However, in several other countries, other disposal options have been considered. One of these considerations is the disposal of spent fuel elements in “deep boreholes”. Within the scope of the research project CREATIEF (Investigations on the opportunities and risks of the disposal of heat-generating radioactive waste and spent fuel elements in deep boreholes, funded by the Federal Ministry for Economic Affairs and Energy, project numbers: 02E11526A and 02E11526B), “deep boreholes” were considered to be boreholes with a depth of 3,000 to 5,000 m from ground level where the emplacement area is located in crystalline rock (crystalline bedrock, Figure 1).
This article gives an overview of the results of the research project CREATIEF. He has undergone a peer review process.
1 Aim of the research project CREATIEF
The disposal of high-level heat-generating waste and/or spent fuel elements in boreholes with a depth of 3,000 m to 5,000 m is a disposal option that the German Commission on the Storage of High-Level Radioactive Waste (Endlagerkommission) broached due to the discussion that has been underway in the USA in recent years (1). Preliminary considerations and/or concepts regarding deep borehole disposal exist in Sweden and the USA and have been made in Germany by Gesellschaft für Anlagen- und Reaktorsicherheit (GRS). Within the scope of the research project CREATIEF, the option “disposal in deep boreholes” was studied further with the following three aims:
- analysis and description of the assumptions made and boundary conditions used in previous research reports/studies;
- description of the key aspects of deep borehole disposal and illustration of potential for improvement; and
- conceptual assessment of the opportunities and risks of deep borehole disposal.
2 Legal requirements
First, the concepts for disposal in deep boreholes developed in Sweden by Svensk Kärnbränslehantering AB (SKB), in the USA by Sandia National Laboratories (SNL), and in Germany by GRS were described and critically assessed. Furthermore, the requirements stipulated in the relevant legal regulations in Germany (2, 3, 4 etc.) were listed and the extent to which disposal of radioactive waste in deep boreholes can meet these requirements was presented. However, a detailed study if and how far specific boundary conditions of disposal in deep boreholes comply with the current legal provisions and requirements is feasible to only a limited extent as these provisions refer to disposal in a mine. Thus, these legal requirements would have to be revised or redrafted in such a way that they also apply to disposal in deep boreholes. For detailed information, please refer to the research report (5).
3 Assumptions about the geological conditions
Within the scope of this research project, two geologic profiles were derived, which served as references for geologic conditions that may be considered as candidates for final disposal in Germany (Figure 2). Disposal takes place at depths between 3,000 and 5,000 m in presumably fractured but stable crystalline rock. Just like for disposal in mines, it has to be demonstrated that radionuclides are not able to migrate from the disposal area at all or only in negligible amounts. Thus, when it comes to disposal in deep boreholes, the existence of a containment providing rock zone (CRZ) has to be demonstrated as well. As disposal takes place in probably fractured, crystalline rock, a CRZ can presumably only be accounted for in the form of a superimposing clay or rock salt layer (Type Bb according to the report of the Arbeitskreis Auswahlverfahren Endlagerstandorte (AkEnd), Working Group Selection Procedure for Repository Sites (6)). Even though these layers have a lower stability, they are considered for final disposal in, e. g., France and Switzerland, because of their tightness. The clay or salt layer must be so widespread (Figure 1) that even with circulation no or only negligible amounts of radionuclides can escape during the reference period of 1 million years or, that they, in the case of Type Bb as defined by AkEnd, circulate the more flatly spread area of the CRZ. Especially in the area of the clay or salt layer, borehole seals have to be installed (Figure 1).
4 The state of conventional deep drilling technology
After the geologic profiles for Germany had been developed, the state of the art in the field of conventional deep drilling in the oil and gas industry for drillings to depths of 5,000 m with as wide a diameter as possible was investigated. In crystalline rock, boreholes can currently be drilled down to 5,000 m with diameters up to 17.5″ (44.5 cm). For larger boreholes, larger roller bits would have to be developed. Alternatively, drilling techniques in hard rock, e. g., the electric impulse method (7), would have to be developed or further developed. It is assumed that a borehole diameter of about 35.4″ (90 cm), as described as expedient in the GRS report (8), would reduce the number of boreholes to an acceptable amount (presumably 31). However, such large borehole diameters up to a depth of 5,000 m are currently not technically feasible. Adapting deep drilling equipment for drilling in hard rock and for diameters considerably larger than 17.5″ (44.5 cm) as necessary for disposal boreholes with depths of 5,000 m would require considerable developmental and testing work. The particular challenges are to provide the large-sized bit with the necessary contact pressure (drill rod design), to continuously clean the cuttings from the borehole (capacity of the pumps), to manage the heavy drill string (development and engineering of a special deep drilling rig), and to develop a well design that can cope with a minimal drilling diameter in the first drilling section (lean casing drilling (9) or mono bore method (10)).
