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Safety issues on laser megajoule facility
Fusion Engineering and Design 69 (2003) 625 /629 www.elsevier.com/locate/fusengdes
Safety issues on laser megajoule facility Ph. Joyer a,*, M. Dupont b, H.P. Jacquet b a
Commissariat a` l’e´nergie atomique, Centre d’e´tudes scientifiques et techniques d’Aquitaine, BP2, 33114 Le Barp, France b Commissariat a` l’e´nergie atomique, Centre d’e´tudes Ile de France, BP12, 91680 Bruye`res le Chaˆtel, France
Abstract The Laser Megajoule (LMJ) is a facility dedicated to inertial confinement fusion experiments. Building construction phase should start in the first half of year 2003. Use of a deuterium /tritium mixture in the experiments and high neutron yield generation induces hazards such as contamination of equipments inside the target chamber and activation of materials. Calculations have been undertaken to estimate dose rates levels in the facility. Optimization is under development on ALARA principle basis to reduce activation and workers exposure. Decontamination is an other important safety issue for LMJ; different processes are available and have to be considered (lasers, foams . . .). Radioactive wastes will be generated and their disposal has to be taken into account. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Safety; Laser megajoule; Fusion
1. Introduction Laser Megajoule (LMJ) will produce over 600 shots per year including 12 yields shots generating 1 /1019 14 MeV each. Up to 4 /1021 neutrons are expected over full operation of LMJ. Such neutron yields will generate high neutron radiation levels during a shot and gamma radiation between shots due to activation of all materials introduced in the experimental area. Nevertheless, LMJ remains an experimental facility inside which access of operators will be needed for maintenance, implementation of diagnostics or experimental configuration changes. Activation is an important issue for next
* Corresponding author. Tel.: /33-557-044-533; fax: /33557-045-463. E-mail address: [email protected] (Ph. Joyer).
fusion facilities [1 /3]. Calculations have been performed for LMJ to estimate consequences of neutron activation and gamma doses rates during operation and maintenance periods. The goal of the paper is not to describe computation codes used but to discuss preliminary results and paths of activation and doses rates reduction. Use of tritium in the experiments will induce contamination of all equipments introduced in the target chamber and is an other important issue for future fusion devices [4]. Decontamination may be required to reduce tritium level for maintenance purposes of internal equipments and diagnostics. Lasers are mainly considered for processes studies. Of course, the interest is important owing to the lack of radioactive effluents or liquid wastes and it is well adapted to large surfaces. Nevertheless, the chamber is not the unique equipment to be decontaminated. Due to the complexity of some
0920-3796/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0920-3796(03)00191-1
Ph. Joyer et al. / Fusion Engineering and Design 69 (2003) 625 /629
626
elements and contamination features, other processes such as foams have been studied and it is assumed that several techniques could probably be necessary to meet all LMJ decontamination needs. Finally, radioactive wastes will be generated. As for other fusion facilities, wastes will be mainly activated and some others will also be contaminated.
2. Facility overview Fig. 1 gives an upper view of LMJ building. The experimental area is located in the center of the building between two large laser bays. Total length of the building is around 300 m. Thirty lasers bundles of eight beams each penetrates the experimental bay, approximately a 65 m diameter cylinder, through a 2 m thick concrete wall. The 240 beams are then distributed by quads to north and south hemisphere of a 10 m diameter target chamber and will be focused on a deuterium tritium target inserted at the center of the chamber. Fig. 2 below gives a more detailed view of the target chamber and its main equipments surrounded by a concrete cylinder of 33 m diameter and 1 m thick. The outer surface of the target chamber is covered with a 40 cm concrete shielding. Sixty openings over a total of 260 are necessary for quads beams penetrations.
Fig. 1. LMJ facility.
Fig. 2. Detailed view of the target chamber area.
Diagnostics will be implemented all around the target chamber mainly at the equatorial level and poles. The inner surface of the chamber will be covered with panels of about 1 m2 each to capture materials vaporized during shots, materials ablated by X-rays emission during a shot and to avoid pollution of final optics by reemission of captured materials from shot to shot.
3. Activation calculations 3.1. Assumptions Calculations require use of computer codes, creation of a model describing the facility and composition of materials. 3-D Tripoli code has been used for neutron transport coupled to Fispact for neutron activation and dose rates calculation. An integrated model has been created including main large equipments of target bay: the concrete building, the chamber and its shielding, the structure, the frequency conversion systems, equipments such as target positioners, diagnostics manipulators. Diagnostics are not defined at present time. Data used in the calculations take into account a simplified geometry for each equipment and specifications of main materials. Aluminum is preferred and will be used, at least, for the chamber, the structure and intermediate floors.
