High-level Radioactive Waste Package Research Articles

Effects of inherent spatial variability of rock properties on the thermo-hydro-mechanical responses of a high-level radioactive waste repository

In France, a deep geological disposal for high level radioactive waste (HLW) and intermediate-level long-lived radioactive waste (ILW-LL) called Cigéo is planned to be constructed in a deep Callovo-Oxfordian claystone (COx) formation. The heat released from the HLW packages leads to a temperature increase in the host rock, which causes a pore pressure increase essentially due to the difference between the thermal expansion of the pore water and the solid skeleton. The low permeability of the COx and its relative rigidity inhibit the discharge of the induced pressure build-up. Moreover, thermal loading also induces thermo-mechanical stresses within the formation. The French National Radioactive Waste Management Agency (Andra) participated in the DECOVALEX-2019 project (Task E) to model the Thermo-Hydro-Mechanical (THM) responses of a benchmark exercise inspired of a HLW repository based on the French concept. This paper aims at studying the THM behavior of the HLW repository by taking into account the inherent spatial variability of the properties of the host rock through thermo-poro-elastic numerical simulations. Only the most influential parameters were selected to perform the spatial variability analysis by using Monte Carlo simulations coupled with the Random Finite Element Method. The selection of the most influential parameters was based on two parametric sensitivity analyses consisting in computing the Sobol indices, which determine the contribution of each input parameter to the overall model output variance. The temporal evolution of the Terzaghi effective stress in the far field is defined as indicator of the influence of each parameter on the THM response.

The Role of Crystalline Rock for Disposal of High-Level Radioactive Waste (HLW)

The stress conditions in the host rock of HLW repositories, altered by construction and heat generated by the waste can cause large changes in groundwater flow, and failure of the repository. These shortcomings reduces the barrier role of the rock and the required effective isolation of HLW has to be provided by engineered barriers of which clay-embedded metal canisters are most important. The paper describes the performance of the near field rock of HLW packages according to two types of repositories: very long subhorizontal holes with large diameter (1.9 m) bored or blasted at 400-600 m depth, and bored 4km deep holes with clay/concrete seals in the upper 2km parts and HLW packages separated by clay/concrete seals in the lower 2km parts. The most important difference between the two concepts is the much higher groundwater transmissivity of the rock in the shallow repository. A common feature is the need for rock support by filling the space between waste containers and rock with clay mud after installing the containers in shallow repositories, and by keeping the holes in deep repositories filled with clay mud from the start of boring the holes and throughout the installation of clay/concrete seals. Focus is on the role of rock discontinuities on the hydraulic and mechanical performances of the repository host rock at construction and under seismic and tectonic impact. Comparison of the two concepts shows that the one implying deep-disposal is superior but requires preparative steps in the waste placement phase.

Experimental Design for in situ Characterization of the Callovo-Oxfordian Pore Water Composition at 85 °C

When emplaced into deep geological disposal cells, the high-level radioactive waste packages will induce a transitory temperature increase in the surrounding geological environment. High temperatures will have an influence on the composition of water that will flow into the cells and come in contact with the disposal materials. An in situ experiment which aims at characterizing the Callovo- Oxfordian pore water at 85°C has been running since 2012. This paper presents the design of the experiment and the predictive thermo-hydro-mechanical (THM) modeling results.

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 Japan's 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 Japan's 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.

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 Japan's 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.

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

Overpack, a high-level radioactive waste package for geological disposal, seals vitrified waste and in line with Japan's 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, (I)

Overpack, a high-level radioactive waste package for geological disposal, seals vitrified waste and in line with Japan's 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.

Deliquescence of NaCl-NaNO3, KNO3-NaNO3, and NaCl-KNO3 salt mixtures from 90 to 120°C.

We conducted reversed deliquescence experiments in saturated NaCl–NaNO3–H2O, KNO3–NaNO3–H2O, and NaCl–KNO3–H2O systems from 90 to 120°C as a function of relative humidity and solution composition. NaCl, NaNO3, and KNO3 represent members of dust salt assemblages that are likely to deliquesce and form concentrated brines on high-level radioactive waste package surfaces in a repository environment at Yucca Mountain, NV. Discrepancy between model prediction and experiment can be as high as 8% for relative humidity and 50% for dissolved ion concentration. The discrepancy is attributed primarily to the use of 25°C models for Cl–NO3 and K–NO3 ion interactions in the current Yucca Mountain Project high-temperature Pitzer model to describe the nonideal behavior of these highly concentrated solutions.

