Wim M.G.T. van den Broek
Delft University of Technology, Faculty of Applied Earth Sciences
Mijnbouwstraat 120, 2628 RX Delft, the Netherlands
Phone (31)-15-2786065, Fax (31)-15-2781189
E-mail w.m.g.t.vandenbroek@ta.tudelft.nl

Three techniques are available for disposal of radioactive waste in rock salt: a salt-mine repository for all waste categories, deep boreholes drilled from the surface for the high-level waste and a salt cavity for the low-level waste. The possibilities, advantages and drawbacks of deep boreholes and salt cavities are described and discussed, and the characteristics of these methods are compared with those of a salt-mine repository. Furthermore it is investigated whether disposal of canisters with high-level waste in deep boreholes can be realized in such a way that these canisters are retrievable. Finally the properties of a pressurized salt cavity, with which relatively large depths can be reached, are summarized. It is concluded that both the deep-boreholes option and the salt-cavity option are full-fledged disposal techniques and that, in principle, incorporation of waste retrievability for the deep-boreholes option is feasible.

Concerning underground disposal of radioactive waste in the Netherlands, the attention is almost exclusively focused on rock salt as a geological host medium. Currently also studies are carried out on clay formations, but rock salt remains the most obvious choice. About the national situation it is noted that the amount of radioactive waste which has to be buried or stored in due time is relatively small, because only a few per cent of the national electricity demand originates from nuclear power. To this is added that, since 1993, there is a government regulation stating that radioactive waste, in case it is disposed in the underground, must be buried in such a way that retrieval of the waste remains possible. As it is supposed that waste retrieval can be realized best with a mine repository, currently only disposal in a salt-mine or a clay repository is considered in the national research program, while in the past also other technical solutions were studied. In the neighboring countries (Belgium, France, Germany and the United Kingdom), mine repositories in different host media are considered or under construction; a requirement for retrievability of the waste, however, is currently only effective in France.

It is regrettable that - with the attention exclusively focused on mine repositories - the potential of rock-salt formations for other technical solutions is neglected. It is for this reason that three alternative possibilities for waste disposal in rock-salt formations are subsequently reviewed in this paper: (i) disposal of high-level waste in deep boreholes, (ii) disposal of high-level waste in deep boreholes with the option to retrieve the waste and (iii) disposal of low-level waste in a salt cavity. Prior to this review the salt-mine repository will be briefly discussed.

The lay-out of a salt-mine repository is very similar to that of a conventional mine, constructed for the production of dry salt. Two shafts lead to an underground system of galleries. Two mine levels can be distinguished: an upper level with excavated rooms for accommodating the low-level waste (LLW) and a lower level with dry vertical boreholes for the high-level waste (HLW). The rooms for the LLW have volumes of the order of about 100,000 m³ each, the number of rooms being determined by the total amount of LLW to be buried. In a permanent-disposal scenario the rooms will be completely filled up with LLW-drums together with crushed salt. In the case of waste retrievability a different procedure will be necessary. The dry boreholes for the HLW have a length of at least 100 m each and in a permanent-disposal scenario these are, after they have been provided with HLW-canisters, sealed with a salt plug. Because of the heat generation of the HLW, the mutual distances of the boreholes must not be too small, so that the temperature rise in the salt remains limited. In the case of waste retrievability the design has to be adapted, e.g. in the sense that the boreholes are provided with a casing. For more detailed information on the salt-mine repository we refer to the Van Hattum en Blankevoort report (1) and Prij (2) for the permanant-disposal option and to Van den Broek et al. (3) for the implications of waste retrievability.

The pressure inside a salt-mine repository is atmospheric. Consequently, with increasing depth, the salt-convergence rate increases and this limits the attainable depth to less than 1000 m. Salt formations can be roughly divided into: salt domes, salt pillows and salt layers. Normally the top of the formation will be lower for a pillow than for a dome, with the top of a layer at an even larger depth. Hence, with respect to salt-formation type, the choice for the salt-mine repository is limited and this explains why the suggested location for a salt-mine repository is usually a salt dome.

