LLW/ILW DISPOSAL IN THE MORSLEBEN REPOSITORY

Peter W. Brennecke
Bundesamt für Strahlenschutz
Salzgitter, Germany

ABSTRACT

In the former German Democratic Republic short-lived low and intermediate level radioactive waste with rather low concentrations of alpha emitters originating from the production and application of radioisotopes in research, medicine and industry was disposed of in the Morsleben repository, an abandoned salt mine. Until 1991, a total volume of solid and solidified radioactive waste of about 14,500 m³ (5.1 * 10⁵ ft³) and about 6,200 spent sealed radiation sources were disposed of. In total, an activity of 1.8 * 10¹⁴ Bq was emplaced (as of 1991). Subsequent to German unity in October 1990, the Morsleben facility has the status of a federal repository in the sense of section 9a (3) of the German Atomic Energy Law. The operational license is limited by law until June 30, 2000. Resumption of emplacement operations took place in January 1994. Until June 30, 2000, according to present plannings, a waste volume of about 40,000 m³ (1.4 * 10⁶ ft³) is envisaged to be disposed of. According to the waste acceptance requirements, a maximum activity of alpha emitters was estimated to be about 10¹³Bq, that of beta/gamma emitters to be about 10¹⁶Bq; the activities actually to be expected until June 30, 2000, are lower in the order of up to two magnitudes. The operation of the Morsleben repository is an important step in the final realization of a proper radioactive waste management system in the Federal Republic of Germany.

INTRODUCTION

Since the early sixties the radioactive waste disposal policy in the Federal Republic of Germany has been based on the decision that all types of radioactive waste are to be disposed of in deep geological formations. The Bundesamt für Strahlenschutz (BfS, Federal Office for Radiation Protection), among other things, is legally responsible for the establishment and operation of federal installations for radioactive waste disposal.

According to this approach, BfS performs planning and site investigation work for the repository projects Konrad and Gorleben and acts as applicant in the respective licensing procedures (1). Subsequent to German unity in October 1990, BfS became the licensee for the Morsleben repository. This facility was operated in the former German Democratic Republic. It now has the status of a federal repository and is the only one presently being in operation in Germany.

THE MORSLEBEN REPOSITORY

In the former German Democratic Republic short-lived low and intermediate level radioactive waste with rather low concentrations of alpha emitters originating from the operation of nuclear power plants and the application of radionuclides in research, medicine and industry was disposed of in the Endlager für radioaktive Abfälle Morsleben (ERAM, Morsleben repository for radioactive waste). This facility is an abandoned salt mine, located near the village of Morsleben, north of the highway from Braunschweig to Berlin, just at the borderline of the federal states Niedersachsen and Sachsen-Anhalt.

Selection of an Abandoned Salt Mine

Until 1969, this mine operated by one shaft had produced potash and rock salt. The salt was excavated in room and pillar mining at four levels in a depth between 385.5 m (1,265 ft) and 506m (1,660 ft) so that mine openings resulted with a maximum length of about 100m (328 ft) and a maximum width and height of about 30m (98ft), respectively. In total, a volume of about 7.6 * 10⁶ m³ (2.7 * 10⁸ ft³) was excavated. Of this, about 5.5 * 10⁶ m³ (1.9 * 10⁸ ft³) are still accessible, i. e. not backfilled.

In parallel to the start of operation of the first East German nuclear power plant at Rheinsberg in 1966, radioactive waste management work was initiated. By the end of the sixties, various concepts were considered for managing radioactive waste originating from the operation of this and additionally planned nuclear power plants as well as from the production and application of radioisotopes. The favored option was the construction and operation of a central disposal facility (2). In the beginning of the seventies, the abandoned salt mine Bartensleben, located near the village of Morsleben, was selected from 10 salt mines considered as potential repositories, after appropriate site and suitability investigations. Apart from safety-related aspects, this decision was influenced by the available volume of excavated cavities, the connection with the shaft Marie in a neighboring village and the possibility of excavating new disposal rooms.

Subsequent to the site license in 1972, the license to construct the repository and to perform research and development investigations for the necessary transport, handling and emplacement technologies was granted in 1974 to the Volkseigenes Kombinat Kernkraftwerke Bruno Leuschner in Greifswald. Thus, test operation of the repository started in 1978. After a period of commissioning, the first operational license was granted in 1981. The second license for continuous operation was issued on April 22, 1986; this license is still effective.

