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The ISX-JET beryllium limiter experiment
422
Journal
THE
ISX-.JETBERYLLIUM LIMITEZRE~E~MENT
of Nuclear Materials 128&129 (1984) 422-424
*
P.H. EDMONDS, P. MIODUSZEWSKI, J.B. ROBERTO Uok Ridge ~~~~o~oldragon, Oak Ridge, Tennessee, US.4 ** R.D. WATSON and M.F. SMITH Sandia Narionnl Laboratories, Albuquerque, New Mexico, USA f
Key words: beryllium limiter test
A devotion is given of an experiment that has been designed for the ISX-B tokamak at the Oak Ridge National Laboratory using beryBium aa the primary plasma limiter.
1. fntrodu&on An experiment has been designed for the ISX-B tokamak at the Oak Ridge National Laboratory using beryllium as the primary plasma limiter. Beryllium is a very attractive material for limiter applications, with low atomic number and adequate thermal and mechanical properties (11. A significant disadvantage of beryllium is the health hazard associated with the inhalation of dust snug the metal or its compounds and special procedures are required for any experiment containing significant quantities of the material. It is proposed to install a beryllium limiter in the JET experiment prior to the high power heating phase of operation and a primary reason for the experiment described in this paper is to establish operating experience and an understanding of the plasma material interactions before committing the JET experiment to this material. Although single properties like thermal shock resistance or tritium retention can be investigated in laboratory experiments, synergistic effects cannot be predicted and can only be studied in the proper plasma environment. Therefore, a necessary test prior to application in a large device like JET has to be a tokamak test. Among all limiter materials tested and used so far there is no candidate compatibie with all requirements: good thermo-mechanicai properties, favorable plasma materials interaction characteristics, stable materials properties under neutron irradiation and adequate technology basis (fabrication, bonding, etc.) [2]. Beryllium is a metal which combines low atomic number with good
Research sponsored by the Jaint European Torus under DOE Project No. ER-D-89-339 and JET Contract No. JD3/0771 with the Union Carbide Corp. Operated by Martin Marietta Energy Systems, Inc. under Contract No. DE-ACOS-8~~1~ with the US Department of Energy, Office of Fusion Energy. This work was performed at Sandia National Laboratories, supported by the US Department of Energy, under Contract No. DE-ACO476DFW789.
0022-3115/84/$03.00 @ Elsevier Science Pub&hers B.V. ~North-HoIland Physics Publishing Division)
thermal properties. The melting point is 1283’ C which is rather Iow compared to graphite, but the good thermal conductivity makes beryllium a candidate for an actively cooled limiter with long pulse operation. Beryllium can be operated in a temperature range around 500 OC where hydrogen retention is low. The corresponding temperatures for graphite are in the temperature regime where chemical erosion is high. This gives beryllium an advantage over graphite in DT-burning devices where the tritium inventory has to be kept at a minimum. In fully nuclear machines problems must be anticipated with beryflium as well as with graphite. Helium generation, Iargely from fn, 2n) reactions with high energy neutrons, has substantial effects on mechanical properties and swelling of beryllium. It is estimated that 3720 at ppm He per MW year/m2 would be generated in a fusion spectrum (17 cm3 of helium at STP per cm3 of belles) (31. Graphite, on the other hand, has the problem that the relatively good thermal conductivity is rapidly reduced at fairly low irradiation levels ( < 1 dpa) [4]. Whether or not either one of these two materials will qualify for application in power producing reactors cannot be decided on the basis of the present data.
2. Limiter design The rail limiter assembly is shown in fig. 1. It consists of the beryllium rail, the stainless steel backing plate, the heat transfer system and the positioning unit. The surface profile of the limiter has been designed for a constant normal surface power loading with a power e-folding length of 2 cm. The model used a one-dimensional approxi~tion and is only valid at the tangent point to the plasma edge. The geometry of the beryllium limiter is the same as the titanium carbide coated carbon rail limiters installed on the JSX-3 experiment. The significant differences are that the beryllium limiter is constructed from 12 segments 2.5 cm wide, alternate segments are scored with 1 mm wide and 1 cm deep
P.H. Edmondr et al. / The ISX-
COOLINQ
TUBES
tHERMOCOU?LES
l El.l.ows
CERAMIC
BREAK
AIR CYLINDER
v*cuuY
JET beryllium limiter experiment
423
DOWTHERM-LF. The maximum operating temperature of the system is limited to less than 3OO“C by the packing material used in the pump. System operation is continuous and a flow meter with a AT block will allow measurement of the power delivered to the limiter. In support of the limiter design, thermal analysis, stress analysis and electron beam thermal fatique tests have been performed. The main result was to support the decision to tessellate the surface of alternate limiter segments with slots 1 cm deep on a 1 cm grid spacing in order to relieve the thermal stresses by allowing the surface to expand in an unconstrained manner.
