Selection of Plasma Facing Components for ITER and ITER Operational plan V. Chuyanov (Thanks to Y. Shimomura ) ITPA Garching May

Content Current ITER plans. Possible changes of PFCs Pro and con. Main limitations. Logic of decision making process Conclusions

The Technical Requirements for ITER Demonstrate Q>10 with inductively-driven plasmas (not precluding ignition) Aim at steady state with Q>5 Demonstrate integration of essential fusion technologies Test components for future reactors including blankets and high heat flux components (neutron flux>0.5 MW/m 2, total fluence>0.3 MWa/m 2 ) ITER is the first experimental fusion reactor; Flexibility is required in the early phase to cope with uncertainties of projection, to explore a variety of operation regimes and to incorporate progress in R&D. In the later phase, more emphasis is on stable and reliable operation to facilitate engineering tests.

ITER Operation Programme

Operation Plan The reference operation plan is a working assumption for preparation of hard and soft ware. The plan must and will be modified with new results or considerations. -H-phase: Mainly for machine commissioning with Ip up to 15MA (Expected duration ~ y) -D-phase include discharges with a limited amount of tritium: Major characteristics of plasma-wall interaction will be clarified in this phase as well as other plasma characteristics. (Expected duration y) The following program will strongly depend on these results as well as other R&D.

H-plasma H-plasma is not appropriate to simulate plasma wall interactions for those in the later phases It is difficult to achieve H-mode of H-plasma. L-mode plasma has different characteristics from H-mode plasma. Study of L-mode H- plasma will not give very useful information for further operation of ITER except in specific areas. Predicted plasma parameters are far below those of D- and DT- plasma. H-phase is mainly for machine commissioning with plasma current. H-Operation is not an operation phase of ITER as a nuclear facility.

Current plans 3CfC divertor sets and 2W sets are currently planned ( not a strong limitation) 1st CfC for H-phase, 2nd CfC for DD and Low Duty DT. What information we can get during H- phase for selection of HHF material for Divertor ?

Is it very difficult to get useful information about retention in H phase The retention factor could be measured only if it is very large and recovery factor is very low. 10 shots/day x 150 days/year = 1500 shots. If 10 g/shot, the total amount of injected hydrogen = 15 kg. Much more hydrogen will be injected during discharge cleaning. Therefore, global retention could not be measured. A retention factor could be measured in a very long pulse operation, e.g. 1000s with lower plasma current, but conditions are very different from later phases. This might give a lower limit of retention factor. If the retention factor is very large, prepare the W targets for the next step. Install the spare divertor cassettes with W targets and prepare for the next step. All W targets might not be ready for the 3rd year of the operation. In this case, extend H-phase or do some D-experiment (+ small T) with the first divertor set (CfC + a few W). If this is not a case, prepare the next CfC targets (keep the reference scenario). If enough money is available, prepare both.

What can be done in DD phase with limited amount of tritium ? Tritium retention and removal can be accurately measured by using a small amount of tritium Major plasma wall interaction characteristics can be clarified except changes of material characteristics after long time operation including effects of material mixture. Demonstration of >100 MW burn with a short pulse is possible. Major characteristics will be clarified for full DT burn except long or ss burn with collective effect of alpha particles. (Issues of steady state operations can be only studied with very high heating power such as 140 MW in total which is difficult to be achieved without DT burn.) The next material for the divertor target and other program will be decided.

What changes are possible and when The ITER project is starting now the procurement process. Long lead components- magnets, the vacuum vessel must be procured first. The procurement of in vessel components and external systems may be started somewhat later ( depending on the component) There is still some but very limited time to incorporate in the design latest results of the fusion The most difficult and controversial issue is the selection of materials for plasma facing components – the first wall and diverter.

Current ITER baseline. The current ITER baseline selects : –beryllium as a material for the first wall; – CFC for strike points of the Diverter, and – tungsten for the dome and upper parts of diverter plates. The choice is an obvious compromise between different requirements and is a target of a strong criticism. There are several other suggestions like an all carbon machine, an all tungsten machine, or all possible combinations. The very existence of so different proposals shows that now there is no obvious best choice. At the same time the project logic dictates the necessity to establish a clear time table of project decisions which has to take into account the time necessary for technological development, design, procurement, production and installation of different plasma facing components.

The technological development for the current baseline is more or less finished. –All the parties involved in the procurement of the diverter and the blanket are already in the process of “Qualification” in which they must demonstrate that they are capable of the production of the plasma facing components within the required specifications. –This qualification process is being carried out according to the present design of ITER and has to be repeated if the design were to change. In accordance with current plans the procurement agreements of the diverter PFC shall be launched in February If a different, say, tungsten PFC for strike points will be selected, the qualification process must be started in May 2007.

