Disposal Depth Research Articles

Uranium migration lengths in Opalinus Clay depend on geochemical gradients, radionuclide source term concentration and pore water composition

Abstract. Safety assessments of highly radioactive waste disposal sites are done based on simulation of radionuclide migration lengths through the containment providing rock zone. For a close to real case situation, the present model concept established for uranium is derived from the hydrogeological evolution and geochemical and mineralogical data measured at the deep geothermal borehole Schlattingen including the effect of geo-engineered barriers on the source term. In the Schlattingen area, the Opalinus Clay is tectonically undeformed compared to the Mont Terri anticline and represents the geochemical and temperature conditions at the favoured disposal depth. The geochemical conditions are more or less constant with slightly decreasing concentrations of pore water components towards the footwall aquifer. Uranium migrates less compared to the Opalinus Clay system at Mont Terri, where gradients of pore water geochemistry towards the embedding aquifers are more pronounced. This means, stable geochemical conditions with no or low concentration gradients are to be favoured for a safe disposal since migration lengths strongly depend on spatial and temporal variation of the hydrogeological and geochemical conditions within the host formation. The engineered barriers reduce the source term concentration what, in turn, is associated with a decrease in uranium migration. Stable geochemical conditions further enable the application of the Kd approach to estimate the impact of the barriers. The hydrogeological system must always be considered when quantifying radionuclide migration.

Deep borehole disposal of intermediate-level waste

Abstract. Around the world, deep borehole disposal is being evaluated for intermediate-level waste (ILW), high-level waste (HLW), spent nuclear fuel (SNF), separated plutonium waste and some very high specific activity fission product waste. In Australia, long-lived ILW from research reactors and radiopharmaceutical production represents the principal waste stream that requires deep geologic disposal. Whilst the Australian government has not yet made a decision on its preferred strategy for ILW disposal, deep borehole disposal of small volumes of ILW would be a more cost-effective and modular solution compared to a conventional geologic disposal facility (GDF). CSIRO, ANSTO and SANDIA have created an international partnership to execute a full-scale borehole research, development and demonstration (RD&D) project in Australia. The project will demonstrate the technical feasibility of the long-term safety of borehole disposal in deep geological formations. The execution of this project could also demonstrate options for nuclear waste disposal that would reduce proliferation risks, potentially up to the termination of compliance with international safeguards requirements. The RD&D includes demonstration of surface handling and waste/seal emplacement capabilities, basic research on foundational science areas, and full-scale field testing in both a deep characterization borehole and a larger-diameter (0.7 m or 27.5 inch) 2000 m deep demonstration borehole. The multi-barrier system designed for such a deep disposal borehole concept places much less reliance on engineered barriers at the disposal zone to achieve safety as compared to a conventional GDF. It rather relies on geological features for waste containment. The concept being explored uses disposal containers with primary waste packages, such as vitrified waste canisters, inside; to be both cost effective and fit for purpose, such a container could have a mild steel-based structural component with copper coating. A critical review of six coating technologies showed that cold spray has the greatest advantages, such as minimal porosity and compressive residual stress. The RD&D has delivered novel enabling tools that assist with site screening, borehole design and post-closure safety assessments. For instance, an automated geological fault mapping and meshing tool was developed that assists with ranking the suitability of potential disposal sites based on proximity to faults. New codes were developed for better representation of fault zones in 2D/3D numerical flow and transport models, while also being more efficient to execute. Post-closure safety assessments tested the sensitivity of long-term safety with respect to disposal depth, rock permeability and sorption. Heat transport calculations explored the sensitivity of temperature evolution within the borehole to parameters such as heat load, borehole depth, geothermal gradients and rock thermal conductivity. For verification of host rock tightness while also demonstrating the absence of recent groundwater, a new noble gas analytical facility has been established for measuring rare noble gases in mineral fluid inclusions as indicators of very old pore fluids.