In the Van Hattum en Blankevoort report (1) a procedure is proposed for the disposal of HLW-canisters in deep boreholes, drilled from the surface of the earth. In concise form this procedure is as follows:

• Boreholes are drilled to the desired depth, which was limited in the mentioned report to 1500 m. While drilling in the salt formation, the drilling fluid must contain a high concentration of dissolved salt to prevent dissolution of salt around the borehole. Simultaneously with the drilling operations a casing string has to be installed down to about 50 m below the top of the salt formation.
• HLW-canisters are lowered into the borehole. A possible way to realize this is by simply allowing them to sink in the borehole. The presence of the borehole fluid ensures that the downward canister velocities will be small. After emplacement of a number of canisters, the fluid around the canisters is replaced by cement, which is subsequently allowed to harden. Then a next batch of canisters is brought in, cement is brought in, etc. A difference with emplacement in boreholes in the salt-mine repository is, that the canisters will be provided with an overpack for extra protection during emplacement.
• After the filling operation a 10 m concrete plug is installed on top of the canisters, and then the borehole fluid will be removed. The sealing of the borehole between the last canister and the casing shoe can be realized with crushed salt, concrete or bitumen or with a combination of these materials.

A few years ago Van den Broek et al. (4) carried out a study on new developments concerning disposal of radioactive waste in deep boreholes and in a salt cavity. The results of this study give rise to some comments on the procedure as given above and on disposal of waste in deep boreholes in general:

Drilling of boreholes for the oil and gas industry, also in salt formations, has been done numerous times. Concerning the attainable depth, factors such as drilling-fluid density, formation strength and casing-installation scheme play a role. The depth limit is certainly much lower than 1500 m; even at a depth of 3 km no technical difficulties are to be expected. Consequently, for deep-boreholes disposal there are few limitations concerning use of different salt-formation types.

In the last couple of years many developments on drilling technology took place. Directional and horizontal drilling are extensively practiced nowadays. For waste disposal in deep boreholes this yields extra options in the sense that spreading of the boreholes by using configurations with directionally drilled holes is perfectly feasible. Use of horizontal or nearly horizontal boreholes in these configurations, however, is not advisable because downward transport of the canisters is in these cases not assisted by the gravity force.

An often-practiced technique in the oil and gas industry is the removal of a part of the casing over a specific height by milling, and transportation of the cuttings to the surface via circulation of the borehole fluid. At the same time a part or all of the cement at the outside of the casing is also removed. In the case of disposal of HLW-canisters, this procedure gives rise to the option to provide the borehole with casing (should this be considered necessary for the stability or integrity of the borehole), and to remove the casing, or a part of it, prior to the actual disposal of the canisters. Consequently, it is possible to make use of the favorable properties of a cased borehole, but with the result that in the disposal section no (or relatively little) casing material is present between the waste canisters and the salt. In Fig. 1 the results of a milling procedure are sketched. In the milled-away section e.g. a sealing plug can be installed.

For abandonment of boreholes in the oil and gas industry, a frequently applied sealing method is emplacement of three cement plugs, with a length of about 150 m each, in those parts of the borehole near which impermeable layers (e.g. shale) are present. To ensure contact between such layers and the cement plugs, the casing is milled away over the borehole sections in question. In the rest of the borehole, borehole fluid is left in place: this has the advantage of giving extra (hydrostatic) pressure, which would not be present in case the rest of the borehole was filled with solid material.

In the Van Hattum en Blankevoort report (1) also the costs of different disposal methods were investigated. The outcome was influenced by parameters as e.g. waste amounts, but in general the combination of deep boreholes and a salt cavity was economically more attractive than a salt-mine repository. The outcome for relatively small amounts of waste will be even more unfavorable for the salt-mine repository, because of the high costs of the basic facilities necessary for a repository (especially the shafts).

Summarizing on HLW-diposal in deep boreholes we note that this method has a number of important advantages, of which are mentioned: ability to reach much larger depths than the salt-mine repository, reliability because of the very extensive experience with drilling/casing operations gained in the oil and gas industry, and economically attractive, in particular for relatively small amounts of HLW.

Up till now, disposal in deep boreholes has always been considered only for permanent disposal. In this paragraph it will be tried to investigate the feasibility of disposal in deep boreholes with the option to retrieve the waste. An essential requirement for retrieval is a sufficient degree of accessibility of the HLW-canisters for a certain time period. The only technical measure which can be taken to realize this objective is to install a casing not only above the disposal region but over the entire length of the borehole. Furthermore the disposal length will have to be much shorter than in the case of permanent disposal. This requirement is connected with the following consideration. In the borehole the canisters are stacked, and the first installed canister will have to carry the combined weight of the other canisters. Information on the strength of HLW-canisters yielded, that one canister is expected to be able to take the weight of 19 canisters, which leads to a total number of 20 canisters per borehole (5). The mentioned figure is not completely reliable, and this is one of the reasons that providing each canister with a sufficiently strong overpack is thought to be a necessity. The function of this overpack is threefold: canister protection during emplacement, canister protection during retrieval and protection against the combined weight of canisters installed later on. The overpack will have to be provided with a fishing neck (6), see Fig. 2, so that emplacement and retrieval of the canisters can be realized with normal fishing practices as used in the oil and gas industry. On the actual emplacement of the canisters it is noted that uncontrolled sinking in the borehole fluid seems less attractive than controlled sinking with the help of a wireline. After emplacement the borehole will be closed with a removable plug. Retrieval of the canisters can be realized by carrying out the emplacement operation in a reversed order.

