Glasses for immobilization of low- and intermediate-level radioactive waste

Reprocessing of spent nuclear fuel (SNF) for recovery of fissionable elements is a precondition of long-term development of nuclear energetics. Solution of this problem is hindered by the production of a great amount of liquid waste; 99% of its volume is low- and intermediate-level radioactive waste (LILW). The volume of high-level radioactive waste (HLW), which is characterized by high heat release, does not exceed a fraction of a percent. Solubility of glasses at an elevated temperature makes them unfit for immobilization of HLW, the insulation of which is ensured only by mineral-like matrices. At the same time, glasses are a perfect matrix for LILW, which are distinguished by low heat release. The solubility of borosilicate glass at a low temperature is so low that even a glass with relatively low resistance enables them to retain safety of under-ground LILW depositories without additional engineering barriers. The optimal technology of liquid confinement is their concentration and immobilization in borosilicate glasses, which are disposed in shallow-seated geological repositories. The vitrification of 1 m³ liquid LILW with a salt concentration of ～300 kg/m³ leaves behind only 0.2 m³ waste, that is, 4–6 times less than by bitumen impregnation and 10 times less than by cementation. Environmental and economic advantages of LILW vitrification result from (1) low solubility of the vitrified LILW in natural water; (2) significant reduction of LILW volume; (3) possibility to dispose the vitrified waste without additional engineering barriers under shallow conditions and in diverse geological media; (4) the strength of glass makes its transportation and storage possible; and finally (5) reliable longterm safety of repositories. When the composition of the glass matrix for LILW is being chosen, attention should be paid to the factors that ensure high technological and economic efficiency of vitrification. The study of vitrified LILW from the Kursk nuclear power plant with high-power channel reactors (HPCR; equivalent Russian acronym, RBMK) and the Kalinin nuclear power plant with pressurized water reactors (PWR; equivalent Russian acronym VVER) after their 14-yr storage in the shallow-seated repository at the MosNPO Radon testing ground has confirmed the safety of repositories ensured by confinement properties of borosilicate matrix. The most efficient vitrification technology is based on cold crucible induction melting. If the content of a chemical element in waste exceeds its solubility in glass, a crystalline phase is formed in the course of vitrification, so that the glass ceramics become a matrix for such waste. Vitrified waste with high Fe; Na and Al; Na, Fe, and Al; Na and B is characterized. The composition of frit and its proportion to waste depends on waste composition. This procedure requires careful laboratory testing.     

Rn and CO2 soil–gas geochemical characterization of thermally altered clays at Orciatico (Tuscany,Central Italy)

The physical properties of clay allow argillaceous formations to be considered geological barriers to radionuclide migration in high-level radioactive-waste isolation systems. As laboratory simulations are short term and numerical models always involve assumptions and simplifications of the natural system, natural analogues are extremely attractive surrogates for the study of long-term isolation. The clays of the Orciatico area (Tuscany, Central Italy), which were thermally altered via the intrusion of an alkali-trachyte laccolith, represent an interesting natural model of a heat source which acted on argillaceous materials. The study of this natural analogue was performed through detailed geoelectrical and soil–gas surveys to define both the geometry of the intrusive body and the gas permeability of a clay unit characterized by different degrees of thermal alteration. The results of this study show that gas permeability is increased in the clay sequences subjected to greater heat input from the emplacement of the Orciatico intrusion, despite the lack of apparent mineral and geotechnical variations. These results, which take into consideration long time periods in a natural, large-scale geological system, may have important implications for the long-term safety of underground storage of nuclear waste in clay formations.     

Clay as sealing material in nuclear waste repositories

Nuclear waste from thermal plants poses a lasting risk to the biosphere because of its long radioactive life. The planned definitive storage place for it is in deeply buried repositories. Such repositories would need to be both impermeable to water, and plastic during deformation, in order to avoid the formation of cracks that may allow water in. One of the clay minerals, smectite, has these two properties and is an ideal candidate as a sealing material or even host rock for nuclear waste repositories. The chemical stability of smectite in the repository environment is sufficient to maintain good sealing properties during the active life of the relevant radionuclides.     

A natural analogue for the long-term corrosion of copper nuclear waste containers—reanalysis of a study of a bronze cannon

The study of natural analogues is often used to support long-term predictions of the safety of geological disposal of used nuclear fuel. The case of a bronze cannon, submerged in seawater-saturated clay sediments for a period of 310 a, is reassessed with reference to a corrosion mechanism developed for copper nuclear waste containers in a conceptual disposal vault in the Canadian Shield. Redox reactions between Cu, Fe and dissolved O₂ are believed to have played an important role in the corrosion of the bronze cannon and also form an essential part of the corrosion mechanism proposed for nuclear waste containers. Limited rates of mass transport through compacted clay are also important in determining the corrosion behaviour of both the cannon and the containers.     

