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Performance and remote maintenance of attachment schemes for Plasma Facing Components
Fusion Engineering and Design 58 – 59 (2001) 463– 467 www.elsevier.com/locate/fusengdes
Performance and remote maintenance of attachment schemes for Plasma Facing Components J. Palmer a,*, S. Chiocchio b, C. Damiani c, M. Irving c, D. Maisonnier a, E. Martin b, A. Poggianti d, M. Siuko e, A. Turner f a EFDA, Boltzmannstr. 2, D-85748 Garching, Germany ITER Joint Work Site, Boltzmannstr. 2, D-85748 Garching, Germany c ENEA, CR Brasimone, CP 1, I-40032 Camugnano (Bo), Italy d ENEA, 6ia Martiri di Montesole 4, I-40129 Bologna, Italy e IHA, Korkeakoulunkatu 2, Box 589, FIN-33101 Tampere, Finland f NNC Limited, Booths Hall, Chelford Road, Knutsford, Cheshire WA16 8QZ, England, UK b
Abstract The divertor design for the ITER-FEAT fusion reactor is based on cassettes which comprise a reusable body and three sacrificial Plasma Facing Components (PFCs) expected to be replaced in a hot-cell a number of times during machine lifetime. Central to this maintenance approach are the PFC-to-cassette attachments which must be readily assembled/disassembled by remote handling methods and withstand severe mechanical and thermal loading conditions during machine operation. This paper describes the facilities, equipment and methods used to carry out extensive testing of two attachment schemes, shear keys and multi-links, in order to assess their in-service performance and suitability to remote maintenance operations. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Remote maintenance; Attachment schemes; Plasma facing components
1. Introduction The divertor assembly for the ITER-FEAT fusion reactor consists of 54 rail-mounted cassettes located in the bottom region of the vacuum vessel. These cassettes comprise a stainless steel body, designed for the full lifetime of ITER, onto which are mounted three sacrificial Plasma Facing Components (PFCs). The latter are designed to be replaced a number of times during the lifetime of the machine to allow for component failure, ero* Corresponding author.
sion of the armour or to implement alternative divertor geometries. Central to this maintenance approach are the PFC-to-cassette attachments which must be readily assembled/disassembled by remote handling methods and withstand severe mechanical and thermal loading conditions during machine operation. During the ITER EDA two such attachment concepts have been considered. The first is based on separate shear keys inserted and locked into keyways integral with the cassette body and PFC. The second is the so called ‘‘multi-link’’ scheme which employs a set of target-to-cassette connecting links secured by an
0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 1 ) 0 0 2 5 2 - 6
464
J. Palmer et al. / Fusion Engineering and Design 58–59 (2001) 463–467
expanded linkpin to form a hinge. The design rationale behind these alternative schemes is described in Ref. [1]. In order to qualify these PFC attachment schemes with regard to in-service performance and suitability to remote maintenance operations, an extensive test program has been developed and executed by the EU-HT in a dedicated test facility (the Divertor Refurbishment Platform, DRP) located at the ENEA Research Centre in Brasimone, Italy (see Fig. 1). This facility has been used to simulate, at full scale, PFC assembly/disassembly and inspection/metrology on the cassette mock-up without direct human intervention.
2. Shear key remote handling trials The test pieces used during these trials represent the lower attachment hinge-type keys which comprise a cylinder connected by a solid tie-bar to a dovetail (see Fig. 2). The cylinder assembly is locked into the PFC-side keyway by two integral aluminium –bronze wedges which radially expand interfacing locking segments. The dovetail is similarly locked into the cassette-side keyway by a single wedge. Insertion and extraction of the key, and in particular insertion of the wedges to a prescribed load, is carried out using purpose built water hydraulic tooling [2]. Following a number of re-
Fig. 2. Shear key attachment.
furbishment test campaigns involving stepwise improvements to the refurbishment procedures and further development of the operating environment, the critical insertion and extraction operations could be achieved under fully remote conditions. It should be noted however, that during earlier trials, jamming of the key in either of the two keyways was a common occurrence due to the low key/keyway clearances of 0.5 mm. This was overcome by the development of a complex system of sensors (e.g. laser pointers, inclinometers, range sensors) and a comprehensive data acquisition system to ensure that the key aligned perfectly with both keyways during the insertion/ extraction processes.