As far as the state of the art in deep drilling technology is concerned, it can be said that almost all previous developments aimed at developing and exploiting oil and gas deposits. Oil and gas can usually be found in the pores of sedimentary rock. The boreholes are optimized in such a way that – taking into account the high safety requirements – the costs are kept low, while at the same time, maximum extraction of the raw materials can take place without damaging the deposit. Typical final diameters in oil and gas drilling are thus between 4 and 8½”, i. e., approximately 10.2 cm to 21.6 cm.
For disposal in deep boreholes, all drilling work is finished before radioactive material is delivered for emplacement. Thus, the borehole can be drilled and inspected without radiation protection restrictions. Not until the quality of the borehole has been confirmed is the borehole approved for disposal. The emplacement building will then be erected above the drilling base. After this the radioactive waste will be delivered.
5 Disposal container
Based on the waste amounts to be disposed of which consists of
- spent fuel of nuclear power plants;
- spent fuel of prototype and research reactors;
- vitrified waste from reprocessing of spent fuel of nuclear power plants; and
- structural components of spent fuel of nuclear power plants
taken from the National Waste Management Plan 2022 (11) – the disposal containers were considered. Regarding disposal at depths between 3,000 and 5,000 m, the particular requirements for disposal containers are related to
- robustness against all possible loads;
- temperature resistance;
- resistance to the drilling fluid in the borehole; and
- requirements regarding the dimensions of the container due to the borehole diameter.
In the disposal area, temperatures between approximately 100 and 160 °C are to be expected. On account of the system, the temperature limit of 100 °C at the container surface as currently stipulated in the Standortauswahlgesetz (2) cannot be met in deep borehole disposal. The pressure on the container as considered in the research project CREATIEF is a result of the load of the stacked containers (Figure 1) and the hydrostatic pressure of the liquid column (during operation, the borehole must be filled with fluid for stability reasons). The rock pressure is not taken into account in this concept, as it was assumed that the borehole casing together with the fluid-filled borehole will withstand the rock pressure until the borehole seal is fully functional. Based on the requirements listed above, the boundary conditions for a container were derived and used to roughly calculate the dimensions of the container. According to the calculations, the containers for a 17.5″ borehole will have an outer diameter of 26.5 cm and an inner diameter of 17.5 cm. For a 35.4″ borehole, the outer diameters will be 63.5 cm, the inner diameters 43.5 cm (Figure 3). Based on data found regarding steel corrosion, the use of K2CO3 or of a Na2CO3 solution as drilling fluid is to be preferred over fluids containing chloride or bromide. Austenitic steel type -X6CrNiMoTi17-12-2 is one example of material that is recommended for the container.
6 Number of boreholes versus borehole diameter
Two concepts were considered for the disposal in deep boreholes. They are described below.
6.1 Concept 1: Borehole diameter 17.5″ (44.5 cm) at a depth of 5,000 m
Taking into account a maximum borehole diameter of 17.5″, the container may only have an outer diameter of 26.5 cm (inner diameter 17.5 cm) due to the necessary borehole casing and the required annular space (Figures 3, 4). The length of the container was assumed to be 5.6 m based on the length of the fuel rods of spent fuel elements from power reactors. If the inner diameter is used to 70 to 80 % to store fuel rods, the number of containers is expected to be between 23,000 and 27,000. If one borehole is filled with 180 containers, approximately 130 to 150 boreholes would be required. The containers described above can be stacked to a height of approximately 1,000 m. A larger stack height, e. g. 2,000 m, was not possible in the design, as this would have led to an excessive, meaningless wall thickness in relation to the inner diameter of the container. Here, there is potential for further optimization.