The stress-corrosion-cracking model for high-level radioactive-waste packages

A stress-corrosion cracking (SCC) model has been adapted for performance prediction of high-level radioactive-waste packages to be placed in the proposed Yucca Mountain repository. For these waste packages, the outer barrier material is the highly corrosion-resistant alloy UNS-N06022 (alloy 22). The tensile stress is principally from weld-induced residual stress. To account for crack propagation, the slip dissolution/film rupture (SDFR) model is adopted to provide mathematical formulae for prediction of the crack growth rate. Once the crack growth rate at an initiated SCC is determined, it can be used to determine the time to through-wall penetration for the waste package. This paper presents the development of the SDFR crack growth rate model. A seismic-damage-related SCC crack opening area density model is also briefly described.

Deliquescence of NaCl–NaNO[sub 3], KNO[sub 3]–NaNO[sub 3], and NaCl–KNO[sub 3] salt mixtures from 90 to 120 °C

We conducted reversed deliquescence experiments in saturated NaCl–NaNO3–H2O, KNO3–NaNO3–H2O, and NaCl–KNO3–H2O systems from 90 to 120°C as a function of relative humidity and solution composition. NaCl, NaNO3, and KNO3 represent members of dust salt assemblages that are likely to deliquesce and form concentrated brines on high-level radioactive waste package surfaces in a repository environment at Yucca Mountain, NV. Discrepancy between model prediction and experiment can be as high as 8% for relative humidity and 50% for dissolved ion concentration. The discrepancy is attributed primarily to the use of 25°C models for Cl–NO3 and K–NO3 ion interactions in the current Yucca Mountain Project high-temperature Pitzer model to describe the nonideal behavior of these highly concentrated solutions.

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

Overpack, a high-level radioactive waste package for geological disposal, seals vitrified waste and in line with Japan's 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.

Materials performance issues for high-level radioactive waste packages

The disposal of spent nuclear fuel and high-level radioactive waste in a geologic repository requires materials performance over time periods unprecedented in any previous engineering application. Corrosion-resistant barriers of nickel-based and titanium alloys are currently proposed for the engineered barrier system of a repository that may be located at Yucca Mountain, Nevada, where a site 305 m underground (but still well above the water table) is being studied for its potential suitability to host a repository. Prediction of the performance of these materials over the 10,000 year regulatory period in an evolving, high-humidity, moderate-temperature, mildly oxidizing environment poses great challenges.

In-Situ Observation of the Alpha/beta Cristobalite Transition Using High Voltage Electron Microscopy

ABSTRACTA high temperature water vapor phase is expected to persist in the vicinity of high level radioactive waste packages for several hundreds of years. We have begun an investigation of the structural and chemical effects of water on cristobalite because of its abundance in the near field environment. A high voltage transmission electron microscope (HVEM) investigation of bulk synthesized α-cristobalite to be used in single phase dissolution and precipitation kinetics experiments revealed the presence β cristobalite, quartz and amorphous silica, in addition to α-cristobalite. Consequently, this apparent metastable persistence of β-cristobalite and amorphous silica during the synthesis of α-cristobalite was investigated using a heating stage and an environmental cell installed in the HVEM that allowed the introduction of either dry CO2or a CO2+ H2O vapor. Preliminary electron diffraction evidence suggests that the presence of water vapor affected the α-β transition temperature. Water vapor may also be responsible for the development of an amorphous silica phase at the transition that may persist over an interval of several tens of degrees. The amorphous phase was not documented during the dry heating experiments.

Critique of the National Academy of Sciences Study of the Isolation System for Geologic Disposal of Radioactive Waste

The 1983 report of the Waste Isolation Systems Panel of the National Academy of Sciences (referred to as NAS-83) introduces a solubility limited dissolution (SLD) theory to estimate release rates from high-level radioactive waste packages. It is pointed out that this theory, as presented, should apply equally well to each grain of average rock, but that when applied to that problem, it overpredicts the observed dissolution rate of SiO/sub 2/ by seven orders of magnitude. The SLD theory also predicts that cesium and other trace elements are leached out of rock grains orders of magnitude more rapidly than the SiO/sub 2/; it is shown that this is clearly contrary to the experimentally observed situation. Other shortcomings of the NAS-83 treatment are pointed out. Modifications to the theory that avoid these large discrepancies are suggested; when applied to the waste problem, they pose some very important questions that should be answered before proceeding with waste management problems. For example, they suggest that reprocessing may reduce the hazards from waste by a factor of 10 million, and that synroc may be millions of times more secure against leaching than waste glass.