Demonstration Tests for Direct Disposal of Spent Fuel. W. Filbert, W. Bollingerfehr DBE TECHNOLOGY GmbH

Transcription

1 Demonstration Tests for Direct Disposal of Spent Fuel W. Filbert, W. Bollingerfehr DBE TECHNOLOGY GmbH Eschenstraße 55, Peine, Germany, , Eschenstraße 55, Peine, Germany, , Abstract In Germany the reference concept for the disposal of heat generating radioactive waste considers the emplacement of vitrified waste canisters in deep vertical boreholes inside a salt mine. Whereas spent fuel will be disposed of in self shielding the 65 t heavy carbon steel POLLUX casks in horizontal drifts of the disposal zones in the salt dome in a depth of 870 m. The space between casks and drift walls will be backfilled with crushed salt. The transport, the handling and the emplacement of POLLUX cask were subject of successfully performed demonstration and in situ test in the nineties. The design and the results of the demonstration tests will be presented. This includes also the concept and the test results of the world wide unique nine years lasting in situ tests in a URL in salt in which the TM behaviour of the rock salt and the backfill material were measured and compared with predictive calculations. 1. INTRODUCTION Rock salt was selected in the early 1960s as the preferred host rock for a repository for heat generating waste in Germany due to the unique geohydrologic, thermal, and geomechanical properties as a self-healing impermeable rock. In Northern Germany, a large number of salt domes with huge dimensions, many of them principally suitable to host a repository, exist. In 1977, at the end of a time consuming selection process, the salt dome in Gorleben was selected out of a group of preselected salt domes for further exploration. Until October 2000, when the Federal Government and the nuclear industry agreed to stop further underground exploration at the Gorleben site for at least 3 and max. 10 years, almost all necessary data to describe the suitability of a repository site were collected. Exploration from surface was accomplished, 2 shafts constructed and the first square of the underground exploration mine excavated accompanied by geotechnical and geophysical monitoring. canisters with vitrified highlevel radioactive waste (HLW) are to be emplaced in up to 300 m deep boreholes with a diameter of 60 cm. In order to facilitate the fast encapsulation of the waste by the host rock (rock salt), the boreholes are not to be furnished with lining. The POLLUX casks, the 65-tonne heavy carbon steel casks, will be laid down on the floor of a horizontal drift. The space between the casks and the drift walls will be backfilled with crushed salt. 2. REFERENCE CONCEPT FOR DISPOSAL OF HEAT GENERATING WASTE AND SPENT FUEL The appropriate reference concept for the disposal of heat-generating radioactive waste (Fig. 1) comprises the emplacement of canisters containing vitrified waste in deep vertical boreholes, whereas spent fuel will be disposed of in selfshielding POLLUX casks in horizontal drifts inside a salt mine [1]. The disposal zones for both, spent fuel and vitrified waste, are to be constructed at a depth of 870 m. In the disposal concept, which allows a temperature of max. 200 C at the contact between waste canister and host rock, unshielded Fig. 1: German reference concept for disposal of heat generating waste Obtaining a license to construct a repository in Germany requires previous demonstration to the competent authority that the level of protection (dose or risk) can be met to a high level of confidence. In the case of waste canister transport and handling systems, the fulfillment of the regulatory requirements can be provided

