Radioactive Waste Canisters Research Articles

Investigation on Suitability of Argillaceous Rocks of Vindhyan Super Group as Host Rock for Deep Geological Repository using Thermal Analysis

Argillaceous rocks are being considered as a potential host rock for deep geological repository (DGR) for hosting vitrified high level radioactive waste canisters by many countries worldwide. Numerical simulation of thermal evolution in the repository environment is an important study for the long term safety performance assessment of a DGR. In this study, thermal dissipation in the near field area of a conceptual repository in Ganurgarh shales from Bhander Group of Vindhyan Super Group, which is the thickest sedimentary succession of India, has been simulated using commercial software FLAC3D, which solves the governing heat diffusion equation using explicit and implicit finite difference methods. Model parameters like thermal conductivity, specific heat, density of the shales are generated in the laboratory. From the analysis of time dependent temperature profile it is observed that maximum temperature of 70.5oC is attained at canister surface after 22 years of heating for a heat loadings of 500 W/overpack. Since the maximum temperature is well below the permissible limit of temperature (100oC), the heat load of the source is increased to 700 W/overpack and in this case the simulated value of maximum temperature is 93­oC. Maximum temperatures at other locations within the near field region are also within the permissible limit.

Radiation Damage Effects in Chlorite Investigated Using Microfocus Synchrotron Techniques

A detailed understanding of the mechanisms and effects of radiation damage in phyllosilicate minerals is a necessary component of the evaluation of the safety case for a deep geological disposal facility (GDF) for radioactive waste. Structural and chemical changes induced by α-particle damage will affect the performance of these minerals as reactive barrier materials (both in the near and far-field) over time scales relevant to GDF integrity. In this study, two examples of chlorite group minerals have been irradiated at α-particle doses comparable to those predicted to be experienced by the clay buffer material surrounding high-level radioactive waste canisters. Crystallographic aberrations induced by the focused 4He2+ ion beam are revealed via high-resolution, microfocus X-ray diffraction mapping. Interlayer collapse by up to 0.5 A is prevalent across both macrocrystalline and microcrystalline samples, with the macrocrystalline specimen displaying a breakdown of the phyllosilicate structure into loosely ...

Novel method for analyzing transport parameters in through-diffusion tests

Buffer materials such as bentonite are vital for absorbing radionuclide leakage and retarding migration from radioactive waste canisters. The diffusion coefficient and the retardation factor are the predominant properties controlling the diffusion-reaction process in a buffer material. Diffusion experiments combined with Crank's graphical method are a well-established process for determining asymptotic diffusion coefficients. However, the inaccuracy of the diffusion coefficient that results from the subjective judgement of the late-time linear part of the cumulative concentration data in Crank's graphical method will deteriorate the estimate of the retardation factor. A novel parameter identification process based on an iterative and analytical method (PIPIAM) is proposed here to obtain the diffusion coefficients and porosity of bentonite using concentration data. The results of PIPIAM and the graphical method are compared through an error analysis of concentration. The results show that PIPIAM outperforms the graphical method in terms of the error reduction of the concentration and the uncertainty decrease of the estimated parameters. The proposed method is thus a good alternative for acquiring transport parameters for use in safety assessments of nuclear waste repositories.

The Investigation of Twin Tunnel Stability: Effect of Spacing and Diameter

ABSTRACT The underground excavation became the backbone of developing as well as developed country due to unavailability of surface area and increase demand of space for transportation, water channeling, waste disposal and storage. Except for transportation, the crystalline rocks (high strength, low porosity and less permeability) are considered most suitable and safe space for the storage of radioactive waste as well as petroleum. The role of spacing between twin tunnel and tunnel’s diameter in its stability was carried out using finite element method. Not much research has been done on twin tunnel spacing in hard rock. In the present study, a total of ninety simulations were performed to investigate critical spacing of the twin tunnel in granitic rock mass. Twin tunnel diameter is varied from the 2 m to 10 m and their spacing varied from 0.2 to 2 times of diameter. It was concluded that minimum spacing should be 0.8 times of the tunnel diameter. It was also found that small diameter tunnel effects only near field but large diameter tunnel effect near field as well as far field. The vertical hole, approximately 3m diameter, containing radioactive waste canister should be drilled at the interval of more than 2.4m.