The philosophy on retrievability is, that the possibility to be able to retrieve the waste for a certain time period is thought to be a benificial technical measure providing a number of advantages. The ultimate aim, however, remains permanent disposal. Consequently, at some point of time it will be decided to refrain from retrieval of the waste. Then the borehole can be closed in a more thorough manner. After removal of the plug, the space around the canisters in the borehole will be filled with cement. For borehole abandonment a number of possibilities are thinkable, e.g.:

• Emplacement of a limited number of sealing (cement) plugs in the cased borehole.
• Milling away of the casing over some tens of meters in that part of the borehole between the top of disposal region and a point of, say, 50 m below the top of the salt formation, circulating out the debris with drilling fluid and emplacing a plug (crushed salt, concrete or cement) directly in the salt formation and other seals higher in the borehole, in the manner already described in the previous paragraph. It is noted that then a situation is reached which is similar to that for permanent disposal in deep boreholes, the two differences being (i) less canisters per borehole and (ii) the presence of casing material over the length of the disposal region.

Summarizing on HLW-disposal in deep boreholes with the option to retrieve the waste, it appears that the technique of disposal in deep boreholes may be adapted in such a way that retrieval of the HLW-canisters remains possible during a certain time period. Ultimately a situation in the underground very similar to permanent disposal in deep boreholes will be reached. From an economic point of view the option of waste retrievability will have negative consequences, mainly because shorter borehole lengths will lead to a larger number of boreholes (however, also in the case of the salt-mine repository a retrievability requirement will give rise to much higher costs). The manner in which waste retrievability is realized is only roughly indicated, and details have still to be worked out.

The technique of disposal of LLW in a salt cavity was described in 1986 in the Van Hattum en Blankevoort report (1). Here two possibilities for the initial cavity contents were distinguished: brine and air at atmospheric pressure. In 1990 a design for disposal of solid chemical waste products was presented by Crotogino (7), in which only a cavity initially filled with atmospheric air was considered. We note that it is not advisable to dispose waste in a brine-filled cavity as, due to long-term pressure effects, the permeability of the salt roof may increase, as was discovered by Kenter et al. (8). At first instance this leaves only the salt cavity at atmospheric pressure as a technical possibility, and this option does not have prominent technical advantages over the salt-mine repository, mainly because there is no significant difference with respect to the maximum depth of about 1000 m which can be reached. Concerning the complete disposal operation we mention the following phases:

• Leaching of the salt cavity.
• Removal of the cavity brine.
• Chemical binding of the remaining brine at the cavity bottom.
• Lowering of the LLW drums etc. until the cavity is completely filled up.
• Sealing of the cavity neck with materials such as bitumen, (wet) crushed salt and/or salt concrete.
• Sealing of the cased borehole.

The maximum depth of the cavity can be increased with a simple technical measure: increasing the gas pressure in the cavity to above atmospheric. Then depths down to about 2000 m are feasible, as was shown in earlier Waste Management papers (9,10). An obvious disadvantage of this design is that difficulties are introduced with respect to the filling-up of the cavity with waste. The main advantage of using a pressurized salt cavity, however, is the following. Once the cavity is filled up with waste, the gas pressure in the cavity can be lowered, ultimately to atmospheric pressure. Then there is a large difference between the lithostatic pressure and the cavity pressure, leading to a relatively high cavity-convergence rate. During this process the cavity contents will be gradually taken up in the salt environment, leading to accelerated isolation and encapsulation of the waste. Fig. 3 gives a sketch of the development of the cavity volume during the different phases.

Summarizing on LLW-disposal in a salt cavity we note that the atmospheric cavity has few advantages in comparison with the salt-mine repository. With the pressurized cavity, however, relatively large depths can be reached and this design offers very favorable isolation characteristics. A disadvantage is the cumbersome way in which cavity filling will have to be carried out.

