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Status of the FRG activities on direct disposal of spent LWR-fuel
Nuclear Engineering and Design 118 (1990) 99-106 North-Holland
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STATUS OF THE FRG ACTIVITIES ON DIRECT DISPOSAL OF SPENT LWR-FUEL K.D. C L O S S 1 a n d H.-J. E N G E L M A N N 2 t Kernforschungszentrum Karlsruhe, Projektgruppe Andere Entsorgungstechniken, P. 0. Box 3640, D-7500 Karlsruhe 1, Fed. Rep. Germany 2 Deutsche Gesellschaft zum Bau und Betrieb yon Endlagern f~r Abfallstoffe, Woltorfer Str. 74, D-3150 Peine 1, FeR Rep. Germany
Received: first version 7 September 1988, revised version 12 May, 1989
As stipulated by the German Atomic Energy Act, reprocessing is the reference waste management route for LWR's in the Federal Republic of Germany (FRG). Spent fuel disposal without reprocessing is being developed to technical maturity for those fuel elements for which reprocessing is either technically not feasible or economically not justifiable. The reference concept for direct disposal is the emplacement of large and heavily-shieldedcasks in drifts of a repository mine located in a salt dome. Moreover, a back-up solution is being pursued which results in smaller canisters which are emplaced in boreholes. The mining authorities have pointed out that the feasibility of direct disposal is to be demonstrated before a license for industrial scale deployment could be granted. Demonstration tests are necessary in the followingareas: shaft transport of large and heavily shielded casks, handling of the casks in the repository and thermal and rock mechanics investigations with respect to the drift emplacement concept. The results of the demonstrations tests as well as the results from layout and optimization studies for a common repository for both reprocessing waste and spent fuel will be available early enough to be incorporated into the licensing procedure for the FRG's first repository for heat-generating nuclear wastes. This means that direct disposal of spent fuel not suitable for reprocessing could be introduced in the future in addition to the reprocessing and recycling waste management concept.
I. Introduction As stipulated by the German Atomic Energy Act, reprocessing is the reference waste management route for LWR's in the Federal Republic of Germany (FRG). After abandoning the construction of a domestic reprocessing plant, the F R G will rely on foreign reprocessing services for the next decades. Spent fuel disposal without reprocessing is being developed to technical maturity for those fuel elements for which reprocessing is either technically not feasible or economically not justifiable. Up to 1984, all funds for R & D in this area were provided by the Federal Ministry of Research and Technology (BMFT). After the technical feasibility of direct disposal had been demonstrated in principle by the Systems Study Alternative Entsorgung [1], the responsibilities were newly assigned: DWK, the German reprocessing company, has to continue cask and spent fuel treatment development work on its own. The Federal Government is supporting work only in the area of its responsibility, namely the repository.
In May 1986, DWK initiated the licensing procedure for a pilot conditioning and encapsulation plant to be built at Gofleben/Lower Saxony directly adjacent to the away-from-reactor (AFR) interim storage facility of DWK [2]. A first construction license for the pilot plant is expected in 1990. The repository-related R & D work is being supported by the BMFT and coordinated by the project group PAE at Kernforschungszentrum Karlsruhe (KfK) Major contributions to the program are made by DBE (Deutsche GeseUschaft zum Bau und Betrieb von Endlagem ftir Abfallstoffe), GSF (Gesellschaft fur Strahlenund Umweltforschung, Institut f't~r Tieflagerung), BGR (Bundesanstalt fiir Geowissenschaften und Rohstoffe), and KfK,
2. Technical concepts The two spent fuel conditioning and encapsulation techniques used in the DWK pilot plant are outlined in fig. 1. The process starts with disassembling the fuel
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K.D. Closs, H.-J. Engelmann / FRG activities on direct disposal
of the disposal galleries. The technique for handling such canisters in a repository has been developed and will be tested soon [3]. This paper will not deal with this topic, but will mainl~¢ concentrate on such aspects associated with the POL]LUX casks. The POLLUX cask which is designed for shipping as well as for interim storage and disposal fulfills the IAEA requirements (Type B(U)). Moreover, it features a corrosion protection layer that acts as a long-term barrier in the repository. The handling and emplacement procedure at the repository is shown in fig. 2. After having been shipped by rail or road to the surface facilities of the repository, the cask passes through the entrance control in the transfer hall. Next, it is placed on a railroad car which is conveyed to the shaft and pushed into the shaft cage. After shaft hoisting the loaded railroad car is pulled from the hoisting cage at the shaft landing. The loaded railroad car is taken over by a mine locomotive at the shaft landing and transferred along the pilot heading to the emplacement site. At a by-pass near the branch of the cross drift, the locomotive is placed behind the railroad car. After recoupling the locomotive pushes the railroad car through the cross drift into the emplacement drift to the intended emplacement position. Fig. 1. Spent fuel conditioning and encapsulation techniques.
assemblies. In the reference concept (shown on the left-hand side) intact consolidated rods from 8 PWR spent fuel assemblies together with the compacted skeletons of the fuel assemblies are placed into the disposal canister. The canister is inserted into a shielding overpack which is designed such that the personnel in the conditioning and encapsulation plant as well as in the repository can handle the package hands-on. Such a package is called POLLUX cask and is disposed of in the drifts of a mined geologic repository. Since handling such heavy (65 Mg) and large (5.5 m long) casks in a repository mine is not state-of-the-art, a back-up solution for spent fuel conditioning and encapsulation resulting in smaller spent fuel packages is being pursued. This concept is shown on the fight-hand side of fig. 1. The POLLUX canisters have the same outer dimensions as the canisters for vitrified reprocessing waste. In this case the disassembled fuel pins have to be cut into pieces of about 1 m in length. The advantage of this concept is that the handling and emplacement techniques in the repository are the same as for vitrified waste, namely the emplacement of the canisters into 300-m-deep boreholes, drilled in the floor
Emplacement Procedure Fig. 2. Handling and emplacement procedure.