Spent fuel corrosion and the impact of iron corrosion – The effects of hydrogen generation and formation of iron corrosion products

Canister designs for deep geological disposal of spent nuclear fuel typically involve large amounts of iron. Canister failure will likely involve corrosion of the canister at the disposal depth several hundred meters underground, where conditions are typically anoxic. Spent nuclear fuel corrosion and dissolution will therefore likely proceed together with anoxic iron corrosion. The iron corrosion can suppress spent fuel corrosion by creation of strongly reducing conditions from Fe(II) formation and generation of large quantities of hydrogen. In an attempt to separate the effect of hydrogen from that of Fe(II) and iron corrosion products, two experiments were performed. In both cases, spent nuclear fuel powders were used in autoclaves filled with simplified granitic groundwater initially pressurized with argon. In the first experiment, the autoclave contained iron powder. Following the initial radionuclide release at start-up, the concentration of redox sensitive radionuclides decreased with uranium reaching UO2 (am) solubility levels within months. Release of non-redox sensitive elements such as Cs ceased after 223 days. Continuous hydrogen generation was observed. XRD and Raman spectroscopy of the iron corrosion products identified magnetite. The second experiment was therefore performed starting with magnetite powder. In this case, the autoclave argon atmosphere remained constant. Radionuclide release was, however, continuous with uranium reaching >2 × 10−5 M after 275 days. The magnetite powder of the second experiment was analyzed and found to be composed of mainly magnetite with a small fraction of hematite. In the iron corrosion experiment, Mo, Tc and U where found to have high iron corrosion product affinities. However, in the magnetite experiment, U, Tc and Mo displayed weak affinity for the corrosion products, indicating presence in the higher oxidation state. The results thus point to hydrogen evolution being the main contributor to the apparent inhibition of radionuclide release in the iron corrosion experiment.

Corrosion of glass used for radioactive waste disposal influenced by iron corrosion products

Iron corrosion products of the steel canister in the nuclear waste container are highly silica sorptive materials and very much expected to delay formation of the protective layer (gel) on the nuclear glass interface. This study is focusing on the role of the iron carbonates (ankerite), which is probably one of iron corrosion products and already exists in the clay structure of the disposal depth. Simulating nuclear borosilicate glass samples (alternative to the R7T7-type) samples are planned to leach in different series of experimental models with dissimilar quantities of ankerite. Batch experiments all will be cultivated in synthetic groundwater; where different pH (4-9) values and temperatures (35, 50, 90 °C) are considered. In this paper the state of the art of the radioactive waste glass corrosion in the circumstances of deep geological disposal will focus on the role of ankerite as one of the iron corrosion products and clarify how it is expected to show delay in achieving the Si saturation status and accelerate glass alterations.

Corrosion of glass used for radioactive waste disposal – state of the art

Iron corrosion products of the steel canister in the nuclear waste container are highly silica sorptive materials and very much expected to delay formation of the protective layer (gel) on the nuclear glass interface. This study is focusing on the role of the iron carbonates (ankerite), which is probably one of iron corrosion products and already exists in the clay structure of the disposal depth. French SON68 inactive reference glass (alternative to the R7T7- type) samples are planned to leach in different series of experimental models with dissimilar quantities of natural ankerite. All batch experiments will be cultivated in synthetic Callovo-Oxfordian groundwater; where different pH (4-9) values and temperatures (35, 50, 90 °C) are considered. In this paper the state of the art of the radioactive waste glass corrosion in the circumstances of deep geological disposal were extensively studied.

Derivation of a speciic activity limit for plutonium for near surface disposal a case study at a potential site in northwest China

Based on the safety assessment framework and site-specific characteristic investigations in northwest China, an approach to deriving the specific activity limit of 239Pu is applied to establish a proposed value. Our analyses, in conjunction with the results of other previous studies, allow for the following conclusions: (1) As an intrusion scenario with a feature of minimal site-dependence and pervasive applicability, the drilling scenario can be used as the limiting scenario for the post-closure period; (2) Given a dose limit of 5 mSv per year, a derived specific activity of 287 Bqg-1 (at a disposal depth shallower than 5 m) for 239Pu is obtained through the formulation of models and subsequent calculations. It is suggested that both our approaches to deriving the limit and the results can be effectively applied to establish acceptance criteria of long-lived transuranic nuclides, for the particular disposal facility; and (3) From the standpoint of exploring the approach for limit derivation, the intrusion scenario and the corresponding exposure evaluation can be the focus of concern in the study area. It is implied that, in arid regions, a leaching scenario may lead to a more complex evaluation, with unnecessary effort, and can be virtually excluded.