Advances in geochemical research for the underground disposal of high-level, long-lived radioactive waste in a clay formation

One of the options for the long-term confinement of high-level, long-lived radioactive waste is the disposal in deep geological formations. In France, this option is particularly studied in a 155-Ma-old Jurassic clay formation located in the eastern part of the Paris Basin. Fifteen years of research and the construction of an underground research laboratory have provided a large set of data that allows the feasibility of the geological disposal to be evaluated from a scientific standpoint. Geochemical aspects of this research are of major importance because they provide essential information on both the characteristics of the geological medium and the long-term behaviour of the waste and the engineered system.     

Reversed mining and reversed-reversed mining: the irrational context of geological disposal of nuclear waste

Man does not only extract material from the Earth but increasingly uses the underground for storage and disposal purposes. One of the materials that might be disposed of this way is high-level nuclear waste. The development of safe disposal procedures, the choice of suitable host rocks, and the design of underground facilities have taken much time and money, but commissions in several countries have presented reports showing that — and how — safe geological disposal will be possible in such a way that definite isolation from the biosphere is achieved. Political views have changed in the past few years, however, and there is a strong tendency now to require that the high-level waste disposed of will be retrievable. Considering the underlying arguments for isolation from the biosphere, and also considering waste policy in general, this provides an irrational context. The development of new procedures and the design of new disposal facilities that allow retrieval will take much time again. A consequence may be that the high-active, heat-generating nuclear waste will be stored temporarily for a much longer time than objectively desirable. The delay in disposal and the counterproductive requirement of retrievability are partly due to the fact that earth-science organisations have failed to communicate in the way they should, possibly fearing public (and financial) reactions if taking a position that is (was?) considered as politically incorrect. Such an attitude should not be maintained in modern society, which has the right to be informed reliably by the scientific community.     

High-level waste disposal,ethics and thermodynamics

Moral philosophy applied to nuclear waste disposal can be linked to paradigmatic science. Simple thermodynamic principles tell us something about rightness or wrongness of our action. Ethical judgement can be orientated towards the chemical compatibility between waste container and geological repository. A container-repository system as close as possible to thermodynamic equilibrium is ethically acceptable. It aims at unlimited stability, similar to the stability of natural metal deposits within the Earth’s crust. The practicability of the guideline can be demonstrated.     

Use of solution-mined caverns in salt for oil and gas storage and toxic waste disposal in Germany

The need for storage caverns for oil and gas, and repositories for toxic chemical waste is increasing world-wide. Rock salt formations are particularly suitable for the construction of cavities for such purposes. Owing to its favourable geomechanical properties, rock salt remains stable over long periods of time without support, and it can be shown that the geological barrier of the host rock remains intact for a remarkably long time.

Safety analysis must be made for each proposed site based on site-specific data. The methods of doing this are well known and related technical recommendations exist in Germany. These recommendations apply to the planning, construction, operation and post-operational management of salt caverns used for the underground disposal of hazardous wastes. In particular, geotechnical site-specific safety verification, as required by the government's technical regulations on wastes (TA-Abfall) under the section “Underground Disposal”, is required. This safety verification must cover the entire system comprising the waste, the cavern and the surrounding rocks. For this purpose geomechanical models have to be developed. The steps which must be taken when carrying out geological engineering site explorations and when determining geotechnical parameters are discussed. In addition, recommendations are made for the design and construction of underground repositories.

For liquid-filled caverns, long-term sealing from the biosphere is of particular interest. In this instance it must be shown that the natural increase in pressure in the closed cavity due to long-term convergence does not exceed the fracture pressure. A special filled test (scale 1:1) has been performed to study this.     

铀矿床作为高放废物处置库天然类似物的国内外研究进展

铀矿床具有目前国际上开发的高放废物深地质处置概念的相似特征，对其开展天然类比研究，是认识核素在地质环境中迁移行为的一种有效方法。本文阐述了高放废物深地质处置库天然类似物的一般概念及天然类比研究拟解决的科学问题。重点介绍了30年来国内外在天然类比研究中取得的主要成果、认识以及国内外研究进展。     

核废物处置安全评价的地质类比研究

本文简要论述了利用天然地质类似物（或类比体），类比评价核废物处置系统的安全性能。地质类比研究是安全评价核废物处置系统的重要方法之一，也是地质学科的新研究领域。目前已有的地质类比研究对象主要有火山玻璃，陨石玻璃，粘土矿物，铁陨石，铜矿床，铀矿床，含含Ｔｈ和ＲＥＥ的铁矿床，天然核反应堆（一种特殊的铀矿床）和火成岩接触带等。最后对我国今后从事同类研究提出了若干建议。     

An analysis of geological conditions when choosing a site for toxic waste storage

This paper substantiates the necessity of developing an integrated specialized engineering geological approach to the problem of siting toxic waste storage facilities. The fact that the ground stratum of a storage base can serve as a natural geochemical barrier, preventing diffusion of pollutants from the storage into a water-bearing horizon, underlies such an approach. The screening capacity of the ground stratum depends on a number of natural factors that are laid out, estimated, and analyzed in the paper. It is suggested that these results should be reflected on a map of special engineering-geological zoning, on which areas with different conditions of groundwater protection are determined. This permits one to determine storage locations most rationally and to solve a number of problems of environmental protection in the course of the operation of such a facility.     