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K.D. Closs, H.-J. Engelmann / FRG activities on direct disposal
At the emplacement position, the cask is lifted from the railroad car by an emplacement device which is designed for lifting and depositing the cask on the floor after removal of the wagon. The railroad car is pulled out of the emplacement drift all the way back to the surface for reloading. The drift section accommodating the emplaced cask is backfilled immediately with crushed salt.
3. R & D program direct disposal The overall aim of the program is to develop direct disposal of spent fuel to technical maturity. With respect to the reference concept, the mining authorities have pointed out that the feasibility of direct disposal is to be demonstrated before a license for deployment on an industrial scale could be granted. Thus, the main thrust of the program lies with the demonstration of the safe and reliable handling of large and heavy casks in a repository on a 1 : 1 scale. Emphasis is also laid on the investigation of special thermal and rock mechanics aspects of the drift emplacement concept as well as on the layout and optimization of a repository for both reprocessing waste and conditioned spent fuel. Demonstration tests are necessary in the following areas: (1) Shaft transport with loading and unloading of the hoisting cage will be simulated in an aboveground test rig with inactive casks. (2) Machinery and devices for the transport of the casks into the disposal drift as well as for lifting and lowering the cask at the disposal position are to be developed and tested. (3) Thermal and rock mechanics investigations for the drift emplacement concept will be performed with electrically heated casks in the Asse underground laboratory. In the following, these demonstration tests are described in some detail. Moreover, the overall aim and some preliminary results of the repository layout and optimization activities are stated. 3.1. Simulation of shaft transport
The realization of direct disposal with POLLUX casks depends on the feasibility to lower up to 85 Mg (65 Mg POLLUX cask + 20 Mg railroad car) in a shaft hoisting facility. Shaft hoisting facilities with such a payload have not been built yet worldwide. As a first step the main components of such a shaft hoist were designed preliminarily. The important design parameters are given in table 1.
Table 1 Design parameters for the hoisting facility General data hoisting system
hoist allocation conveyingspeed in the case of heavy load (normal load) number of breaking force generators payload in the case of heavy load (normal load)
koepe hoist cage and counterweight tower-mounted 5 m/s (12 m/s) 24×2 85 Mg (30 Mg)
Hoist multiple rope hoist 2 direct-current shunt-wound motors rated power approx.
direct drive 2 x 2800 kW
Hoisting cage available width available length weight
max. 2600 mm max. 6400 mm approx. 38 Mg
8 ropes
Hoisting ropes head rope (type of manufacture) i.e. flat-strand rope (approx. 51 mm) tail rope (type of manufacture) i.e. flat-strand rope
The hoist system chosen is a tower-mounted friction hoist with 8 ropes, using a cage and a counterweight. The conveying speed for normal load up to 30 Mg is 12 m/s, whereas it is only 5 m / s when the POLLUX casks are lowered. The hoist is directly driven by two directcurrent shunt-wound motors with a power of 2800 kW each. The hoisting cage has a width of 2.6 m and a length of 6.4 m allowing the POLLUX casks to be transported in the shaft in a horizontal position. In order to demonstrate the maturity of the technology a study about facilities in operation was carried out. The basic data of 110 hoisting systems worldwide were collected. 93 of these systems were analyzed in more detail and compared with the specifications of the 85Mg-system. The sources of the data were constructional and reference specifications as well as publications in the technical literature. The conclusion of this investigation was that for almost all the essential components of the hoisting system comparable installations are in operation from which their suitability for this application could be demonstrated. For some components that must meet special requirements if they are employed for nuclear waste disposal, this has still to be demonstrated [4]. In order to provide such evidence a full-scale test is to be
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K.D. Closs, 1t.-J. EngelmanP, "FRG activities on direct disposal
Fig. 4. Rail transport of POLLUX casks.
c a 0 a
b o t t o m
Fig. 3. Diagrammatic sketch of the test rack for demonstrating the shaft transport.
- minimize the overall weight of the transport vehicle in order to reduce the payload of the shaft hoisting system. Several alternatives were investigated, and, finally, a four-axle railroad car with parabola springs was chosen as shown in fig. 4. The main advantages of this design are the design is state-of-the-art in all important components, - the bogie wheel system is standard used by the Deutsche Bundesbahn (German Federal Railroad), - the load for the system w h e e l / t r a c k is in the same range as that of the Deutsche Bundesbahn, - a reduction in height seems possible when designing the vehicle in detail. In fig. 5, the emplacement device for the P O L L U X cask is shown. It is intended to use electrically driven screw jacks. Those devices for lifting heavy loads are state-of-the-art, but they have to be adapted to the special conditions of a repository mine in a salt formation. Development is mainly necessary with respect to reducing the height of the device and, thus, the height of -
carried out. It is planned to construct and test the following components in the empty generator building of a decommissioned power station: cage, diagonal arrangement of the guide rails intermediate cage bottom support for loading, - mechanical arrestment systems for overrunning, - charging unit, rail transport vehicle with test cask. A diagrammatic sketch of the planned test rack which is now under design and construction is given in fig. 3. -
-
-
-
3.2. Drift handling test
In another test, the handling of the large and heavy P O L L U X casks in the limited space of a repository mine is to be demonstrated. For this purpose the railroad car as well as the emplacement device will be designed and tested. Figure 4 is a schematic view of the rail transport with the P O L L U X loaded on the car. Major development efforts are devoted to - minimize the height of the car in order to minimize drift cross sections, - allow transport with small track radius in order to utilize the given salt volume in an optimal way,
Fig. 5. Emplacement device for POLLUX casks.