Emplacement Techniques and Waste Amounts

Since the beginning of the seventies, the Morsleben facility has been used for the disposal of radioactive waste. Emplacement took place at a depth of 506 m (1,660 ft) on the fourth level. The concept of waste handling and disposal has basically been based on an adjustment of waste characteristics and appropriate emplacement techniques. Thus, solid low level waste in 200 litre drums was stacked. Stacking was accomplished by a forklift. Intermediate level radioactive waste and spent sealed radiation sources were dumped into non-accessible cavities by means of remote controlled equipment. In addition, liquid radioactive waste was solidified in situ using lignite filter ash as binding agent.

Until 1991, radioactive waste with a total volume of approximately 14,500 m³ (5.1 * 10⁵ ft³) and about 6,200 spent sealed adiation sources were disposed of. In total, an activity of 1.8 10¹⁴ Bq was emplaced (as of 1991). The activity of alpha emitters amounted to 1.6 * 10¹¹ Bq, and the activity of beta/gamma emitters to 1.77 * 10¹⁴Bq. The waste was mainly delivered by combined rail-and-road-transport using standardized freight containers.

RESUMPTION OF OPERATION

Subsequent to German unity in October 1990, the Morsleben facility has the status of a federal repository in the sense of section 9a (3) of the German Atomic Energy Law. Responsibility for operation is with the BfS; the Deutsche Gesellschaft zum Bau und Betrieb von Endlagern für Abfallstoffe mbH (DBE, German Company for the Construction and Operation of Repositories for Waste), being the main contractor of BfS, has been charged with the actual operation of this facility. The operating license is limited by law until June 30, 2000.

Thus, since October 1990, many discussions and negotiations took place, concentrating on the possibilities to continue operation of the Morsleben repository. As an outcome, it was decided to continue waste disposal and to cover the repository's operational costs until mid 2000, amounting to the order of magnitude of DM 500 millions (approximately U.S. $ 3.1 * 10⁸), by the emplacement costs for a total waste volume of about 40,000 m³ (1.4 * 10⁶ ft³).

The utilities signed up to dispose of a volume of 25,000 m³ (8.8 * 10⁵ ft³) radioactive waste originating from the operation of nuclear power plants. The Energiewerke Nord GmbH, the successor company of the East German utility, signed up a waste volume of 10,000 m³ (3.5*10⁵ ft³) originating from the operation and decommissioning of the shut- down Rheinsberg and Greifswald nuclear power plants. Finally, a waste volume of 5,000 m³ (1.7 * 10⁵ ft³) was assigned for those wastes originating from the collecting depots of the federal states, nuclear research establishments or other waste generators.

According to the waste acceptance requirements, a maximum activity of alpha emitters was estimated to be about 10¹³ Bq, that of beta/gamma emitters to be about 10¹⁶Bq. Compared to that, the activities actually to be expected until June 30, 2000, are in both cases lower in the order of up to two magnitudes.

The emplacement of radioactive waste in the Morsleben repository was stopped by court order in February 1991. Decisive steps towards resumption of disposal operations were the judgement of the Federal Administrative Court in June 1992 stating that the Morsleben repository can continue operation as a federal installation, and the judgement of the Superior Administrative Court in the federal state of Sachsen-Anhalt in December 1993 stating that radioactive waste originating from the old federal states can also be disposed of in this facility. In particular these juridicial decisions paved the way and BfS intensified its efforts to resume waste disposal. After all preparatory work had been completed, first emplacement of radioactive waste from the shut-down Greifswald nuclear power plant took place on January 13, 1994.

RADIOACTIVE WASTE ORIGINS AND ARISINGS

In Germany, 19 nuclear power plants with a gross generating capacity of about 22 GW are presently in operation. 13 of them are PWRs, six are BWRs. Reprocessing of spent fuel elements originating from these plants takes place abroad in French and British facilities. Basic and applied investigations are performed in several nuclear research establishments, uranium enrichment and fuel element fabrication facilities are operated, and nuclear facilities are decommissioned and dismantled. Finally, there are a lot of smaller waste generators, e. g. universities, industrial companies, hospitals, medical centers, the German Federal Armed Forces, and pharmaceutical and biomedical companies (note: radioactive waste originating from the smaller generators is generally handed over to the collecting depots of the federal states).

