Characterising and modelling the excavation damaged zone in crystalline rock in the context of radioactive waste disposal

This paper describes current knowledge about the nature of and potential for thermo–hydro–mechanical–chemical modelling of the excavation damaged zone (EDZ) around the excavations for an underground radioactive waste repository. In the first part of the paper, the disturbances associated with excavation are explained, together with reviews of Workshops that have been held on the subject. In the second part of the paper, the results of a DECOVALEX [DEmonstration of COupled models and their VALidation against EXperiment: research funded by an international consortium of radioactive waste regulators and implementers (http://www.decovalex.com)] research programme on modelling the EDZ are presented. Four research teams used four different models to simulate the complete stress–strain curve for Avro granite from the Swedish Äspö Hard Rock Laboratory. Subsequent research extended the work to computer simulation of the evolution of the repository using a ‘wall-block model’ and a ‘near-field model’. This included assessing the evolution of stress, failure and permeability and time-dependent effects during repository evolution. As discussed, all the computer models are well suited to sensitivity studies for evaluating the influence of their respective supporting parameters on the complete stress–strain curve for rock and for modelling the EDZ.     

Electrical analogy for modelling thermal regime and moisture distribution in sandy soils

The soil mass is subjected to temperature variation due to several human activities (viz. tanks storing heated fluids, buried cables and pipelines, air-conditioning ducts, disposal of nuclear and thermal power plant wastes etc.), which result in heat-induced migration of the moisture in it. Though several studies have been conducted in the past to investigate the mechanism of heat migration through the soil mass, a methodology for ‘real-time measurement of the variations in temperature, flux and moving moisture front, in tandem, with respect to space' has rarely been attempted. In this context, extensive laboratory investigations were conducted to measure real-time flux and temperature variations in the sandy soils, and the validation of results has been done by employing an equivalent electrical circuit programme, LTspice. Subsequently, a mathematical model PHITMDS (i.e. Prediction of Heat-Induced Temperature and Moisture Distribution in Soil) has been developed and its utility and efficacy, for predicting the depth-wise temperature and heat-induced moisture migration, due to evaporation, in terms of position of moving moisture front in the sandy soil has been critically discussed and demonstrated.     

Aqueous geochemistry of radioactive waste disposal

Present plans for disposing of high-level radioactive waste include converting it to a stable solid form, enclosing it in metal canisters, placing the canisters in mined cavities several hundred meters below the surface, and filling and sealing all entries to the cavities. The chief hazard in such disposal is dissolution of radionuclides from the waste in the ground water that will eventually fill the cavity and may carry the dissolved material to surface environments. To prevent or delay release of radionuclides in this manner, the form of the waste must be made as insoluble as possible, the canister metal must be resistant to corrosion, both the cavity filling and surrounding rock must be impermeable and highly sorbent, and the repository must be located at a site where ground water is scanty and slow-moving and its path to the ground surface is long. The effectiveness of these barriers in isolating the waste can be predicted for long times in the future by models based on experimentally determined solubilities and sorption coefficients and on measured rates of ground-water movement. Much uncertainty is involved in such predictions, and additional research is needed to lower the uncertainty. But there seems little question that a well-located and well-engineered repository of this sort will provide adequate protection against harm from radioactive releases far into the future.     

Geochemical influences on the design, construction and operation of a nuclear waste vault

The design, construction and operation of a vault for used nuclear fuel in crystalline rock may be influenced by a number of geochemical factors. During the period when a site is being selected, information is needed regarding the rock type, heterogeneities in its composition and the mineralogy of permeable zones, because these will cause variations in thermal conductivity, strength and radionuclide sorptive properties of the rock. These factors may affect decisions regarding depth of vault construction, tunnel dimensions and spacing of panels and waste containers. During site characterization, borehole drilling from the surface and subsequent hydraulic testing will introduce both chemical and microbiological contaminants that may further influence this decision.During vault construction, the geochemistry of the rock may cause changes to the design and construction methods used. High salinity fluids in the rock matrix could limit the ability of radar surveys to detect fractures in the surrounding rock and may also cause unacceptably high total dissolved solids loadings in water discharged from the facility. The presence of toxic, corrosive or radioactive constituents in inflowing groundwater may require grouting or development of treatment facilities both above and below ground. The use of explosives will cause high organic and nitrate loadings in service water as well as the possible impregnation of these chemicals in the disturbed wall-rock surrounding an excavation. These chemicals may remain despite cleaning efforts and act as nutrients to promote microbial activity in the post-closure phase. In the operational phase, inflow of increasingly saline groundwater may affect predicted container corrosion-rates and buffer and grout stabilities following vault resaturation and increase in temperature. This, in turn, may affect designs for container materials and spacing. Salt accumulations and microbial growths on vault walls may extend into the rock mass and require skimming of wall-rock before closure.This paper describes geochemical influences that might be anticipated in the disposal of nuclear waste with examples from the construction and operation of Canada's Underground Research Laboratory, Manitoba, in a granitic batholith on the Canadian Shield.     