Ph. Joyer et al. / Fusion Engineering and Design 69 (2003) 625 /629
3.2. Activation results and optimization After a full yield shot, a week will be the minimum time required before access to the target bay to allow radioactive decay of short half-lives radionuclides. Fig. 3 gives first ambient doses rates calculations at one location near the chamber after full operation distributed in 12 yield shots per year. Dose rates are dominated by 60Co. At high neutron energies, this nuclide is produced on 60Ni through (n, p) reactions and on 63Cu through (n, a) reaction. At lower energies, it is produced on 59Co through (n, g) reactions. Outside target chamber, 60Co is mainly due to thermal capture. Due to the high doses rates expected, activation has to be reduced to limit workers exposure. Boron will be added to the chamber concrete shielding but has not been included in the calculations. Non negligible gain may be expected. Even if it remains high steel amounts in several equipments outside the chamber as in target positioners, frequency converters or diagnostics manipulator, other equipments will be fabricated with Al 5083 alloy and will contribute nevertheless to about 30% in 60Co amount as shown on Fig. 4.
627
Chamber contribution is reduced owing to its concrete shielding. Choice of materials is of high importance for LMJ. As mentioned above, 60Co is produced on nickel, cobalt and copper which are impurities in the aluminum alloy. Calculations have been made on Al 5083 specifications basis and take into account 500 ppm nickel, 500 ppm cobalt and 0.1% copper. Impurity levels limitation could be required to reduce dose rates levels in the target bay and studies are under development on ALARA principle to determine the best gain that can be expected.
4. Contamination levels expected and decontamination processes for LMJ All in-vessels equipments, mainly the target chamber, the inner wall panels, the final optics (debris shields, phase strips and chamber windows), the target positioners and diagnostics including their insertion systems will be contaminated with tritium unburnt in the experiments, target debris and front end diagnostics vaporized during shots. For maintenance, equipments will be
Fig. 3. Dose rates near the chamber after full operation.
628
Ph. Joyer et al. / Fusion Engineering and Design 69 (2003) 625 /629
Fig. 4. Contribution of main equipments in
removed and transferred under confinement to dedicated areas located outside LMJ building. No contamination will be allowed in the target bay in normal operation. Tritium quantities involved in LMJ experiments will be limited to a few tens of mg/year. Up to 300 mg of a deuterium /tritium mixture will be used in full yields shots targets. Tritium contamination levels higher than 500 Bq/cm2 per shot are then expected in LMJ vessel depending on tritium burning rate and tritium amount not removed by the vacuum system. At present time it is not required to decontaminate the target chamber itself. Contamination will be mainly captured on the inner wall panels and on debris shield optics. It is planned to change all panels once a year and it is rather difficult to estimate, at the moment, residual contamination of the chamber after panels removal. Acidic foams have been used for decontamination of Phebus target chamber, a CEA laser facility decommissioned in 1999, and have been implemented in view of their potential use for LMJ chamber [5]. To reduce radioactive effluents generation, the foam has been sprayed inside the chamber through an hemispherical pipe equipped with nozzles and the chemical solution has been recycled during operation. Good decontamination results were obtained. Contamination of the chamber is mainly a codeposition of tritium and target debris. Tritium is
60
Co generation near the chamber.
mainly trapped in the deposit (90%), 7% is non fixed contamination removable with swipes and about 3% is trapped in the aluminum alloy itself. Even if foams are not used for LMJ target chamber decontamination, the process may be interesting for other complicated equipments such as panels. In addition, lasers efficiency tests will be made on remaining Phebus samples to take into account surface contamination profile and check that contamination will not be fixed in depth in the alloy.
5. Wastes generation During LMJ operation, equipments will be reused as much as possible, to reduce wastes quantities. All materials installed in the target bay and in the switchyards rooms (inner spaces between the two concrete walls of 1 and 2 m each) will be only activated. After full operation and eventual temporary storage, activated wastes will be mainly lower than 100 Bq/g which is the upper limit in France for very low radioactive wastes. As aluminum will be preferred, much of them should be lower than 10 Bq/g especially for wastes coming from switchyards rooms. Optics are mainly pure silicon which deactivate rapidly. At present time, no clearance levels are available in France and all
Ph. Joyer et al. / Fusion Engineering and Design 69 (2003) 625 /629
these wastes will be eliminated as very low level radioactive wastes. In-chamber equipments will be activated and contaminated wastes. Chamber windows and phase strips should only be contaminated after silicon radioactive decay but, as mentioned in Section 4, it is difficult to estimate contamination levels for these optics. As aluminum will be preferred, 26Al will be produced through (n, 2n) reactions on 27Al. In all wastes, 26Al levels will be lower than 1 Bq/g.