3. Multi-link remote handling trials
Fig. 1. DRP hot-cell environment.
The ‘multi-links’ scheme for attaching the PFC target to the cassette body involves a series of interconnecting links secured to the target and cassette by internally expanded hollow linkpins (see Fig. 3). The pin is expanded using purpose built water hydraulic tooling [3]. Alignment of the array of target-side links with the cassette-side housing is achieved by an alignment tool attached to the target prior to assembly. Removal of the pins is achieved by drilling-out the central core to leave a thin shell which can easily be pushed out of the hole. Initial trials have shown that the remotisa-
J. Palmer et al. / Fusion Engineering and Design 58–59 (2001) 463–467
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tion of the assembly process was relatively straightforward and has been achieved without the need for the sophisticated operator assistance developed for the shear key tests. The pin removal process, although conceptually sound, is less reliable at present and requires further development.
4. Shear key technological tests For the shear keys, the principal operating criterion is for the key locking components to provide even and gap free contact in the keyway. This was initially assessed by monitoring the distribution of the forces applied to each cylinder wedge when both were simultaneously inserted to equal depths. The total applied load was found to be somewhat unevenly distributed, the extreme distribution being 60:40, which is likely to be an effect of machining tolerances. In general, extraction forces were found to be approximately 80% of the insertion force. To assess the integrity and security of the attachment during differential thermal movements, the cylindrical end of the key was secured within an aluminium– bronze support block and cyclically articulated over an angular range of 9 0.1° (see Fig. 4). Two shear key test assemblies, designated ‘‘key A’’ and ‘‘key B’’, were tested, the target test duration being 4000 cycles. For key A,
Fig. 3. The multi-link scheme. 1, Target; 2, cassette; 3, pin; 4, link.
Fig. 4. Articulation test set-up.
the test was terminated after only 1500 cycles because the articulation force had fallen to below half its original value. Repetition of the test produced a similar result after only 600 cycles. In both cases one of the two cylinder wedges was found to be loose; a different wedge in each case. Conversely, for key B, the articulation force was found to rise steadily during the test, almost doubling after 6500 cycles. Inspection of key A cylinder wedges and locking segments identified a mismatch between the angled faces of the order of 0.05° which is outside the design specification of 0.02°. This may have contributed to the unsatisfactory performance of the assembly and illustrates the criticality of manufacturing accuracy to the shear key design. For key B, where the angled faces were within specification, post-test localised damage was evident on the key body and the keyway. In addition, the combined wedge extraction force for the cylinder was found to be approximately 8000 N (compared to the 4500 N insertion force) which aligns with the increase in articulation force. The reasons for the damage and the tightening of the wedges during testing have not specifically been identified. To assess the ability of the keys to withstand large in-service loads during plasma disruptions, cyclic loading tests were carried out with the keys mounted in inconel support blocks. Each key was subjected to 100 cycles over load ranges of +50 to −350 kN for key A and +250 to − 50 kN for key B. Results indicate that the keys function elastically within these load ranges, but wedge
J. Palmer et al. / Fusion Engineering and Design 58–59 (2001) 463–467
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extraction measurements differed significantly from those obtained during the preliminary insertion/extraction cycles (see Table 1). It is evident that for key B the cylinder wedge extraction load was extraordinarily large. This is possibly due to an effect of keyway deformation and/or relative movement between the locking components and the key body during the predominantly tensile loading cycle. It is also evident that, during the same test, the dovetail wedge for key B had loosened somewhat. These observations not only have an implication with regard to attachment security but also have an impact on the design of tooling, particularly with regard to maximum load capacity for extraction. Heat evacuation capability was assessed using a test cylinder (identical to the cylindrical head of the key) locked inside a water-cooled thick-walled aluminium– bronze tube. The whole assembly was housed within a vacuum chamber and heated by a cartridge heater located along the central axis of the cylinder. The test measurements indicate that, although the heat evacuation appears to be nonuniform along the length of the cylinder, the thermal in-service performance of the shear key is expected to be satisfactory, with an average temperature of the key being nominally 100 °C greater than that of the keyway.
