Hidekazu Asano

Long-term integrity of waste package final closure for HLW geological disposal, (I) Points at issue concerning 1,000 years containment capability of overpack

Overpack, a high-level radioactive waste package for geological disposal, seals vitrified waste and in line with Japans waste management program is required to isolate it from contact with groundwater for 1,000 years. In this study, based on the structure of carbon steel overpack presented in JNCs H12 Report, the basic requirements for welding and non-destructive examination methods presumed to be necessary to seal the overpack lid and to examine the weld, were investigated and categorized into two separate groups of technical and theoretical issues. Through discussions about these issues from the viewpoint of the long-term integrity required of the overpack, an incremental approach was identified consisting of (1) experimental study on applicability of welding and NDE methods, (2) introduction of evaluation methods for the obtained data, and (3) Presentation of promising methods for overpack final closure including lid structure, appropriate welding and NDE methods. This led to the provision of a methodology that would be able to ensure the duration of containment for the overpack final closure weld and is discussed in the conclusion of this paper, as the 1st report.

Long-Term Integrity of Waste Package Final Closure for HLW Geological Disposal, (V) Applicability of MAG Welding Method to Overpack Final Closure

Overpack, a high-level radioactive waste package for Japans geological disposal program, is required for preventing the sealed vitrified waste from contact with groundwater for 1,000 years. In this study, metal active gas (MAG) welding, a typical arc welding method, was evaluated for its applicability in sealing a carbon steel overpack lid with a thickness of 190 mm. Welding conditions and parameters were examined with multilayer welding for three different groove depths. Welded joint tests including the observation of macro- and microstructures were conducted, and mechanical properties were within tentatively applied criteria. Measurements and numerical calculations for residual stress were also conducted, and residual stress distribution tendencies were discussed. These test results were compared with those previously reported for tungsten inert gas welding (TIG) and electron beam welding (EBW). MAG welding possesses the potential to complete overpack lid closure with a maximum groove depth of 190 mm, but the applicability of MAG welding to overpack final closure should be discussed from the viewpoint of the presence of slag inclusions possibly induced in the weld metal.

Long-Term Integrity of Waste Package Final Closure for HLW Geological Disposal, (III)

Overpack, a high-level radioactive waste package for geological disposal, seals vitrified waste and in line with Japans waste management program is required to isolate it from contact with groundwater for 1,000 years. In this study, TIG (Tungsten Inert Gas) welding method, a typical arc welding method and widely used in various industries, was examined for its applicability to seal a carbon steel overpack lid with a thickness of 190 mm. Welding conditions and welding parameters were examined for multi-layer welding in a narrow gap for four different groove depths. Weld joint tests were conducted and weld flaws, macro- and microstructure, and mechanical properties were assessed within tentatively applied criteria for weld joints. Measurement and numerical calculation for residual stress were also conducted and the tendency of residual stress distribution was discussed. These test results were compared with the basic requirements of the welding method for overpack which were pointed out in our first report. It is assessed that the TIG welding method has the potential to provide the necessary requirements to complete the final closure of overpack with a maximum thickness of 190 mm.

Long-term integrity of waste package final closure for HLW geological disposal, (III) : Applicability of electron beam welding to overpack final closure

Overpack, a high-level radioactive waste package for Japans geological disposal program, is required to isolate the sealed vitrified waste from contact with groundwater for 1,000 years. In this study, EBW (Electron Beam Welding), a typical application of high energy density beams widely used in various industries, was examined for its applicability to sealing a carbon steel overpack lid with a thickness of 190 mm. Welding conditions and welding parameters were examined for single-pass welding in a narrow area for three different penetration depths. Weld joint tests including macro- and microstructure, and mechanical properties were conducted and assessed within tentatively applied criteria for weld joints. Measurement and numerical calculation for residual stress were also conducted and the tendency of residual stress distribution was discussed. These test results were compared with the basic requirements of the welding method for overpack, which were pointed out in our first report. The induced void and cold shut inside the weld joint and surface roughness were also discussed for their improvement and evaluation, which need to be established to assure the long-term integrity of overpack lid closure.

Aqueous dissolution of silver iodide and associated iodine release under reducing conditions with FeCl2 Solution

An empirical and analytical study was performed on the aqueous dissolution of silver iodide (AgI) to release iodine under reducing conditions with Fe2+ in order to understand the fundamental chemical and/or physical behavior of potential radioactive iodine waste forms under geological disposal conditions. Aqueous dissolution tests of AgI powder in FeCl2 solutions (10−6 M to 10−3 M) were performed in a glove box purged with a gas mixture (Ar + 5% H2). The test results showed that AgI dissolves to release iodine at extremely slow rates, being controlled by a diffusion process in any FeCl2 solution. The comparison with thermodynamic calculations based on redox equilibria suggested that the AgI dissolution proceeds by redox reaction between Ag+ and Fe2+; however, it was far from the thermodynamic equilibrium. These results suggested that the form of AgI itself has a potential to immobilize iodine for a long time even under the disposal conditions. Solid-phase analysis for the reacted AgI by using SEM/EDS showed a certain amount of silver (maybe metallic silver) precipitated at the surface. On the basis of these results and discussion, a potential mechanism for the actual AgI dissolution was proposed as follows. The AgI dissolution proceeds by redox reaction between Ag+ and Fe2+ to release I−, which results in the precipitation of metallic silver as a reduction product of Ag+ at the AgI surface to form a thin layer covering the AgI surface. The silver layer evolves to be protective against the transport of reactant species, by which the further dissolution to reach the equilibrium is suppressed. Consequently, the dissolution proceeds at extremely slow rates, being controlled by a diffusion process.