K.D. Closs, H.-J. Engelmann / FRG activities on direct disposal
103
the drifts. This is advantageous for thermal reasons, mine stability, and economics. 3.3. Thermal simulation test A third demonstration test deals with the behavior of the rock salt and the crushed salt backfill under the specific conditions of the drift emplacement concept. For this test, two parallel drifts are excavated at the 800-m-level of the Asse underground laboratory, as shown in fig. 6. Each drift accommodates three electrically heated casks with an electric power of 10 kW, each to simulate the decay heat of the spent fuel. The dummy casks have the same dimensions and the same weight as the original POLLUX casks. The space surrounding the casks will be backfilled with crushed salt. Different types of measuring instruments are installed in the crushed salt and surrounding strata in order to record and evaluate all relevant parameters like temperatures as a function of time at selected points on the cask surface, in the backfill material, and in the surrounding rock salt, - compaction, porosity, and permeability of the backfill material in the thermally exposed emplacement drifts, - stresses and strains in the rock salt as well as drift convergence in the thermally exposed emplacement drifts. The main objective of this experiment on a 1 : 1 scale is to verify thermal and thermomechanical computer codes in order to have tested codes available in a future licensing procedure. -
demonstration of backfllllno technique
Fig. 7. Backfillingby slinger truck.
For the backfilling of emplacement drifts for POLLUX casks, only pneumatic backfilling and slinger backfilling are suitable. Other techniques like gravity or hydraulic backfilling cannot be applied because the necessary density cannot be met or the addition of water should be avoided in a repository. In order to decide which of the two suitable backfilling techniques will be used in the thermal simulation test and, later on, in a repository, comprehensive screening tests were performed in the Asse mine on a 1:1 scale. The two techniques were compared and evaluated considering safety, technical and economic aspects [5]. As far as the backfill quality is concerned, no marked differences existed between the two techniques. A mobile slinger truck as shown in fig. 7 was chosen as reference for the main test mainly due to its higher flexibility compared to a stationary pneumatic backfilling unit, its higher reliability, its lower costs, and its lower dose burden to the plant personnel. In a fourth demonstration test, the "Active Handling Test", experiments with a neutron source will be performed in the Asse underground laboratory. The aim of this test is to measure the effect of neutron backscattering in rock salt during handling casks and canisters with spent fuel in the drifts and galleries of a repository. The time schedule for the demonstration tests is outlined in fig. 8. The "Simulation Test of Shaft Transport" which is of special importance for the realization of direct disposal with POLLUX casks will be finished by the end of 1990. All other test will last somewhat longer, but all data will be available by the end of 1993 at the latest. 3. 4. Systems analysis dual-purpose repository
Ifl-si! backlllieO OlSpOSel oriTI
Fig. 6. Test field for the thermal simulation test of drift emplacement.
Beside the aforementioned demonstration tests, the layout and optimization of all aboveground as well as subsurface process steps with regard to a common repository for both reprocessing waste and conditioned spent fuel is necessary [6]. Different mixes of spent fuel
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K.D. Closs, H.-J. Engelmann / FRG activities on direct disposal
Concept Variants
DemonstrationTest: Simulation el Shaft Transport
Reprocessing Plant
I Encapsulation Plant ]
-- Studyon ShelfTransport
-- Preparation -- Test Drift Handling.Test -- Preparation -- AbovegronndTest -- Untergrmind Test
HLW Canister
Interim Storage
d
I
Thermal Simulation Test -- Evaluationof BonkfllllngTonhnlque -- Preparatlen -- Test Active Handling Test -- Preparation -- Test vlnterim Result
Interim Storage
Repository
Borelmle Emplacement
I
Combined Boreholo/Drift Emplacement
Drift Emplacement
Fig. 9. Systems analysis dual-purpose repository.
Fig. 8. Time schedule for demonstration tests (4/88).
that is going to be disposed of directly and reprocessed, respectively, are the basis for this systems analysis "Dual-Purpose Repository". Concept variations are shown in fig. 9. They are generated by varying cooling times prior to disposal, by conceiving different conditioning and canister concepts, and finally by taking into consideration a number of repository design alternatives and emplacement techniques. The set of repository alternatives, as outlined in fig. 10, will include either vertical borehole or drift emplacement as well as a combination of both concepts. Emplacement drifts and boreholes will be located either in different sectors of the model salt dome or in one common sector. Emplacement of heavy disposal casks will even be studied for drifts being arranged at different levels of the repository. Conceptual design of the various repository and emplacement concepts will be accompanied by geothermal and thermomechanical calculations. The pros and cons of the system variants will be determined by quantifying the performance criteria "expenditure until technical maturity", "radiological safety during routine operation", long-term safety of the repository", and "cost". The final evaluation which is scheduled for mid-1989 will also include the areal requirements of the disposal concepts and aspects of international safeguards.
An interim result of the thermal repository layout is shown in fig. 11. For an annual amount of 500 and 200 Mg of spent LWR fuel that is reprocessed and disposed BoraholeEmpimnuM:
~rlml i
Orm Emplacement:
Bin
R~lng
~a
SpentFuel
k ~ l e + 0tlRE m
gonn~ ~10re:
-m! i
km~ueWng ~ m
$1md Fml Fig. 10. Emplacement concepts.