Construction of a large storage for critical waste

The paper describes the construction of a long-term storage for critical waste. The base impermeability layer consisted of natural clay, whereby an additional 130,000 m³ of clay were placed on the existing 70,000 m² clay stratum. The method of of placement and the permeability coefficients obtained are described.     

Geoscientific R&D for high level radioactive waste disposal in Sweden — current status and future plans

SKB (Svensk Kärnbränslehantering AB) is responsible for all handling, transport and storage of the nuclear wastes outside the Swedish nuclear power stations. According to Swedish law, SKB is responsible for an R&D-programme needed to take care of the radwastes. The programme comprises, among others, a general supportive geo-scientific R&D and the Äspö Hard Rock Laboratory (HRL) for more in-situ specific tasks.

Sweden is geologically located in the Fennoscandian shield which is dominated by gneisses and granitoids of Precambrian age. The Swedish reference repository concept thus considers an excavated vault at ca. 500 m depth in crystalline rocks. In this concept (KBS-3), copper canisters with high level waste will be emplaced in deposition holes from a system of tunnels. Blocks of highly compacted swelling bentonite clay are placed in the holes leaving ample space for the canisters. At the final closure of the repository, the galleries are backfilled with a mixture of sand and bentonite. This repository design aims to make the disposal system as redundant as possible. Although the KBS-3 concept is the reference concept, alternative concepts and/or repository lay-outs are also studied. The main alternative, currently under development at SKB, is disposal in boreholes with depths of 4–5 km. The geoscientific research will to a great extent be guided by the demands posed by the performance and safety assessments, as well as the constuctability issues. Some main functions of the geological barrier are fundamental for the long-term safety of a repository. These are: bedrock mechanical stability, a chemically stable environment as well as a slow and stable groundwater flux. The main time-table for the final disposal of long-lived radioactive waste in Sweden foresees the final selection of the disposal system and site during the beginning of next decade.     

Confinement matrices for low- and intermediate-level radioactive waste

Mining of uranium for nuclear fuel production inevitably leads to the exhaustion of natural uranium resources and an increase in market price of uranium. As an alternative, it is possible to provide nuclear power plants with reprocessed spent nuclear fuel (SNF), which retains 90% of its energy resource. The main obstacle to this solution is related to the formation in the course of the reprocessing of SNF of a large volume of liquid waste, and the necessity to concentrate, solidify, and dispose of this waste. Radioactive waste is classified into three categories: low-, intermediate-, and high-level (LLW, ILW, and HLW); 95, 4.4, and 0.6% of the total waste are LLW, ILW, and HLW, respectively. Despite its small relative volume, the radioactivity of HLW is approximately equal to the combined radioactivity of LLW + ILW (LILW). The main hazard of HLW is related to its extremely high radioactivity, the occurrence of long-living radionuclides, heat release, and the necessity to confine HLW for an effectively unlimited time period. The problems of handling LILW are caused by the enormous volume of such waste. The available technology for LILW confinement is considered, and conclusion is drawn that its concentration, vitrification, and disposal in shallow-seated repositories is a necessary condition of large-scale reprocessing of SNF derived from VVER-1000 reactors. The significantly reduced volume of the vitrified LILW and its very low dissolution rate at low temperatures makes borosilicate glass an ideal confinement matrix for immobilization of LILW. At the same time, the high corrosion rate of the glass matrix at elevated temperatures casts doubt on its efficient use for immobilization of heat-releasing HLW. The higher cost of LILW vitrification compared to cementation and bitumen impregnation is compensated for by reduced expenditure for construction of additional engineering barriers, as well as by substantial decrease in LLW and ILW volume, localization of shallow-seated repositories in various geological media, and the use of inexpensive borosilicate glass.     

A review of the ANDRA''s research programmes on the thermo-hydromechanical behavior of clay in connection with the radioactive waste disposal project in deep geological formations

This paper presents the different studies realized or launched by ANDRA in collaboration with different contractors, including laboratory and in situ experiments, as well as physical and numerical modelizations, related to the thermo-hydromechanical behavior of clays and clayey materials. Clays are considered as both potential host rocks and sealing materials, among other geological formations and materials, respectively.