EIS study on aerobic corrosion of copper in ground water: influence of micro-organisms

The nuclear waste disposal concept in Finland and Sweden is based on a multi-barrier system, where the solid spent fuel is placed in a cast iron container that is further enclosed in a copper canister. The copper canisters will be placed vertically in holes drilled in a deep bedrock which are then filled with a bentonite clay. The near-field environment of bentonite clay and groundwater is then expected to maintain integrity of the copper canisters for at least 100 000 years. The ground water at the final disposal depth contains micro-organisms with a vast metabolic potential, and it is known that micro-organisms may induce or accelerate the corrosion of metals. Therefore, it is of vital importance to understand the surface interactions of copper and the expected near-field environment. In this study, the results from two types of experiments that investigate the corrosion of copper in oxic ground water environment with and without micro-organisms and electrochemical impedance spectroscopy (EIS) as one of the key research methods are reported. First, long-term immersion experiments with a range of in-situ electrochemical measurements were designed to simulate the initial oxic stage of the deep geological nuclear waste repository in the presence of bentonite. Second, electrochemical laboratory tests were planned to improve understanding on the interactions between copper and the micro-organisms in ground water environment: copper specimens were incubated with micro-organisms for different time periods and then subjected to electrochemical measurements to provide data. The results collected from these two test series are reported and discussed here by focusing on the EIS data and its role in interpreting the surface processes. It is evident that EIS helps in understanding the influence of micro-organisms on the copper degradation process.

In situ strength and failure mechanisms of migmatitic gneiss and pegmatitic granite at the nuclear waste disposal site in Olkiluoto, Western Finland

The primary goals of the experimental work described in this paper were to establish the in situ spalling/damage strength of the intact rock, to establish the state of the in situ stress, and for the work to act as a Prediction–Outcome exercise within the context of confirmatory underground site investigations being conducted at the nuclear waste disposal site at Olkiluoto, in Western Finland. To establish the in situ mechanical properties, Posiva (the Finnish implementer) formulated the in situ Posiva Olkiluoto Spalling Experiment (POSE) in 2009. The outcome of this experiment was that rock failure mainly occurred due to structurally controlled factors, rather than being dictated solely by the expected location of the maximum stress. The POSE experiment also showed that the onset of fracture initiation in the Olkiluoto rock occurs at 40 MPa, and the rock mass strength is ca. 90 MPa-compared to the mean laboratory value of 104 MPa. In view of these observations made at the projected disposal depth, the vertical disposal concept, KSB-3V, will involve initiation of new fractures, although the horizontal disposal concept would be less affected by such fracture initiation. However, neither of the disposal concepts is expected to suffer any major rock mass failure, and the vertical disposal holes are not particularly sensitive to fracture initiation if the disposal tunnels are oriented within 30° trend of the major principal stress direction.

Evaluation of the State of Disposal Polygons of Solid Domestic Waste by the Variation in the Organic Component

The results of the investigation into the determination of the stages of the living cycle by the example of waste disposal objects of the Perm krai using the physicochemical methods of analysis are presented. Indicator characteristics for the evaluation of the degree of decomposition and stage of waste biodegradation relative to the age and variation over the disposal depth are presented: pH, BOD5/COD ratio, and organic carbon/ash content. The biodestruction stages of waste with the disposal age of 1–3 years, 2–6 years, more than 10 years, and more than 30 years are determined. It is established that the BOD5/COD ratio at used polygons of solid domestic waste (SDW) and dumps is 0.45–0.54 and more, at reclamated polygons—0.21–23, and at closed polygons—0–0.15. A decrease in COD and BOD5 in the water extract of waste, organic carbon content, and ash content in waste are revealed with an increase in the disposal time and over the depth of waste disposal. Based on the selected indicator characteristics for the evaluation of the stage of the living cycle of SDW disposal polygons, we can predict the emission potential of waste and develop technical measures for a decrease in the negative environmental effect of SDW polygons.