5. Multi-link technological tests The first step in qualifying the ‘‘multi-link’’ attachment scheme was the identification of a reference geometry based on ITER-FEAT re-
Table 1 Load test insertion/extraction forces Key A Cylindera Insertion 5000 N Extraction 5694 N a
Both wedges.
Key B Dovetail
Cylindera
Dovetail
4500 N 3618 N
5000 N 15 283 N
4500 N 968 N
Table 2 Multi-link reference geometry Linkpin material Link/housing material Linkpin outer diameter Linkpin inner diameter Link/housing thickness Link/housing depth Link/housing pitch Number of links/target
Aluminium–bronze Stainless steel 316L 35 mm 17.5 mm Nominally 14 mm 110 mm 28 mm Four assemblies of six links
quirements (see Table 2). This was followed by an extensive test program involving ‘free-pin’ and ‘plate-pin’ test assemblies to define link/linkpin clearances, linkpin/housing clearances and associated swaging mandrel diameter to achieve the required joint characteristics. In this case, the principal operating criterion relates to the ability to perform the swaging process in a reliable and repetitive manner to provide uniform contact between the pin and the fixation holes. This is accompanied by a secondary requirement for the link to rotate around the pin, rather than the pin rotate in the cassette-side fixation hole, in order to facilitate re-use of the latter without re-machining. These tests showed that for 14 mm thick plates, pin/hole diametrical clearances (prior to expansion) of 0.509 0.02 mm and 0.459 0.02 mm resulted in post expansion torques of nominally 15 and 55 Nm, respectively, with an 18.8 mm mandrel. These values were chosen as references for the test program. Tests were carried out on a structure representative of a single link from the multi-link array. The implications of in-service loading was established by simultaneously applying loads up to 75 kN (which represents the equivalent ambient-temperature design load) in conjunction with 9 1° articulation of the link. Results showed that the joint remained elastic over a test duration of 100 cycles, but the torque required to articulate the joint was found to increase during the first 70 cycles of the test. The latter implies a change in the tribological characteristics of the joint, an aspect which requires further investigation. Thermal and electrical tests on the single link test piece will also be carried out.
J. Palmer et al. / Fusion Engineering and Design 58–59 (2001) 463–467
6. Conclusions The DRP facility is proving to be very effective in simulating a hot cell environment for testing divertor cassette maintenance scenarios. Cassette refurbishment trials involving the shear key and multi-link attachment schemes have demonstrated the basic feasibility of PFC replacement using both approaches. Shear key replacement however, requires a greater degree of operator assistance and skill than is required for multi-link. Technological tests, in particular for the shear keys, have illustrated the value of extensive endurance testing of entirely representative test pieces in the qualification of a design choice. Tests to date have not demonstrated the security of the shear keys during in-service operation or provided
467
positive confirmation of damage-free operation. If this design choice is to be pursued, further work in this area will be necessary. In the case of the multi-link scheme, although the tests indicate a good strength margin with respect to operating conditions, tribological issues require further investigation. References [1] S. Chiocchio, et al., The attachment system for the ITER divertor plasma facing components, 20th SOFT Marseille, 1998. [2] M. Siuko, et al., Water hydraulics in ITER divertor maintenance, 20th SOFT Marseille, 1998. [3] M. Siuko, et al., Tool prototypes for replacing divertor cassette plasma facing components for ITER, this symposium.