2 by means of 1:1-scale demonstration and reliability tests. The transport, handling and emplacement techniques of the POLLUX cask were subject to successful demonstration and in-situ tests performed in the 1990s. As a result, the Atomic Energy Act was amended accordingly in For waste canisters with high level reprocessing waste this demonstration is still pending. 3. DIRECT DISPOSAL OF SPENT FUEL IN POLLUX -CASKS The first steps for direct disposal of spent fuel in POLLUX casks are: loading of spent fuel into transport und storage flasks after spent fuel has sufficiently cooled down in the pools of the nuclear power plant transfer of the flasks into an interim storage at the site of the nuclear power plant interim storage of flasks until conditioning or final disposal is possible transport of flasks to the conditioning facility or the final disposal site conditioning by separating fuel rods from structural parts of the fuel element final disposal of fuel rods and structural parts To minimise costs of transport and interim storage, large casks, mainly of the CASTOR V-type, are used. These contain up to 19 fuel elements from PWR and 52 fuel elements from BWR. The cask body is made of ductile cast iron, neutron shielding is achieved by polyethylene bars assembled in uniformly distributed drillings in the cask wall. The pilot conditioning plant at the Gorleben site was completed in License for operation was granted in According to the license the plant will be operated in a stand-by modus to accept and repair casks when necessary. As of today, the throughput is limited to 35 thm per year. Fig. 2 shows the hot cell where fuel assemblies are separated into fuel rods and structural parts. This, and their subsequent packing into cans is necessary to ensure sub-criticality in the repository. Source GNS Fig. 2: Hot cell of the pilot conditioning plant The cans containing the fuel rods are inserted into the POLLUX cask. The POLLUX cask was developed as a triple-purpose cask for transport, storage and final disposal. The safety analysis report and the licensing documents according to the regulations of the Atomic Energy Act were submitted to the licensing authority and its independent expert for obtaining the flask approval certificate according to the transport regulations (type B(U)F) and the storage license according to the acceptance criteria of the Gorleben interim store. A drop test programme was carried out in Fig. 3 shows the basic design of the POLLUX final disposal cask. Fig. 3: POLLUX -cask for final disposal of fuel rods from spent fuel

3 It consists of the shielding cask with an inscrewed lid and an inner cask with bolted primary and welded secondary lid. The fuel rods are inserted into the POLLUX -cask in cans. The cylindrical wall and the bottom of the inner cask are made of fine-grained steel and extruded in one piece. The body of the shielding cask also consists of one piece and is made of ductile graphite iron. Two rows of boreholes in the shielding casks wall contain neutron moderator material. A prototype cask has undergone tests in the Pilot Conditioning plant in Gorleben. The cask will contain fuel rods from up to 10 PWR fuel assemblies or fuel rods from up to 30 BWR-fuel assemblies. not be given for components to be designed to meet specific, stringent repository requirements. Their applicability has to be confirmed within the scope of specialpurpose demonstration tests. 4. SIMULATION OF SHAFT TRANSPORT IN A TEST FACILITY Based on a conceptual design for a 85 t payload shaft hoisting facility to lower down spent fuel and also high level waste to a repository the R&D effort required to reach its licensing maturity has been estimated. Shaft hoisting equipment for the transport of radioactive waste in a repository mine must be designed to perform properly under normal service conditions and, in addition, to meet stringent safety requirements. Its failure may result in a heavy hazard for the personnel or even in a release of radioactive substances into the facility and environment. Such events must be safely prevented, and hence the shaft hoisting equipment must be designed according to the state-of-the-art of science and technology to fulfill appropriate safety requirements. The planned system assumes that radioactive waste is transported in casks that satisfy IAEA's regulations for the Safe Transport of Radioactive Materials at the level of a type-b certificate. The repository facilities were designed under special consideration of the following accidents [2]: drop of waste package during the loading process; mechanical impact on a waste package during the transport into the mine; drop of waste package during its transport; drop of heavy loads onto waste packages. A detailed analysis of all relevant parts and components of the shaft hoisting system regarding transferability of the state-of-the-art to payloads of up to 85 t was carried out considering a significant number of reference plants. Results of this study which proves the application maturity of essential components by reference plants were reported in [3]. Such an evidence can ITEM DESCRIPTION 1 shaft safety gate 1 2 air lock gate 1 3 lift gate 1 with interlocking 4 transport car 5 car catch 1 6 air lock gate 2 7 weigh bridge I. Unloading direction II. Loading direction 8 shaft safety gate 2 9 loading device 1 10 lift gate 2 with interlocking 11 cage rests hoisting cage 13 unloading device 1 14 car catch 2 Fig. 4: Shaft hoisting facility The safety devices have to be constructed in one to one scale to carry out the test program. For this purpose a test stand (see Fig. 3) was built in a power plant, using the foundations of a decommissioned turbine.