처분 심도의 지하수 유량이 처분공에서 누출될 것으로 가정된 방사성핵종의 이동에 끼치는 영향 평가

Using results of groundwater flow system modeling for a hypothetical deep geological repository site, a distribution of groundwater flow rates at the disposal depth was analyzed and a method of applying this distribution to a safety assessment for a disposal of radioactive wastes was suggested. The distribution of groundwater flow rates was produced by hydraulic heads simulated from regional and local scale groundwater flow models for the hypothetical disposal site. The flow rates at the locations where deposition holes would be located were estimated. These rates were normalized by the maximum of the flow rates in order to probabilistically illustrate a possibility of canister failures at the deposition holes. From the normalized distribution, probabilistic expectations for mass discharges of radionuclides released from the canisters assumed to be failed were calculated and compared with those deterministically estimated under the assumption that the canisters at the same deposition holes were definitely failed. The suggested method can be contributed to constructing a methodology for safety assessment of a geological repository by reflecting natural conditions of a disposal site in more detail.

전기비저항 탐사를 이용한 가축사체 매몰지 특성 분석

가축사체 매몰지로부터 유출되는 침출수는 환경적으로 치명적인 영향을 주는 고농도 오염폐수의 일종이다. 본 연구에서는 효율적인 가축사체 매몰지의 특성을 파악하기 위하여 전기비저항 탐사와 함께 시추조사를 실시하였다. 또한, 가축사체 매몰지에서 샘플링한 침출수의 화학적 분석을 실시하였다. 매몰지 내부에서 5측선의 쌍극자 전기비저항 탐사와 3지점의 시추조사를 실시하였으며, 매몰지 주변의 11지점에서 시추조사를 수행하였다. 전기 비저항 탐사에 의하여 수집된 자료를 이용하여 2차원 역산 모델링을 수행하여 매몰지의 특성(크기, 심도, 형태 등)을 평가하였다. 침출수 분석결과, pH는 7.4에서 6.7로 감소하였고, Eh는 -358 mV에서 -48 mV로 변화하였다. 또한, 가축사체 부패에 의하여 용존 이온이 증가하였다. 전기비저항 탐사 자료의 해석결과, 지표에서 심도 8 m 이내에서 최소 <TEX>$0.64{\Omega}m$</TEX>의 낮은 비저항값을 가지는 영역이 나타났다. 이 지역의 기반암 위치와 가축사체의 매립 깊이를 고려할 때, 매립지 내부에서는 침출수에 의한 기반암의 오염은 진행되지 않은 것으로 나타났다. 전기비저항 탐사의 결과는 시추조사의 결과와 잘 일치 하였으며, 이는 전기비저항 탐사가 가축매몰지의 특성을 효과적으로 묘사한다는 것을 보여주었다. 본 연구에 의하면, 전기비저항 탐사가 가축사체 매몰지의 현황을 조사하는데 적합한 기술로 판단된다. In this study, an electrical resistivity survey and a drilling investigation were conducted at an animal carcass disposal site. Chemical analysis of leachate collected from the site was also performed (sampling times: May 2011 and June 2012). Five lines of dipole-dipole electrical resistivity surveys were carried out, along with drilling investigations at 3 points within the disposal areas and 11 points near the disposal site. Two-dimensional inverse modeling of the collected resistivity data was performed to evaluate the properties (size, depth, and form) of the disposal site. Leachate analysis showed that pH of leachate decreased from 7.4 to 6.7, while Eh changed from -358 mV to -48 mV over time. In addition, dissolved ions increased due to the progression of carcass decomposition. Results of the electrical resistivity survey indicated that low resistivity zones (minimum value, <TEX>$0.64{\Omega}m$</TEX>) existed at a depth of 8 m from the surface. Considering the bedrock location and carcass disposal depth, there was no evidence of bedrock contamination by leachate. The results of the electrical resistivity survey are consistent with those of the drilling investigation, which indicates that electrical resistivity effectively depicted the properties of the disposal site. This study demonstrates that electrical resistivity survey is a suitable technique for investigation of animal carcass disposal sites.

SR-97: Canister Performance under Normal Disposal Conditions

ABSTRACTThe SR-97 disposal canister consists of an inner load bearing cast iron component and an outer corrosion barrier of copper. The mechanical strength of the insert must be such that it withstands, with sufficient safety margins, the combined forces of hydrostatic pressure at the disposal depth (7 MPa) and the bentonite swelling pressure (7 MPa). It must also withstand an increased hydrostatic pressure during a future glaciation. This pressure increase may be as high as 30 MPa. For this load situation, no extra safety margins will be required. Uneven pressure may occur during the water saturation of the buffer and to some extent some uneven pressure may also persist after water saturation. Calculations have been performed for such design basis uneven loads and the present canister design has been shown to withstand them. Under the oxygen-free conditions at the disposal depth, copper will be thermodynamically stable. Only dissolved sulfide will be able to corrode the copper. The supply of sulfide is expected to be low and, consequently, the service life of the canister will be very long. Calculations indicate that no known corrosion processes will lead to canister failure during a shorter time than at least 1 million years.