3. Health hazards
VESSEL ma SEGMENTS
Fig. 1. Cutaway of the rail limiter assembly showing the slotted beryllium segments and the mounting structure.
slots into 1.25 cm square tessellations. The beryllium segments are held against the steel support plate with a pre-load of about 440 N (100 lbs) using belleville washers. This supplies adequate thermal contact for the heat transfer without the risk of damaging the segments with thermal stresses. The bulk temperature of each segment can be monitored using a thermocouple. The limiter position can be remotely controlled with a stroke of 15 cm using an air cylinder. A closed loop heat transfer system is used to control the bulk limiter temperature. The 304 SS cooling/heating tube is brazed to the support plate using OFHC copper as the braze material. Since beryllium has a very low ductility at room temperature, the temperature control system is used to maintain the limiter at approximately 200 ‘C where the ductility is fairly high (fig. 2). The heat transfer fluid is an organic liquid,
100
WD
700
WD
1 tw’r
TEMPERATURE
Fig. 2. Temperature
dependence
of ductility of beryllium.
The primary health hazard associated with beryllium is the inhalation of dust containing beryllium. To protect against the escape of any dust from the main vacuum vessel a filtered air ventilation system has been installed which will maintain the liner at a depressed pressure in the event of a vacuum system failure. In addition careful monitoring will identify the possible occurrence of any spills. Clean up will be with conventional wet wash techniques. The experience of industry is that the beryllium toxicity problem is serious but that the material may be handled safely by taking proper precautions [l].
4. Experimental program The experimental program consists of two tasks: to evaluate the performance of beryllium as a limiter material and to study the effects of beryllium on the plasma properties. Before plasma operation the thermo-mechanical properties of beryllium were determined, including measurements of thermal conductivity, yield strength, tensile strength, surface hardness, and surface topology. After completion of the experiment, the thermo-mechanical analysis will be repeated and in addition, surface analysis techniques will be applied for hydrogen (deuterium) depth profiling as well as for erosion/redeposition studies. Since the thermal load alone was simulated in electron beam tests, comparison with plasma exposure allows us to study synergistic effects. Particle densities and temperatures in the scrape-off layer will be measured with Langmuir probes. For comparison, one Thomson scattering channel and one FIR interferometer chord are available in the scrape-off layer. In addition, a biased calorimeter will measure the ion-saturation current. Particle fluxes will be inferred from density and temperature measurements and, possibly, be measured with a directional Langmuir probe. To study the power flux to the limiter, the following diagnostics will be employed: an infrared camera to 6. LIMITERS;
EXPERIMENT
AND THEORY
424
P.H. Edmonds ef al. / The ISX- JET betylhum limirer experimem
measure the surface temperature as function of space and time, a movable biased calorimeter probe to measure ion saturation current and the power flux using a second infrared camera, and thermocouples to measure the total energy to the limiter during each discharge. Recycling and retention of hydrogen isotopes will be studied with H,-detectors, surface collection probes and a thermal desorption probe. H,-detectors are placed at various locations of the tokamak to measure relative recycling fluxes: near the limiter, in the section that contains the gas puff nozzle, and in a “neutral” sector that represents wall recycling. Hydrogen depth profiling will be performed on limiter segments after completion of the experiment as well as on collector probes after a specified number of shots. The total amount of hydrogen (deuterium) implanted in a given number of discharges will be determined with a thermal desorption probe. Erosion/redeposition studies will be performed with the help of markers applied to several limiter segments. Net erosion will be determined by weight-loss techniques. Surface collection probes will be exposed in positions typical of the limiter and the first wall. The samples will be exposed under specified conditions (wall conditioning, tokamak shots, disruptions) and analyzed in the surface analysis station. Arrays of silicon collection probes are distributed in the torus, the neutral beam injector and in various diagnostics. After completion of the experiment these probes will be analyzed for material migration studies. The effect of the beryllium limiter on the plasma properties will be studied by comparison with a reference plasma run on the Tic-coated graphite rail limiter. Emphasis will be on exploration of the parameter space, comparison of profiles, power balance including heat flux studies in the plasma edge, impurity evolution and transport, and scalings of the confinement parameters with plasma density and current. Special emphasis will be put on spectroscopic studies. One visible spectrome-