Multi-criteria analysis for the identification of waste disposal areas

The municipality of Cachoeiro de Itapemirim (ES) is currently the largest domestic producer and exporter of marble and granite in Brazil. The processing of the rock, especially the sawing results in a large volume of waste, including the abrasives used during sawing and the off-cut pieces. Presently these waste products are disposed of without proper environmental protection and cause pollution to surface water and groundwater resources, including silting, an increase in hardness and increased aluminium values in the groundwater. In order to minimize the harmful effects of these practices on the environment, a research project was undertaken to identify potential disposal sites for the generated waste. A multi-criteria approach was followed during which a database was compiled of all the companies generating waste products and also including a baseline environmental study of the region, integrated analysis (multi-criteria analysis) of the restriction factors and the preliminary identification of possible disposal sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Subterranean microorganisms and radioactive waste disposal in Sweden

In 1987, microbiology became a part of the Swedish scientific program for the safe disposal of high level nuclear waste (HLW). The goal of the microbiology program is to understand how subterranean microorganisms will interact with the performance of a future HLW repository. The Swedish research program on subterranean microbiology has mainly been performed at two sites in granitic rock aquifers at depths ranging from 70 m down to 1240 m, the Stripa research mine in the middle of Sweden and the Äspö Hard Rock Laboratory (HRL) situated on the south eastern coast of Sweden. Some work has also been performed in cooperation with other national or international research groups in Sweden, Canada and at the natural analogue sites Oklo in Gabon and Maqarin in Jordan. The following conclusions are drawn. There is a very high probability of the existence of a deep subterranean biosphere in granitic rock. The documented presence of a deep biosphere implies that relevant microbial reactions should be included in the performance assessment for a HLW repository. A HLW repository will be situated in a subterranean biosphere that is independent of solar energy and photosynthetically produced organic carbon. The ultimate limitation for an active microbial life will be the availability of hydrogen as energy source over time, and hydrogen has indeed been found in most deep groundwaters. Sulphide producing microorganisms are active in environments typical for a Swedish HLW repository, and the potential for microbial corrosion of the copper canisters must be considered. The bentonite buffer around the copper canisters will be a hostile environment for most microbes due to the combination of radiation, heat and low water availability. Discrete microbial species can cope with each of these constraints, and it is theoretically possible that sulphide producing microbes may be active inside a buffer, although the experiments conducted thus far have shown the opposite. Microorganisms have the capability to enzymatically recombine radiolysis oxidants formed by radiation of water. It has earlier been concluded that the migration of radionuclides due to sorption on microorganisms can be neglected. The influence of microbially produced complexing agents remains to be studied at realistic conditions in deep groundwater. Microorganisms have been found in natural alkaline groundwaters, but it could not be conclusively demonstrated that they were in situ viable and growing, rather than just transported there from neutral groundwater. A possible hypothesis based on the obtained results from investigations of natural alkaline groundwaters is that fresh concrete may be a bit too extreme for active life even for the most adaptable microbe – but this remains to be demonstrated.     

Application of integrated methods in mapping waste disposal areas

An integrated suite of environmental methods was used to characterize the hydrogeological, geological and tectonic regime of the largest waste disposal landfill of Crete Island, the Fodele municipal solid waste site (MSW), to determine the geometry of the landfill (depth and spatial extent of electrically conductive anomalies), to define the anisotropy caused by bedrock fabric fractures and to locate potential zones of electrically conductive contamination. A combination of geophysical methods and chemical analysis was implemented for the characterization and management of the landfill. Five different types of geophysical surveys were performed: (1) 2D electrical resistance tomography (ERT), (2) electromagnetic measurements using very low frequencies (VLF), (3) electromagnetic conductivity (EM31), (4) seismic refraction measurements (SR), and (5) ambient noise measurements (HVSR). The above geophysical methods were used with the aim of studying the subsurface properties of the landfill and to define the exact geometrical characteristics of the site under investigation.     