Wante
K.D. Closs, H.-J. Engelmann / FRG activitieson direct disposal LWR: 500 Mg/yr Reproeessing 200 Mg/yr Direct Disposal HTR: 1 MillionSpheres/yr 1,61
~ T~ <_ 200° C I'~1 Ts,. ~ iOOoC
i.............. i iiili
i
2~
iiiiiii i!ii ::::::::::::::::::::::::::: [~ii!!!ii:i:::::::::i
1,03
iiiii!!iiii!iiii!!! iiii!iiiii~ii!ii ¸
iiiii!iiiiii! !iiiiiii!ii
0,92 i!i!i!i!i!i!i!i!!!i!!!!!! iiiiiiiiiSF,i!iliiiil ::::::::::::::::::::::::::::: MLW !!i!i!!!!!~::i:jii:: ...............
iiii::iiHLW::i::i::i
::~::~:;MLW;~:~
MLW HTR HTR A D Combined Borehole/DrHt Emplacement
iiiilili ill
HTR HTR ! ::::::::::::::::::::::::::: BL BL* SL 3 SL 3" Borehole Three Level Drill Emplacement Emplacement • = idvanud
MLWtrlmtm~
Fig. 11. Annual repository area requirements (ha/yr) for heatgenerating wastes.
of directly, respectively, as well as 1 million fuel spheres/yr from high-temperature gas cooled reactors the repository area requirements are compared for different emplacement concepts whereby cooling-times of 30 years for spent fuel (SF) and 40 years for high level reprocessing waste (HLW) have been chosen. Maximum temperatures of 200°C in the package-salt interface for vitrified high-level reprocessing waste or spent fuel and 100°C for beat-generating medium- level reprocessing waste embedded in a concrete matrix served as a design basis. It should be underscored, however, that the absolute values of annual repository area requirements in this figure should not be overstressed, since detailed repository designs lead to areas that are about 25 to 40% larger. MoreoVer, choosing another design basis like avoidance of tensile stresses in the top of the salt dome might somewhat change the absolute values of area requirements. What really counts is the relative difference between the different emplacement concepts. For concept A, both kinds of reprocessing wastes as well as the HTR fuel spheres are disposed of in boreholes, whereas spent LWR fuel encapsulated in POLLUX casks is emplaced in drifts in a separate repository sector. Concept D is similar, but some POLLUX casks with spent LWR fuel are emplaced in galleries above
105
the boreholes for HLW. For the pure borehole emplacement concept BL, shortened LWR fuel rods inserted into POLLUX canisters are disposed of together with HLW canisters in the same boreholes of the repository. SL3 denotes a three level drift emplacement concept where both kinds of reprocessing wastes as well as spent LWR and HTR fuel are placed into large and heavily shielded casks. The two concepts BL* and SL3* differ from BL and SL3 inasmuch as the heat-generating medium-level reprocessing waste, MLW, is no longer embedded in a concrete matrix, but a more heat resistant matrix is used so that this waste form can be coemplaced with HLW and spent fuel. This figure demonstrates that a potential exist for reducing the annual repository area requirements by changing the waste treatment concept for MLW. The overall budget of the repository-related activities is about DM 100 million, spread over the years 1986 to 1993. Out of this sum, DM 80 million are funded by the Federal Ministry of Research and Technology (BMFT), and DM 20 million are contributed by Kernforschungszentrum Karlsruhe. The 1 : 1 scale demonstration tests cost about DM 57 million, whereas DM 14 million will be needed for the systems analysis studies, DM 18 million for an experimental laboratory program to investigate canister corrosion and spent fuel leaching. The rest is earmarked for project management.
4. Conclusion In addition to the realization of the fuel cycle based on reprocessing, spent fuel disposal without reprocessing has been investigated during the last years. After having proven the principal technical feasibility of this waste management route, the program has reached a new phase: During the next years, important aspects of this back-end fuel cycle concept will be demonstrated. For this reason, a pilot conditioning and encapsulation plant will be built at Gorleben and repository related demonstration tests will be performed on a 1:1 scale. The aim of the ongoing program on spent fuel disposal is to incorporate its results in the licensing procedure of the Gorleben repository which will start as soon as the results of the underground investigations prove the suitability of the salt dome. This means that in addition to the reprocessing and recycling waste management concept - direct disposal of spent fuel not suitable for reprocessing could be introduced in the future for completely closing the back-end of the fuel cycle.
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K.D. Closs, H.-J. Engelmann / F R G activities on direct disposal
References [1] Systems Study Alternative Entsorgung, Executive Summary, Kernforschungszentrum Karlsruhe (1985). Systemstudie Andere Entsorgungstechniken, Hauptband (in German), Kernforschungszentrum Karlsruhe (1984). System Study of Alternative Waste Management Technologies, Main Volume, Translated from the German, ORNL/TR-86/31, Oak Ridge National Laboratory (1986). [2] K. Einfeld, Pilot plant for spent fuel conditioning at Godeben, Nuclear Europe 3-4 (1987) 21. [3] T. Rothfuchs, R. Stippler, High level waste disposal project in the Asse salt mine (FRG), Prec. Waste Management '87 Symposium, (Tucson, Arizona, 1987), pp. 89-95.
[4] B. Hartje, C. Schrimpf, W. Weber, Technical maturity of shaft hoisting facilities for the 65 t-heavy casks with spent fuel, Prec. Conf. on Uranium and Electricity, the Complete Nuclear Fuel Cycle, Saskatoon, 1988, to appear. [5] H. J. Engelmann et al., Thermal simulation of drift emplacement for direct disposal of spent fuel, evaluation of the backfilling technique, Prec. Conf. on Uranium and Electricity, the Complete Nuclear Fuel Cycle, Saskatoon, 1988, to appear. [6] R. Papp, K.D. Closs, Results of the German alternative fuel cycle evaluation and further efforts geared toward demonstration of direct disposal, Prec. Waste Management "86 Symposium, Vol. 2 (Tucson, Arizona, 1986) pp. 523-526.
