IFMIF, a Neutron Source for Fusion Material Development

Transcription

1 ICANS-XVIII 18 th Meeting of the International Collaboration on Advanced Neutron Sources April 25-29, 2007 Dongguan, Guangdong, P R China IFMIF, a Neutron Source for Fusion Material Development Pascal Garin 1, on behalf of the IFMIF EVEDA Team 1 Obuchi-MO Bldg. 3F Nozuke 1-3, Obuchi, Rokkasho-mura, Aomori-ken Japan Abstract The design of a fusion reactor by magnetic confinement requires three main categories of knowledge: the understanding of the physics of a burning plasma, (i.e. a plasma where the energy produced by the fusion reactions becomes predominant) and its interaction with the components facing it, all technologies specific to fusion (superconducting magnets, very high flux components, remote handling, tritium cycle) and the knowledge of the materials submitted to an intense neutron flux. IFMIF (International Fusion Materials Irradiation Facility) will be dedicated to the last challenge. Its Engineering Validation and Engineering Design Activities are now starting in the framework of an international agreement between Japan and the European Union called Broader Approach. 1. Background A fusion reactor will produce typically more than 30 dpa per year of damages due to high energy neutrons (the peak being at 14 MeV). ITER, the largest experimental tokamak ever built will produce all along its life a total irradiation of about 3 dpa. Furthermore, its main purpose is the study of a burning plasma, i.e. a plasma where the energy produced by the fusion reactions are much larger than the energy brought from outside to heat it up. The fusion community thus needs a dedicated installation, which will study the damages of these high energy neutrons, in conditions that are relevant with respect to the future fusion power plant. High flux (> 20 dpa/an, 0.5 L) Medium flux (20 1 dpa/an, 6 L) Low flux (< 1 dpa/an, > 8 L) Lithium target Source RFQ DTL HEBT Accelerator (x 2) Test Cell Figure 1. Principle of IFMIF: the two accelerators bring the deuteron beams (125 ma each) to an energy of 40 MeV. The neutrons produced by their interaction with a liquid lithium flow irradiate three 190

2 sets of volumes called High Flux Test Module (irradiation damage ranging between 20 and 50 dpa/year), Medium Flux Test Module (1 to 20 dpa/year) and a low flux region. The International Fusion Materials Irradiation Facility (IFMIF, see Figure above) will answer this specific need. It consists of a set of two parallel deuteron accelerators (40 MeV, 125 ma each, CW) bringing the beams to a liquid lithium target flowing at a velocity of about 15 m/s. The interaction between the deuterons and the lithium generates a flux of neutrons whose spectrum is rather well suited with fusion needs (with the main peak at 14 MeV). The foreseen reactions are: 7 Li(d,2n) 7 Be 6 Li(d,n) 7 Be 6 Li(n,T) 4 He Deuteron energies: 32, 36 and 40 MeV Current: ma Beam footprint: mm 2 Three sets of test cells will host the material samples, with damage rates ranging from 50 dpa per year to a few dpa per year for the lowest part of the test facilities. The overall available irradiated volume is 8 liters Neutron n-flux Flux density Density [10 [10 s 10-1 s -1 cm cm -2-2 MeV -1-1 ] Initial CDA Design (1996) Current Present Design (2003) DEMO fusion Fusion Reactor reactor High Flux Volume Neutron Energy energy [MeV] [MeV] Figure 2. Calculated neutron spectrum in the High Flux Test Volume. Thanks to the optimization of the irradiation conditions (insertion of tungsten neutron spectrum shifter for example), a better fit of the neutron spectrum expected in the fusion demonstrator can be obtained. After several conceptual phases leading to the principles recalled above, the Engineering Validation and Engineering Design Activities (EVEDA) are about to start in the framework of a bilateral collaborative effort between the European Union and Japan, called Broader Approach. The Engineering Validation and Engineering Design Activities of IFMIF aims at producing a detailed, complete and fully integrated engineering design of the International Fusion Materials Irradiation Facility (hereinafter referred to as IFMIF ) and all data necessary for future decisions on the construction, operation, exploitation and decommissioning of IFMIF, and to validate continuous and stable operation of each IFMIF subsystem. They consist mainly of two parts: The Engineering Validation of the major parts of IFMIF thanks to the design, manufacture and test of three prototypes: o A Prototype Accelerator, with output energy of about 10 MeV. This includes: The ion source The low energy beam transport section The radiofrequency quadrupole The first section of the drift tube linac All necessary diagnostics to characterize the beam The control command and auxiliaries systems 191