The study of a high-level or long-lived nuclear waste disposal concept is necessarily a step-by-step procedure, with two main objectives: on one hand, ensuring a good disposal design, including feasibility of the vaults and their stability during the operating phase with the eventual continuation of the phase of waste retrievability, i.e. on a long period of time not shorter than one hundred years. On the other hand, the objective is to preserve the long-term clay properties with regard to radionuclide behavior, in particular those properties which influence their containment capacity.

Swelling clays as an engineered barrier material have been considered by ANDRA for a long time. Buffer material as an interface between the nuclear waste packages and the geological barrier and tight core of dams in drifts and shafts are the two types of constructions for which the clay conditioning and performance have been investigated. In both of these applications, the understanding of the THM behavior of the swelling clay is required. In the first case the THM load parameters are much more severe and the geochemical imbalances are of greater consequences.

R & D programmes are necessarily integrated into the design process of the disposal concept. The results will allow selection between technical solutions estimated as valuable candidates for validation within the project duration, i.e. 13 years, and those solutions which cannot be. In return the design process indicates what are the more profitable axis for the future R & D efforts.     

Landscaping with waste

For centuries back, man has searched for and utilised geological material resources for many different purposes. These efforts have left many, large wounds or “eyesores” in bedrock and soil strata, that have destroyed the appearance of the original landscape, and which now often serve as waste dumps and scrap sites. Worked-out gravel and sand pits, as well as extinct quarries and open-pit mineral mines, can extend over wide areas of land and reach considerable depths. Depths which subsequently have become particularly attractive as waste storage sites, especially when these offer short transport distances to suppliers in major, built-up areas and communities.

This paper gives some examples as to how Stockholm has replaced worked-out overburden and rock masses from eskers and rock strata with waste masses, simultaneously restoring land formation into its original condition through landscaping. Specific engineering-geological survey methods, control systems, as well as risk and sequel analyses, relating to long-term interplay between rock, soil strata and groundwater, with particular consideration to surface construction and property development planning and isostatic uplift, will be presented.     

Simulation of thermal stress influence on the Boom Clay kerogen (Oligocene,Belgium) in relation to long-term storage of high activity nuclear waste: I. Study of generated soluble compounds

Closed pyrolyses were performed on the Boom Clay kerogen to simulate the weak thermal stress applied during the in situ CERBERUS heating experiment (80 °C for 5 a). Two stronger thermal stresses, encompassing the range generally considered for the long-term disposal of high-activity nuclear waste (80 °C for 1 ka and 120 °C for 3 ka), were also simulated. Quantitative and qualitative studies were carried out on the products thus generated with a focus on the C₁₂₊ fraction, especially on its polar components. It thus appeared that the soluble C₁₂₊ fractions generated during these simulation experiments comprise a wide variety of polar O- and/or N-containing compounds, including carboxylic acids and phenols. The nature and/or the relative abundance of these polar compounds exhibit strong variations, with the extent of the thermal stress, reflecting the primary cracking of different types of structures with different thermal stability and the occurrence of secondary degradation reactions. These observations support the idea that the compounds, generated upon exposure of the Boom Clay kerogen to a low to moderate thermal stress, may affect the effectiveness of the geological barrier upon long-term storage of high-activity nuclear waste.     

Problems of site selection for nuclear waste disposal in a loess-covered hill environment in Hungary

Problems of the long-term storage of nuclear waste produced by the Paks power plant have recently come to the fore in Hungary. After an extensive debate between investors and the local population the decision makers took the side of those having opposed the establishment of the waste disposal site in the initially proposed environment. Several studies have been conducted to support both pros and cons. Although the idea was rejected finally, this debate has proven that comprehensive research prior to decision making is indispensable in similar cases.Regretfully, the academic staff of the GRIHAS was invited to participate in the expertise too late, in 1988, to investigate the geomorphological and social-economic environmental conditions of the proposed site. The latter cannot be neglected since the settlement pattern, demographic structure, character of economic activities, the level and state of the infrastructure — though indirectly — exert a long-term impact on the circumstances of the operation of the site, the living conditions of the people working there and on the use of the environment. Any harmful effect from the secondary wastes to be deposited could endanger the security of the local population, that is why a thorough geographical analysis based on field research in the immediate and wider surroundings of the site is necessary already in the phase of decision preparation.     

核废物地质处置中岩石选用现状及其开发研究

核废物处置已成为当务之急,其目的是要保证人类与环境的安全。普遍采用的方法是地质处置,即保寻安全封闭的天然屏障。为此,岩石的研究就至关重要。目前,各国选用的岩石不一,并不断变化,主要为石盐、粘土和花岗岩。开发研究总趋势是建立地下实验室,进行现场试验。