Performance and remote maintenance of attachment schemes for Plasma Facing Components J. Palmer a,*, S. Chiocchio b, C. Damiani c, M. Irving c, D. Maisonnier a, E. Martin b, A. Poggianti d, M. Siuko e, A. Turner f a EFDA, Boltzmannstr. 2, D-85748 Garching, Germany ITER Joint Work Site, Boltzmannstr. 2, D-85748 Garching, Germany c ENEA, CR Brasimone, CP 1, I-40032 Camugnano (Bo), Italy d ENEA, 6ia Martiri di Montesole 4, I-40129 Bologna, Italy e IHA, Korkeakoulunkatu 2, Box 589, FIN-33101 Tampere, Finland f NNC Limited, Booths Hall, Chelford Road, Knutsford, Cheshire WA16 8QZ, England, UK b
Abstract The divertor design for the ITER-FEAT fusion reactor is based on cassettes which comprise a reusable body and three sacrificial Plasma Facing Components (PFCs) expected to be replaced in a hot-cell a number of times during machine lifetime. Central to this maintenance approach are the PFC-to-cassette attachments which must be readily assembled/disassembled by remote handling methods and withstand severe mechanical and thermal loading conditions during machine operation. This paper describes the facilities, equipment and methods used to carry out extensive testing of two attachment schemes, shear keys and multi-links, in order to assess their in-service performance and suitability to remote maintenance operations. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Remote maintenance; Attachment schemes; Plasma facing components
1. Introduction The divertor assembly for the ITER-FEAT fusion reactor consists of 54 rail-mounted cassettes located in the bottom region of the vacuum vessel. These cassettes comprise a stainless steel body, designed for the full lifetime of ITER, onto which are mounted three sacrificial Plasma Facing Components (PFCs). The latter are designed to be replaced a number of times during the lifetime of the machine to allow for component failure, ero* Corresponding author.
sion of the armour or to implement alternative divertor geometries. Central to this maintenance approach are the PFC-to-cassette attachments which must be readily assembled/disassembled by remote handling methods and withstand severe mechanical and thermal loading conditions during machine operation. During the ITER EDA two such attachment concepts have been considered. The first is based on separate shear keys inserted and locked into keyways integral with the cassette body and PFC. The second is the so called ‘‘multi-link’’ scheme which employs a set of target-to-cassette connecting links secured by an
0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 1 ) 0 0 2 5 2 - 6
464
J. Palmer et al. / Fusion Engineering and Design 58–59 (2001) 463–467
expanded linkpin to form a hinge. The design rationale behind these alternative schemes is described in Ref. [1]. In order to qualify these PFC attachment schemes with regard to in-service performance and suitability to remote maintenance operations, an extensive test program has been developed and executed by the EU-HT in a dedicated test facility (the Divertor Refurbishment Platform, DRP) located at the ENEA Research Centre in Brasimone, Italy (see Fig. 1). This facility has been used to simulate, at full scale, PFC assembly/disassembly and inspection/metrology on the cassette mock-up without direct human intervention.
2. Shear key remote handling trials The test pieces used during these trials represent the lower attachment hinge-type keys which comprise a cylinder connected by a solid tie-bar to a dovetail (see Fig. 2). The cylinder assembly is locked into the PFC-side keyway by two integral aluminium –bronze wedges which radially expand interfacing locking segments. The dovetail is similarly locked into the cassette-side keyway by a single wedge. Insertion and extraction of the key, and in particular insertion of the wedges to a prescribed load, is carried out using purpose built water hydraulic tooling [2]. Following a number of re-
Fig. 2. Shear key attachment.
furbishment test campaigns involving stepwise improvements to the refurbishment procedures and further development of the operating environment, the critical insertion and extraction operations could be achieved under fully remote conditions. It should be noted however, that during earlier trials, jamming of the key in either of the two keyways was a common occurrence due to the low key/keyway clearances of 0.5 mm. This was overcome by the development of a complex system of sensors (e.g. laser pointers, inclinometers, range sensors) and a comprehensive data acquisition system to ensure that the key aligned perfectly with both keyways during the insertion/ extraction processes.