6. Conclusion High doses rates are expected inside the target bay after full yields shots and are dominated by 60 Co even though aluminum would be preferred as steels. At the moment, it remains a lot of steel (vacuum pumps and valves), and diagnostics are not, at present time, defined. Few paths of activation reduction are studied and should lead to significant decreases. Nevertheless results already indicate that an accurate control of materials introduced in the experimental bay will be required. For large equipments, it could be required to limit 59Co in aluminum alloys below specification values of 500 ppm. 63Cu is also an important 60Co contributor and it could be interesting to limit its amount. Foams have been tested and may be interesting for LMJ decontamination needs. Comparison tests with lasers processes will be undertaken.
629
LMJ will generate high amounts of very low levels radioactive wastes. Most of them should be cleared if clearance levels could be applied. Wastes issues are limitation of radioactive wastes and disposal of activated and tritiated wastes.
Acknowledgements The authors wish to thank M. Messaoudi and his team from Assystem Company for their involvements in activation calculations.
References [1] J.F. Latkowski, Occupational dose estimates for the National Ignition Facilities, Inertial Fusion Sciences and Applications 99, Elsevier, Paris, 1999, pp. 831 /838. [2] M.J. Loughlin, R.A. Forrest, J.E.G. Edwards, Neutron activation studies on JET, Nucl. Eng. Des. 58 /59 (2001) 967 /971. [3] G. Martin, A. Le Luyer, F. Saint Laurent, Activation observation on the Tore Supra Tokamak, Nucl. Eng. Des. 58 /59 (2001) 973 /977. [4] C.H. Skinner, G. Federici, Tritium issues in next step devices, International Conference on Advanced Diagnostics for Magnetic and Inertial Fusion, Varenna, Italy, September 3 /7, 2001. [5] B. Fournel, S. Angot, P. Joyer, Decontamination of Phebus experimental target chamber using sprayed foams, Proc. 10th Int. Conf. on Nuclear Energy, Washington April, 2002.
Safety issues on laser megajoule facility Ph. Joyer a,*, M. Dupont b, H.P. Jacquet b a
Commissariat a` l’e´nergie atomique, Centre d’e´tudes scientifiques et techniques d’Aquitaine, BP2, 33114 Le Barp, France b Commissariat a` l’e´nergie atomique, Centre d’e´tudes Ile de France, BP12, 91680 Bruye`res le Chaˆtel, France
Abstract The Laser Megajoule (LMJ) is a facility dedicated to inertial confinement fusion experiments. Building construction phase should start in the first half of year 2003. Use of a deuterium /tritium mixture in the experiments and high neutron yield generation induces hazards such as contamination of equipments inside the target chamber and activation of materials. Calculations have been undertaken to estimate dose rates levels in the facility. Optimization is under development on ALARA principle basis to reduce activation and workers exposure. Decontamination is an other important safety issue for LMJ; different processes are available and have to be considered (lasers, foams . . .). Radioactive wastes will be generated and their disposal has to be taken into account. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Safety; Laser megajoule; Fusion
1. Introduction Laser Megajoule (LMJ) will produce over 600 shots per year including 12 yields shots generating 1 /1019 14 MeV each. Up to 4 /1021 neutrons are expected over full operation of LMJ. Such neutron yields will generate high neutron radiation levels during a shot and gamma radiation between shots due to activation of all materials introduced in the experimental area. Nevertheless, LMJ remains an experimental facility inside which access of operators will be needed for maintenance, implementation of diagnostics or experimental configuration changes. Activation is an important issue for next
* Corresponding author. Tel.: /33-557-044-533; fax: /33557-045-463. E-mail address: [email protected] (Ph. Joyer).
fusion facilities [1 /3]. Calculations have been performed for LMJ to estimate consequences of neutron activation and gamma doses rates during operation and maintenance periods. The goal of the paper is not to describe computation codes used but to discuss preliminary results and paths of activation and doses rates reduction. Use of tritium in the experiments will induce contamination of all equipments introduced in the target chamber and is an other important issue for future fusion devices [4]. Decontamination may be required to reduce tritium level for maintenance purposes of internal equipments and diagnostics. Lasers are mainly considered for processes studies. Of course, the interest is important owing to the lack of radioactive effluents or liquid wastes and it is well adapted to large surfaces. Nevertheless, the chamber is not the unique equipment to be decontaminated. Due to the complexity of some
0920-3796/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0920-3796(03)00191-1
Ph. Joyer et al. / Fusion Engineering and Design 69 (2003) 625 /629
626
elements and contamination features, other processes such as foams have been studied and it is assumed that several techniques could probably be necessary to meet all LMJ decontamination needs. Finally, radioactive wastes will be generated. As for other fusion facilities, wastes will be mainly activated and some others will also be contaminated.
