Peter W. Brennecke, Georg Arens, Almut Hollmann
Bundesamt für Strahlenschutz, Salzgitter, Germany
ABSTRACT
According to the German approach to disposal, all types of radioactive waste have to be disposed of in a repository constructed and operated in deep geological formations only. This especially includes radioactive waste containing transuranic elements and other alpha emitters. Such waste originates in particular from reprocessing of spent fuel elements, basic and applied research and development work as well as from uranium enrichment and fuel element fabrication.
In Germany, the Bundesamt für Strahlenschutz (BfS, Federal Office for Radiation Protection) is legally responsible for waste disposal. For the safety assessment of a repository it is required to demonstrate that the protection objectives of disposal in the operational and post-closure phase of a repository are fulfilled. With respect to possible releases of long-lived radionuclides in the post-closure phase, the barrier functions of the site-specific geological formations have in particular to be investigated and evaluated, among other things, by means of radionuclide-specific sorption and desorption experiments.
Disposal of alpha bearing waste is predominantly envisaged for the Konrad and Gorleben repository projects. The results of the Konrad site-specific safety assessments have been converted into a system of waste acceptance requirements covering the disposal of radioactive waste with transuranic elements and other alpha emitters. As to the emplacement of such waste, an assessment of the disposability shows that all respective waste packages with negligible heat generation can basically be emplaced in the planned Konrad repository. In addition, for the disposal of all types of radioactive waste including alpha bearing waste the Gorleben repository project is still under investigation.
INTRODUCTION
The objective of disposal in repositories is to ensure that radioactive waste is handled and stored in such a way that the protection of man and the environment from harm caused by the ionizing radiation of the waste is guaranteed. Consequently, the planning and construction of a repository must be carried out in such a way that this objective will be met in its operational and post-closure phase. Specific challenges will occur provided that alpha bearing waste is intended to be disposed of. Thus, the very long half-lives and other disposal-related properties of respective radionuclides must be taken into account when demonstrating the observance of the objective of disposal.
GERMAN RADIOACTIVE WASTE DISPOSAL POLICY
In the Federal Republic of Germany, the use of nuclear energy especially started with the operation of the first nuclear power plant in 1960. Since the early sixties, i. e. from its very beginning, the German radioactive waste disposal policy has been based on the decision that all kinds of radioactive waste are to be disposed of in deep geological formations. Thus, vitrified fission product solution from reprocessing and spent fuel elements as well as spent sealed radiation sources and miscellaneous waste from small waste generators are affected by this decision. It also applies to alpha bearing waste originating in particular from reprocessing facilities, nuclear research establishments or the nuclear fuel cycle industry. Near-surface disposal or shallow land burial is not practiced in Germany because of high population density, climatic conditions and existing appropriate deep geological formations.
COMPETENCIES AND RESPONSIBILITIES
The disposal of radioactive waste in a repository is in particular governed by the following specific acts and regulations:
The protection objectives of radioactive waste disposal in a repository are prescribed by the Atomic Energy Act and the Radiation Protection Ordinance. The Federal Mining Act regulates all aspects concerning the operation of a disposal mine. The Safety Criteria specify the measures to be taken in order to achieve the objective of disposal and define the principles by which it must be demonstrated that this objective has been reached.
The peaceful use of nuclear energy in Germany is regulated by the Atomic Energy Act. On September 5, 1976, its Fourth Amendment was enacted. It provided the legal basis for the disposal of radioactive waste. According to section 9 a of this act, the Federal Government has to establish installations for the engineered storage and disposal of radioactive waste, i.e. disposal of radioactive waste is assigned to the Federal Government as a sovereign task. On November 1, 1989, this competency was assigned to the Bundesamt für Strahlenschutz (BfS, Federal Agency for Radiation Protection). Accordingly, the BfS is responsible for the establishment and operation of those federal installations, acting on behalf of the Federal Government.