As to the waste arisings, the BfS carries out an annual inquiry into the amounts of unconditioned and conditioned radioactive waste in Germany. According to the 1995 inquiry the amount of conditioned LLW/ILW was about 63,000 m³ (2.2 * 10⁶ ft³) on December 31, 1995. The annual arisings of conditioned LLW/ILW amounts on average to 4,300 m³ (1.5 * 10⁵ ft3).

Since 1979, i. e. after the stop of waste emplacement in the Asse salt mine, radioactive waste has been placed in surface-based temporary storage facilities. Of this, a considerable part is and still will be shipped to the Morsleben repository in order to be disposed of. Thus, the Morsleben facility receives waste from a wide range of origins and/or sources, in particular from nuclear power plants, research institutions ant the collecting depots of the federal states.

MORSLEBEN WASTE ACCEPTANCE REQUIREMENTS

The operation of the Morsleben repository is regulated by the license issued on April 22, 1986, and by further documents pertinent to it. As the license is still effective, these documents represent the legally binding framework which must be adhered to. Thus, the Morsleben waste acceptance requirements prepared by BfS (3) include both boundary conditions prescribed in the license and additional regulations, in particular self-restrictions, results of supplementary safety assessments which keep to the above-mentioned framework and instructions by the self-surveillance of the Morsleben repository. According to this, at first sight, the Morsleben requirements appears to be rather complicated.

The Morsleben waste acceptance requirements which from part of the operational licence and its additional documents are based on a categorization of radioactive waste to be disposed of and the respective emplacement techniques. The waste categorization considers both the waste types according to basic properties and the so-called radiation protection groups according to the radiological characteristics. Thus, the waste acceptance requirements clearly distinguish between requirements on solid radioactive waste (type A1 waste) and on sealed radiation sources (type A3 waste). Liquid radioactive waste (type A2 waste) is no longer accepted. BfS has stopped in situ-solidification of that waste as a self-imposed restriction (4). The classification due to the radiation protection groups S1 to S5 accounts for the dose rate at the unshielded surface of solid wastes, and for the activity of spent sealed radiation sources.

The Morsleben requirements are basically structured as follows:

1. Classification of radioactive waste,
2. General requirements on radioactive waste to be disposed of,
3. Requirements on type A1 waste (solid waste),
4. Requirements on type A3 waste (sealed radiation sources),
5. Radionuclides to be declared,
6. Delivery of radioactive waste.

The requirements on both type A1 and type A3 waste specify, among other things, quality characteristics of the waste form, waste containers/packagings and limitations of permissible activity concentrations. As to the activities per waste package, it should be pointed out that, due to the license of 1986, the activity concentration of alpha emitters is limited to 4 * 10⁸Bq/m³. With regard to beta/gamma emitters, radiation protection groups S1 to S5 were established representing bandwidths of permissible activity concentrations from below 4 * 10⁹ Bq/m³ up to 4 10¹³ Bq/m³. Radioactive waste assigned to S1 and S2 is stacked whereas waste assigned to S3, S4 or S5 has to be dumped. In addition to this, supplementary safety assessments were performed for the normal operation of the repository, assumed incidents, nuclear criticality safety and long-term radiological aspects, thus concretizing the operational licence of 1986 by introducing radionuclide-specific activity limitations. The most restrictive limitations of both sets of requirements on permissible activity concentrations must be met.

The Morsleben waste acceptance requirements (3) have been in force since emplacement operations were resumed. Some changes and amendments (e.g., increase of gross volumes of usable drums, increase of permissible waste package masses, limitation of activity concentrations of radionuclides relevant to long-term safety) were introduced in March and September 1995.

A revised version of the Morsleben requirements was issued in August 1996 (5) and became effective on September 15, 1996. In addition to the above-mentioned changes and amendments, particularly the following requirements have been implemented:

1. cancellation of the reduction of the local dose rate for waste assigned to radiation protection group S2,
2. revised activity concentration limiting values resulting from the incident analysis due to legal modifications of the Störfallberechnungsgrundlagen (Incident Calculation Fundamentals),
3. cancellation of the limitation of the activity concentrations of radionuclides being relevant to long-term safety dating from March 1995,
4. addition of further radionuclides resulting from safety-related assessments of assumed incidents and long-term radiological effects including pe rmissible activity concentrations and the obligation to declare,
5. addition of further waste containers and packagings, e. g. overpacks with volumes up to 600 litres, cylindrical concrete containers of type I and type II, the modified drum container of type FC 50 M as well as the primary containers of type PC 120/2 and type PC 120/4.