Reductive precipitation of neptunium on iron surfaces under anaerobic conditions

Reductive precipitation of the radiotoxic nuclide 237Np from nuclear waste on the surface of iron canister material at simulated deep repository conditions was investigated. Pristine polished as well as pre-corroded iron specimens were interacted in a deoxygenated solution containing 10–100 μM Np(V), with 10 mM NaCl and 2 mM NaHCO3 as background electrolytes. The reactivity of each of the two different systems was investigated by analyzing the temporal evolution of the Np concentration in the reservoir. It was observed that pre-oxidized iron specimen with a 40 μm Fe3O4 corrosion layer are considerably more reactive regarding the reduction and immobilization of aqueous Np(V) as compared to pristine polished Fe(0) surfaces. 237Np immobilized by the reactive iron surfaces was characterized by scanning electron microscopy as well as synchrotron-based micro-X-ray fluorescence and X-ray absorption spectroscopy. At the end of experiments, a 5–8 μm thick Np-rich layer was observed to be formed ontop of the Fe3O4 corrosion layer on the iron specimen. The findings from this work are significant in the context of performance assessments of deep geologic repositories using iron as high level radioactive waste (HLW) canister material and are of relevance regarding removing pollutants from contaminated soil or groundwater aquifer systems.

Influence of heat transfer on corrosion behaviour of materials for radioactive waste canisters in synthetic bentonite pore water and bentonite suspension

Abstract This study is focused on an influence of heat transfer on corrosion behaviour of structural materials for radioactive waste canister in synthetic bentonite pore water and bentonite suspension. Experiments were carried out in special cell on cylindrical sample of carbon steel, copper, titanium-palladium alloy and two types of stainless steel. Samples were subjected in synthetic bentonite pore water to sixty-fold higher heat transfer compared to real repository, and approximately thirty-fold higher in bentonite suspension. Corrosion behaviour was observed in two-electrode setup with platinum wire as counter electrode by means of electrochemical impedance spectroscopy. Results showed negligible effect of heat transfer compared to surface temperature.

A Model to Predict Stress Corrosion Cracking of Welded Stainless Steel Canister

ABSTRACTFabricated stainless steel structures are susceptible to stress corrosion cracking (SCC), despite being placed in chloride-containing natural water or humid atmospheres. The present paper describes a model that can define the conditions under which SCC is initiated and propagated, based on analyses of actual SCC incidents induced at welded flanges of cylindrical stainless steel structures.Whenever the vitrified radioactive waste canister storage conditions deviate from normal and appropriate conditions due to earthquakes or tsunamis, the exposed canisters are expected to suffer SCC within 400 hours to 7 years, according to the analytical results obtained such as degree of sensitization, residual stress distribution, chloride ion concentration, and temperature.

Demonstration of a transmission nuclear resonance fluorescence measurement for a realistic radioactive waste canister scenario

Transmission nuclear resonance fluorescence (NRF) is a promising method for precision non-destructive assay (NDA) of fissile isotopes—including 239Pu—in spent fuel while inside a storage canister. The assay, however, could be confounded by the presence of overlapping resonances from competing isotopes in the canister. A measurement is needed to demonstrate that transmission NRF is unaffected by the shielding material. To this end, we carried out a transmission NRF measurement using a mono-energetic γ-ray beam on a proxy target (Al) and absorbing material simulating a realistic spent fuel storage canister. Similar amounts of material as would be found in a possible spent fuel storage canister were placed upstream: concrete, stainless steel (SS 304), lead (as a proxy for U), and water. An Al absorption target was also used as a reference. These measurements demonstrated that the canister material should not significantly influence the non-destructive assay.

Microbial processes relevant for the long-term performance of high-level radioactive waste repositories in clays

Abstract The primary purpose of the present work was to qualitatively evaluate the relevance of microbial activity for the long-term performance of a deep geological repository for high-level radioactive waste and spent nuclear fuel utilizing clay and to identify which safety-relevant processes and properties can be potentially influenced by this activity. Deterioration of clay properties accompanying destabilization and destruction of clay mineral structure as a result of microbial actions can be considered as the primary microbial impact on clay. The present analysis identified eight clay properties essential for maintaining safety functions of containment and retardation of the disposal system – swelling pressure, specific surface area, cation exchange capacity, anion sorption capacity, porosity, permeability, fluid pressure and plasticity – which can potentially be influenced by microbial processes in clay-based materials and claystone within a repository. Radioactive waste canisters and over-packs made from cast metal or steel represent a further component of the engineered barrier system which can be strongly affected by microbial activity in the clay buffer or in the adjacent host rock. This work should provide a basis for a follow-up quantitative estimation of the maximum possible effects of microbial processes on the barrier system of a deep geological repository in clay.