For the establishment of a repository, pursuant to section 9 b of the Atomic Energy Act, the initiation of a plan-approval procedure, i.e. a special kind of a licensing procedure, has to be applied to the respective licensing authority. BfS is the authorized applicant. It is the objective of the plan-approval procedure to examine a project which is important for the region concerned, weighing and balancing the interests of the body responsible for the project and public and private interests affected by the planning in one procedure and to reach a decision which is legally binding in relation to third parties. The plan-approval procedure includes, among other things, the participation of all authorities concerned and a public hearing. The procedure is terminated by the plan-approval decision, i. e. the license. This decision embraces the so-called integration effect, whereby it replaces all other approvals. With respect to the integration effect, section 9 b of the Atomic Energy Act defines a specific commitment regarding the Federal Mining Act according to which the integration of mining law under atomic law is excluded. Thus, the legal competencies for the licensing of the construction and operation of a repository are regulated in such a way that two procedures must be performed: the procedure under atomic law on the one hand and the procedure under mining law on the other.
SAFETY CRITERIA
The basic aspects which must be taken into account to achieve the objective of disposal are compiled in the 'Safety Criteria for the Disposal of Radioactive Waste in a Mine' (1). Their scope implies all types of radioactive waste to be disposed of. The safety criteria qualitatively specify the measures to be taken in order to achieve the protection objective of disposal and define the principles by which it must be demonstrated that this objective has been reached, i. e. technical measures and methods of procedure are to be adjusted to one another. The importance of the site selection, the system consisting of geology/repository/waste packages, the multibarrier concept, and the use of state-of-the-art technology are emphasized. The following criteria are considered to be the most important ones:
The safety criteria permit a certain latitude of judgement. Such margins gradually diminish in the realization of a repository project. This process is predominantly determined by a site-specific safety assessment within the scope of which the required safety of the repository must quantitatively be demonstrated including the derivation of requirements on the design of the facility as well as on the waste packages to be disposed of.
Nevertheless, the protection objective can only be achieved by an iterative process drawing together more and more detailed information obtained as the respective repository project progresses through its various phases of investigation, planning, detailed design and performance assessment, thus assuming more and more concrete forms.
ALPHA BEARING WASTE
An essential prerequisite for the development of waste management and disposal strategies or the planning and construction of repositories is the provision of a realistic data base. Data on the origin, type and expected amount or radioactive waste are therefore necessary. According to the German disposal policy, this especially includes radioactive waste containing transuranic elements and other alpha emitters.
Origin and Type of Waste
In Germany, alpha bearing waste in particular originates from
The term "alpha bearing waste" covers a wide range of various materials which are quite different according to the nature and quantity of the radionuclides associated with them. The major bulk of such waste consists of a wide variety of solids and liquids contaminated with long-lived alpha emitters. In addition, this waste may contain fission products. Typical waste types (primary wastes) are:
Waste Conditioning
Conditioning of alpha bearing waste includes processing and/or packing of the waste, eventually after a pretreatment or a sorting. Various strategies and techniques are applied. The selection of a conditioning process is dependent upon factors like the requirements for temporary storage and disposal, acceptance of the process, and volume of the resulting waste packages. Furthermore, the necessity to minimise the volume of the conditioned waste because of in former times lacking repository capacity stimulated the development of new and advanced conditioning techniques.
Primary alpha bearing waste must be collected and pretreated in such a way that it is suitable for the selected conditioning process. Principal pretreatment methods are, e.g., decontamination, compression, evaporation or incineration.
Especially the incineration is attractive for all types of combustible waste. Solid or liquid waste including alpha bearing waste is incinerated (2). The cementation of alpha bearing waste is the most well-known immobilization process being widely applied. It is used for the solidification of liquids, the embedding of solids as well as the grouting of voids in scrap, rubble or filters. In addition, bituminization is used. The high-pressure compaction with 1500 Mg to 2000 Mg compactors is an advantageous development to minimize alpha bearing waste amounts. Solid materials are compacted to a stable pellet. This technique is applied to, e.g. metallic materials, paper, plastics, rubble and even ashes from the incineration of organic radioactive waste.
Radioactive waste has to be packed for handling, transportation and storage. The necessary quality of a packaging is dependent on the type of waste and its radionuclide inventory. Sheet steel, reinforced concrete and cast iron are common as packaging material. Cylindrical and box-shaped packages of different sizes and weights are being used. A standardization of packages has successfully been realized in order to harmonize the equipment as well as the repository-related handling and emplacement techniques.