Engineering geological assessment of a proposed waste disposal site in coastal southwestern Ghana

 An engineering geological investigation has been undertaken of a proposed landfill site in coastal southwestern Ghana which is characterized by torrential rains, high relative humidity, shallow groundwater conditions and almost surrounded by a ridge. The study shows that the site has some desirable characteristics such as availability of suitable soils for the construction of the capping and bottom liners of the landfill, a large tract of land for landfilling operation and its location close to a major highway. However, the groundwater table was shallow and there was a high potential for leachate intrusion into and consequent contamination of nearby groundwater abstraction wells and surface potable water sources thereby constituting serious threat to life. Construction costs could be prohibitive as extensive dewatering and backfilling of vast portions of the project site would be required to enable the provision of a separation between the shallow water table and the bottom of the landfill. Received: 20 January 1998 · Accepted: 26 May 1998     

Evaluating the risk of climate change to nuclear waste disposal

A hierarchy of models is being developed to represent the changes in climate that could occur in the next 10,000 years at proposed nuclear waste repository sites in the U.S. Three levels of modeling of the global aspects of climate change are included. At the broadest level a multitude of theoretical representations are being considered, most based upon the Milankovitch theory. A set of at least 150 situations will be examined, and those of concern for site stability will be screened for more thorough analysis at the next level of detail. The screening criteria include estimation of the probability of the event; the level of probability which must be considered (0.0001) requires use of the most detailed paleoclimatic records available. Uncertainty in the results will be evaluated by comparison of model reconstructions to the paleoclimatic record and by Monte Carlo analyses.This paper was presented at Emerging Concept, MGUS-87 Conference, Redwood City, California, 13–15 April 1987.     

Evaluation of waste disposal site using the DRASTIC system in Southern Korea

As a systematic approach to waste disposal site screening for groundwater pollution protection, the DRASTIC system developed by the US Environmental Protection Agency (USEPA), was introduced at Younggwang County in Korea. Hydrogeological spatial databases for the system include information on depth to water, net recharge, aquifer media, soil media, topographic slope, hydraulic conductivity and lineament. Using the databases, the DRASTIC system and a GIS, the regional groundwater pollution vulnerability of the study area was assessed. The fracture density extracted from lineament maps was added to the DRASTIC system to take into account the preferential migration of contaminants through fractures. From the results of the study, a degree of groundwater pollution vulnerability through the study area was easily interpreted, and waste disposal sites could be screened for groundwater protection.     

Thermal constraints on disposal of heat-emitting waste in argillaceous rocks

Although the post-closure thermal phase of a repository containing heat-emitting radioactive wastes is likely to be of comparatively short duration when judged against performance assessment time-scales, the effects of temperature increase and associated thermal gradients within an argillaceous host-rock are likely to be wide-ranging and include: (a) geomechanical effects on the rock-mass and engineered structures (linings, emplacement boreholes, etc.), (b) perturbation of groundwater pressures and flow by ‘aquathermal mechanisms’, (c) possible near-field effects on mass transport properties and mechanisms (e.g. coupled flow phenomena), and (d) effects on the chemical evolution of the near-field environment. Maximum allowable temperature in a repository is likely to be constrained by host-rock and/or clay buffer/backfill thermal behaviour. Given the diverse range of the thermal responses, the identification of a primary constraint on near-field temperature and the specification of a maximum allowable temperature remains problematic.     

高放废物地质处置系统核素迁移模型研究

高放废物地质处置系统核素迁移研究是高放废物安全处理和处置的重要内容.从整个系统的角度出发,把高放废物地质处置系统核素迁移过程分为三种模式:工程屏障中的释放-扩散模型;地质屏障中的对流-弥散模型和在生物圈中的分区传递模型.在分区传递模型中利用转移系数描述核素在各分区之间的迁移.通过对每种模型核素迁移机理的分析得出概化模型,并给出了相应的数学描述.     

Glass-iron-clay interactions in a radioactive waste geological disposal: An integrated laboratory-scale experiment