The advantage of concept 1 is that a further development of the drilling technology is not necessary and that the state of the art in deep drilling technology can be used. However, concept 1 has the disadvantage that a relatively large number of boreholes is required. Furthermore, concept 1 cannot accommodate the radioactive waste from reprocessing, which is already vitrified, as this would require that the containers have an inner diameter of at least 43 cm. Thus, only the fuel rods of spent fuel elements from power reactors could be emplaced and an additional repository for the waste from reprocessing would be required.
6.2 Concept 2: Borehole diameter 35.4″ (90 cm) at a depth of 5,000 m
For concept 2, a borehole diameter of 35.4″ at a depth of 5,000 m was assumed (8). Taking into account the thickness of the borehole casing and the necessary annular space, the container can have an outer diameter of 63.5 cm (inner diameter: 43.5 cm) and can be stacked to a height of 2,000 m. The length of the container is 5.6 m. Based on these assumptions, a total of 11,000 containers would be required. If 363 containers were emplaced per borehole, 31 boreholes would be required.
The advantage of concept 2 is that with 31 holes, the number of boreholes is considerably lower than in concept 1. The disadvantage is that without considerable further developments in the deep drilling equipment, this concept cannot be implemented.
7 Containment providing rock zone
In the case of disposal in deep boreholes, the existence of a CRZ has to be demonstrated just like in the case of disposal in a mine. As emplacement takes place in crystalline rock, which is very likely fractured, an overlying sealing layer of clay or salt is necessary (Figures 1, 2). This CRZ corresponds to Type Bb according to AkEnd. How the existence of a Type Bb CRZ is to be demonstrated – mathematically/through exploration – is yet unclear. This applies to both disposal in a mine and disposal in deep boreholes.
8 Safety and safety demonstration concept
A safety and safety demonstration concept for the disposal in deep boreholes was developed to some extent; i. e. reference was made to the concept of GRS presented in (8). The safety requirements issued by BMU in 2010 (3) stipulate the general protection goals and safety principles. The safety concept describes how the safe enclosure of the radioactive waste is to be ensured for one million year in the case of disposal in deep boreholes. To ensure enclosure of the radioactive waste in a defined rock zone as far as possible, the following requirements are defined:
- The radioactive waste must be contained in this rock zone in such a way that it remains in situ and, at best, only minimal quantities of substances are able to exit this rock zone (containment efficacy).
- The geologic barrier in conjunction with the technical seals has to ensure enclosure of the radioactive waste.
- Any pore water present in the CRZ may not participate in the hydrological cycle outside of the CRZ.
- The integrity of the CRZ has to remain intact throughout the reference period of 1 million years.
The safety demonstration concept describes how the requirements stipulated in the safety concept are verified. Concerning Type Bb CRZ, evidence concerning the following points is to be provided or the following points are to be assessed:
- thickness of the salt/clay barrier;
- integrity of the geological barrier;
- integrity and effectiveness of the geotechnical seals (qualified sealing in the clay and salt layer area);
- exclusion of criticality;
- containment of the radionuclides in and/or below the CRZ;
- analysis of the radiological consequences in the biosphere in case radionuclides are released;
- human intrusion scenarios.
9 Emplacement and retrieval concept
Furthermore, an emplacement and retrieval concept was developed. After the borehole has been completed, the drilling rig can be removed. Then, an emplacement plant is assembled on the surface. The emplacement plant has to include an emplacement device with shielding feature (lock) above the borehole. The disposal container is transported to the borehole inside a transfer cask. There, it is placed above the emplacement borehole in vertical position (Figure 5).
Throughout the entire process, the shielding device protects the personnel against radiation. The disposal container can then be emplaced in the borehole by means of rods or ropes made of steel or fibers (Figure 5). The processes related to retrieval are similar. While retrieval during the operating period of a borehole is considered to be possible in this concept, recovery of the disposal containers does not seem possible according to current information. Waste recovery means the retrieval of the waste after sealing and closure of the borehole. The period of time in which recovery is to be possible under current legal requirements is 500 years. Thus, it should be legally reassessed if the recovery requirement for the disposal in deep boreholes can be given up in the rules and regulations.
10 Sealing and closure of the boreholes
Concerning the backfilling and closure of the emplacement boreholes, various possible materials have been listed in the research paper (5). So far, concept studies – e. g., (12, 13) – have mentioned bentonite, bitumen/asphalt, cement as well as salt suspensions and eutectic molten salt, and barite as backfilling or sealing materials. First considerations about how to feed the material into the boreholes or voids have been presented. However, all these technologies still need to be developed and tested with regard to the special conditions in repository boreholes. There are basically two options for installing the backfill materials:
- The backfill materials are installed in the borehole fluid.