Long-Term Integrity of Waste Package Final Closure for HLW Geological Disposal, (IV) Influence of Welding and Prediction of Long-Term Integrity of Weld Joint

Overpack, a high-level radioactive waste container for geological disposal in the Japanese waste management program, is required to isolate the sealed vitrified waste from contact with groundwater for 1,000 years. This means that the weld joint between the body and the lid of the overpack is also required to endure for this period of time. The authors have already started studies on a methodology guaranteeing the long-term integrity of the lid closure, the applicability of several welding and non-destructive examination methods for the weld joint and the corrosion properties of the weld joint. In this paper, through review of the obtained test results, the influences of welding were summarized. Considering the presence of weld flaws, crack growth phenomenon was estimated by fracture mechanics assessment, while the critical crack length and the maximum tolerable flaw size were discussed. Moreover, referring to the Fitness for Service for nuclear power plant components, a method guaranteeing the long-term integrity of the lid closure was discussed and proposed with consideration to the physical and chemical environmental conditions peculiar to the overpack.

Extended two-phase flow model with mechanical capability to simulate gas migration in bentonite

Abstract Long-term gas migration through clays cannot be simulated by conventional two-phase flow models alone owing to the presence of material deformation. In this article, an extended two-phase flow model that incorporates mechanical effects is proposed. The model allows the formation of preferential pathway and considers the relation between pore moisture and pore deformation. It was carried out with the intention of avoiding the complexity of a fully coupled thermal, hydraulic and mechanical modelling. In the new model, porosity, permeability, swelling pressure and pathways formation threshold depend on the water saturation. The model is validated on different gas injection experiments with controlled flow rate and controlled pressure. Some experiments are well known in the literature; some are new. In each case, an inverse approach is used to identify the model parameters. The results confirm that, depending on the type of bentonite (MX80, Avonlea, KunigelV1), modelling the gas migration could require the existence of a pressure-induced saturation-depending preferential pathway. In laboratory-scale experiments, the model leads to an accurate evaluation of the long-term gas migration trends, including not just the gas migration stage but also the water re-saturation level. In a field-scale experiment, the behaviour of the model in a realistic context is revealed.

Laboratory gas injection tests of compacted bentonite buffer material for TRU waste disposal

Abstract The Radioactive Waste Management Funding and Research Center (RWMC) is leading a research programme to evaluate the gas transport mechanisms through a TRU (TRans-Uranium) waste disposal facility in Japan and acquire information on gas migration properties. In this paper we describe a series of laboratory gas injection tests using the bentonite adopted for use in Japanese TRU disposal, as well as an attempted visualization of the gas migration path inside the bentonite used as a buffer. By building a conceptual model from the results of these tests, the characteristics of gas migration through to breakthrough for bentonite can now be better understood in the context of Japanese TRU disposal. Thanks to the outcome of this research project, advanced knowledge may be applied to the conceptualization of TRU waste disposal.

Long-Term Integrity of Waste Package Final Closure for HLW Geological Disposal, (VI) Consistency of the Structural Integrity Evaluation Model for the Weld Joint

Overpack, a container of high-level radioactive waste for Japans geological disposal program, is required to prevent the sealed vitrified waste from coming into contact with groundwater for 1,000 years. This means that the same duration and function arerequired for the weld joint between the body and lid of the overpack. The authors have initiated studies on a methodology evaluating the long-term integrity of the lid closure, including welding tests and non-destructive examination tests, and proposed a structural integrity evaluation model for the lid closure as a basis for fracture mechanics assessment. The model makes two main assumptions consistent with six prerequisites. Thisarticle examines the consistency of the mechanical strength of the lid closure (one of the two assumptions) with its prerequisites, through a review of the test results of corrosion behavior and numerical calculations of neutron irradiation embrittlement for weld joints. Furthermore, a comparison of the critical crack lengths at different penetration depths was conducted, and an estimation of the maximum tolerable flaw size, which serves as a form of an acceptance criterion after adding a safety factor for the size of the weld flaw, was made. Based on these results, the consistency of the proposed evaluation model was discussed and identified.

Development of an Iodine Immobilization Technique by Low Temperature Vitrification With BiPbO2I

This paper describes low temperature vitrification process with BiPbO2 I (BPI) as a promising immobilization technique in which Iodine-129 reacts with BiPbO2 NO3 (BPN) to form BPI, which is then solidified into a lead-boron-zinc glass matrix (PbO-B2 O3 -ZnO) using a low temperature vitrification process. Studies with EPMA, STEM-EDS and XRD found that iodine, lead and zinc were homogeneously dispersed in the waste form, and that there were no residual crystalline minerals in the amorphous glass matrix. Leaching tests conducted under typical geological disposal conditions show that iodine dissolves congruently with the BPI glass matrix in simulated seawater, whereas it dissolves incongruently in alkaline Ca(OH)2 solutions. This is due to retention within an altered surface layer.Copyright