Possible future changes of PFCs In cases of necessity (for example a short time of life or a damage by disruptions ) it is foreseen to exchange the diverter cassettes up to eight times during the life of ITER. Expected change time for the cassettes is ~ 4 months. In the same time only several blanket modules can be exchanged. A change of all blanket modules will take ~ 2 years. With the current design of the first wall it looks very difficult to test more then 2 different first walls during the expected ( 20 years) life of the machine.

Pro an Con CFC +Excellent thermo-mechanical properties (no melting under transient heat load) + Permissible contamination level; order of 1% +A core radiation is low, while divertor radiation loss is effective to maintain semi-detached operation –Large erosion / Short lifetime –High tritium retention (co-deposition). May be not acceptable. CFC offers excellent flexibility in plasma operation which is essential in the initial phase of ITER experiment. CFC does not satisfy requirements of long lifetime and low tritium retention in the later phase of ITER operations. W +Low sputtering erosion +Low tritium retention -Permissible contamination level: order of 0.01% -Melting/Irregular surface High-Z is challenging but could be workable in well established operations and could offer a long lifetime of the divertor target. DEMO relevance is not a necessary condition for selection of PFC for the initial phase of operation but must be considered as an important factor at the latest stages of ITER experimental program.

Long list of Pro&Con leads to a simple conclusion: At this moment there is no simple choice of the plasma facing materials that can guarantee a success throughout the life of ITER. A research program must be established with –the parallel ( CFC/ W ) development of hardware options, –targeted investigations of the in-vessel material behaviour and –several decision points for hardware changes.

Important considerations for decision making process : Taking into account that production of High Heat Flux components of diverter will take at least 4 years –(currently, the time allocated for the production of the first divertor is 5.5 years) ITER must start procurement of the second set of HHF components immediately after start of the operational phase ( when money become available) and better earlier. Because the information about the real retention will not be available at this moment, it is desirable to order 2 sets of HHF elements – CFC and W ones. It is also desirable to order at the same time the second set of cassettes to be able to keep refurbishing of the first activated set of cassettes out of critical path of the second set installation. Technology of the tungsten HHF components and qualification of producers must be prepared in advance.

Can be the first divertor HHF components made of W ? ITER has developed a design for the Be first wall and the divertor with CFC HHF elements and the tungsten dome and upper parts. –Technology has been developed for the design and qualification of producers is in progress. Design of divertor with all tungsten PFCs is not very different. –Technology has been developed but qualification process has not been started yet. The change from CFC to tungsten may be done now without change of ITER schedule, but the qualification of producers must be started this summer. The cost of this additional qualification is not prohibitive- < 1000 kIUA for 2 parties involved in diverter PFC production.

The situation with changing of the first wall looks different. –At this moment there is no technology for the tungsten first wall and there is no design which can permit a relatively fast exchange. With current design the exchange will take as minimum 2 years and will produce a lot of waste. With current design it is impossible to consider more than one change which can be done only after first 10 years of operation. No advantages to start with the CFC first wall have been identified. The risk to start with the all tungsten machine is even bigger than the risk to start with the tungsten divertor. At this moment the Be first wall looks like the only possible choice for the first 10 years of operation ( if B and W are compatible in ITER conditions) However, it is important to consider now a possible design of the tungsten first wall that can be installed without total removal of shielding blocks and to introduce the necessary changes in the current shielding block design permitting such installation

Decision making process. A general time table for decision making process looks as following : –The baseline choice must be done this summer. Taking in account many uncertainties it is necessary also to identify a back up option. If the CFC will not be fully rejected and kept as the first or as the back up option it is necessary to consider immediately a design change permitting to bake divertor up to ~ 350C. –Independently of what will be selected as a base line ( CFC or W divertor PFC s) the same parallel process will be needed to support the main and back up options and resolve the following questions : - long term consequences of carbon contamination, ( initial assessment – it is not a problem) -tritium removal -Be/W compatibility -ELM suppression, -disruption prevention/mitigation.

-Initial results of these - Results of ASDEX- U full tungsten experiments - JET Be/W results will be a basis for the final selection of the option to be installed first. The final choice must be done latest in 2010 before fabrication of the diverter targets for the first diverter will be started.

The second diverter will be installed before the beginning of tritium phase or in its very beginning. The selection of material for its target plates must be done latest just after the beginning of hydrogen phase. –If tungsten does not work, it will be discovered already in the hydrogen phase. –To discover that with CFC the co-deposition is a real problem DD plasmas will be necessary. –If ITER will wait for this information, some delay of the tritium phase will be inevitable. Therefore if money will be available it is reasonable to start manufacturing of two new sets PFCs – W and CFC – immediately after beginning of operations. The third divertor may be installed in the middle of tritium phase. The selection of the target plates for this change must be as late as possible, say in the beginning of the tritium phase. Both, the second and the third installations can be financed by operational budget.

Time table of PFC selection