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STATUS OF THE FRG ACTIVITIES ON DIRECT DISPOSAL OF SPENT LWR-FUEL K.D. C L O S S 1 a n d H.-J. E N G E L M A N N 2 t Kernforschungszentrum Karlsruhe, Projektgruppe Andere Entsorgungstechniken, P. 0. Box 3640, D-7500 Karlsruhe 1, Fed. Rep. Germany 2 Deutsche Gesellschaft zum Bau und Betrieb yon Endlagern f~r Abfallstoffe, Woltorfer Str. 74, D-3150 Peine 1, FeR Rep. Germany
Received: first version 7 September 1988, revised version 12 May, 1989
As stipulated by the German Atomic Energy Act, reprocessing is the reference waste management route for LWR's in the Federal Republic of Germany (FRG). Spent fuel disposal without reprocessing is being developed to technical maturity for those fuel elements for which reprocessing is either technically not feasible or economically not justifiable. The reference concept for direct disposal is the emplacement of large and heavily-shieldedcasks in drifts of a repository mine located in a salt dome. Moreover, a back-up solution is being pursued which results in smaller canisters which are emplaced in boreholes. The mining authorities have pointed out that the feasibility of direct disposal is to be demonstrated before a license for industrial scale deployment could be granted. Demonstration tests are necessary in the followingareas: shaft transport of large and heavily shielded casks, handling of the casks in the repository and thermal and rock mechanics investigations with respect to the drift emplacement concept. The results of the demonstrations tests as well as the results from layout and optimization studies for a common repository for both reprocessing waste and spent fuel will be available early enough to be incorporated into the licensing procedure for the FRG's first repository for heat-generating nuclear wastes. This means that direct disposal of spent fuel not suitable for reprocessing could be introduced in the future in addition to the reprocessing and recycling waste management concept.
I. Introduction As stipulated by the German Atomic Energy Act, reprocessing is the reference waste management route for LWR's in the Federal Republic of Germany (FRG). After abandoning the construction of a domestic reprocessing plant, the F R G will rely on foreign reprocessing services for the next decades. Spent fuel disposal without reprocessing is being developed to technical maturity for those fuel elements for which reprocessing is either technically not feasible or economically not justifiable. Up to 1984, all funds for R & D in this area were provided by the Federal Ministry of Research and Technology (BMFT). After the technical feasibility of direct disposal had been demonstrated in principle by the Systems Study Alternative Entsorgung [1], the responsibilities were newly assigned: DWK, the German reprocessing company, has to continue cask and spent fuel treatment development work on its own. The Federal Government is supporting work only in the area of its responsibility, namely the repository.
In May 1986, DWK initiated the licensing procedure for a pilot conditioning and encapsulation plant to be built at Gofleben/Lower Saxony directly adjacent to the away-from-reactor (AFR) interim storage facility of DWK [2]. A first construction license for the pilot plant is expected in 1990. The repository-related R & D work is being supported by the BMFT and coordinated by the project group PAE at Kernforschungszentrum Karlsruhe (KfK) Major contributions to the program are made by DBE (Deutsche GeseUschaft zum Bau und Betrieb von Endlagem ftir Abfallstoffe), GSF (Gesellschaft fur Strahlenund Umweltforschung, Institut f't~r Tieflagerung), BGR (Bundesanstalt fiir Geowissenschaften und Rohstoffe), and KfK,
2. Technical concepts The two spent fuel conditioning and encapsulation techniques used in the DWK pilot plant are outlined in fig. 1. The process starts with disassembling the fuel
0 0 2 9 - 5 4 9 3 / 9 0 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V.
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K.D. Closs, H.-J. Engelmann / FRG activities on direct disposal
of the disposal galleries. The technique for handling such canisters in a repository has been developed and will be tested soon [3]. This paper will not deal with this topic, but will mainl~¢ concentrate on such aspects associated with the POL]LUX casks. The POLLUX cask which is designed for shipping as well as for interim storage and disposal fulfills the IAEA requirements (Type B(U)). Moreover, it features a corrosion protection layer that acts as a long-term barrier in the repository. The handling and emplacement procedure at the repository is shown in fig. 2. After having been shipped by rail or road to the surface facilities of the repository, the cask passes through the entrance control in the transfer hall. Next, it is placed on a railroad car which is conveyed to the shaft and pushed into the shaft cage. After shaft hoisting the loaded railroad car is pulled from the hoisting cage at the shaft landing. The loaded railroad car is taken over by a mine locomotive at the shaft landing and transferred along the pilot heading to the emplacement site. At a by-pass near the branch of the cross drift, the locomotive is placed behind the railroad car. After recoupling the locomotive pushes the railroad car through the cross drift into the emplacement drift to the intended emplacement position. Fig. 1. Spent fuel conditioning and encapsulation techniques.
assemblies. In the reference concept (shown on the left-hand side) intact consolidated rods from 8 PWR spent fuel assemblies together with the compacted skeletons of the fuel assemblies are placed into the disposal canister. The canister is inserted into a shielding overpack which is designed such that the personnel in the conditioning and encapsulation plant as well as in the repository can handle the package hands-on. Such a package is called POLLUX cask and is disposed of in the drifts of a mined geologic repository. Since handling such heavy (65 Mg) and large (5.5 m long) casks in a repository mine is not state-of-the-art, a back-up solution for spent fuel conditioning and encapsulation resulting in smaller spent fuel packages is being pursued. This concept is shown on the fight-hand side of fig. 1. The POLLUX canisters have the same outer dimensions as the canisters for vitrified reprocessing waste. In this case the disassembled fuel pins have to be cut into pieces of about 1 m in length. The advantage of this concept is that the handling and emplacement techniques in the repository are the same as for vitrified waste, namely the emplacement of the canisters into 300-m-deep boreholes, drilled in the floor
Emplacement Procedure Fig. 2. Handling and emplacement procedure.
