Radiation damage in fission and fusion reactor materials

1. Radiation damage in fission and fusion reactor materials Kai Nordlund Accelerator laboratory University of Helsinki

2. The guilty parties • University of Helsinki: • Kai Nordlund, Emppu Salonen, Krister Henriksson, Petra Träskelin, Niklas Juslin, Juhani Keinonen • University of Illinois: • Mai Ghaly, Bob Averback, Pascal Bellon • MPI für Plasmaphysik, Garching • Chung Wu • University of Liverpool: • Fei Gao

3. Contents • Overview of types of primary state of reactor damage • Fission reactors • Fusion reactors • Our approach to studying it: MD • What is MD? • Capabilities and limitations of MD today • Brief history of MD for irradiation effects • Direct production of dislocation loops in bulk cascades • Surface effects during heavy ion bombardment

4. Radiation damage in fission reactors • Neutron-induced damage in structural materials: • Practically purely bulk damage • Primary state of damage caused by a few MeV neutrons which give a recoil energy of 50 – 100 keV to sample atoms • These recoil atoms then cause damage • The primary damage becomes on long time scales dislocation loops and voids, which degrade the mechanical properties of the material

5. Radiation damage in fusion reactors • Neutron-induced damage in structural materials: • As in fission reactors, but recoil atoms are produced by 14 MeV neutrons and have a broader spectrum • He produced in reactions • Damage at the reactor first wall: • Ions escape from the hot plasma and erode the wall of the reactor

6. Our activities • In my group we study: 1. Irradiation effects in semiconductors 2. Ion, neutron and plasma modification of metals 3. Plasma-wall interactions in fusion reactors 4. Nanocluster interactions with surfaces 5. Carbon nanotubes • Main working tool: Molecular dynamics computer simulations • Also now DFT and KMC

7. What is molecular dynamics? • Central idea: simulate atom motion • Solve Newtons equations of motion numerically • Forces derived from classical or quantum mechanical models

8. Give atoms initial positions r0 Calculate forces F = - V(r)and a = F/m Move atoms: r = r + v Dt +1/2 a Dt2 + correction terms Advance time: t = t +Dt Repeat until done The MD algorithm, slightly simplified

9. Force calculation • All other steps except the force calculation can usually be made arbitrarily accurate • Forces between atoms can be obtained from • quantum mechanical models • classical interatomic potentials • Quantum mechanical models • Very slow, number of atoms usually limited to a few hundred • Good for studying small defects • But mostly too slow to study irradiation effects

10. Capabilities: classical models • Number of atoms ~ 5 million • System size ~ 40 x 40 x 40 nm • Time scale ~ 1 ns • Materials that can be modelled (for irradiation effects) • Almost all commonly available pure elements • Some common binary compounds and alloys • In my group we have simulated irradiation effects in • FCC: Al, Cu, Ag, Au, Ni, Pd, Pt, Pb, CuAu, CoCu, NiCu, … • BCC: Mo, W, Fe, FeCr • HCP: Co • Other: C, Si, Ge, C nanotubes, Ga, a-C:H, SiGe, GaAs, GaN, …

11. Limitations • Number of atoms ~ 5 million • System size ~ 40 x 40 x 40 nm • Time scale ~ 1 ns • But these are actually not so serious for irradiation effects • Main limitation: interatomic potential availability • If there is no suitable potential available for your material of interest, what can you do? • Create a potential - but needed effort ~ 0.3 - 3 person-years • Use some model systems expected to behave qualitatively like your real system

12. Surface irradiation High-E ion Bulk irradiation Collision cascade simulations • Principle: initiate ion movement in or outside simulation cell with enough atoms, follow what happens until system has cooled down • Heat removed at cell borders • Electronic stopping included

13. Example: surface event • 30 keV Xe -> Au

14. Example: bulk event • 10 keV recoil in Cu3Au

15. Diaz de la Rubia et al. , PRL 1987 Brinkman, 1954 History: first MD of heat spike • In 1987 Diaz de la Rubia, Averback, Benedek and King showed using MD that heat spikes in metals behave much like the prediction in 1954 by Brinkman

16. Diaz de la Rubia and Gilmer, PRL 1995 History: phase changes • In 1995 Diaz de la Rubia and Gilmer showed that irradiation in semiconductors can directly induce amorphization

17. Ghaly and Averback, PRL 1994 Nordlund et al., Nature 1999 History:surface effects • Between 1994 and 1999 we have shown how a nearby surface can alter the damage production picture from that predicted by bulk models

18. Contents revisited • Overview of types of primary state of reactor damage • Fission reactors • Fusion reactors • Our apprach to studying it: MD • What is MD? • Capabilities and limitations of MD today • Brief history of MD for irradiation effects • Direct production of dislocation loops in bulk cascades • Surface effects during heavy ion bombardment

19. Vacancy cluster production in metals • Vacancy clusters are typically produced in the center of the cascade • The vacancies are actually pushed inwards by the advancing resolidification front and cluster there

20. Interstitial cluster production in metals • Interstitial clusters can also be produced directly in a cascade due to the recrystallization • "Liquid isolation mechanism" [Nordlund et al, PRB 57 (1998) 7556]

21. Vacancy cluster shape? • The vacancy clusters typically are disordered, but already directly close to the shape of an dislocation loop • With some annealing (even room temperature enough) these become well-ordered dislocation loops

22. Interstitial cluster shape? • Also interstitial clusters can be directly in the shape of interstitial dislocation loops • These will then subsequently affect the mechanical properties of the wall material

23. Irradiation of metal surfaces • But what happens when a surface is present?

24. Results: crater formation • Formation of a crater when a 50 keV Xe ion hits a Au surface [Nordlund, Physics World 14, 3 (2001) 22]

25. Comparison to experiment Final crater shape Simulated TEM image Experimental TEM image [Donnelly, PRB 56 (1997) 13599]

26. Comparison to experiment • Crater radius as a function of energy

27. Damage inside material • The “liquid” formed by the incoming ion can extend very deep into thematerial

28. Damage inside material • Thus there can be large defective crystal zones not only at the surface but also very deep in the sample [Nordlund et al, Nature 398 (1999) 6722]

29. Difference in damage production • Comparison of damage production in bulk and surface events:

30. Conclusions on metal irradiation • The surface can have a huge effect on damage production even at energies of the order of 100 keV!

31. How about lower energies? • We saw that heavy ions or neutrals with energies of the order of 50 keV produce really massive erosion of metals • But you plasma physics people will take care that such ions will never hit the fusion reactor first wall at such high energies • The limit for possible run-off erosion is when Y > 1 • For realistic divertor materials, what is the energy limit for that? • And could atom clusters enhance the effect?

32. Cluster sputtering in W? • We studied both self-ion and self-cluster sputtering in Mo or W • Our result: • Cluster sputtering enhancements can occur in Mo and W • But if the incoming energies are < 2 keV the yields are < 1 => no run-off effect [J. Nucl. Mater. 15 (2003) 5845]

33. Lighter ions? • But the main erosion issue for the plasma-wall community is of course the possible erosion by low-energy light ions (H, He) at the divertor • There are several important ways in which these can cause erosion: • Physical sputtering of Be, C and W • Chemical sputtering of Be and C • Chemical sputtering of mixed materials (e.g. WC) • Bubble formation and blistering • Only the first of these is well understood! • The following three talks will discuss the latter three issues!

34. Conclusions • Molecular dynamics simulations can be useful for understanding lots of different radiation damage issues in reactor materials