[en] Vitrification is one of the recommended immobilization routes for nuclear waste, and is currently implemented on an industrial scale in several countries, notably for high-level waste. To optimize and extend the scope of nuclear waste vitrification, research is being conducted to specify suitable glass formulations and develop more effective processes. Vitrified nuclear waste often contains several multivalent species whose oxidation state can impact the properties of the melt and of the final glass. The redox control of nuclear glass is often advantageous: in many case it could improve the glass melting and increase the amount of waste in the final glass. In the glass making industry, the oxidation state of glass is optimized in order to improve the glass properties, notably refinement and colour. In this case the aimed redox state is obtained by the appropriate choice of raw materials. Redox control of nuclear glass is more complex because several redox species are contained in this glass. Furthermore, raw materials are partly driven by the chemical and physical properties of the waste that must be vitrified and specific constraints due to the active process. Different techniques of glass redox control developed for nuclear waste vitrification are reviewed. (authors)

[en] Glass was selected to immobilize nuclear waste because the capacity of the glass network to incorporate a very wide range of chemical species. Until now, efforts to optimize glass matrices have focused on their chemical composition or melting temperature. Vitrification studies in recent years have shown that the redox state of the glass, i.e. the oxidation state of the multivalent species it contains, can significantly modify the properties of the melt and of the final glass. Examples are discussed to highlight the advantages of optimizing the glass redox state to facilitate industrial fabrication processes and improve the resulting glass quality. (author)

[en] In the context of the management of radioactive wastes, iodine constitutes a special case owing to its volatility that does not enable its vitrification into conventional borosilicate glasses. Silver phosphate glasses are good candidates, since they can be melted at low temperature and they can accommodate large AgI quantity within their glass network. To improve their thermal characteristics, Al₂O₃ is added in the glass formulation in order to increase the glass network reticulation. But only a limited amount can be used, since phase separation is observed through the formation of Al(PO₃)₃ crystals. We show that Bi₂O₃ can be used as an efficient alternative to Al₂O₃. DSC curves indicate that the crystallization peak is almost suppressed, and ³¹P NMR shows the formation of bismuth phosphate groups but no formation of any crystalline phase. These effects are discussed in terms of cationic field strength and polarizability. (authors)

[en] The kinetics of iron oxidation in sodium borosilicate melts have been determined as a function of time from 710 to 1570 K for four compositions of constant SiO_2 content and B_2O_3 contents ranging from 0 to 22 mol%. The iron redox ratios have been measured by iron K-edge XANES spectroscopy as a function of time at constant temperature. The redox kinetics become slower with increasing B_2O_3 content. Near the glass transition range, they are controlled by the diffusion of network modifying cations toward the surface of the melt whereas they are limited at super liquidus temperatures by the diffusion of free Na"+ ions to charge compensate Fe"3"+ in tetrahedral coordination, along with the diffusion of oxygen. (authors)

[en] 250 cubic meters of high-level liquid waste from reprocessed U-Mo-Sn-Al spent fuel, used in Gas Cooled Reactors (GCR), were produced during the mid-1960's at La Hague facility. These 'UMo' solutions are less radioactive than the current fission product concentrates coming from ongoing reprocessing activities, but are very rich in molybdenum and phosphorus whose contents make the molten glass quite corrosive and require a high-temperature glass formulation to obtain sufficiently high waste loading factors (12% in molybdenum oxide). Hence the use of the Cold Crucible Induction Melter (CCIM) technology to process such solutions has been deemed a good opportunity for AREVA to meet its performance expectations. In addition to being very corrosive, the UMo waste is quite challenging to process as the molybdenum has a strong tendency to stick in the calciner and causes clogging issues in off-gas treatment equipment. Therefore, the process and technological qualifications were deployed in order to address these specific issues. UMo solutions processing in the La Hague CCIM has started in January 2013 and is currently ongoing. During this period (from 2013 to 2015), many data have been collected to confirm the process parameters that were defined during the qualification of this innovating process. Even if some difficulties occurred, operations teams experience along with engineering and R and D support allowed managing them. This paper presents the start-up methodology and the feedback from the first years of UMo solutions processing with the CCIM technology at La Hague site. Lessons learned are presented with the difficulties encountered and the solutions implemented, emphasizing the benefits of a close integration between R and D, engineering and operations teams. (authors)