Modelling the thermomechanical volume change behaviour of compacted expansive clays

Compacted expansive clays are often considered as a possible buffer material in high-level deep radioactive waste disposals. After the installation of waste canisters, the engineered clay barriers are subjected to thermohydromechanical action in the form of water infiltration from the geological barrier, heat dissipation from the radioactive waste canisters, and stresses generated by clay swelling under almost confined conditions. The aim of the present work is to develop a constitutive model that is able to describe the behaviour of compacted expansive clays under these coupled thermo-hydromechanical actions. The proposed model is based on two existing models: one for the hydromechanical behaviour of compacted expansive clays and another for the thermomechanical behaviour of saturated clays. The elaborated model has been validated using thermo-hydromechanical test results on compacted MX80 bentonite. Comparison between the model prediction and the experimental data shows that this model is able to reproduce the main features of volume changes: heating at constant suction and pressure induces either expansion or contraction; the mean yield stress changes with variations of suction or temperature.

Corrosion of iron: A study for radioactive waste canisters

The purpose of this study is to examine the risks of atmospheric corrosion of steel waste canisters following their deep geological disposal in the temperature range from 303 to 363K. The work was performed using iron samples deposited as thin films on a quartz crystal microbalance (QCM) and disposed in a climatic chamber. The experiments showed that, in the temperature under study (298–363K), the mass increase due to the formation of oxide/hydroxide rose sharply above 70% RH, as is commonly observed at room temperatures, indicating that the phenomenon remains electrochemical in nature. Ex situ Raman spectrometric analyses indicate the formation of magnetite, maghemite and oxyhydroxides species in the 298–363K temperature range, and for oxygen contents above 1vol.%, whereas only Fe3O4 and γ-Fe2O3 are detected at 363K. In this work, the kinetics of the rust growth is discussed, on the bases of the rate of mass increase and of the composition of the rust, as a function of the climatic parameters and the oxygen content of the atmosphere.

Lead/carbon steel galvanic corrosion evaluation

In a study aimed to select a carbon steel for its possible use as a liner for a high level radioactive waste canister, galvanic corrosion tests of lead/carbon steel galvanic couples in groundwater and in seawater were performed at 75°C. Lead behaved anodically due to a current polarity inversion phenomenon, and therefore corroded preferentially. This polarity inversion is temperature dependent. Under the same conditions, commercial lead showed higher corrosion rates in groundwater than high purity lead. No current polarity inversion was detected in galvanic tests performed either in bentonite suspensions in groundwater or in seawater.

Nondestructive evaluation of low-level radioactive waste canisters for free-water content : Hornibrook, C. Electric Power Research Institute, Palo Alto, California (United States), NP-6502, 96 pp. (Jun. 1991)

Nondestructive evaluation of low-level radioactive waste canisters for free-water content : Hornibrook, C. Electric Power Research Institute, Palo Alto, California (United States), NP-6502, 96 pp. (Jun. 1991)

Analysis of the Radiative and Conductive Heat Transfer Characteristics of a Waste Package Canister

The effects of stabilizers on the heat transfer properties and temperature distribution inside a radioactive waste canister are studied. The two-dimensional energy equations are solved numerically. (AIP)

Natural analogues for radwaste disposal: Elemental migration in igneous contact zones

Chemical studies of various igneous rocks intrusive into many different rocks have been examined to see if elemental transfer has taken place. Systems examined include: granite intrusive into shale and siltstone; basalt intrusive into granodiorite, diorite, rhyolite and dolomite; diabase intrusive into rhyolite and extrusive over arkose; rhyolite extrusive over basalt; lamprophyre intrusive into evaporites; monzonite intrusive into tuff and andesite; and quartz monzonite intrusive into metasedimentary rocks. The mechanism for elemental transport in these systems is due in part to diffusion and in part to chemical reaction between intrusive and host rocks. Evidence for transport of elements from intrusive into intruded rocks is noted for several elements. In some cases, the distribution of some elements is mineralogically controlled, and variations in Co, Cr, REE's, Ni, Cu, Zr, Mn, Ti, Rb, Sr, U and Th can be attributed to more-or-less specific reactions. In a few contact zones, there has been limited migration of Cs, but usually only within 0.5-1.0 m of the igneous rock contact. Thermal effects of the igneous intrusions are to reset some mineral isotopic systematics (loss of 40Ar, redistribution of ★87Sr, 18O), but otherwise the whole-rock chemistry usually remains unchanged. Collectively, the data for elemental behavior in the vicinity of the igneous contacts studied show that elemental migration of redistribution is often restricted to within 0.5-1.0 m of the contact, thus suggesting that for hypothetically stored radioactive waste canisters, emplaced at much lower temperatures, little if any elemental transport would take place in the event of breaching of the waste package.

The Theoretical Evaluation of Localised Corrosion in Radioactive Waste Canisters.