Waste Amounts
According to the latest inquiry into the amounts of radioactive residues and conditioned radioactive waste arising in Germany, the total primary waste volume was about 30,600 m3 on December 31, 1995. Of this, a portion of 21.6% was produced by the nuclear fuel cycle industry, 16.1% by the nuclear research establishments and 0.7% originates from reprocessing. The total volume of conditioned radioactive waste amounted to about 63,000m3 at the end of 1995. Of this, a major contribution was made by the nuclear research establishments (45.0%), whereas smaller contributions were made by reprocessing (18.0%) and the nuclear fuel cycle industry (3.9%). Nevertheless, about 38% and about 67% of the existing radioactive primary waste and conditioned radioactive waste amounts, respectively, may contain considerable alpha emitter concentrations.
Waste Disposal
According to the German disposal concept, all radioactive waste has to be emplaced in a repository constructed and operated in deep geological formations. As liquid and gaseous wastes are excluded from disposal in such a mine, only solid or solidified radioactive waste is accepted. At present, three sites are considered for disposal (3):
According to these planning data and boundary conditions the major bulk of alpha bearing waste is intended to be disposed of in the planned Konrad and Gorleben repositories. The emplacement of such waste in the Morsleben repository is restricted by the permissible alpha emitter activity concentrations being limited to 4 * 108 Bq/m3. Thus, radioactive waste with only minor alpha emitter concentrations can be disposed of in this facility.
SAFETY ASSESSMENTS
In order to demonstrate the safety of the planned Konrad and Gorleben repositories in the operational and post-closure phase, site-specific safety assessments have to be carried out covering the following aspects:
This work has to be based on detailed site-specific geological and hydrogeological data, a sufficiently detailed concept of the repository including the planned mode of operation, and data concerning the types, quantities and properties of the waste packages to be disposed of.
Within the realization of a repository project, among other things, the above-mentioned safety assessments have to address all types of radioactive waste intended for disposal in this facility. According to the German approach, this comprises alpha bearing waste and non alpha bearing waste. Therefore, basic data on both are to be introduced into the site-specific safety assessment. Specific investigations only concentrating on alpha bearing waste or on non alpha bearing waste are neither intended nor performed. Nevertheless, properties of radioactive waste especially containing transuranic elements and other alpha emitters relevant to disposal must particulary be addressed. Thus, key elements to demonstrate the safety to alpha bearing waste disposal are assessments on criticality safety as well as on possible releases of long-lived radionuclides on the water path including their sorption and desorption behavior.
CRITICALITY SAFETY
Generally, criticality calculations depend very much upon the boundary conditions chosen (e.g., supposed scenario, geometry of fissile material zone, reflection and moderation). Thus, in order to simplify the calculations, a model and very conservative boundary condition have been chosen, e. g. a fissile material zone with spherical geometry and a reflector with a strong backscattering effect, with reference to the post-closure phase.
For the planned Konrad repository, it has been investigated within the framework of the comprehensive safety assessment whether critical assemblies in the buffer hall (surface) or in a disposal room (underground) could occur during the operational phase of this facility or whether a criticality incident could be caused in the post-closure phase by water access to the emplaced waste packages and by leaching of the whole fissile material inventory.
Concerning the disposal of spent nuclear fuel, some preliminary criticality investigations have been performed regarding the behavior of fuel element bundles in the cases of direct disposal and disposal of bundles disassembled into single rods. A more detailed criticality analysis will be part of future safety assessments for the planned Gorleben repository.
LIMITATION OF THE PERMISSIBLE MASS
OF FISSILE
MATERIAL
To ensure safety of the planned Konrad repository a mass limitation of fissile materials in the cross-section of a disposal room and per waste package is necessary and has been derived within the site-specific safety assessment (4). As far as uranium is concerned, it is reasonable to examine separately uranium of lower and higher enrichment, that is, less than 5 wt% and more than 5 wt% U-233 or U-235.