Prior to delivery of the primary containers of type PC 120/2 and type PC 120/4 an agreement with BfS is necessary. Concerning this matter, it should be pointed out that both primary container shave successfully passed the respective procedures of the self-surveillance of the Morsleben repository as well as inactive testing and handling on-site but the procedure referring to the Federal Mining Law is still pending.

All changes and amendments were properly checked. Only those modifications have been included in the revised Morsleben waste acceptance requirements which are within the scope of the operational license issued on April 22, 1986, and passed the relevant procedures of the self-surveillance of the Morsleben repository.

EXPERIENCE IN THE OPERATION OF THE MORSLEBEN REPOSITORY

Radioactive Waste Disposed Of

Since emplacement operations were resumed, it has been the objective to increase the amounts of waste packages which have been delivered and disposed of each month in order to attain the anticipated total volume of about 40,000m³ (1.4 * 10⁶ ft³) to be disposed of by the end of June, 2000. During the first months of 1994, comparatively low amounts of waste were disposed of. This was due to initial difficulties which had to be surmounted, and to the cooperation of all institutions involved which had to become attuned to each other. Approximately since mid 1994, a gradual increase of the waste volume delivered has been recorded. Since December 1994, an average waste package volume of about 400m³ (1.4 * 10⁴ ft³) has been disposed of per month.

From January 13, 1994, through January 25, 1997, a) 11,408 m³ (4.0 * 10⁵ ft³) solid wastes of the type A1 waste in 52,755 pieces of 200 litre, 280 litre and 400 litre drums as well as in 23 pieces of cylindrical concrete containers of type I and type II, and b) 372 pieces of spent sealed radiation sources of the type A3 waste were disposed of in the Morsleben repository. The total volume is made up of 10,258 m³ (89.9%) operational waste from nuclear power plants as well as from the shut-down plants in Rheinsberg and Greifswald, 518 m³ (4.6 %) from the collecting depots of the federal states, and 632 m³ (5.5%) from nuclear research establishments and other waste generators.

The wastes originating from nuclear power plants are mainly miscellaneous waste, cemented evaporator concentrate and resins as well as high force compacted waste. The wastes originating from other waste generators are also miscellaneous waste as well as compacted waste, cemented evaporator concentrate or solution, and sealed radiation sources. The assignment of the solid waste disposed of to the different radiation protection groups shows that 89.2% belongs to the S1 group, 9.8% to the S2 group, and 1% to the S3 group. 148 pieces of the spent sealed radiation sources disposed of are assigned to the S1 group, 90 pieces to the S2 group, 63 pieces to the S3 group, and 71 pieces to the S4 group.

The total activity emplaced during the above-mentioned period of time is split up into

1. 3.0 * 10¹⁰Bq for alpha emitters, and
2. 3.9 * 10¹³Bq for beta/gamma emitters.

The following radionuclides contribute significantly to the total activity:

A more detailed radionuclide-specific analysis of the activities being disposed of clearly shows that, e.g.

1. the total alpha emitter activity of nuclear power plant operational and decommissioning wastes as well as of wastes originating from nuclear research establishments, nuclear fuel cycle industry and the German Federal Armed Forces is governed by the activities of Am-241, Cm-244, Pu-240, and Pu-239, respectively, whereas
2. the total alpha emitter activity of radioactive waste originating from the collecting depots of the federal states is governed by the activity of Am-241.

Waste Acceptance Requirements

It should be pointed out the Morsleben waste acceptance requirements describe/represent a safety-related envelope or framework the waste packages to be disposed of must comply with. There are no specific requirements on a specifically characterized waste stream. These requirements were elaborated in such a way that a flexible system of requirements has been established. Such a system includes several alternatives and different options for the waste packages which ensure the required level of safety for the repository. The waste generators thus have the possibility of applying and fulfilling those requirements which are specifically applicable to the waste packages produced by them.