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K.D. Closs, H.-J. Engelmann / FRG activities on direct disposal
At the emplacement position, the cask is lifted from the railroad car by an emplacement device which is designed for lifting and depositing the cask on the floor after removal of the wagon. The railroad car is pulled out of the emplacement drift all the way back to the surface for reloading. The drift section accommodating the emplaced cask is backfilled immediately with crushed salt.
3. R & D program direct disposal The overall aim of the program is to develop direct disposal of spent fuel to technical maturity. With respect to the reference concept, the mining authorities have pointed out that the feasibility of direct disposal is to be demonstrated before a license for deployment on an industrial scale could be granted. Thus, the main thrust of the program lies with the demonstration of the safe and reliable handling of large and heavy casks in a repository on a 1 : 1 scale. Emphasis is also laid on the investigation of special thermal and rock mechanics aspects of the drift emplacement concept as well as on the layout and optimization of a repository for both reprocessing waste and conditioned spent fuel. Demonstration tests are necessary in the following areas: (1) Shaft transport with loading and unloading of the hoisting cage will be simulated in an aboveground test rig with inactive casks. (2) Machinery and devices for the transport of the casks into the disposal drift as well as for lifting and lowering the cask at the disposal position are to be developed and tested. (3) Thermal and rock mechanics investigations for the drift emplacement concept will be performed with electrically heated casks in the Asse underground laboratory. In the following, these demonstration tests are described in some detail. Moreover, the overall aim and some preliminary results of the repository layout and optimization activities are stated. 3.1. Simulation of shaft transport
The realization of direct disposal with POLLUX casks depends on the feasibility to lower up to 85 Mg (65 Mg POLLUX cask + 20 Mg railroad car) in a shaft hoisting facility. Shaft hoisting facilities with such a payload have not been built yet worldwide. As a first step the main components of such a shaft hoist were designed preliminarily. The important design parameters are given in table 1.
Table 1 Design parameters for the hoisting facility General data hoisting system
hoist allocation conveyingspeed in the case of heavy load (normal load) number of breaking force generators payload in the case of heavy load (normal load)
koepe hoist cage and counterweight tower-mounted 5 m/s (12 m/s) 24×2 85 Mg (30 Mg)
Hoist multiple rope hoist 2 direct-current shunt-wound motors rated power approx.
direct drive 2 x 2800 kW
Hoisting cage available width available length weight
max. 2600 mm max. 6400 mm approx. 38 Mg
8 ropes
Hoisting ropes head rope (type of manufacture) i.e. flat-strand rope (approx. 51 mm) tail rope (type of manufacture) i.e. flat-strand rope
The hoist system chosen is a tower-mounted friction hoist with 8 ropes, using a cage and a counterweight. The conveying speed for normal load up to 30 Mg is 12 m/s, whereas it is only 5 m / s when the POLLUX casks are lowered. The hoist is directly driven by two directcurrent shunt-wound motors with a power of 2800 kW each. The hoisting cage has a width of 2.6 m and a length of 6.4 m allowing the POLLUX casks to be transported in the shaft in a horizontal position. In order to demonstrate the maturity of the technology a study about facilities in operation was carried out. The basic data of 110 hoisting systems worldwide were collected. 93 of these systems were analyzed in more detail and compared with the specifications of the 85Mg-system. The sources of the data were constructional and reference specifications as well as publications in the technical literature. The conclusion of this investigation was that for almost all the essential components of the hoisting system comparable installations are in operation from which their suitability for this application could be demonstrated. For some components that must meet special requirements if they are employed for nuclear waste disposal, this has still to be demonstrated [4]. In order to provide such evidence a full-scale test is to be
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K.D. Closs, 1t.-J. EngelmanP, "FRG activities on direct disposal
Fig. 4. Rail transport of POLLUX casks.
c a 0 a
b o t t o m
Fig. 3. Diagrammatic sketch of the test rack for demonstrating the shaft transport.
- minimize the overall weight of the transport vehicle in order to reduce the payload of the shaft hoisting system. Several alternatives were investigated, and, finally, a four-axle railroad car with parabola springs was chosen as shown in fig. 4. The main advantages of this design are the design is state-of-the-art in all important components, - the bogie wheel system is standard used by the Deutsche Bundesbahn (German Federal Railroad), - the load for the system w h e e l / t r a c k is in the same range as that of the Deutsche Bundesbahn, - a reduction in height seems possible when designing the vehicle in detail. In fig. 5, the emplacement device for the P O L L U X cask is shown. It is intended to use electrically driven screw jacks. Those devices for lifting heavy loads are state-of-the-art, but they have to be adapted to the special conditions of a repository mine in a salt formation. Development is mainly necessary with respect to reducing the height of the device and, thus, the height of -
carried out. It is planned to construct and test the following components in the empty generator building of a decommissioned power station: cage, diagonal arrangement of the guide rails intermediate cage bottom support for loading, - mechanical arrestment systems for overrunning, - charging unit, rail transport vehicle with test cask. A diagrammatic sketch of the planned test rack which is now under design and construction is given in fig. 3. -
-
-
-
3.2. Drift handling test
In another test, the handling of the large and heavy P O L L U X casks in the limited space of a repository mine is to be demonstrated. For this purpose the railroad car as well as the emplacement device will be designed and tested. Figure 4 is a schematic view of the rail transport with the P O L L U X loaded on the car. Major development efforts are devoted to - minimize the height of the car in order to minimize drift cross sections, - allow transport with small track radius in order to utilize the given salt volume in an optimal way,
Fig. 5. Emplacement device for POLLUX casks.