- The borehole is pumped dry and the backfill materials are installed dry. Whether this possibility exists depends on the dimensioning of the casing.
11 Opportunities and risks of disposal in deep boreholes
Subsequently, the opportunities and risks of disposal in deep boreholes were assessed. Some of them are described below.
The drilling fluid in the borehole has a shielding effect against radionuclides and a decelerating effect when the containers are emplaced, both of which are presumably positive for disposal in deep boreholes. Thus, it is allocated to the “Chances” category. Compared with a repository mine, the total volume of voids to be excavated is much lower. In addition, a lower damage zone is assumed. Furthermore, sealing of the individual boreholes can start directly after the containers have been emplaced and the sealing zone is longer than in a repository mine. In this case, it may well be possible to insert the borehole seal into a dry borehole. Drilling a borehole with a final diameter of 17.5″ is feasible with the state of the art of deep drilling technology. In addition, the development of a technology for the sinking of boreholes with a larger diameter of up to 90 cm at depths of up to 5,000 m in crystalline rock is not excluded. However, this development would have to be funded by public resources, as there is no direct need for such a technology within the raw materials industry.
However, the assessment of the opportunities and risks shows significant risks for the disposal in deep boreholes. This includes the fact that, according to the current state of the art, not all heat-generating waste can be emplaced in a 17.5″ (44.5 cm) borehole. The disposal of canisters (diameter: 43 cm) with waste from reprocessing (CSD-C, CSD-B and CSD-V) is not possible in deep boreholes with a final diameter of 17.5″. Accordingly, a repository, e. g. in a mine, must also be built. The permanent presence of the drilling fluid, which must be selected specifically for the casing used and the repository containers, necessitates the choice of suitable corrosive-resistant container materials and the permanent tightness of all containers in a borehole until the borehole is sealed. Other risk topics like retrieval during the operating period or disaster management require considerable research and development effort, the result of which is open. In addition, for the safety assessment of deep borehole disposal, temperature or criticality calculations are considered to be urgently necessary. As yet, no studies have been published.
11.3 Results of the opportunities/risks reflection
From the consideration of the opportunities and risks, no statement can currently be made as to whether a final disposal in deep boreholes is a sensible alternative to final disposal in a mine and thus remains worth further investigating. There are too many open issues that would require very extensive and time-consuming research and development work. In addition, there are two points, the solution of which represents a very considerable challenge in each case and whose solution is considered rather unlikely by the project team even with generous financial support in the coming years. These two points concern:
- The demonstration of containment providing rock zone Type Bb according to the AkEnd report (extensive overlying salt layer/clay layer): For the disposal of radioactive waste at depths between 3,000 and 5,000 m in crystalline bedrock, it must be demonstrated that the bedrock is covered by an extensive, sufficiently thick salt rock or claystone layer that is impermeable enough that no groundwater that has been in contact with the waste can reach the biosphere or comes into contact with the aquifers of the cap rock or – if the latter has happened – is so diluted that the contamination remains below a fixed limit value for a period of one million years (reference period). When all these requirements are met, the overlying salt or clay layer is a containment providing rock zone Type Bb according to the AkEnd report. A characteristic of Type Bb is that the disposal area is not completely surrounded by the containment providing rock zone. Currently, there are no scientific publications or research projects that point out a way how the existence of a Type Bb could be demonstrated. The project team greatly doubts that such demonstrations may ever be carried out, as considerably more exact investigations would have to be carried out regarding the permeability of the subsoil over an extensive area around the disposal area than are required for disposal in a Type A CRZ (where the CRZ encloses the entire disposal area).
- Recovery of the high-level radioactive waste from deep boreholes during a period of up to 500 years after repository closure: According to the current debate and to the legal provisions, it must be possible to recover the containers with high-level radioactive waste and spend fuel elements for a period of 500 years after repository closure. To be able to do so, recovery techniques need to be developed that are able to recover a container from a depth of, e. g. 4,500 m, where the container is possibly deformed due to the rock pressure that acted on it for 500 years. The project team greatly doubts that the development of such recovery techniques may ever be possible even with generous financial support.
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