Tritium retention buildup towards pulses in ITER PFCs and dust

1. Tritium retention buildup towards pulses in ITER PFCs and dust W.M. SHU, S. Ciattaglia and M. Glugla ITER Organization Acknowledgement to ITER TF on Tritium inventory 9th Hydrogen Workshop, Salamanca, June 2-3

2. fully-removed lids by FIB fabrications big blisters small blisters partial-removed lids by FIB observations New findings: two kinds of blistersformed on W by low energy D plasma Re-crystallized W; 38 eV, 1026 D/m2, around 500 K;W.M.Shu, Appl. Phys. Lett., 92, 211904 (2008). Blistering occurs at W for energy well below the displacement threshold. The lowest energy to produce Frenkel pair is 940 eV for D → W. For the small blisters, internal blister was a hole or pit, but the maximum height against diameter reached 0.7, which is one-order of magnitude greater than that reported before. 9th Hydrogen Workshop, Salamanca, June 2-3

3. cross-section of a blister cross-section of a blister crack/void along grain boundary crack/void along grain boundary New features of blisters Re-crystallized W; 38 eV, 1026 D/m2, around 500 K;W.M.Shu, Appl. Phys. Lett., 92, 211904 (2008). By conventional definition, blisters are plastic dome-shaped buildings where a lenticular cavity is included between the blister lid and the bulk material. For most cases of big blisters, there was no hollow lid formed, but a crack/void at the grain boundary underneath the blister. 9th Hydrogen Workshop, Salamanca, June 2-3

4. Various shapes of big blisters Re-crystallized W; 38 eV, 1026 D/m2, around 500 K;W.M.Shu, et al., PSI Conference. (b) (a) (c) (d) 9th Hydrogen Workshop, Salamanca, June 2-3

5. Bursting release and retention ratio Re-crystallized W; 38 eV, W.M.Shu,et al., PSI Conference. 1026 D/m2, 500 K 2×1026 D/m2, 400 K There is a peak around 500 K. Retention ratio at 775 K and 380-440 K is smaller than 10-7 and 5×10-6, respectively. Busting release peaks were found in the TDS curve, indicating bursts of some blisters. 9th Hydrogen Workshop, Salamanca, June 2-3

6. Fully-recrystallized W Partially- recrystallized W Annealed W Single W (111) 38 eV D ions, 315 K 1026 1027 In comparison with the data of J. Roth et al. (ICFRM 2007) W.M.Shu,et al., Nucl. Fusion 47 (2007) 201; Phys. Scr T128 (2007) 96. Smaller retention ratio was found in the higher fluence region at lower energy. 9th Hydrogen Workshop, Salamanca, June 2-3

7. Calculation by J. Roth et al. (ICFRM 2007) 700 g limit (1000 g(limit in VV) – 120 g (in cryopump) – 180 g (others)) 750 discharges In the calculation, the retention ratio in W was assumed to be around 10-3, due to their higher energy (200 eV) and lower fluence (max. 1025 ions/m2). 9th Hydrogen Workshop, Salamanca, June 2-3

8. Assumptions made in this calculation 1. Area, flux and temperature: (1) Divertor (strike points): 3 m2, 1×1024 DT atoms/m2/s, 775 K (2) Divertor (other target area except strike points): 47 m2, 1×1023 DT atoms/m2/s, 775 K (3) Divertor (others): 100 m2, 1×1022 DT atoms/m2/s, 775 K (not considered in [1]) (4) First wall: 700 m2, 1×1020 DT atoms/m2/s, 380-440 K (750 m2 in [1]) 2. Retention ratio (retention against fluence) in W PFCs: (1) Divertor (at 775 K): 5×10-7 [2] 3. Constant retention in Be due to implantation: 7×1020 DT atoms/m2 [3] 4. Breading in Be first wall: Tritium inventory I (appm) = 280F - 2350[1 - exp(-0.1F)]; [3] where F(MWa/m2): neutron fluence. 5. Sputtering yield of Be first wall: 4×10-2 atoms/ions, half is dust [4] 6. Retention ratio of tritium in Be: 4×10-2 [4] 7. Producing rate of W dust (700 kg in 106 s): 2.3×1021 atoms/s [5] 8. Retention ratio in W dust: 1×10-6 [5] [1] J. Roth, et al., “Tritium Inventory in ITER: Laboratory data,” presented at the 1st meeting of ITER DCR 131 (In Vacuum Vessel Tritium Control), Oct.16, 2007. [2] W.M. Shu, et al., Fusion Eng. Des. (in press). [3] R.A. Anderl, et al., J. Nucl. Mater.273, 1 (1999). [4] GSSR III [5] W.M. Shu and S. Ciattaqlia, internal discussion. 9th Hydrogen Workshop, Salamanca, June 2-3

9. 700 g limit & codeposits T inventory at case 1: full tungsten divertor The main contribution is from the Be first wall initially, but Be dust will be the controlling factor after 200 seconds. 9th Hydrogen Workshop, Salamanca, June 2-3