Can CO 2 hydrate deposited in the ocean always reach the seabed?

Equations for the densities of seawater in the abyssal zone and of CO 2 hydrate are derived based on measured densities of seawater at intermediate depths and on the hydrate crystalline structure. Predictions from the derived equations agree excellently with the measured densities of seawater and CO 2 hydrate. It is found that CO 2 hydrate contains fewer than the stoichiometric number of CO 2 molecules, and thus the density of CO 2 hydrate is smaller than the value widely used in the literature. Therefore, albeit denser than seawater above the abyssal zone, CO 2 hydrate may become less dense than abyssal seawater. A neutral buoyancy depth may exist in the abyssal zone in about 60% of the areas of the Atlantic, Pacific and Indian Oceans. In these areas, CO 2 hydrate deposited in the ocean cannot reach the seabed. The most economic depth for hydrate disposal is the minimum depth for hydrate formation, which is about 500 m below the surface in many oceans. Since CO 2 hydrate is unstable in seawater which is unsaturated with CO 2, no permanent hydrate phase will exist in the ocean where the concentration of CO 2 is negligible. If the disposal depth is chosen at 500 m and the sizes of hydrate particles are not very large, then in more than 90% of the areas of the three oceans (excluding adjacent seas), hydrate particles deposited in the ocean will dissolve fully before reaching the neutral buoyancy depth or the seabed, if the neutral buoyancy depth does not actually exist.

Influence of disposal depth on the size of CO2 droplets produced from a circular orifice

Abstract Disposal of liquid, anthropogenic CO2 in intermediate-depth ocean waters has been accepted as a means to reduce atmospheric concentrations of this greenhouse species and mitigate global warming. Since the CO2-seawater system is hydrodynamically unstable, liquid CO2 effluent in the ocean will break up into droplets. This paper discusses the influence of both the discharge orifice and properties of CO2 and seawater on the size of CO2 droplets produced. It is found that the droplet size is affected strongly by disposal depth. This suggests that determination of the behavior of CO2 discharged in the ocean must consider not only the orifice configuration, but also fluid properties at the disposal site.

The effectiveness of marine CO 2 disposal

We consider the relative effectiveness of engineered systems for collection and marine disposal of CO 2 from fossil-fuel-fired power plants using comparisons of the trend with time of CO 2 concentration in the atmosphere from systems with and without marine disposal. Since the retention time for CO 2 increases rapidly with depth of disposal, from a few years in the mixed layer up to several centuries as the depth increases beyond 1000 m, deep oceans have been considered as potential storage sites for CO 2. However, CO 2 collection and disposal consumes energy and produces extra CO 2. We show that some of this extra CO 2 reaches the atmosphere, so that atmospheric concentrations from systems with marine disposal ultimately exceed those from systems without controls. In some circumstances, they do so rapidly, making marine disposal less favorable than direct atmospheric release. The Global Warming Potential (GWP) can be used to compare different systems, but results for GWP highlight the difficult issue of determining what time scales are important in considering options to reduce concerns about global warming.

Assessing the effectiveness of marine CO 2 disposal

We consider the relative effectiveness of systems for collection and marine disposal of CO 2 from fossil fuel fired power plants using comparisons of the trend with time of CO 2 concentration in the atmosphere from systems with and without marine disposal. The retention time for CO 2 increases rapidly with depth of disposal, from a few years in the mixed layer up to several centuries as the depth increases beyond 1000 m, suggesting that deep oceans should be considered as potential storage sites for CO 2. However, CO 2 collection and disposal consumes energy and produces extra CO 2. We show that some of this extra this CO 2 reaches the atmosphere, so that atmospheric concentrations from systems with marine disposal ultimately exceed those from systems without controls. In some circumstances they do so rapidly, making marine disposal less favorable than direct atmospheric release. Global Warming Potential can be used to compare systems. This highlights the difficult issues involved in determining what time scales are important in considering options to reduce concerns about global warming.