A E Costley ITERO, Cadarache, France 12th Meeting of the ITPA Diagnostic Topical Group

1. DUST: SOME RECENT THOUGHTS A E Costley ITERO, Cadarache, France 12th Meeting of the ITPA Diagnostic Topical Group Princeton Plasma Physics Laboratory, 26 - 30 March, 2007 ITER_D_25VMPK 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007

2. Two kinds of dust and two kinds of hazards Hot dust and cold dust Explosive and radiological hazards When examine materials in ITER, and potential events (window breaks etc) can show that if amounts of dust are below certain amounts then both hazards can (almost completely) be avoided. Have to limit hot dust to 6 Kg and cold dust to 400 Kg 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007

3. The problem is that these are global quantities and we can only make local measurements. Hence, in addition to validating measurement have to validate method of extrapolation as well, and these validations have to be to ‘safety class’. A ‘conventional’ approach is to: • build an understanding of dust production and transport • through measurement and modelling; • develop methods of measurement; • develop methods of removal; • develop methods of extrapolation; etc and aim to validate at the safety level. 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007

4. Europe is following this approach and there is a very comprehensive, funded programme in progress. All these aspects are under investigation and deliverables are expected on all of them this year. 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007

5. In-VV H-dust safety issue solution strategy guidelines ongoing R&D – further R&D necessary Diagnostics (inventory measurement), Removal Dust production In VV Hydr -Dust reaction 3D model Mobilization Factor during Accidents Mitigation system • Model definition** • How much, where, when • Transport • Mobilization factor • R&D validation plan • Feasibility study * • In-VV inerting • Cath.Recombiner • in VVPSS Review of potential **methods Selection R&D development plan Data base completion** Model development R&D validation plan Exper. in Tokamak & test beds (e.g. ILW-JET) Model validation Diagn-Removal devel. Tests on Tokamak Integration into ITER Detailed Design and validation Model validation • Dust prediction • validated code • Operating instructions • Dust • Diagnostics • Removal techniques Validated code for dust behavior in accident Mitigation system Design and validation • On going under ITAs • ** On going, an overall ITER task is under preparation 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007

6. On the basis of this research it may be possible to build a credible method of extrapolation that could be validated during the Hydrogen phase of ITER operation. In this case the measurements required would be specific local measurements, and these measurements would have to be safety class. Not impossible - but it could require many probes and measurements on the machine. 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007

7. Likely Deposition Locations From K Itami 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007

8. From K Itami 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007

9. An ‘alternative approach’ would be to separate the hot dust/explosion and cold dust/radiological hazards, and to take advantage of specific features of fusion devices. For example, for the explosion hazard: • dust inventory is completely separated from air by the design of • the device, • failures of different independent components are required to • create conditions for explosion, • practically each of these failures will automatically shutdown • the plasma. • maximum penetration failure likely to be limited to < 1 m2 • modelling shows that dilution by 50% with an inert gas is enough • to prevent explosion. 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007

10. This opens the possibility of avoiding explosion conditions by having a detection system that detects an air or water ingress and triggers the injection of a large quantity of inert gas. Obviously, it will be necessary to prove that explosion conditions will be avoided, that is the injection rate is high enough. Also, the system would have to have some Intelligence - it should not fire off every time there is an air or water ingress. In this case the safety class measurement is of the air or water ingress. 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007

11. For the cold mobilizeable dust, this could in principle be measured by mobilizing it. It is a possible to envisage a system whereby a worst case air ingress is tested by injection of an inert gas and the dust mobilised is measured (eg by laser scattering techniques). It could also be pumped out filtered and measured. M Wykes, UKAEA 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007

12. In which case the safety class measurements would be the measurement of the mobilised dust (locally and globally). A few local measurements would probably be needed to decide when the mobilized dust measurement should be made. This approach is attractive because it connects the measureable quantity with the hazardous quantity and would less invasive to the machine than many local measurements. Moreover recent site specific studies indiacted that limits on cold dust could be too low by perhaps an order of maginitude. These approaches are being discussed and developed with the ITER IT through a dialogue involving Safety, First Wall, Vacuum, FS&T and Diagnostics divisions. There is urgency because we have to prepare a credible methodolgy for inclusion in the ITER Prelim. Safety Report to be released in the summer. 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007

13. Measurement of Divertor Erosion, Dust and Tritium retention. (from K Itami) These topics are linked: Erosion - > dust and/or deposits Deposits -> Ti retention Hydrogen isotope ratio in both of Be and C deposits in VV ranges from X = 0.1 to X= 0.4. (Results from tokamak experiments) In the worst case with X=0.4 and D:T=1:1 in co-deposits, only 11 kg of C or 8 kg of Be retain 450 g of tritium 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007

14. REQUIRED PARAMETERS GROUPED ACCORDING TO • THEIR OPERATIONAL ROLE Expect to meet measurement requirements; performance not yet known;expect not to meet measurement requirements. 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007

15. SUMMARY • The hazards are known, the limits defined. • Extensive R&D is in progress on a ‘conventional approach”. • Discussion and dialogue between relevant groups in ITER on • an ‘alternative approach’. This could result in a change in the • quantities to be safety qualified. • The measurements required will depend on the strategy • adopted. • Decisions are needed soon. IPSR due ~summer this year. 12th Diagnostic TG ITPA Mttg, PPPL, 26 - 30 March, 2007