AbstractUnder repository conditions, it is likely that radioactive waste canisters will be subjected to both uniform and localised corrosion. Localised corrosion can take many forms depending on the precise physical and chemical environment of the metal at the time of attack, but generally the rates of penetration are much greater than those associated with uniform corrosion. The most likely forms under aerated repository conditions are pitting and crevice corrosion. To ensure adequate radionuclide containment an allowance for these rates must be included in the final canister dimensions. It is considered the best way to predict long term penetration rates is to develop mathematical models, which include all the physical and chemical processes necessary to describe the system and use experimentally determined input parameters relating to these processes. These models must then draw on further experimental data for validation over short timescales. In this paper, we discuss several techniques of modelling long term pit propagation in waste canisters. The complexity of the problem has lead to a number of physical and chemical approximations in the modelling. We investigate the applicability and ranges of validity of several of the more common approximations, both in our own models and in the literature, and compare the predictions with experimental pit growth rates. An investigatation of the sensitivity of the models to the various empirical input parameters indicates which need to be determined most accurately.

The pore-water and effective-stress response of clay sediments to a heat source

The pore-water and effective-stress response of clay sediments to the heat output of a radioactive waste canister is studied. It is shown that thermal expansion of the clay pore water (as opposed to buoyancy forces) can be the dominant force driving the pore-water flow field. Analytic solutions for the pore-water flow field and temperature field are compared favourably with a numerical solution including both thermal expansion and buoyancy driving terms. These analytic solutions are used to derive an idealized effective-stress response of the clay, both with and without yielding. It is shown that the effective-stress field induced in the clay by the pore water decreases inversely with distance and can therefore extend to large distances.

High pressure-high temperature laboratory apparatus for the measurement of deep sea sediment physical properties

Laboratory equipment has been built which will measure the permeability and thermal conductivity of deep-sea sediments at their in-situ conditions of hydrostatic pressure, temperature, and void ratio. The apparatus has the capability of uniaxially consolidating a sediment sample to simulate compaction within the sediment column, while exposing the specimen to hydrostatic pressures ranging from atmospheric to 62 MPa and to temperatures from 22 to 220°C. The equipment includes a hypodermic needle mounted vertically through the base of the pressure vessel from which thermal conductivity is determined by the needle probe method. The system also features a combination of dead-weight testers which produces a small hydraulic gradient across the sample and permits the measurement of sediment permeability at large hydrostatic pressures. The physical property data generated from this apparatus will be important in understanding the mechanisms of heat transfer through the ocean floor and in analysing the coupled flow of heat and pore fluid in the vicinity of a heat source, such as a radioactive waste canister, buried in the seabed.

Very Rough Preliminary Estimate of the Colloidal Sodium Induced in Rock Salt by Radioactive Waste Canister Radiation

ABSTRACTVery rough estimates have been made of the total amount, the formation rate and spatial distribution of the Na metal colloid particles induced in rock salt adjacent to four types of radioactive waste canisters. A number of extrapolations were required. Salt immediately adjacent to a lightly shielded, 2.16 kW, high level waste canister could be converted entirely to colloidal Na (and presumably chlorine gas) in 200-400 years. The total Na metal formed will be 250-300 kg. A heavily shielded, 3.3 kW, spent fuel canister will convert roughly 0.3 percent of the salt at the canister surface to colloidal Na and the total sodium metal will be roughly 0.5 kg. Even at the lowest colloid levels the Na metal formed should greatly influence interactions between canisters and the surrounding salt, particularly if brine enters.

Thermal Conductivity of Bentonite/Quartz High-Level Waste Package Backfill

ABSTRACTA mixture of bentonite clay and quartz sand is being considered for use as a waste-package backfill, the material placed between a radioactive-waste canister and the repository host rock. Compacts of bentonite/quartz with weight-percent ratios of 100/0, 70/30, 50/50 and 30/70 were made at room temperature under a pressure of 100 MPa (15 ksi). Upon initial heating, the thermal conductivity of the 70/30 compact rose from 1.20 W/m·K at 298 K to 1.32 W/m·K at 373 K. After further heating to 473 K, it fell to 1.10 W/m·K, reflecting the loss of interlamellar water from the bentonite. The conductivity of the now-dehydrated compact was reproducible through several heating and cooling cycles, varying from 1.15 W/m·K at 573 K to 1.03 W/m·K at 298 K. The other mixtures were qualitatively similar to the 70/30: the 100/0, 50/50 and 30/70 dehydrated compacts displayed conductivities of 0.59, 1.06 and 0.83 W/m.K,respectively, at 298 K. Measured densities ranged from 1.98 to 2.12 g/cc. Combined geometric-mean and Maxwell models for thermal conduction in composite systems predict the measured results reasonably well. An analysis of the impact of backfills on high-level waste (HLW) package design indicates that no significant thermal penalty is imposed.