ter will be set up to monitor lines of beryllium and other in-vessel components on the limiter. The existing ISX spectrometers will be used to determine the content of beryllium and other impurities in the plasma. Detection of the fully stripped species (in particular beryllium) will be attempted via charge-exchange methods. The parameters of the standard discharge to be used for the fluence test and documented for the TIC rail limiter are: B, = 14 kG, I, = 120 kA and FJ, = 4.7 x 1O’j cm-3. Edge plasma measurements from both the Thomson scattering profiles and from Langmuir probe data suggest a particle flux to the limiter of more than 1019 cm -2 s-1 . The operating envelope available for this particular configuration is essentially that bounded by the “DITE” limits and the q+ = 3 line, as described in a Hugill diagram.
5. Conclusion Beryllium is a potential candidate for high heat flux limiter applications. A disadvantage in its use in experimental facilities is the associated health hazard. An experiment is in progress in the ISX-B tokamak to test beryllium as a limiter material in a short pulse neutralbeam heated tokamak. This experiment is a precursor to the possible installation of beryllium limiters in the JET experiment.
References [l) Workshop on Beryllium for Plasma-Side Applications, Germantown, Maryland, July, 1983. (21 R.W. Corm, J. Nucl. Mater. 78-77 (1978) 105. [3] F.W. Wiffen, Bulk Properties; Low-Z Materials, in: Ch.
VII, FED/INTOR ICRW-82-17. [4] US FED/INTOR Report (1982).
Journal
THE
ISX-.JETBERYLLIUM LIMITEZRE~E~MENT
of Nuclear Materials 128&129 (1984) 422-424
*
P.H. EDMONDS, P. MIODUSZEWSKI, J.B. ROBERTO Uok Ridge ~~~~o~oldragon, Oak Ridge, Tennessee, US.4 ** R.D. WATSON and M.F. SMITH Sandia Narionnl Laboratories, Albuquerque, New Mexico, USA f
Key words: beryllium limiter test
A devotion is given of an experiment that has been designed for the ISX-B tokamak at the Oak Ridge National Laboratory using beryBium aa the primary plasma limiter.
1. fntrodu&on An experiment has been designed for the ISX-B tokamak at the Oak Ridge National Laboratory using beryllium as the primary plasma limiter. Beryllium is a very attractive material for limiter applications, with low atomic number and adequate thermal and mechanical properties (11. A significant disadvantage of beryllium is the health hazard associated with the inhalation of dust snug the metal or its compounds and special procedures are required for any experiment containing significant quantities of the material. It is proposed to install a beryllium limiter in the JET experiment prior to the high power heating phase of operation and a primary reason for the experiment described in this paper is to establish operating experience and an understanding of the plasma material interactions before committing the JET experiment to this material. Although single properties like thermal shock resistance or tritium retention can be investigated in laboratory experiments, synergistic effects cannot be predicted and can only be studied in the proper plasma environment. Therefore, a necessary test prior to application in a large device like JET has to be a tokamak test. Among all limiter materials tested and used so far there is no candidate compatibie with all requirements: good thermo-mechanicai properties, favorable plasma materials interaction characteristics, stable materials properties under neutron irradiation and adequate technology basis (fabrication, bonding, etc.) [2]. Beryllium is a metal which combines low atomic number with good
Research sponsored by the Jaint European Torus under DOE Project No. ER-D-89-339 and JET Contract No. JD3/0771 with the Union Carbide Corp. Operated by Martin Marietta Energy Systems, Inc. under Contract No. DE-ACOS-8~~1~ with the US Department of Energy, Office of Fusion Energy. This work was performed at Sandia National Laboratories, supported by the US Department of Energy, under Contract No. DE-ACO476DFW789.