10. Tritium retention at the case of large wall flux The averaged tritium retention estimated is 0.056 g T/discharge. In the calculation, averaged D-T flux at the first wall was assumed to be 7×1022 DT atoms/s, the same as that used by Roth. However, Philipps [1] argued that the most recent value of the averaged D-T flux increased to 3-5×1023 DT atoms/s. If the same assumptions are used, the averaged tritium retention will increase to 0.24-0.4 g T/discharge for the case of large wall flux. [1] V. Philipps, “T–retention from present experiments and further validation,” presented at the 4th meeting of ITER DCR 131 (In-Vacuum Vessel Tritium Control), March 12, 2008. 9th Hydrogen Workshop, Salamanca, June 2-3

11. Baking at 623 K to release major portion of tritium in Be codeposits M.J. Baldwin, et al., J. Nucl. Mater.337-339, 590 (2005). D/Be (a) and O/Be (b) ratios for deposited material collected on Ta (grey symbols), Mo (dotted symbol) and W (white symbols) deposition probe coupons as a function of coupon temperature. 9th Hydrogen Workshop, Salamanca, June 2-3

12. Tritium retention after baking at 623 K Baking at 623 K of divertor after 1750-3000 discharges should be performed to release tritium from the Be dust that is located around divertor region. The DT/Be ratio could decrease from 4×10-2 to less than 10-2 after baking. Thus, the averaged tritium retention finally will be 0.06-0.1 g T/discharge if baking is taken into account. 9th Hydrogen Workshop, Salamanca, June 2-3

13. Tritium buildup in the first 5 years’ operation ~ 0.06 g T/discharge [1] Project Integration Document PID, Jan. 2007, ITER Organization, Editor: J. How. 9th Hydrogen Workshop, Salamanca, June 2-3

14. Permeation of tritium in CuCrZr (castellation) at 623 K Permeation flux: =2pDLSP1/2/ln(dout/din) 3.7×10-7 g-T/h for inner divertor; 5.7×10-7 g-T/h for outer divertor; 9.4×10-7 g-T/h in total. 0.09 mg in 100 h. Graph considers bulk diffusion only, not grain boundary diffusion or leakage. If the transport properties of hydrogen in CuCrZr are the same as that in Cu, tritium permeation through CuCrZr pipes without W armor will reach the steady state within one hour. 9th Hydrogen Workshop, Salamanca, June 2-3

15. Permeation of tritium in large SS pipes at 623 K Permeation flux at steady state: =2pDLSP1/2/ln(dout/din) 1.3×10-10 g-T/h In 100 hours’ baking: 7×10-10 g-T in total. Graph considers bulk diffusion only, not grain boundary diffusion or leakage. The steady state will be reached in more than one month, and tritium permeation will be negligibly small in 100 hours’ baking. 9th Hydrogen Workshop, Salamanca, June 2-3

16. Permeation of tritium in small SS pipes at 623 K Permeation flux at steady state: =2pDLSP1/2/ln(dout/din) 2.8×10-10 g-T/h In 100 hours’ baking: 3×10-8 g-T in total. Graph considers bulk diffusion only, not grain boundary diffusion or leakage. The steady state will be reached in one day, but tritium permeation will be negligibly small in comparison with that of CuCrZr. 9th Hydrogen Workshop, Salamanca, June 2-3

17. 700 g limit 350 days for continuous operation T inventory at case 2: full tungsten PFCs W divertor is always the major component for T retention. Tritium retention in continuous operation: 2 g /day (9 mg / discharge) 9th Hydrogen Workshop, Salamanca, June 2-3

18. 700 g limit In comparison with that by J. Roth for the case of Full W PFCs The retained amount calculated by this work is smaller than that by Roth, because of the lower retention ratio in higher fluence region. 9th Hydrogen Workshop, Salamanca, June 2-3

19. Summary • In the case of full W divertor and Be first wall, tritium in Be dust (including codeposits) will be the controlling factor after 200 s of discharge. The averaged tritium retention finally will be 0.06-0.1 g T/discharge for the case of large wall flux if baking is taken into account. • If baking at 623 K is performed, permeation through CuCrZr pipes located at castellation will be predominant. Considering bulk diffusion only, the total permeation will be 0.09 mg in 100 hours’ baking. • In the case of full W FPCs, the major contribution to the inventory is from the divertor, and the averaged tritium retention will be 9 mg/discharge (2 g/day for continuous operation). • More accurate calculation should be performed by considering the effects of simultaneous H and He plasma on W blistering and dust producing. 9th Hydrogen Workshop, Salamanca, June 2-3

20. T inventory at case 3: CFC+W divertor 700 g (~700 discharges) & codeposits 9th Hydrogen Workshop, Salamanca, June 2-3

21. Some issues related to in-vessel removal of T by oxidation • Highly tritiated water processing  DCR-140 • Corrosion highly tritiated water is very corrosive even to stainless steel due to the radiochemical formation of peroxides and radicals • Radiolysis and tritiated polymer formation  re-deposition and accumulation of tritiated polymers formed in the gas mixture of tritiated water vapour, tritium, CO and CO2 is unavoidable • Oxidation of Be first wall  tritiated water moisture produced during oxidation may react with beryllium • Wall conditioning  implications for after-oxidation wall conditioning to be evaluated • Increased tritium retention in Be co-deposits  Oxidised Be codeposits are found to retain larger amount of T than pure Be codeposits • Evaluation of safety related issues (such as dust-related) required to determine compatibility with ITER safety requirements 9th Hydrogen Workshop, Salamanca, June 2-3