4 65 t (POLLUX ) loaded with spent fuel. Here, emphasis was put on the development and construction of components, such as an emplacement device, a transport cart and a mining locomotive. Their capabilities of working under normal operating conditions and under conditions of operational disturbances were demonstrated at a full scale aboveground test facility in order to guarantee the safe handling of waste packages. Fig. 6 shows the mining locomotive, transport cart, emplacement device and the dummy cask. ITEM COMPONENT DESCRIPTION 1. Shaft safety gate 2. Loading device 3. Transport car with inactive cask 4. Air lock gate 5. Cage rests with track bridge 6. Hoisting cage 7. Car catch Fig. 5: Test stand The deterministic approach considers some components important to safety (shaft safety gate, shaft airlock gate and cage rests) to be under certain conditions the last mechanical safety device to prevent a drop of heavy loads into the shaft or to limit an overwinding of the hoisting cage ("SELDA" Strain Energy Linear Ductile Arrestor). All the safety devices were designed fabricated and tested according to the applicable specific requirements, which include mining regulations, ordinances and other regulations of the responsible mining authority [4] and under consideration of the high weight of a loaded transport car of 85 t loading and unloading operations were performed at the test stand. Occurrence of operational disturbances were simulated. All components were successful tested. 5. TESTS FOR DRIFT DISPOSAL WITH THE POLLUX -CASK IN AN ABOVEGROUND TEST FACILITY These tests performed in 1994/1995 were aimed at demonstrating the feasibility of rail-bound handling, horizontal transportation, and drift emplacement of self-shielded spent fuel disposal casks with a weight of Fig. 6: Components of the disposal system The battery-operated transport and disposal locomotive is state-of-the-art. It is constructed in socalled open order with heading and trailing cabin and is the result of efforts to improve mining locomotives from ergonomic, operational and safety-technological points of view. The transport cart was built with a special carriage to shift the center of gravity of the POLLUX cask to a lower position. The emplacement device for drift disposal (ELVIS) was made of components that are used under comparable conditions in other technical fields and was designed for a load of 65 t. The dummy cask which was used for the tests has all the features of the POLLUX -cask disposal operations have been successfully performed at the test stand in Peine. 6. TSDE IN SITU EXPERIMENT SET-UP IN THE ASSE MINE The Thermal Simulation of Drift Emplacement (TSDE) in situ experiment in the Asse mine in Germany lasting for almost nine years is such an outstanding example for a representative underground experiment. Within the framework of the BAMBUS project [5], the temperature and stress dependent compaction behavior of crushed salt backfill was studied by improved numerical modelling and post-test analyses of the TSDE backfill. A second topic was the investigation of the excavation disturbed zone (EDZ) evolution around openings in salt. Thus, permeability measurements as a function of distance to openings were per-

5 formed at various locations in the Asse mine. The entire BAMBUS work program consisted of in situ investigations in the Asse research mine, laboratory studies on retrieved material samples (backfill, instruments and corrosion specimen), modeling exercises and desk studies. The TSDE experiment [6] was set up in a test field in the anticlinal core of the Asse salt dome inside the Stassfurt Halite of the Zechstein Series. The test field comprised two parallel test drifts on the 800-m level and several observation and access drifts on the 800m and 750m -levels (Fig. 7). The geometrical dimensions of the test drifts (3.5-m-hight, 4.5-m-width and 70-mlength) as well as the nominal heater power of each cask (6.4 kw) were selected in order to represent real repository conditions. Fig. 7: TSDE-test field in the Asse mine at the 800 m- level In each test drift three electrically heated casks (type POLLUX : 1.5 m diameter, 5.5 m length, 65 Mg mass) were deposited. The remaining space between the casks and between casks and the drift walls was backfilled with crushed salt once a cask has been completely installed. About twenty cross sections in and around the drifts were equipped with measuring instruments applicable for the harsh environment (backfill emplacement procedure by slinger technology and high temperatures) to monitor the rock salt and backfill material behaviour over time. The instruments were installed either via boreholes in the rock or around and between the heated casks in the backfill material. Figure 8 shows the installation of the instruments and the electrically heated casks. Fig. 8: Installation of instruments and electrically heated casks in a test drift The scientific investigation program performed during the test operation included geotechnical (temperature, deformation, and stress) measurements in the backfill and the surrounding rock salt. In September 1990 the heaters were switched on and after almost 8 and a half year of operation, heating was terminated in January With regard to an acceptable temperature level for mining activities an additional year was spent for cooling down the rock and air temperature in the test drift environment before dismantling and removal of instrumentation and heaters could start. Thus, backfill excavation and sampling as well as dismantling of heaters and instrumentation began in August 2000 and lasted until May A Post-test investigation program comprised Reliability cheque of instruments Results of backfill investigations Accuracy of model calculations Retrievability desk study Corrosion investigation Investigations on the excavation damaged zone (EDZ) As an essential safety relevant process the thermomechanical behavior of crushed salt backfill was investigated in an almost nine years in-situ experiment simulating the direct disposal of spent fuel in self-shielding disposal POLLUX casks in a repository in salt. To verify the predictive capabilities needed for long-term safety analyses, the in-situ measurement results were compared with model predictions based on preceding laboratory investigations. To confirm measuring and modeling results one of the two backfilled drifts was uncovered after termination of the experiment. The