K.D. Closs, H.-J. Engelmann / FRG activities on direct disposal
103
the drifts. This is advantageous for thermal reasons, mine stability, and economics. 3.3. Thermal simulation test A third demonstration test deals with the behavior of the rock salt and the crushed salt backfill under the specific conditions of the drift emplacement concept. For this test, two parallel drifts are excavated at the 800-m-level of the Asse underground laboratory, as shown in fig. 6. Each drift accommodates three electrically heated casks with an electric power of 10 kW, each to simulate the decay heat of the spent fuel. The dummy casks have the same dimensions and the same weight as the original POLLUX casks. The space surrounding the casks will be backfilled with crushed salt. Different types of measuring instruments are installed in the crushed salt and surrounding strata in order to record and evaluate all relevant parameters like temperatures as a function of time at selected points on the cask surface, in the backfill material, and in the surrounding rock salt, - compaction, porosity, and permeability of the backfill material in the thermally exposed emplacement drifts, - stresses and strains in the rock salt as well as drift convergence in the thermally exposed emplacement drifts. The main objective of this experiment on a 1 : 1 scale is to verify thermal and thermomechanical computer codes in order to have tested codes available in a future licensing procedure. -
demonstration of backfllllno technique
Fig. 7. Backfillingby slinger truck.
For the backfilling of emplacement drifts for POLLUX casks, only pneumatic backfilling and slinger backfilling are suitable. Other techniques like gravity or hydraulic backfilling cannot be applied because the necessary density cannot be met or the addition of water should be avoided in a repository. In order to decide which of the two suitable backfilling techniques will be used in the thermal simulation test and, later on, in a repository, comprehensive screening tests were performed in the Asse mine on a 1:1 scale. The two techniques were compared and evaluated considering safety, technical and economic aspects [5]. As far as the backfill quality is concerned, no marked differences existed between the two techniques. A mobile slinger truck as shown in fig. 7 was chosen as reference for the main test mainly due to its higher flexibility compared to a stationary pneumatic backfilling unit, its higher reliability, its lower costs, and its lower dose burden to the plant personnel. In a fourth demonstration test, the "Active Handling Test", experiments with a neutron source will be performed in the Asse underground laboratory. The aim of this test is to measure the effect of neutron backscattering in rock salt during handling casks and canisters with spent fuel in the drifts and galleries of a repository. The time schedule for the demonstration tests is outlined in fig. 8. The "Simulation Test of Shaft Transport" which is of special importance for the realization of direct disposal with POLLUX casks will be finished by the end of 1990. All other test will last somewhat longer, but all data will be available by the end of 1993 at the latest. 3. 4. Systems analysis dual-purpose repository
Ifl-si! backlllieO OlSpOSel oriTI
Fig. 6. Test field for the thermal simulation test of drift emplacement.
Beside the aforementioned demonstration tests, the layout and optimization of all aboveground as well as subsurface process steps with regard to a common repository for both reprocessing waste and conditioned spent fuel is necessary [6]. Different mixes of spent fuel
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K.D. Closs, H.-J. Engelmann / FRG activities on direct disposal
Concept Variants
DemonstrationTest: Simulation el Shaft Transport
Reprocessing Plant
I Encapsulation Plant ]
-- Studyon ShelfTransport
-- Preparation -- Test Drift Handling.Test -- Preparation -- AbovegronndTest -- Untergrmind Test
HLW Canister
Interim Storage
d
I
Thermal Simulation Test -- Evaluationof BonkfllllngTonhnlque -- Preparatlen -- Test Active Handling Test -- Preparation -- Test vlnterim Result
Interim Storage
Repository
Borelmle Emplacement
I
Combined Boreholo/Drift Emplacement
Drift Emplacement
Fig. 9. Systems analysis dual-purpose repository.
Fig. 8. Time schedule for demonstration tests (4/88).
that is going to be disposed of directly and reprocessed, respectively, are the basis for this systems analysis "Dual-Purpose Repository". Concept variations are shown in fig. 9. They are generated by varying cooling times prior to disposal, by conceiving different conditioning and canister concepts, and finally by taking into consideration a number of repository design alternatives and emplacement techniques. The set of repository alternatives, as outlined in fig. 10, will include either vertical borehole or drift emplacement as well as a combination of both concepts. Emplacement drifts and boreholes will be located either in different sectors of the model salt dome or in one common sector. Emplacement of heavy disposal casks will even be studied for drifts being arranged at different levels of the repository. Conceptual design of the various repository and emplacement concepts will be accompanied by geothermal and thermomechanical calculations. The pros and cons of the system variants will be determined by quantifying the performance criteria "expenditure until technical maturity", "radiological safety during routine operation", long-term safety of the repository", and "cost". The final evaluation which is scheduled for mid-1989 will also include the areal requirements of the disposal concepts and aspects of international safeguards.
An interim result of the thermal repository layout is shown in fig. 11. For an annual amount of 500 and 200 Mg of spent LWR fuel that is reprocessed and disposed BoraholeEmpimnuM:
~rlml i
Orm Emplacement:
Bin
R~lng
~a
SpentFuel
k ~ l e + 0tlRE m
gonn~ ~10re:
-m! i
km~ueWng ~ m
$1md Fml Fig. 10. Emplacement concepts.
