ITER and Fusion Safety Aspects

1. ITER and Fusion Safety Aspects H.-W. Bartels, ITER Prague, 13.November 2006

2. Fusion and nuclear safety D He-4 + 17.6 MeV T n ==> nuclear safety related issues: 1) radioactivity of tritium (~5 kg in reactor) 2) activation from 14 MeV neutrons (~1/2 of activity of PWR)

3. Tritium • Tritium in human body: • fast component t1/2 ~10 days • slow component t1/2 ~ 30 – 300 d • (organically bound tritium) • Half-life: 12.3 a • ß—radiator: <Ee-> = 5.7 keV • range organic matter: 6 µm • Horny layer skin: 70 µm Incorporation: • Inhalation, skin absorption, • Ingestion Very Mobile: HTO, HT Biological half-life:~10 days • Hazard/Bq: tritium <1000*Cs137

4. Activation Products Neutron activation in some elements (5 MW/m2 in 2.5 years) Problematic isotopes: steel: 60Co: t1/2 ~ 5 years 1.2 MeV g-radiation tungsten: 185W: t1/2 ~ 75 days 0.4 MeV b-radiation Favorable elements: - Vanadium (V) - Chromium (Cr) - Titanium (Ti)

5. Schematic Fusion Reactor Build-up

6. ALARA principle lower project release limits Tritium 1 g/a Activated dust 10 g/a Corrosion products 50 g/a Dose limits: vary by country: 0.1-5 mSv/a average natural dose: 2 mSv/a Doses < 1% of natural dose Technical precedence used: tritium plants chemical plants (beryllium) fission reactors (esp. CANDU’s) tokamak (D_T: JET, TFTR) Analysis for last year ITER shielding blanket operation Initial releases small: limited use of tritium low level of activation time delay permeation of tritium into coolant Normal operational effluents

7. Accidents:Design guideline atmospheric release Ultimate safety margin: Evacuation threshold for ground level releases: Tritium: < 100 g Tungsten dust: < 4 kg

8. Accidents: Energy Sources

9. Schematics of computer model for integrated accident analysis

10. Accidents: Pressurization and decay heat In-vessel decay heat driven temperature transient • Accident scenario: • multiple FW failure  • all FW cooling pipes in two toroidal rings damaged  • fast pressurization plasma chamber • pressure limited by suppression system • Maximum pressure ITER: 2 bar • no in-vessel cooling VV cooled by natural circulation

11. H2 + air explosion Accidents: Hydrogen in ITER H2 formation in fusion: chemical reactions with hot plasma facing components, e.g.: Be + H2O  BeO + H2 Combustion wave propagates ~2000 m/s Pressures from 15 to 20 bar

12. Accident: Loss of coolant w/o shutdown

13. 40 kg-H2 20 Accident: Loss of coolant w/o shutdown Ex-vessel LOCA w/o plasma shutdown • In case of on-going plasma burn temperature increase in affected components. • failure of in-vessel components at elevated temperatures • ingress of steam into VV • Be/steam chemical reactions • hazard of hydrogen formation • bypass 1. confinement barrier • ==> plasma burn will be terminated by fusion shutdown system

14. Accident analysis margins • Large margins maintained because: • limitation of radioactive inventories; • inherent plasma termination processes; • long time for component heat-up; • gross structural melting impossible; • multiple layers and lines of defense to implement radioactive confinement; • design tolerant to safety system failures • Maximum doses < 2 mSv (annual natural dose) • ITER accident analysis has confirmed safety potential of Fusion Energy.

15. Accident: Wet bypass • Hypothetical event: • loss of coolant plus 2 failures in one heating or diagnostic line (“wet bypass”) • Analysis results: • plasma chamber pressurizes • opening bypass / suppression tank • transport radioactivity into room • capture tritium, dust in tank, settling of dust by gravity, condensation of HTO in room • cleaning of room after 8 hours: •  ~15 g tritium released

16. Accidents: Loss of cooling Fusion Reactor

17. Is it true ? • V&V: verification & validation • verification • correct coding • comparison between different codes and performers • validation • comparison codes / data

18. Verification thermo-hydraulic codes • Two codes used in ITER: • MELCOR (US) • INTRA (EU) • benchmark: large loss of cooling accident in ITER vacuum vessel • initially some differences, but both below design pressure 5 bar • differences could be explained by different treatment of mixed flow of steam and water • ==> feedback to design: lower pressures for separation of phases, e.g. pressure suppression system on top of vacuum vessel

19. Validation experiments steam vacuum

20. Validation thermo-hydraulic codes • Two codes used for ITER: • MELCOR (US) • INTRA (EU) • Comparison of code results with experimental data of water injection into vacuum vessel • Problem is scaling: length 1/10 of ITER • ==> larger surface/volume

21. H 2 O R S S 3 1 6 S S 3 1 6 B e W alloy SS316 or Copper Support Z B e a m D u c t ( 1 6 0 x 8 0 ) # 1 D - T S o u r c e 600 400 382.4 1 0 1 . 6 2 0 3 . 2 6 0 9 . 8 6 6 0 . 4 5 0 . 8 14 MeV n-Source Experiment

22. Fusion Neutron Source (FNS)

23. ITER Decay Heat R&D -International code validation effort: - Uncertainties < 15% - 14 MeV n-irradiation at FNS at JAERI - Decay heat measurement: sum of ß,g radiation Cu, 7 hours irradiation SS-316, 7 hours irradiation

24. JA dust mobilization experiments

25. Russian hydrogen detonation experiments

26. What if it is not true ?

27. ”No-evacuation” limit and cliff-edge effects • Release assumptions: ground level, duration 1 h, worst case weather • No-evacuation limit (early dose):IAEA, ITER: 50 mSv • ITER “no-evacuation” limit met for tritium release < 90 g, in HTO form • No cliff-edge effect for tritium(For a hypothetical tritium release of 1 kg no-evacuation limit exceeded for < 1 km2) Area [km2]

28. Long term contamination Tritium concentration in soil after contamination

29. Waste volumes fusion reactor • Fusion optimized materials: • V-alloys • steel without Ni, Co • impurities need careful attention (Nb, Ag, Co, U) • Significant part (~30%) of activated material can be cleared • Volume ~1-2 x larger than fission waste (not counting U-mining~1.5 Mm3) • large fractions could be recycled

30. No burden to future generations

31. Conclusion • Normal operation • dose < 1% of natural background • Accidents • source term ~ 1000 times smaller compared to fission • if properly designed: no destruction due to internal accidents • large reactions times • tritium and dust largest hazard >> allowable releases • Waste • volumes comparable (~2 * larger) compared to fission • toxicity 1000 times smaller compared to fission • recycling might be feasible • Safety and environmental features dependent on design

32. Other fusion reactions (1a) D + D  3He + n + 3.3 MeV (1b) D+D  T + p + 4.0 MeV (2a) D + 3He  4He + p + 18.4 MeV (2b) D + T  4He + n + 17.6 MeV (3) p + 11B  3 * 4He + 8.7 MeV (4) p + 6Li  3He + 4He + 3.9 MeV Equations (1a) – (2b) can be summarized as 3D  4He + p + n + 21.6 MeV D in water ~3.3*e-5  energy content 1 liter water ~ 350 l gasoline

33. Advantages of fusion • No radioactive raw material • No chain reactions (small amount of fuel ~1 g in plasma) • Moderate decay heat (large surfaces) • Low biological toxicity and half-life time of activation products • Generates no greenhouse gases (no SO2, NOx)