Bearing these possibilities in mind, it cannot be excluded that the waste generators and conditioners will re-evaluate and optimize/rationalize present conditioning strategies and procedures. Up to now, those strategies and procedures have been determined by the available temporary storage capacities, lacking repositories and the Konrad repository project waste acceptance requirements (6,7). As a consequence, further developments and improvements of conditioning techniques have in particular been aiming at volume reduction and observing the permissible activities per waste package due to the Konrad requirements. From now on, the operation of the Morsleben repository offers potential new developments or modifications of existing conditioning techniques.

Waste acceptance requirements give guidance to the selection of proper waste treatment and conditioning processes. In addition, they constitute the basis for the waste generators or conditioners detailed specification of the waste packages to be disposed of. Thus, from a conditioner's point of view, the range and period of validity of the Morsleben waste acceptance requirements as well as the disposal costs per waste package are of utmost importance. Thus, it is meaningful to analyze these requirements and to adapt conditioning strategies and techniques to these requirements. In addition to technical aspects, due to the Morsleben costs for disposal fixed at DM 12,500 (approximately U.S. $7,800) per m³ (35.3ft³) during the operational lifetime until June 30, 2000, i.e. DM 2,500 (approximately U.S. $ 1,560) per 200 litre drum, it is henceforth possible to select appropriate conditioning procedures also taking economic aspects into account. For example, regarding combustible waste, it could be meaningful to use high-pressure compaction instead of incineration. Decantation and/or immobilization of liquid waste seems to be preferable compared to drying. As to miscellaneous waste (i.e., scrap, protective clothing, insulation material, worn-out equipment, paper, polyvinylchloride, rubble and contaminated soil), processing via high force compaction should be opposed to less expensive treatment using a baling press and resulting disposal costs be compared with. Waste processing and emplacement costs could thus be optimized and the most economical solution be applied. According to present information such an optimization of waste conditioning steps and procedures has already taken place and respective activities are expected to be intensified in future.

Waste Package Quality Assurance

The general objective of waste package quality assurance is to provide adequate confidence of the fulfilment of the Morsleben waste acceptance requirements. In particular, the waste generators and conditioners are responsible for the fulfilment of these requirements and must demonstrate to the BfS that the Morsleben requirements are met.

The performance of quality checking during waste conditioning or non-destructive and/or destructive testing of waste packages is a rather complicated system (8). According to the procedures agreed upon the waste generator or licensee of a nuclear facility applies for conditioning or checking of waste packages in parallel to the BfS and to the respective supervising authority. The main steps in waste treatment, processing and packaging and in particular all quality checking steps are to be laid down in a process control quality plan (PCQP) to be prepared by the waste generator. Complicated steps or procedures of the PCQP are usually described in additional documents, e.g. concerning the determination of radionuclide-specific activities per waste package or the reduction of gas generation by waste drying. The supervising authority checks this plan with regard to the safe operation of the waste generator´s facility and the fulfilment of requirements of any temporary storage facility. BfS checks the PCQP with regard to the fulfilment of the Morsleben waste acceptance requirements.

The fulfilment of the Morsleben waste acceptance requirements must also be checked for those waste packages conditioned in the past without using a PCQP. In that case sampling inspections are performed. Waste packages to be checked are taken as random samples. Additional checks are performed, e.g., if indications to faulty packages result from a visual inspection of the lot.

The experience gained from the performance of quality assurance measures may be summarized as follows (9):