As an alternative for the salt-mine repository for disposal of HLW and LLW, two techniques are available: deep boreholes drilled from the surface for the HLW and a salt cavity for the LLW. Concerning the deep boreholes there are a number of advantages which ensure that this technique is at least equivalent to the salt-mine repository. Of these advantages we mention the much larger depth which can be reached. On sealing we note that, in the oil and gas industry, sealing methods are used which can also be applied for deep boreholes filled with HLW-canisters. To this is added that one of the reasons for the diminished interest in deep boreholes is the current attention for waste retrievability. However, in this paper we have shown that, in principle, retrieval of HLW-canisters from deep boreholes can be realized, provided that some extra technical measures are taken, viz.: a limited amount of waste canisters per borehole, use of canister overpacks with fishing necks and installation of a casing string over the entire length of the borehole.

Also for disposal of LLW in a salt cavity is valid that we have to do with a method with possibilities which are at least equivalent to those of a salt-mine repository. In case pressurization of the cavity during the waste-emplacement phase is considered, these possibilities are even greater, because much larger depths can be reached than for a salt-mine repository. It is admitted, however, that incorporation of waste retrievability in a design for waste disposal in a salt cavity is not possible. On this subject we give the following comments:

• At this moment it is not clear whether a retrievability requirement as valid in the Netherlands is applicable for both HLW and LLW or only for HLW. Therefore the future use of a salt cavity in the national situation cannot be ruled out at this moment.
• In countries where waste retrieval is obligatory (e.g. the U.S.A.), such requirement is only valid for HLW. Consequently, the question can be raised whether it is wise or meaningful to have such a requirement for LLW.

The main conclusion of this paper is, that deep boreholes and salt cavities have many favorable characteristics and are full-fledged disposal techniques. It is therefore recommended that the possibilities of these techniques should be investigated further.

1. VAN HATTUM EN BLANKEVOORT, "Location-Independent Study Concerning the Construction, Management and Sealing of Possible Facilities for the Permanent Disposal of Radioactive Waste in Rock-Salt Formations in the Netherlands", Report Koninklijke Volker Stevin, Beverwijk, 1986 (in Dutch).
2. J. PRIJ, "On the Design of a Radioactive Waste Repository", PhD-thesis, Technical University Twente, Enschede, 1991.
3. W.M.G.T. VAN DEN BROEK, H.C. HEILBRON and M.J.V. MENKEN, "Feasibility of Retrieval of Radioactive Waste from a Salt-Mine Repository: an Overview", Geologie en Mijnbouw, Vol. 75, 1-10, 1996.
4. W.M.G.T. VAN DEN BROEK, M.J.V.MENKEN and D.G.M. VAN OERS, "Aspects of the Deep-Boreholes/Cavity Option" (final report of OPLA project MIJBO-4A/52610), Delft University of Technology, Faculty of Applied Earth Sciences, 1993.
5. W.M.G.T. VAN DEN BROEK, M.J.V. MENKEN, H.C. HEILBRON, J.P.A. ROEST, D.G.M. VAN OERS and H.H. MEIJER, "Retrievability of Radioactive Waste" (final report of OPLA project MIJBO-1A/54595), Delft University of Technology, Faculty of Applied Earth Sciences, 1993.
6. H. RABIA, "Oilwell Drilling Engineering: Principles & Practice", Graham & Trotham, London, 1985.
7. F. CROTOGINO, "Technical Concept for a Hazardous Waste Cavern in Salt in Accordance with the German Regulations on Hazardous Waste", Proceedings Solution Mining Research Institute (SMRI) Fall Meeting, Paris, 1990.
8. C.J. KENTER, S.J. DOIG, H.P. ROGAAR, P.A. FOKKER and D.R. DAVIES, "Diffusion of Brine through Rock Salt Roof of Caverns", Proceedings SMRI Fall Meeting, Paris, 1990.
9. W.M.G.T. VAN DEN BROEK and A.G.M.T.J. MOONEN, "Feasibility of Radioactive Waste Disposal in a Salt Cavity at Medium Depth", Proceedings Symposium on Waste Management, Tucson, Vol. 1, 593-596, 1994.
10. R.J. HAARSMA and W.M.G.T. VAN DEN BROEK, "Convergence Behavior of Pressurized Salt Cavities", Proceedings (CD-ROM) Symposium on Waste Management, Tucson, 1995.

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