Permissible Mass of Fissile Material in the Cross-section of Disposal Room
Conservative boundary conditions were chosen for the assessment of criticality safety during the post-closure phase. For instance, a reflector of normal concrete 30 cm thick, and hence a strong reflector of neutrons, is assumed in the criticality calculations. A sphere and also a hemisphere have been assumed for the shape of the fissile material zone. Only the salt content of the deep underground water, which has the effect of decreasing the reactivity owing to its content of chlorine and which has been determined by measurements, has been considered in the calculations.
Additionally, the calculations are based on selective leaching of the whole of the fissile material from the waste package matrix by water access and its accumulation and concentration in the area of an assumed hollow in the disposal room level.
Assuming a spherical geometry of the fissile material zone, the criticality calculations have shown that the infinite multiplication factor is below 1 in the case of low enriched UO2 with a mass of 13 kg U-235, and in the case of high enriched UO2 with a mass of 2 kg U-235. The relevant mass for low enriched U-233 is 4 kg, for high enriched U-233 1.1 kg, for Pu-241 0.55 kg and for Pu-239 1.1 kg. With regard to the above mentioned boundary conditions, these values are limiting for the masses of wastes to be emplaced in the cross-section of a disposal room (i.e. for a stacking section) and are therefore used to derive requirements on the waste packages.
Permissible Masses of Fissile Material per Waste Package
The volumes of the standardized waste packages which should be used for the disposal of radioactive wastes in the planned Konrad repository vary between 0.7 and 10.9 m3. Hence, the number of waste packages which can be stacked in the cross- section of disposal room depends upon the dimensions of the respective container type.
Thus, taking into account the maximum number or waste packages in the cross-section of a disposal room, the next step is to calculate the permissible masses of fissile material per waste package. Because an exceeding of the permissible concentrations cannot be excluded in individual cases, criticality safety must also be guaranteed if the masses are doubled. This leads to a limitation of the permissible mass of fissile materials to 45 wt% of the smallest critical spherical mass.
CRITICALITY SAFETY CONSIDERATIONS CONCERNING DISPOSAL OF SPENT FUEL
General criticality with regard to the direct disposal of spent fuel elements was examined several years ago. It has been shown that, from the criticality safety point of view, direct disposal is a possible option.
Another option which is part of the disposal strategy for the planned Gorleben repository is the emplacement of rods (i. e. disassembled fuel bundles) packaged in canisters. For preliminary criticality considerations it is assumed that the rods remain structurally intact such that the enclosure of the radioactive material is ensured. A tight package of fuel rods influences the essential parameters for criticality safety. The criticality considerations were restricted to the determination of the infinite multiplication factor; water access to the fuel rods was assumed, conservatively neglecting the fact that the realistic moderator in the repository is a salt solution or brine.
As a result it has been calculated that the fuel elements are more reactive than close packed bundles of fuel rods owing to the near optimal moderator to fuel ratio. Hence, the disassembling of the fuel elements and the tight package of the rods may be favorable in respect of criticality safety.
LONG-TERM SAFETY ASPECTS
Konrad Repository Project
On the basis of an evaluation of the geological and hydrological situation, it is assumed that in the post-closure phase of the planned Konrad repository, formation water (i.e. water originating from the host rock formation) will come in contact with the waste packages. The transition of radionuclides from the waste into the formation water, their migration via the water-path from the repository through the geosphere have been studied as a part of the safety assessment of the post-closure phase. The flow paths and travel times of the groundwater were determined by geohydraulic model calculations. They formed the basis for determining the release of radionuclides from the repository to the biosphere. Important parameters for the water movement in the geosphere are the hydraulic characteristics (hydraulic gradient, hydraulic conductivity and effective porosity) of the individual stratigraphical units. They show large variation due to regional and local differences in the conditions during the formation of the rocks and their diagenesis. Moreover, within an additional hydrogeological model, the co-called fault zone model, zones of higher permeability along important tectonic faults were taken into account in order to evaluate their influence on the water movement and travel times.
Particular attention was given to groundwater measurement results indicating an increase in salinity with depth up to about 220 g/l. Therefore, the flow and radionuclide transport are strongly coupled. This coupling causes significant nonlinearities in the flow and transport equations and poses significant challenges for numerical simulation. As a result, due to the downward increasing salinity, even in the presence of strongly disturbed zones in the geological barrier, there is no negative effect on the long-term safety of the Konrad site.