3. Multi-link remote handling trials
Fig. 1. DRP hot-cell environment.
The ‘multi-links’ scheme for attaching the PFC target to the cassette body involves a series of interconnecting links secured to the target and cassette by internally expanded hollow linkpins (see Fig. 3). The pin is expanded using purpose built water hydraulic tooling [3]. Alignment of the array of target-side links with the cassette-side housing is achieved by an alignment tool attached to the target prior to assembly. Removal of the pins is achieved by drilling-out the central core to leave a thin shell which can easily be pushed out of the hole. Initial trials have shown that the remotisa-
J. Palmer et al. / Fusion Engineering and Design 58–59 (2001) 463–467
465
tion of the assembly process was relatively straightforward and has been achieved without the need for the sophisticated operator assistance developed for the shear key tests. The pin removal process, although conceptually sound, is less reliable at present and requires further development.
4. Shear key technological tests For the shear keys, the principal operating criterion is for the key locking components to provide even and gap free contact in the keyway. This was initially assessed by monitoring the distribution of the forces applied to each cylinder wedge when both were simultaneously inserted to equal depths. The total applied load was found to be somewhat unevenly distributed, the extreme distribution being 60:40, which is likely to be an effect of machining tolerances. In general, extraction forces were found to be approximately 80% of the insertion force. To assess the integrity and security of the attachment during differential thermal movements, the cylindrical end of the key was secured within an aluminium– bronze support block and cyclically articulated over an angular range of 9 0.1° (see Fig. 4). Two shear key test assemblies, designated ‘‘key A’’ and ‘‘key B’’, were tested, the target test duration being 4000 cycles. For key A,
Fig. 3. The multi-link scheme. 1, Target; 2, cassette; 3, pin; 4, link.
Fig. 4. Articulation test set-up.
the test was terminated after only 1500 cycles because the articulation force had fallen to below half its original value. Repetition of the test produced a similar result after only 600 cycles. In both cases one of the two cylinder wedges was found to be loose; a different wedge in each case. Conversely, for key B, the articulation force was found to rise steadily during the test, almost doubling after 6500 cycles. Inspection of key A cylinder wedges and locking segments identified a mismatch between the angled faces of the order of 0.05° which is outside the design specification of 0.02°. This may have contributed to the unsatisfactory performance of the assembly and illustrates the criticality of manufacturing accuracy to the shear key design. For key B, where the angled faces were within specification, post-test localised damage was evident on the key body and the keyway. In addition, the combined wedge extraction force for the cylinder was found to be approximately 8000 N (compared to the 4500 N insertion force) which aligns with the increase in articulation force. The reasons for the damage and the tightening of the wedges during testing have not specifically been identified. To assess the ability of the keys to withstand large in-service loads during plasma disruptions, cyclic loading tests were carried out with the keys mounted in inconel support blocks. Each key was subjected to 100 cycles over load ranges of +50 to −350 kN for key A and +250 to − 50 kN for key B. Results indicate that the keys function elastically within these load ranges, but wedge
J. Palmer et al. / Fusion Engineering and Design 58–59 (2001) 463–467
466
extraction measurements differed significantly from those obtained during the preliminary insertion/extraction cycles (see Table 1). It is evident that for key B the cylinder wedge extraction load was extraordinarily large. This is possibly due to an effect of keyway deformation and/or relative movement between the locking components and the key body during the predominantly tensile loading cycle. It is also evident that, during the same test, the dovetail wedge for key B had loosened somewhat. These observations not only have an implication with regard to attachment security but also have an impact on the design of tooling, particularly with regard to maximum load capacity for extraction. Heat evacuation capability was assessed using a test cylinder (identical to the cylindrical head of the key) locked inside a water-cooled thick-walled aluminium– bronze tube. The whole assembly was housed within a vacuum chamber and heated by a cartridge heater located along the central axis of the cylinder. The test measurements indicate that, although the heat evacuation appears to be nonuniform along the length of the cylinder, the thermal in-service performance of the shear key is expected to be satisfactory, with an average temperature of the key being nominally 100 °C greater than that of the keyway.