0022-3115/84/$03.00 @ Elsevier Science Pub&hers B.V. ~North-HoIland Physics Publishing Division)
thermal properties. The melting point is 1283’ C which is rather Iow compared to graphite, but the good thermal conductivity makes beryllium a candidate for an actively cooled limiter with long pulse operation. Beryllium can be operated in a temperature range around 500 OC where hydrogen retention is low. The corresponding temperatures for graphite are in the temperature regime where chemical erosion is high. This gives beryllium an advantage over graphite in DT-burning devices where the tritium inventory has to be kept at a minimum. In fully nuclear machines problems must be anticipated with beryflium as well as with graphite. Helium generation, Iargely from fn, 2n) reactions with high energy neutrons, has substantial effects on mechanical properties and swelling of beryllium. It is estimated that 3720 at ppm He per MW year/m2 would be generated in a fusion spectrum (17 cm3 of helium at STP per cm3 of belles) (31. Graphite, on the other hand, has the problem that the relatively good thermal conductivity is rapidly reduced at fairly low irradiation levels ( < 1 dpa) [4]. Whether or not either one of these two materials will qualify for application in power producing reactors cannot be decided on the basis of the present data.
2. Limiter design The rail limiter assembly is shown in fig. 1. It consists of the beryllium rail, the stainless steel backing plate, the heat transfer system and the positioning unit. The surface profile of the limiter has been designed for a constant normal surface power loading with a power e-folding length of 2 cm. The model used a one-dimensional approxi~tion and is only valid at the tangent point to the plasma edge. The geometry of the beryllium limiter is the same as the titanium carbide coated carbon rail limiters installed on the JSX-3 experiment. The significant differences are that the beryllium limiter is constructed from 12 segments 2.5 cm wide, alternate segments are scored with 1 mm wide and 1 cm deep
P.H. Edmondr et al. / The ISX-
COOLINQ
TUBES
tHERMOCOU?LES
l El.l.ows
CERAMIC
BREAK
AIR CYLINDER
v*cuuY
JET beryllium limiter experiment
423
DOWTHERM-LF. The maximum operating temperature of the system is limited to less than 3OO“C by the packing material used in the pump. System operation is continuous and a flow meter with a AT block will allow measurement of the power delivered to the limiter. In support of the limiter design, thermal analysis, stress analysis and electron beam thermal fatique tests have been performed. The main result was to support the decision to tessellate the surface of alternate limiter segments with slots 1 cm deep on a 1 cm grid spacing in order to relieve the thermal stresses by allowing the surface to expand in an unconstrained manner.
3. Health hazards
VESSEL ma SEGMENTS
Fig. 1. Cutaway of the rail limiter assembly showing the slotted beryllium segments and the mounting structure.
slots into 1.25 cm square tessellations. The beryllium segments are held against the steel support plate with a pre-load of about 440 N (100 lbs) using belleville washers. This supplies adequate thermal contact for the heat transfer without the risk of damaging the segments with thermal stresses. The bulk temperature of each segment can be monitored using a thermocouple. The limiter position can be remotely controlled with a stroke of 15 cm using an air cylinder. A closed loop heat transfer system is used to control the bulk limiter temperature. The 304 SS cooling/heating tube is brazed to the support plate using OFHC copper as the braze material. Since beryllium has a very low ductility at room temperature, the temperature control system is used to maintain the limiter at approximately 200 ‘C where the ductility is fairly high (fig. 2). The heat transfer fluid is an organic liquid,
100
WD
700
WD
1 tw’r
TEMPERATURE
Fig. 2. Temperature
dependence
of ductility of beryllium.
The primary health hazard associated with beryllium is the inhalation of dust containing beryllium. To protect against the escape of any dust from the main vacuum vessel a filtered air ventilation system has been installed which will maintain the liner at a depressed pressure in the event of a vacuum system failure. In addition careful monitoring will identify the possible occurrence of any spills. Clean up will be with conventional wet wash techniques. The experience of industry is that the beryllium toxicity problem is serious but that the material may be handled safely by taking proper precautions [l].