3 All facilities, including the building at Rokkasho to conduct all tests of the accelerator o The lithium target, at a scale of about 1/3. It includes: The target itself Its remote handling tools The lithium loop and its purification systems All diagnostics to characterize the lithium flow and quality o The test facilities, and validation of the models to be developed. It includes: The full scale high flux test module Irradiation tests in fission reactors Engineering design of the medium flux test modules and test cells The detailed Engineering Design of the entire IFMIF facility, based on the validation process summarized above, and comprising in particular: o A complete description of IFMIF, with: These three subsystems, as well as the buildings with the post irradiation examination cells, auxiliaries, etc. Detailed design of all components Planning for the construction, assembly, tests and commissioning of the whole facility Technical specifications of urgent items to be launched at the start-up of the construction o Candidate site requirements and generic site safety report; o Proposal of an overall program (construction, operation and decommissioning), with corresponding schedule, cost and human needs. 2. Description of IFMIF and its main challenges IFMIF will be an installation at the disposal of the material specialists community. One important objective is the high level of availability and reliability. As described in more detail after many complex systems constitute the whole plant. Each of them will thus have to be carefully evaluated considering not only the necessary optimization of performance and cost, but also its capacity to demonstrate a high reliability. The table below shows examples of reliability targets, chosen such that an overall reliability of 70 %, including the periods of maintenance, is obtained. System Reliability [%] Test facility 97.5 Target facility 95.0 Accelerator 88.0 Conventional 99.5 Central Control System 99.5 Total 80.7 Online/year The Accelerators Generating, accelerating up to 40 MeV and shaping a deuteron beam of 125 ma is probably an exciting challenge, which justifies the validation by the construction and tests of the first section. The ion source development and construction will be based on a prototype built by DAPNIA and called SILHI. Preliminary tests have shown that an excellent reliability can be expected from such a source, based on an electron cyclotron resonance at a frequency of 2.45 GHz at 875 Gauss. Tests performed in 1999 have shown reliability higher than 99.9 % over 5 day CW operation (see Figure below). 192

4 Extracted Current [ma] Duration: 104 hours Availability: % MTTR: 00:02:32 MTBF: 52.0 hours Date and Time Figure 3. View of the SILHI source at Saclay, France with its reliability tests performed in These tests show an availability greater than % with a mean time between failures of 52 hours. After a low energy beam transport with beam shaping and appropriate diagnostics, the beam enters a radiofrequency quadrupole. The ongoing work aims at optimizing this delicate part. The RFQ (with a length of about 10.5 m) could be made of 3 segments, each of them made of 3 individual parts. The RF frequency, common to the DTL will be 175 MHz and generated by 1 MW CW unit power electron tubes. After the matching section the DTL will bring the energy of the beam from 5 MeV, energy at the output of the RFQ to 40 MeV. The reference solution is an Alvarez structure at room temperature. An alternative design, based on a superconducting CH structure is also under consideration, because operation costs could be strongly decreased. Finally, the high energy beam transport section will bring the beams to the lithium target and profile them to the appropriate shape (a rectangle of 200 times 50 mm). Multipole magnets are considered (probably up to duodecapole arrangements), rather than a sweeping technique. Figure shows the result of the first computations of the beam shape at its arrival on the lithium target y (mm) x (mm) Figure 4. Shape of the beam (preliminary calculation integrating errors) showing a sharp profile and a flat top of about mm 2. The stability and flatness of the beam need further optimization work. The combination of the two beams hitting the lithium target lead to a power density of 1 GW /m 2! 2.2 The Liquid Lithium Target The lithium, injected at high speed by a nozzle on a back-plate flows downwards and is then trapped and purified in a dedicated circuit. Four main challenges have been yet identified: The thickness of the lithium flow must be kept homogeneous within ±1 mm. Because of the high speed of the flow (the nominal speed should be 15 m/s, ranging between 10 and 20 m/s). 193

5 Hydrodynamic instabilities should thus be avoided, in order to maintain these characteristics and avoid in any case that the beam hits the back-plate. This back-plate will endure the highest neutron flux (up to 60 dpa per year). Its lifetime will evidently not cover the full operation of IFMIF, planned to last more than 20 years. A periodic replacement is thus foreseen, probably once a year. Remote handling will be necessary because of the high activation of this area. Two technologies are being studied: o A welded solution, the replacement being made by milling the external lips of the plate; o A plate bolted, with the advantage of an easier replacement, but the uncertainty about the o ring lifetime and tightness. The lithium purification system is also a concern, as the highest purity is required, in order to avoid in particular a rapid erosion of the nozzle; nitrogen content will be minimized down to a few ppm. Because of the very high activation and irradiation in this zone, the diagnostics, necessary to monitor the interaction between the deuteron beam and the lithium flow, will have to be remotely positioned and the transmission of the signals between the monitored area and the detectors will be carefully studied. Li inlet Flow straightener Measurement and Observation of Li surface Contraction nozzle D J : mm W J : 100 mm R W : 0.25 m U 0 : 20 m/s For nozzle exit R W : m Figure 5. Schematic view of the liquid lithium target. After purification, the lithium is injected on the back-plate thanks to a nozzle, which contracts the flow width from 250 down to 25 mm with an intermediate thickness of 62.5 mm. The width, perpendicular to the Figure is 260 mm (the prototype to be built during the EVEDA phase will have a width of 100 mm, all other characteristics being the same as the actual one. 2.3 The Test Facilities As mentioned in the introduction, the test facilities are divided in three volumes, with irradiation doses raging from 50 dpa down to 1 dpa. The Figure below illustrates a schematic view of the whole set, with the delicate mechanical compatibility. Remote maintenance tools are not illustrated, but form a dedicated section of the target and test facilities. Thanks to regular exchange with the material community (and in particular via the so called users group), a more precise definition of the experiments is expected (irradiation conditions, homogeneity, temperature range and accuracy, objectives in terms of reliability) as well as possible new experiments, not yet defined (in particular in the low flux area). 194