On the basis of the two hydrogeological models three-dimensional groundwater calculations were performed for a wide range of parameter sets and neglecting the salinity of the groundwater. In addition, two-dimensional groundwater calculations were carried out including density effects. According to the results it was be concluded that water travel times from the planned underground facilities (disposal rooms) of the Konrad repository to the biosphere may vary between 300,000 years and more than 10 million years. The groundwater calculations were supported by analyses of environmental isotopes and noble gases in brines from the Konrad mine. They prove that these brines contain a large fraction of concentrated salt solutions from evaporite formation with halite deposition 150 million years ago or earlier. This indicates a very low exchange of deep groundwater with meteoric water and confirms the conservatism of the calculated minimum groundwater travel time of about 300,000 years. Based on this travel time radiation exposures were calculated for times up to 10 million years.
It is to be expected that the radionuclide inventories for the Konrad repository will be limited despite the proved conservatism of the conceptual model and data set of the safety case and calculated radiation exposures after 300,000 years up to 10 million years after closure. Likely the inventory of I-129 will be limited to 110 kg and the inventory of U-238 to 150 Mg. The licensing authority is justifying these limitations by the calculated organ doses which exceed approximately 50% of the dose limitations. Especially for the latter the acceptable inventory will be low compared with the radioactive natural background in the host rock.
The results of the respective Konrad safety assessments have finally been converted into a system of waste acceptance requirements covering the disposal of radioactive waste with transuranic elements and other alpha emitters (5).
Gorleben Repository Project
Because of its favorable properties rock salt is the main long term barrier of the planned repository in the Gorleben salt dome. In this facility all types of radioactive waste are envisaged to be emplaced, in particular heat generating waste (spent fuel elements, reprocessing waste). Nevertheless, the possible release of radionuclides in the post-closure phase has been investigated in a conservative approach assuming that brines may intrude into the backfilled parts of the repository. Both the creation of pathways for water in anhydrite horizons due to thermomechanical effects caused by the heat generating waste and the existence of brine inclusions in the rock salt were considered.
Possible releases of radionuclides via the water-path are assessed and the respective dose rates calculated. Comprehensive experimental investigations on site-specific samples provide the necessary data base for the performance of a quantitative long term safety assessment for the Gorleben repository project. The sorption and desorption experiments have been focused on the influence of natural groundwater colloids on the sorption and transport behavior of radionuclides, especially on elements such as Np, Pu, Am and Cm. The latest experiments supplement previous investigations, aiming to deepen the understanding of the sorption behavior and the underlying sorption mechanisms (6). The results in general confirm the sorption data of radionuclides and their dependence on the influencing parameters as, e.g., Eh, ph, natural and artificial complexing agents as humic substances and EDTA and colloid formation.
With the completion of the experiments on migration, characteristic sorption data for the relevant elements are now available for 56 sediment and ground water systems. The quantification of chemical variables which affect sorption was made possible by means of a targeted variation of the input parameters. The investigations into the radionuclide sorption for Np, Pu and Am have shown that sorption is generally higher in cohesive sediments compared to sandy sediments, i. e., in the range of orders of magnitude. An influence of humic substances on the sorption characteristics of radionuclides is identifiable, particularly in the sandy systems. Where the content in humic substances increases, sorption by sediments decreases. The redox potential of the sediment and ground water systems constitutes, for Np sorption, the basis of the most significant parameters. Where reductive conditions were present and redox potential was low, a higher level of sorption by the sediment was measured for Np.
In order to complete the migration experiments, microbiological investigations were carried out, in particular aiming at
Since microorganisms were found to be present in very low numbers, these presumably play only a secondary role within the dispersion of radionuclides. Beyond this, due to the non-sterile conditions of the sorption experiments, an influence due to microorganisms was detected.
The results of these investigations contribute to the data base for model calculations still to be performed, aiming at the demonstration of a long term safety of the Gorleben site.
CONCLUDING REMARKS
As to the disposal of alpha bearing waste an assessment of the disposability shows that all respective waste packages with negligible heat generation can basically be emplaced in the planned Konrad repository. In addition, for the disposal of all types of radioactive waste including alpha bearing waste the Gorleben repository project is still under investigation.
REFERENCES