Wante
K.D. Closs, H.-J. Engelmann / FRG activitieson direct disposal LWR: 500 Mg/yr Reproeessing 200 Mg/yr Direct Disposal HTR: 1 MillionSpheres/yr 1,61
~ T~ <_ 200° C I'~1 Ts,. ~ iOOoC
i.............. i iiili
i
2~
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0,92 i!i!i!i!i!i!i!i!!!i!!!!!! iiiiiiiiiSF,i!iliiiil ::::::::::::::::::::::::::::: MLW !!i!i!!!!!~::i:jii:: ...............
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MLW HTR HTR A D Combined Borehole/DrHt Emplacement
iiiilili ill
HTR HTR ! ::::::::::::::::::::::::::: BL BL* SL 3 SL 3" Borehole Three Level Drill Emplacement Emplacement • = idvanud
MLWtrlmtm~
Fig. 11. Annual repository area requirements (ha/yr) for heatgenerating wastes.
of directly, respectively, as well as 1 million fuel spheres/yr from high-temperature gas cooled reactors the repository area requirements are compared for different emplacement concepts whereby cooling-times of 30 years for spent fuel (SF) and 40 years for high level reprocessing waste (HLW) have been chosen. Maximum temperatures of 200°C in the package-salt interface for vitrified high-level reprocessing waste or spent fuel and 100°C for beat-generating medium- level reprocessing waste embedded in a concrete matrix served as a design basis. It should be underscored, however, that the absolute values of annual repository area requirements in this figure should not be overstressed, since detailed repository designs lead to areas that are about 25 to 40% larger. MoreoVer, choosing another design basis like avoidance of tensile stresses in the top of the salt dome might somewhat change the absolute values of area requirements. What really counts is the relative difference between the different emplacement concepts. For concept A, both kinds of reprocessing wastes as well as the HTR fuel spheres are disposed of in boreholes, whereas spent LWR fuel encapsulated in POLLUX casks is emplaced in drifts in a separate repository sector. Concept D is similar, but some POLLUX casks with spent LWR fuel are emplaced in galleries above
105
the boreholes for HLW. For the pure borehole emplacement concept BL, shortened LWR fuel rods inserted into POLLUX canisters are disposed of together with HLW canisters in the same boreholes of the repository. SL3 denotes a three level drift emplacement concept where both kinds of reprocessing wastes as well as spent LWR and HTR fuel are placed into large and heavily shielded casks. The two concepts BL* and SL3* differ from BL and SL3 inasmuch as the heat-generating medium-level reprocessing waste, MLW, is no longer embedded in a concrete matrix, but a more heat resistant matrix is used so that this waste form can be coemplaced with HLW and spent fuel. This figure demonstrates that a potential exist for reducing the annual repository area requirements by changing the waste treatment concept for MLW. The overall budget of the repository-related activities is about DM 100 million, spread over the years 1986 to 1993. Out of this sum, DM 80 million are funded by the Federal Ministry of Research and Technology (BMFT), and DM 20 million are contributed by Kernforschungszentrum Karlsruhe. The 1 : 1 scale demonstration tests cost about DM 57 million, whereas DM 14 million will be needed for the systems analysis studies, DM 18 million for an experimental laboratory program to investigate canister corrosion and spent fuel leaching. The rest is earmarked for project management.
4. Conclusion In addition to the realization of the fuel cycle based on reprocessing, spent fuel disposal without reprocessing has been investigated during the last years. After having proven the principal technical feasibility of this waste management route, the program has reached a new phase: During the next years, important aspects of this back-end fuel cycle concept will be demonstrated. For this reason, a pilot conditioning and encapsulation plant will be built at Gorleben and repository related demonstration tests will be performed on a 1:1 scale. The aim of the ongoing program on spent fuel disposal is to incorporate its results in the licensing procedure of the Gorleben repository which will start as soon as the results of the underground investigations prove the suitability of the salt dome. This means that in addition to the reprocessing and recycling waste management concept - direct disposal of spent fuel not suitable for reprocessing could be introduced in the future for completely closing the back-end of the fuel cycle.
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K.D. Closs, H.-J. Engelmann / F R G activities on direct disposal
References [1] Systems Study Alternative Entsorgung, Executive Summary, Kernforschungszentrum Karlsruhe (1985). Systemstudie Andere Entsorgungstechniken, Hauptband (in German), Kernforschungszentrum Karlsruhe (1984). System Study of Alternative Waste Management Technologies, Main Volume, Translated from the German, ORNL/TR-86/31, Oak Ridge National Laboratory (1986). [2] K. Einfeld, Pilot plant for spent fuel conditioning at Godeben, Nuclear Europe 3-4 (1987) 21. [3] T. Rothfuchs, R. Stippler, High level waste disposal project in the Asse salt mine (FRG), Prec. Waste Management '87 Symposium, (Tucson, Arizona, 1987), pp. 89-95.
[4] B. Hartje, C. Schrimpf, W. Weber, Technical maturity of shaft hoisting facilities for the 65 t-heavy casks with spent fuel, Prec. Conf. on Uranium and Electricity, the Complete Nuclear Fuel Cycle, Saskatoon, 1988, to appear. [5] H. J. Engelmann et al., Thermal simulation of drift emplacement for direct disposal of spent fuel, evaluation of the backfilling technique, Prec. Conf. on Uranium and Electricity, the Complete Nuclear Fuel Cycle, Saskatoon, 1988, to appear. [6] R. Papp, K.D. Closs, Results of the German alternative fuel cycle evaluation and further efforts geared toward demonstration of direct disposal, Prec. Waste Management "86 Symposium, Vol. 2 (Tucson, Arizona, 1986) pp. 523-526.