2. Facility overview Fig. 1 gives an upper view of LMJ building. The experimental area is located in the center of the building between two large laser bays. Total length of the building is around 300 m. Thirty lasers bundles of eight beams each penetrates the experimental bay, approximately a 65 m diameter cylinder, through a 2 m thick concrete wall. The 240 beams are then distributed by quads to north and south hemisphere of a 10 m diameter target chamber and will be focused on a deuterium tritium target inserted at the center of the chamber. Fig. 2 below gives a more detailed view of the target chamber and its main equipments surrounded by a concrete cylinder of 33 m diameter and 1 m thick. The outer surface of the target chamber is covered with a 40 cm concrete shielding. Sixty openings over a total of 260 are necessary for quads beams penetrations.
Fig. 1. LMJ facility.
Fig. 2. Detailed view of the target chamber area.
Diagnostics will be implemented all around the target chamber mainly at the equatorial level and poles. The inner surface of the chamber will be covered with panels of about 1 m2 each to capture materials vaporized during shots, materials ablated by X-rays emission during a shot and to avoid pollution of final optics by reemission of captured materials from shot to shot.
3. Activation calculations 3.1. Assumptions Calculations require use of computer codes, creation of a model describing the facility and composition of materials. 3-D Tripoli code has been used for neutron transport coupled to Fispact for neutron activation and dose rates calculation. An integrated model has been created including main large equipments of target bay: the concrete building, the chamber and its shielding, the structure, the frequency conversion systems, equipments such as target positioners, diagnostics manipulators. Diagnostics are not defined at present time. Data used in the calculations take into account a simplified geometry for each equipment and specifications of main materials. Aluminum is preferred and will be used, at least, for the chamber, the structure and intermediate floors.
Ph. Joyer et al. / Fusion Engineering and Design 69 (2003) 625 /629
3.2. Activation results and optimization After a full yield shot, a week will be the minimum time required before access to the target bay to allow radioactive decay of short half-lives radionuclides. Fig. 3 gives first ambient doses rates calculations at one location near the chamber after full operation distributed in 12 yield shots per year. Dose rates are dominated by 60Co. At high neutron energies, this nuclide is produced on 60Ni through (n, p) reactions and on 63Cu through (n, a) reaction. At lower energies, it is produced on 59Co through (n, g) reactions. Outside target chamber, 60Co is mainly due to thermal capture. Due to the high doses rates expected, activation has to be reduced to limit workers exposure. Boron will be added to the chamber concrete shielding but has not been included in the calculations. Non negligible gain may be expected. Even if it remains high steel amounts in several equipments outside the chamber as in target positioners, frequency converters or diagnostics manipulator, other equipments will be fabricated with Al 5083 alloy and will contribute nevertheless to about 30% in 60Co amount as shown on Fig. 4.
627
Chamber contribution is reduced owing to its concrete shielding. Choice of materials is of high importance for LMJ. As mentioned above, 60Co is produced on nickel, cobalt and copper which are impurities in the aluminum alloy. Calculations have been made on Al 5083 specifications basis and take into account 500 ppm nickel, 500 ppm cobalt and 0.1% copper. Impurity levels limitation could be required to reduce dose rates levels in the target bay and studies are under development on ALARA principle to determine the best gain that can be expected.
4. Contamination levels expected and decontamination processes for LMJ All in-vessels equipments, mainly the target chamber, the inner wall panels, the final optics (debris shields, phase strips and chamber windows), the target positioners and diagnostics including their insertion systems will be contaminated with tritium unburnt in the experiments, target debris and front end diagnostics vaporized during shots. For maintenance, equipments will be
Fig. 3. Dose rates near the chamber after full operation.