[en] Highlights: • Aluminophosphate glass composition for nuclear iodine immobilization. • Up to 28 mol% AgI could be incorporated in these glasses without volatilization. • AgI is inserted in the aluminophosphate glasses and does not form clusters. • AgI does not induce any modification of the glass polymerization but only an expansion of the network. -- Abstract: Silver aluminophosphate glasses have been investigated as matrices for the immobilization of radioactive iodine. In this study, up to 28 mol% AgI have been incorporated without volatilization thanks to a low temperature synthesis protocol. Alumina was added in the composition in order to increase the glass transition temperature for a better thermal stability in a repository conditions. Two series of glasses have been investigated, based on AgPO₃ and Ag₅P₃O₁₀ compositions, and with 0–5 mol% Al₂O₃. We report ³¹P, ²⁷Al and ¹⁰⁹Ag NMR study of these glasses, including advanced measurement of the connectivities through {²⁷Al}–³¹P cross-polarization and ³¹P–³¹P double-quantum correlation. We confirm that AgI is inserted in the aluminophosphate glasses and does not form clusters. AgI does not induce any modification of the glass polymerization but only an expansion of the network. Despite no evidence for crystallization could be obtained from XRD, NMR revealed that the introduction of AgI induces an exclusion of alumina from the network, leading to the crystallization of aluminophosphate phases such as Al(PO₃)₃ or AlPO₄. As a consequence, despite NMR gives evidence for the presence of aluminophosphate bonds, only a limited effect of alumina addition on thermal properties is observed

[en] Among platinoid fission products, ruthenium is a key element during the vitrification of high-level waste because it is poorly soluble in the glass matrix. It occurs mainly as RuO₂ particles and sometimes as Ru⁰ particles in the glass melt and final cooled glass. Because the physical properties of ruthenium are very different from those of glass, the RuO₂ particles change the glass melt properties such as the rheology, thermal conductivity and electrical conductivity. In addition, because the Ru⁰ properties are even more distinct from the glass properties, glass melts can be more sensitive to the presence of Ru⁰ particles. Thus, ruthenium speciation, specifically the RuO₂ reduction into Ru⁰, was investigated in air and glass environments, and the experimental results were compared to thermodynamic calculations (Calphad method). Under air conditions, the thermodynamic modeling and experimental results are in good agreement. They show a reduction of RuO₂ at 1403 °C and 1405 ± 7 °C, respectively. The experimental results indicate that the Ru⁰ particle formation may result from an autocatalysis mechanism, which involves the gaseous species of ruthenium. In the investigated glass, the first Ru⁰ particles appear at approximately 1280 °C. At 1280 °C < T < 1400 °C, only few particles are observed attributed to minor reduction phenomena: local redox reaction with iron or disproportionation reaction. At higher temperatures (T > 1400 °C), a much more significant reduction takes place. This one is attributed to the decomposition under temperature and oxygen pressure conditions, predicted by thermodynamic calculations. Based on the results obtained under air conditions, it is assumed that the reduction mechanism involves dissolved ruthenium species.

[en] Very early in its history, nuclear industry has taken care of the future of its wastes. Cementation processes for medium-level activity wastes, vitrification processes for minor actinide solutions and fission products are now proven technologies. The conditioning of wastes is just one ink in the full chain of the waste management process. However, this link is of prime importance because the future of the waste depends on the way it is conditioned. Reciprocally, the storage and disposal largely rely on the confidence given to the behaviour of waste packages with time. The leading role of France in the domain of radioactive wastes conditioning is a strong and valorizable asset at the international industrial plan, but also in terms of social acceptance by showing to the public that technical solutions exist. This monograph takes stock of the conditioning of nuclear wastes and describes the researches in progress, the stakes and the recent results obtained by the CEA (French atomic energy commission). Content: 1 - introduction: waste volumes and fluxes, management strategy, conditioning; 2 - Decontamination and treatment processes for effluent and technological waste; 3 - Glass, a waste conditioning matrix for the long term: Glass packages and manufacturing processes; Nuclear glass formulation, structure and properties; Long-term behavior of glasses; Cold crucible vitrification; 4 - Current conditioning systems for low and intermediate level waste: Cements as confining materials; Bitumens; Metallic structure waste conditioning; 5 - Researching alternative matrices and processes for waste treatment and conditioning: Plasma benefits for incineration / vitrification waste treatment; The Shiva process; Alternative confining matrices; Confinement of waste from pyrochemical processes: status of research; 6 - Spent fuel: a possible confining matrix?: Spent fuel initial characteristics; Spent fuel evolution in a dry storage facility; Modelling the long-term spent fuel behavior; Spent fuel containers in long-term storage and direct disposal concepts; Spent fuel storage and direct disposal: results and outlooks; 7 - general conclusion: Conditioning: a major asset for nuclear waste management; Glossary; Index