4. Experimental program The experimental program consists of two tasks: to evaluate the performance of beryllium as a limiter material and to study the effects of beryllium on the plasma properties. Before plasma operation the thermo-mechanical properties of beryllium were determined, including measurements of thermal conductivity, yield strength, tensile strength, surface hardness, and surface topology. After completion of the experiment, the thermo-mechanical analysis will be repeated and in addition, surface analysis techniques will be applied for hydrogen (deuterium) depth profiling as well as for erosion/redeposition studies. Since the thermal load alone was simulated in electron beam tests, comparison with plasma exposure allows us to study synergistic effects. Particle densities and temperatures in the scrape-off layer will be measured with Langmuir probes. For comparison, one Thomson scattering channel and one FIR interferometer chord are available in the scrape-off layer. In addition, a biased calorimeter will measure the ion-saturation current. Particle fluxes will be inferred from density and temperature measurements and, possibly, be measured with a directional Langmuir probe. To study the power flux to the limiter, the following diagnostics will be employed: an infrared camera to 6. LIMITERS;
EXPERIMENT
AND THEORY
424
P.H. Edmonds ef al. / The ISX- JET betylhum limirer experimem
measure the surface temperature as function of space and time, a movable biased calorimeter probe to measure ion saturation current and the power flux using a second infrared camera, and thermocouples to measure the total energy to the limiter during each discharge. Recycling and retention of hydrogen isotopes will be studied with H,-detectors, surface collection probes and a thermal desorption probe. H,-detectors are placed at various locations of the tokamak to measure relative recycling fluxes: near the limiter, in the section that contains the gas puff nozzle, and in a “neutral” sector that represents wall recycling. Hydrogen depth profiling will be performed on limiter segments after completion of the experiment as well as on collector probes after a specified number of shots. The total amount of hydrogen (deuterium) implanted in a given number of discharges will be determined with a thermal desorption probe. Erosion/redeposition studies will be performed with the help of markers applied to several limiter segments. Net erosion will be determined by weight-loss techniques. Surface collection probes will be exposed in positions typical of the limiter and the first wall. The samples will be exposed under specified conditions (wall conditioning, tokamak shots, disruptions) and analyzed in the surface analysis station. Arrays of silicon collection probes are distributed in the torus, the neutral beam injector and in various diagnostics. After completion of the experiment these probes will be analyzed for material migration studies. The effect of the beryllium limiter on the plasma properties will be studied by comparison with a reference plasma run on the Tic-coated graphite rail limiter. Emphasis will be on exploration of the parameter space, comparison of profiles, power balance including heat flux studies in the plasma edge, impurity evolution and transport, and scalings of the confinement parameters with plasma density and current. Special emphasis will be put on spectroscopic studies. One visible spectrome-
ter will be set up to monitor lines of beryllium and other in-vessel components on the limiter. The existing ISX spectrometers will be used to determine the content of beryllium and other impurities in the plasma. Detection of the fully stripped species (in particular beryllium) will be attempted via charge-exchange methods. The parameters of the standard discharge to be used for the fluence test and documented for the TIC rail limiter are: B, = 14 kG, I, = 120 kA and FJ, = 4.7 x 1O’j cm-3. Edge plasma measurements from both the Thomson scattering profiles and from Langmuir probe data suggest a particle flux to the limiter of more than 1019 cm -2 s-1 . The operating envelope available for this particular configuration is essentially that bounded by the “DITE” limits and the q+ = 3 line, as described in a Hugill diagram.
5. Conclusion Beryllium is a potential candidate for high heat flux limiter applications. A disadvantage in its use in experimental facilities is the associated health hazard. An experiment is in progress in the ISX-B tokamak to test beryllium as a limiter material in a short pulse neutralbeam heated tokamak. This experiment is a precursor to the possible installation of beryllium limiters in the JET experiment.
References [l) Workshop on Beryllium for Plasma-Side Applications, Germantown, Maryland, July, 1983. (21 R.W. Corm, J. Nucl. Mater. 78-77 (1978) 105. [3] F.W. Wiffen, Bulk Properties; Low-Z Materials, in: Ch.
VII, FED/INTOR ICRW-82-17. [4] US FED/INTOR Report (1982).