628
Ph. Joyer et al. / Fusion Engineering and Design 69 (2003) 625 /629
Fig. 4. Contribution of main equipments in
removed and transferred under confinement to dedicated areas located outside LMJ building. No contamination will be allowed in the target bay in normal operation. Tritium quantities involved in LMJ experiments will be limited to a few tens of mg/year. Up to 300 mg of a deuterium /tritium mixture will be used in full yields shots targets. Tritium contamination levels higher than 500 Bq/cm2 per shot are then expected in LMJ vessel depending on tritium burning rate and tritium amount not removed by the vacuum system. At present time it is not required to decontaminate the target chamber itself. Contamination will be mainly captured on the inner wall panels and on debris shield optics. It is planned to change all panels once a year and it is rather difficult to estimate, at the moment, residual contamination of the chamber after panels removal. Acidic foams have been used for decontamination of Phebus target chamber, a CEA laser facility decommissioned in 1999, and have been implemented in view of their potential use for LMJ chamber [5]. To reduce radioactive effluents generation, the foam has been sprayed inside the chamber through an hemispherical pipe equipped with nozzles and the chemical solution has been recycled during operation. Good decontamination results were obtained. Contamination of the chamber is mainly a codeposition of tritium and target debris. Tritium is
60
Co generation near the chamber.
mainly trapped in the deposit (90%), 7% is non fixed contamination removable with swipes and about 3% is trapped in the aluminum alloy itself. Even if foams are not used for LMJ target chamber decontamination, the process may be interesting for other complicated equipments such as panels. In addition, lasers efficiency tests will be made on remaining Phebus samples to take into account surface contamination profile and check that contamination will not be fixed in depth in the alloy.
5. Wastes generation During LMJ operation, equipments will be reused as much as possible, to reduce wastes quantities. All materials installed in the target bay and in the switchyards rooms (inner spaces between the two concrete walls of 1 and 2 m each) will be only activated. After full operation and eventual temporary storage, activated wastes will be mainly lower than 100 Bq/g which is the upper limit in France for very low radioactive wastes. As aluminum will be preferred, much of them should be lower than 10 Bq/g especially for wastes coming from switchyards rooms. Optics are mainly pure silicon which deactivate rapidly. At present time, no clearance levels are available in France and all
Ph. Joyer et al. / Fusion Engineering and Design 69 (2003) 625 /629
these wastes will be eliminated as very low level radioactive wastes. In-chamber equipments will be activated and contaminated wastes. Chamber windows and phase strips should only be contaminated after silicon radioactive decay but, as mentioned in Section 4, it is difficult to estimate contamination levels for these optics. As aluminum will be preferred, 26Al will be produced through (n, 2n) reactions on 27Al. In all wastes, 26Al levels will be lower than 1 Bq/g.
6. Conclusion High doses rates are expected inside the target bay after full yields shots and are dominated by 60 Co even though aluminum would be preferred as steels. At the moment, it remains a lot of steel (vacuum pumps and valves), and diagnostics are not, at present time, defined. Few paths of activation reduction are studied and should lead to significant decreases. Nevertheless results already indicate that an accurate control of materials introduced in the experimental bay will be required. For large equipments, it could be required to limit 59Co in aluminum alloys below specification values of 500 ppm. 63Cu is also an important 60Co contributor and it could be interesting to limit its amount. Foams have been tested and may be interesting for LMJ decontamination needs. Comparison tests with lasers processes will be undertaken.
629
LMJ will generate high amounts of very low levels radioactive wastes. Most of them should be cleared if clearance levels could be applied. Wastes issues are limitation of radioactive wastes and disposal of activated and tritiated wastes.
Acknowledgements The authors wish to thank M. Messaoudi and his team from Assystem Company for their involvements in activation calculations.
References [1] J.F. Latkowski, Occupational dose estimates for the National Ignition Facilities, Inertial Fusion Sciences and Applications 99, Elsevier, Paris, 1999, pp. 831 /838. [2] M.J. Loughlin, R.A. Forrest, J.E.G. Edwards, Neutron activation studies on JET, Nucl. Eng. Des. 58 /59 (2001) 967 /971. [3] G. Martin, A. Le Luyer, F. Saint Laurent, Activation observation on the Tore Supra Tokamak, Nucl. Eng. Des. 58 /59 (2001) 973 /977. [4] C.H. Skinner, G. Federici, Tritium issues in next step devices, International Conference on Advanced Diagnostics for Magnetic and Inertial Fusion, Varenna, Italy, September 3 /7, 2001. [5] B. Fournel, S. Angot, P. Joyer, Decontamination of Phebus experimental target chamber using sprayed foams, Proc. 10th Int. Conf. on Nuclear Energy, Washington April, 2002.