6 analysis of uncovered backfill material confirmed the reduction of porosity and consequently an increase of the isolation capability with time. Another important result was, that modeling of the whole test under consideration of end effects required the application of complex 3D-models. Post-test analyses both of the compacted backfill material and the measuring instruments confirmed the accuracy and reliability of the applied models and measuring equipment. For quality assurance, sensor calibration before and after an in-situ test has been done. In-situ test and post-test investigations revealed that the robust gauge design and the used sensors were very successful despite of their harsh environment application. Most failures were caused by damaged measuring lines. The re-calibration results revealed a high reliability of the applied sensors and a low sensor drift. After up to 12 years of operation, the linearity of most gauges was still within the manufacturer's respective limit of tolerance. The applied measuring systems proved to be suitable for the long-term monitoring of a final repository. Post-test analysis on retrieved corrosion specimens showed that the average corrosion rate of all materials was negligibly low. Thus, there are several container materials available which could be recommended for the application as waste package material. 7. SUMMARY AND CONCLUSION Demonstration tests in a 1:1 scale for the safe shaft transport and drift disposal of self shielding POLLUX -casks were performed in the nineties. The simulation of shaft transport shows that all the successfully tested safety devices are now state of the art and can be used for the design and construction of shaft hoisting systems of a repository for high level waste and spent fuel. The tests for drift disposal were also successfully performed. It could be shown that the technical equipment allows reliable and safe disposal operation. All the developed equipment in both projects is since that time state of the art. demonstration tests Simulation of shaft transport and tests for drift disposal the safety concept of complete and safe waste containment by the host rock salt and the technical feasibility of POLLUX transport and emplacement concept. 8. REFERENCE [1] H.-J. Engelmann, et al.: 1995, Systemanalyse Endlagerkonzepte, Abschlussbericht, Hauptband, DEAB T 59 [2] B. Hartje, C. Schrimpf, W. Weber: "Technical Maturity of Shaft Hoisting Facilities for the 65 t Heavy Casks with Spent Fuel", Proc. Int. Symp. on Uranium and Electricity, Saskatoon, Canada (1988) [3] H.-J. Engelmann, et al.: "Demonstration Tests for the Simulation of Shaft Transport", Waste Management '93, Tucson, Arizona (1993) [4] Rundschreiben des BMI vom RS- AGK /2, Sicherheitskriterien für die Endlagerung radioaktiver Abfälle in einem Bergwerk, FRG, 1983 [5] W. Bechthold, et al.: Backfilling and sealing of underground repositories for radioactive waste in salt (BAMBUS Project). Nuclear Science and Technology, EUR EN. Luxembourg: European Commission, 1999 [6] W. Bollingerfehr, T. Rothfuchs, et al.: The BAMBUS project in the Asse mine full scale testing of the direct disposal of spent fuel elements in salt formations and investigations of the excavation disturbed zone around a repository, DisTec 2004 The TSDE experiment and the post-test studies within the framework of the BAMBUS project helped to substantially improve the understanding of processes related to backfill and host rock behavior in a repository in rock salt. Thus, the basis was extended for optimizing the repository design and construction and for predicting the long-term performance of the most important barriers in a repository in salt. Summarizing it can be stated that the TSDE in situ experiment and in particular the post-test analysis within the BAMBUS project, confirmed as well as the