Glass-iron-clay setups were reacted at 90 °C for 6-18 months to investigate the coupled interactions between glass alteration, Fe corrosion and clay transformation. The reacted interfaces were probed at the microscopic level using complementary characterization methods (scanning electron microscopy coupled with energy-dispersive X-ray analysis, micro-Raman spectroscopy, micro X-ray diffraction, micro X-ray fluorescence spectroscopy, and micro X-ray absorption near-edge structure spectroscopy). The 10-μm thick Fe foil was fully corroded within 10 months, exposing glass to the pore solution. Iron corrosion led to the formation of a layer containing mostly magnetite, siderite and Fe-rich phyllosilicates with one tetrahedral and one octahedral sheet (TO) or two tetrahedral and one octahedral (TOT) sheet per layer. The clay in contact with this corrosion layer was enriched in siderite (FeCO₃). Glass alteration resulted in the formation of a gel layer whose thickness increased with reaction time (from 20 μm after 6 months to 80 μm after 18 months) and a thin layer of secondary precipitates that concentrated lanthanides, P, and Mo. Assuming conservative behavior of Zr, the Si molar concentration in the gel is about 57% that in the glass. Glass dissolution remained at a rate close to the initial dissolution rate r₀. The data are consistent with glass dissolution sustained by the uptake of dissolved Si and charge-compensating cations on secondary (corrosion) products, thus maintaining the gel porosity open and facilitating the leaching of easily soluble elements.     

A comparative study of waste landfill disposal in different countries

This study draws attention to the fact that natural processes can mobilize thallium （Tl）, a highly toxic metal, which may enter the food chain with severe health impacts on the local human population. A rural area having independent Tl mineralization in southwestern Guizhou, China, was chosen for a pilot study. Tl contents of soils extracted by HNO3 in the study area range from 35-165 mg/kg in soils from the mining area, 14-78 mg/kg in alluvial deposits downstream, and 〈0.2-0.5 mg/kg in soils from the background area. Tl contents in ammonium acetate EDTA-extracted fraction are 0.013-1.3 mg/kg, less than 1% of concentration in HNO3-extracted fraction. The amounts of Tl in NH4Ac-extracted fraction were thought to be more exchangeable and bioavailable, i.e., immediately available to plants and/or available to plant roots over a period of years. Tl concentration in crops exhibits species-dependent preferences. The enrichment of Tl in edible crop species decreases in the following order： green cabbage〉chili〉Chinese cabbage〉rice〉com. The highest level of Tl in green cabbage is up to 500 mg/kg （DW）, surpassing the values of Tl in the soils （13-59 mg/kg）. The enrichment factor for TI in green cabbage is up to 1-10 when considering the HNO3-extracted Tl, but the factor highly rises to 30-1300 while considering the NH4Ac-extracted Tl. The average daily uptake of Tl by the local villagers through consumption of locally planted crops was estimated at about 1.9 mg per person, which is 50 times the daily ingestion from the Tl-free background area.     

The biogeochemical behaviour of U(VI) in the simulated near-field of a low-level radioactive waste repository

Microbial processes have the potential to affect the mobility of radionuclides, including U in radioactive wastes. A range of geochemical, molecular biological and mineralogical techniques were applied to investigate stable element biogeochemistry and U solubility in the simulated “near-field” (or local environment) of a low-level radioactive waste (LLW) repository. The experiments used a microbial inoculum from the trench disposal area of the UK LLW repository at Drigg, Cumbria, England, in combination with a synthetic trench leachate representing the local environment at the Drigg site. In batch culture experiments in the absence of U, a classic redox progression of terminal electron accepting processes (TEAPs) occurred in the order ${\text{NO}}_3^{-}$, Fe(III) and ${\text{SO}}_4^{2-}$ reduction. When 126 μM U was added to the system as U(VI)_aq, up to 80% was reduced to U(IV) by the indigenous microbial consortium. The U(IV) was retained in solution in these experiments, most likely by complexation with citrate present in the experimental medium. No U(VI)_aq was reduced in sterile cultures, confirming that U(VI)_aq reduction was microbially mediated. Interestingly, when U(VI)_aq was present, the progression of TEAPs was altered. The rate of Fe(III) reduction slowed compared to experiments without U(VI)_aq, and SO₄ reduction occurred at the same time as U(VI) reduction. Finally, an experiment where ${\text{SO}}_4^{2-}$-reducing microorgansisms were inhibited by Na molybdate showed no ingrowth of sulfide minerals, but U(VI) reduction continued in this experiment. This suggested that sulfide minerals did not play a significant role in abiotically reducing U(VI) in these systems, and that metal-reducing microorganisms were dominant in mediating U(VI) reduction. Bacteria closely related to microorganisms found in engineered and U-contaminated environments dominated in the experiments. Denaturing gradient gel electrophoresis (DGGE) on 16SrRNA products amplified from broad specificity primers showed that after incubation, differences in diversity and abundance of the microbial culture were observed between U and non-U experiments. These results indicate that the biogeochemistry of the LLW repository near-field stimulates reduction of U(VI)_(aq) to U(IV) under anaerobic conditions and that the fate of reduced U(IV) will depend on the complexants present in LLW systems.