That Contains Noble Metals

Numerical Studies of Processes in an Induction Furnace with a Cold Crucible for Vitrification of High-Level Waste That Contains Noble Metals

Impacts of substitution of Fe2O3 for SiO2 on structure and properties of borosilicate glasses containing MoO3

Borosilicate glass is an effective matrix for vitrification of high-level nuclear waste. However, the solubility of molybdenum is lower. In this paper, the impacts of Fe2O3 replacing SiO2 on the phase, glass structure, and chemical durability of a simplified calcium borosilicate glass containing molybdenum were investigated. According to XRD, Raman, and FTIR spectra, replacing SiO2 with Fe2O3 makes the glass structure polymerize first and then depolymerize. Furthermore, when Fe2O3 replaces SiO2 up to 5 mol%, the samples show a fully amorphous character, and the glasses show a more disordered structure. The solubility of Mo in the simplified calcium borosilicate glass can be enhanced by extending the depolymerization regions and introducing Fe3+ to compensate for the charge of [MoO4]2−. Moreover, the substitution of Fe2O3 can improve the chemical durability of glass waste form. As a result, the borosilicate glass containing 6 mol% Fe2O3 is a potential matrix to immobilize Mo-containing HLW.

Automatic maintenance with robotics of the induction melting plant in a cold crucible

The performance of works within the framework of technological operations of the closed nuclear fuel cycle of an industrial power complex is inevitably associated with the effects of an external factors that have a significant impact on the technological process. Thus, human participation should be minimized, mechanical and electromechanical systems should be included in the process, in particular, means of robotics. When creating a deserted technology, the issue of automation of the process comes to the fore, including with the use of robotic systems that solve transport, service, inspection tasks, including tasks directly related to the technological process. Special requirements are also imposed on the materials used. The processing of the vitrification of high-level waste is being considered in this work. Such a glazing site by the IMCC method was created by specialists of JSC «VNIINM». In this work, the analysis of the current technical process of the work of this site is carried out, proposals for optimizing the performance of individual operations are issued, recommendations for retrofitting the site are given. Based on the test results of the demonstration stand, proposals were formulated for the automation of the glazing section, taking into account the environmental factors that take place on a real installation, and taking into account the results obtained as a result of the mock-up tests.

Melting behaviour of simulated radioactive waste as functions of different redox iron-bearing raw materials

Improved understanding of the mechanisms by which foaming occurs during vitrification of high-level radioactive waste feeds prior to operation of the Waste Treatment and Immobilization Plant at the Hanford Site, USA, will help to obviate operational issues and reduce the duration of the clean-up project by enhancing the feed-to-glass conversion. The HLW-NG-Fe2 high-iron simulated waste feed has been shown to exhibit excessive foaming, and the most recent predictive models overestimate the feed melting rate. The influence of delivering iron as a Fe2+-bearing raw material (FeC2O4·2H2O), rather than a Fe3+ (Fe(OH)3) material, was evaluated in terms of the effects on foaming during melting, to improve understanding of the mechanisms of foam production. A decrease of 50.0 ± 10.8% maximum generated foam volume is observed using FeC2O4·2H2O as the iron source, compared with Fe(OH)3. This is determined to be due to a large release of CO2 before the foam onset temperature (the temperature above which the liquid phases forming are sufficiently viscous to trap the gases) and suppression of O2 evolution during foam collapse. Structural analyses of simulated waste feeds after different stages of melting show that the remaining Fe2+ in the modified feed is oxidised to Fe3+ at temperatures between 600 and 800 °C. This feed was tested in a Laboratory Scale Melter with no excessive foaming or feeding issues. Analysis of the final glass products indicates that the glasses produced using the original HLW-NG-Fe2 feed using Fe(OH)3 and the feed made with FeC2O4·2H2O are structurally similar but not identical: the difference in the structure converges when the glass is melted for 24 h, suggesting a transient structure slightly different to that of the baseline in the glass produced using the reduced raw material.

Characterization and modeling of chemical reactions taking place during the vitrification of high level nuclear waste

Several chemical reactions take place during the vitrification of high-level nuclear waste. The integration of these chemical aspects in process numerical simulation tools is one of the key points in the approach of improving the prediction capabilities of numerical tools today. In this study, based on simultaneous differential scanning calorimetry-thermogravimetry (DSC-TGA) and the run/rerun method, we propose a modeling of the kinetics of chemical reactions taking place when nuclear glass precursors are subjected to constant heating. For the mathematical modeling, a weighted sum of nth order reactions was used to describe the overall mechanism. A hybrid approach coupling the Kissinger and least squares methods was performed to identify the apparent kinetic parameters. Along with the thermal characterization, we also provide some information about the nature of the main reactions through microstructural evolution and evolved gas analyses.

Dissolution behavior of MgO-P2O5 glass system for vitrification of high-level radioactive waste

Dissolution behavior of MgO-P2O5 glass system for vitrification of high-level radioactive waste

A Numerical Study of Sedimentation of Noble Metals during Vitrification of High-Level Waste Using Cold Hearth Melting

A Numerical Study of Sedimentation of Noble Metals during Vitrification of High-Level Waste Using Cold Hearth Melting

Numerical study of noble metals sedimentation during vitrification of high-level radioactive waste in a cold crucible induction melter

Numerical study of noble metals sedimentation during vitrification of high-level radioactive waste in a cold crucible induction melter

Research Progress of Noble Metal Deposition in Vitrification of High-Level Liquid Wastes

Research Progress of Noble Metal Deposition in Vitrification of High-Level Liquid Wastes

Effects of Pretreatment of Ba(NO3)2 on Phase Segregation of Sulfate in High-Level Liquid Waste Vitrification Process

Effects of Pretreatment of Ba(NO3)2 on Phase Segregation of Sulfate in High-Level Liquid Waste Vitrification Process

Study of the volatilization of cesium and rhenium in the waste vitrification process

A side effect of high level waste vitrification at 1100 °C is the volatilization of radioactive cesium (Cs), recaptured further on in the process as a solid aerosol by a dust scrubber. Feedback from industry suggests that Cs combines with technetium (Tc) – simulated here by rhenium (Re), chosen for its similar behavior – to form volatile species. This work characterized Cs volatilization and aerosol formation. The first experiment involved elaborating and analyzing a simplified glass enriched with Cs and Re, which reinforce the volatilization phenomena. The results showed that a demixed liquid phase rich in Cs, Re, Mo, and Na formed in the liquid melt, which is reminiscent of a molybdic phase evoked in the literature. Moreover, analyzing the condensed gasses led to the conclusion that the alkalis and Re volatilize congruently at around 700 °C. In the second experiment, a series of measurements of the gas phase characterized aerosols throughout the process. The size distribution results showed that the particles formed in the furnace were mostly submicronic, and that the aerosols in the calciner and the dust scrubber included nanoparticles and micronic particles. SEM-EDS characterization revealed the composition of a spherical particle from the furnace made of two nested phases: one rich in Cs-Re-O, and the other rich in Na-Mo-O. The analyses performed showed that Cs and Re could be found in every particle sampled throughout the process, and that the micronic particles might originate from the aggregation of submicronic particles because of composition similarities. Thus, the observations led to a better understanding of the behavior of the elements of interest in both the melt and the gas phase.