1. Difficulties in performing quality assurance for waste packages to be disposed of in the Morsleben repository have resulted from the fact that some supervising authorities of the federal states demand participation in respective measures carried out by BfS and its experts. The interests of the waste generators, these supervising authorities and the BfS which in parts clearly differ resulted in different opinions with regard to the execution of waste package quality control thus causing delays. For instance, there are difficulties in the transmission of measurement results between the experts acting on behalf of BfS and on behalf of the respective supervising authority, respectively.
2. About 30 alpha emitters and about 36 beta/gamma emitters shall be declared by the waste generators according to the Morsleben waste acceptance requirements. The main difficulties arise for non well-documented waste packages because the necessity to declare these radionuclides did not exist at the time of that waste's conditioning. In addition, the declaration of the radionuclide-specific inventory per waste package is made difficult by the relatively low activity limitations resulting from the safety assessment of the radiological long-term effects in the post-closure phase.
3. For wastes originating from the operation of nuclear power plants, the radionuclide inventory is usually determined taking correlations between the key radionuclides Co-60 and Cs-137 and respective activation and fission products into account. These correlations are based, e. g., on measurements of the local dose rate of the package, available data of similar waste packages, calculations of the burn-up of the fuel elements, the age of the waste, and world-wide radionuclide-specific measurements of nuclear power plant wastes. This procedure is now performed with the aid of adequate computer programs and less time consuming as compared to the past.
4. For wastes not originating from nuclear power plants, i.e. wastes with heterogeneous composition, it may be essential to determine the radionuclide inventory by non-destructive gamma scanning. In addition, it may be necessary to take samples for analytic measurements, too. Thus, the demonstration that the Morsleben requirements are met demands great efforts for radioactive waste which is not characterized by a rather uniform radionuclide spectrum. This applies in particular to the waste originating from the application of radionuclides in research, medicine and industry.
5. The fulfilment of the Morsleben requirements on the permissible gas generation has also turned out to require great efforts, particularly in case that gas generation cannot be excluded due to the type and treatment/processing of the waste.
6. The process control quality plans turned out to be an appropriate and very easy to handle managing tool for radioactive waste conditioning and checking. Up to now more than 400PCQPs got the approval by BfS. They refer to waste amounts varying between a few and several thousand waste drums.

Despite of the above-mentioned difficulties, restrictions or delays with regard to waste package quality assurance could be avoided to a far extent, so that emplacement operations have hardly been hindered by such occurrences. Therefore, the performance of waste package quality assurance can only be evaluated positive.

Emplacement Operations

During the period of time from January 1994 through January 1997, there have been no difficulties in accepting waste packages released for disposal by BfS. For transport and for safety reasons waste packages have usually been shipped on the last three days of a week. In the village of Morsleben, there are basically no problems with the local communities. Citizens' actions are negligible; from time to time opponents of nuclear energy took actions.

With regard to disposal of radioactive waste, a systematic performance of waste handling and emplacement operations is to be observed. The cavities used for disposal are located on the fourth level of the Morsleben repository. Of the total waste volume emplaced (11,408 m³; 4.0 * 10⁵ ft³), approximately 93.6% were stacked in the western emplacement field and approximately 6.4% were dumped in the southern emplacement field. In December 1995, the backfill of residual voids in the waste package filled parts of the western emplacement field has been started. Filter ashes originating from the operation of coal fired power plants are used as backfill material.

The radiation exposure to the staff and the environment of the Morsleben repository was very low in the above-mentioned period of time. In 1994 the exposure to the staff (category A according to the German Radiation Protection Ordinance) was in average 1.1 mmanSv/yr and in 1995/96 slightly decreased to an average value of 1.0 mmanSv/yr. The discharge of volatile radionuclides such as H-3 or C-14 and aerosols with long-lived radionuclides or Radon daughter radionuclides remain under the permissible limiting values by one order up to four orders of magnitude, respectively.

FUTURE PLANNINGS

Despite its revision in August 1996, it cannot be excluded that additional changes and/or supplementary amendments will be introduced into the Morsleben waste acceptance requirements. At present, the 600 litre drum, the modified drum container of type FC 75 M, the box-shaped container of type KC 1,200 as well as the primary containers of type PC 120/2 and type 120/4 are in the course of passing through the respective procedures

As to future emplacement operations it should be pointed out that the western emplacement field presently be used for stacking of waste packages will be filled by mid 1997. Therefore, preparatory planning work for the eastern emplacement field has already been performed. In this field disposal rooms with a length of up to 75 m (241 ft) and a width and height of 25 m (82 ft), respectively, are in the course of preparation. Emplacement operations are expected to start in the second half of 1997; stacking of solid waste will last until the end of June, 2000. The southern emplacement field will be used for dumping solid waste and spent sealed radiation sources until the end of the operational period in mid 2000.

CONCLUDING REMARKS

Until June 30, 2000, the Morsleben repository will contribute to the proper management (i. e., disposal) of radioactive waste in the Federal Republic of Germany. According in particular to the limit of the alpha emitter activity concentration, the operation of this facility will primarily contribute to the emptying of temporary storage facilities. Thus, the operation of the Morsleben repository contributes to increase the protection of man and the environment by removing radioactive waste from surface-based temporary storage facilities to disposal rooms located at the depth of 506 m (1,660 ft) below the land surface.

REFERENCES

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