Advances in Science and Technology Vol. 73

Abstract: Ceramics based on SiC are characterized by extreme hardness, high thermal conductivity, relatively low thermal expansion and chemical durability. In principle, SiC ceramics can be considered as a long-term stable matrix for final disposal of radioactive waste, such as coated fuel particles (CP) separated from the graphite matrix from spent HTR (high-temperature gas-cooled reactor) fuel pebbles. In the present work, SiC-based ceramic with the embedded UO2 - TRISO (tristructural-isotropic) coated particles was synthesized by the reaction-bonding process. The synthesis was performed in standard SiC crucible. Several physico-mechanical properties of the synthesized samples were investigated. It was shown that the coated particles in the reaction-bonded silicon carbide (RBSiC) matrix are distributed homogeneously. The amount of pores is insignificant and the crippling of the coated particles is not observed. Besides, the junction between CP and RBSiC matrix and between RBSiC matrix and the SiC crucible is very good. For all ceramic components of the synthesized samples, namely, for the UO2-kernels, SiC-layers, SiC crucible wall and for the synthesized RBSiC ceramics, the values of microhardness and fracture toughness were measured and compared with the reference data. The strength properties, such as tensile strength of the synthesized samples, failure mechanism of the reaction-bonded SiC ceramic with embedded UO2 - TRISO coated particles, microstructure of the fracture surfaces, the peculiarities of the coated particles fracture were investigated in detail. Moreover, the diffusion of radioactive tracers (137Cs, 241Am, 36Cl, 3H) across synthesized ceramic was studied and the high safety characteristics of the synthesized ceramic were demonstrated.

Abstract: Cubic oxides with pyrochlore and garnet structures are promising matrices for long-lived actinides immobilization. Their isomorphic capacity with respect to An and REE was determined. To predict the long-term behavior of these matrices under their underground disposal radiation stability of synthetic pyrochlores and garnets was studied. Most of titanate phases have the critical (amorphization) doses close to 0.2 displacements per atom at 298 K. This value is significantly higher for Sn- and Zr-rich pyrochlores. Corrosion behavior of the pyrochlore- and garnet-composed matrices was investigated. The lowest actinides leach rates were observed in water and alkaline solutions most typical for underground waste repositories. Amorphization of the phases has a low influence on their corrosion behavior in solutions. Possibility for joint incorporation of actinides and Tc into zirconate- and titanate-based matrices with the pyrochlore structure is discussed.

Abstract: The cold crucible technology first developed for the treatment of the high level fission products can also be used for the direct treatment of intermediate level wastes. In this case, the wastes can be under the states of liquids or solids. The first experiments carried out for the direct treatment of ionic exchange resins emphasised the requirement of very high temperature on the surface of the glass. When this surface is to cold, the unperfected oxidation lead to a glass containing inclusion such as metallic compounds coming from the reduction of species contained in the waste. Thus, the quality of the glass could be not enough to meet with some specific requirements for long term storage. For few years, the Laboratory of the Innovative Processes has been studied the capability of a cold crucible to involved plasma torches ensuring the high temperature required for a complete oxidation of a large composition of waste. The developments and the assessment of different technological ways lead to build a cold crucible fitted with a bottom inductor together with twin plasma torches above the glass bath. This is the SHIVA process. The researches carried out on this innovative technology have shown the high efficiency of the combination for the treatment of a large variety of solid wastes. The oxidation is complete and the produced glass can be easily poured in a canister. This innovative process provides new perspective of treatment for a large variety of intermediate level waste stored on the ground of nuclear facilities.

Abstract: I-129 is a very long-lived radionuclide that is released to an off-gas stream when spent fuels are dissolved at a reprocessing plant. An iodine filter can capture I-129 in the form of AgI. However, because AgI is unstable under the reducing conditions of a geological repository and I-129 has a very long half-life, I-129 can migrate to the biosphere. These characteristics make I-129 a key radionuclide for the safety assessment of a geological disposal of radioactive wastes generated from a reprocessing plant (TRU wastes). To improve disposal safety, several new waste forms have been developed to confine I-129 for a very long period in order to reduce the leaching of I-129 from radioactive wastes. These new waste forms have technical objectives of solidifying more than 95% of I-129 into the waste form and achieving a leaching rate of less than 10-5/y. Several iodine immobilization techniques have been examined. This paper presents experimental results concerning the treatment process, leaching behavior, modeling, and related elements of these immobilization techniques.

Abstract: Major characteristics of two alumosilicate filters based on fly ash microspheres and porous chamotte were studied for high-temperature trapping of Cs-137 vapours. To determine aerodynamic resistance of the filters, a test facility was designed and built to determine the aerodynamic resistance of the filters. It was shown that porous chamotte had significantly lower resistance than that of the filter based on fly ash microspheres at linear airflows up to 15 cm/sec. Thermal treatment at 1400-15000С made it possible to increase mechanical strength of the filter based on porous chamotte factors of 1,5- 3,4, without any significant changes of other characteristics that exceeded similar parameters for the filter based on fly ash microspheres.

Abstract: The Vitrification Test Rig (VTR) is a full scale non-active waste vitrification plant (WVP), that replicates the lines used for immobilising highly active reprocessing waste at Sellafield in the UK. In the high level waste (HLW) vitrification process, liquid HLW is dried in a rotating tube furnace then mixed with an alkali borosilicate glass frit. This mixture is heated to form a homogeneous product glass that is poured, cooled and stored in steel canisters. The primary function of the VTR is to trial and develop methods to increase the efficiency of high level waste processing at the active WVP. Efficiency gains are mainly achieved by increasing the rate at which the immobilised product is created and by increasing the ratio of HLW to glass frit in the product. The VTR has also been used to investigate the chemistry of various process additions and conditions, the effects of potential fault scenarios, and the processing of dilute waste streams that will be received by WVP in the future. All of these areas have the potential to improve processing efficiency through the optimisation of process conditions and the minimisation of unplanned plant outages. This paper discusses several VTR campaigns that have led to overall improvements of WVP operation.

Abstract: Cold crucible inductive melting is a promising method for production of high-temperature materials. The method is based on direct heating of conductive materials by high-frequency (105-107 Hz) electromagnetic field from an external source. Application of the CCIM to production of vitreous borosilicate and alumino/iron phosphate and ceramic waste forms such as Synroc and its varieties and pyrochlore, murataite and garnet-based ceramics has been successfully demonstrated. Currently a full-scale low level waste vitrification plant based on a 418 mm inner diameter cold crucibles energized from a 1.76MHz/160 kW generators is under operation at SIA Radon. This plant was used for demonstration of feasibility of cold crucible vitrification of Savannah River Site high-iron and high iron/aluminum high level wastes. Numerous ceramic and glass ceramic materials containing high level and actinide waste surrogates such as actinide and actinide/rare earth fractions of high level waste have been successfully produced in the Radon lab- and bench-scale cold crucible based units operated at 5.28 and 1.76 MHz. Large-scale cold crucibles may be applied for vitrification of liquid and solid low and high level wastes whereas small-scale cold crucible may be efficiently used for immobilization of actinide-bearing waste generated from advanced nuclear fuel cycle reprocessing.

Abstract: For more than 30 years Sandia National Laboratories (SNL) has played a key role in the development and implementation of total system analyses of waste management systems in the United States. Two very important applications have been the total system analysis of long term performance that supported (1) the Compliance Certification Application (CCA) for the Waste Isolation Pilot Plant (WIPP) in 1996 and (2) the License Application (LA) for the Yucca Mountain (YM) Repository in 2008.