5. Multi-link technological tests The first step in qualifying the ‘‘multi-link’’ attachment scheme was the identification of a reference geometry based on ITER-FEAT re-
Table 1 Load test insertion/extraction forces Key A Cylindera Insertion 5000 N Extraction 5694 N a
Both wedges.
Key B Dovetail
Cylindera
Dovetail
4500 N 3618 N
5000 N 15 283 N
4500 N 968 N
Table 2 Multi-link reference geometry Linkpin material Link/housing material Linkpin outer diameter Linkpin inner diameter Link/housing thickness Link/housing depth Link/housing pitch Number of links/target
Aluminium–bronze Stainless steel 316L 35 mm 17.5 mm Nominally 14 mm 110 mm 28 mm Four assemblies of six links
quirements (see Table 2). This was followed by an extensive test program involving ‘free-pin’ and ‘plate-pin’ test assemblies to define link/linkpin clearances, linkpin/housing clearances and associated swaging mandrel diameter to achieve the required joint characteristics. In this case, the principal operating criterion relates to the ability to perform the swaging process in a reliable and repetitive manner to provide uniform contact between the pin and the fixation holes. This is accompanied by a secondary requirement for the link to rotate around the pin, rather than the pin rotate in the cassette-side fixation hole, in order to facilitate re-use of the latter without re-machining. These tests showed that for 14 mm thick plates, pin/hole diametrical clearances (prior to expansion) of 0.509 0.02 mm and 0.459 0.02 mm resulted in post expansion torques of nominally 15 and 55 Nm, respectively, with an 18.8 mm mandrel. These values were chosen as references for the test program. Tests were carried out on a structure representative of a single link from the multi-link array. The implications of in-service loading was established by simultaneously applying loads up to 75 kN (which represents the equivalent ambient-temperature design load) in conjunction with 9 1° articulation of the link. Results showed that the joint remained elastic over a test duration of 100 cycles, but the torque required to articulate the joint was found to increase during the first 70 cycles of the test. The latter implies a change in the tribological characteristics of the joint, an aspect which requires further investigation. Thermal and electrical tests on the single link test piece will also be carried out.
J. Palmer et al. / Fusion Engineering and Design 58–59 (2001) 463–467
6. Conclusions The DRP facility is proving to be very effective in simulating a hot cell environment for testing divertor cassette maintenance scenarios. Cassette refurbishment trials involving the shear key and multi-link attachment schemes have demonstrated the basic feasibility of PFC replacement using both approaches. Shear key replacement however, requires a greater degree of operator assistance and skill than is required for multi-link. Technological tests, in particular for the shear keys, have illustrated the value of extensive endurance testing of entirely representative test pieces in the qualification of a design choice. Tests to date have not demonstrated the security of the shear keys during in-service operation or provided
467
positive confirmation of damage-free operation. If this design choice is to be pursued, further work in this area will be necessary. In the case of the multi-link scheme, although the tests indicate a good strength margin with respect to operating conditions, tribological issues require further investigation. References [1] S. Chiocchio, et al., The attachment system for the ITER divertor plasma facing components, 20th SOFT Marseille, 1998. [2] M. Siuko, et al., Water hydraulics in ITER divertor maintenance, 20th SOFT Marseille, 1998. [3] M. Siuko, et al., Tool prototypes for replacing divertor cassette plasma facing components for ITER, this symposium.