6 Shield Plug High Flux Test Module Medium Flux Test Modules Low Flux Irradiation Tubes D+ Lithium Target 2m Lithium Tank Figure 6. Schematic view of the Lithium Target Facilities and the Test Facilities. In the High Flux Test Module (20 to 50 dpa) about 1,000 samples, positioned in temperature controlled boxes, will be irradiated. The Medium Flux Test Module (between 1 and 20 dpa) can contain for example several tests as creep fatigue tests under irradiation, in situ tritium release experiments. The Low Flux Irradiation Tubes are not yet defined and could be used to test ceramic insulators, RF windows, or even superconducting materials. Thermocouples Heating 4 x 3 capsules samples Helium Figure 7. Arrangement of the samples inside the High Flux Test Module capsules. Thanks to the incorporated heating wire and the helium flow, excellent temperature homogeneity can be 195

7 obtained, ensuring good conditions of tests. A dedicated program called Small Specimen Test Technology is launched to master the extrapolation from the very small samples and the bulk material that will be used to build the future fusion devices. 2.4 The conventional Facilities The Figure below summarizes the layout of the systems described above inside the main building. One can distinguish in particular the Post Irradiation Experiment (PIE) facilities, where the samples will be characterized after irradiation. The EVEDA phase aims at providing all engineering documents for the construction of the whole plant, including the generic site safety file, its overall detailed cost and construction planning. Test Modules inside Test Cells Post Irradiation Experiment Facilities Ion Source RF Quadrupole Drift Tube Linac High Energy Beam Transport Li Target m Li Loop Figure 8. Overall view of the IFMIF installation. This artist view shows the main parts of the plant: the two accelerators, the liquid lithium target and its purification loop, the test facilities behind the lithium target and the post irradiation facilities and the surrounding building. 3. Project Organization and Goals The Broader Approach agreement, signed at Tokyo on 5 February 2007 between Japan and the European Union, sets-up three projects: IFERC, the International Fusion Energy Research Center, the construction of a Satellite tokamak, called JT-60SA and IFMIF. Each Project Leader reports to the Steering Committee and is assisted by a Project Committee. In the case of IFMIF, representatives of the materials community could belong to the Project Committee, in order to ensure a tight link between the project and the specialists who will have to define the detailed experimental content. The Project Team, located at Rokkasho, at the North of the Honshu, the main island of the Nippon archipelago, will coordinate the work performed in many European and Japanese laboratories. Two main activities will lead to the detailed definition of the construction file of IFMIF: The Engineering Validation activities, with the design, construction and tests of the three main challenging parts of the project: o The accelerator, up to an energy of about 10 MeV; 196

8 o The lithium target, at a 1/3 scale; o The test sample rigs and the demonstration of the extrapolation capacity between the small samples and the future bulk materials. The Engineering Design Activities, with the detailed engineering description of all upper parts, plus the conventional facilities, the generic site safety file, a construction schedule and detailed cost estimation. The EVEDA phase is foreseen to last 6 years. Conclusion Thanks to the implementation of the Broader Approach agreement in parallel to ITER construction, the fusion community strongly improves the coherence of its overall program, and shortens the time required to design and build the first power plant based on fusion energy. If a suitable site is rapidly chosen for the construction of IFMIF, the start of its operation could be almost simultaneous to ITER operation. A new essential phase of IFMIF is now starting with its engineering activities, which will last 6 years. Main Achievements Required Design Construction Operation Application of results Production and control of long pulse-burning plasma Heat and particles exhaust (plasma facing components) Test of breeding blanket modules for DEMO Net electricity production (full hot breeding blanket) High reliability of operations Qualification of lower activation materials for PROTO Improved economy in electricity production Improved low activation materials Demonstration of a reference low activation steel for DEMO Search for higher performance materials for PROTO Demonstration of waste management and recycling Demonstration of safety management Demonstration of low environmental impact potential Accompanying Programme in Physics & Technology ITER (1/2 GW th ) Material Development (IFMIF) Environment & Safety DEMO (2 GW th ) PRO TO (1.5 G W e ) Large Scale electricity production Years after decision on Next Step Figure 9. Overall view of the fusion program, as imagined today. The two main installations ITER and IFMIF are not at the same status of development, as ITER is now in construction, while only the engineering phase of IFMIF is now starting. Nevertheless, both installations should produce their experimental results relatively simultaneously, provided that a site decision of IFMIF is taken in a reasonable time frame. From Five Year Assessment Report related to the specific programme: Nuclear energy covering the period June 2000 Acknowledgement The Engineering Validation and Engineering Design Activities of IFMIF are one of the three projects of the Broader Approach agreement signed between Japan and the European Union on 5 February 2007 at Tokyo. 197