Basic Effects of Radiation J. M. Perlado Director Instituto de Fusión Nuclear
R&D in Advanced Materials Materials Science Investigating the relationship between structure and properties of materials. Materials Engineering Designing the structure to achieve specific properties of materials. The development of advanced Fe-12Cr-3W-0.4Ti-0.25Y materials 2 O 3 12Cr-0.25Y requires 2 0 3 a combination of both
Motivation Radiation Damage Mechanical properties F/M steels Advanced Burner Reactor Nuclear materials Structural steels (Fe- Cr) / nuclear fuels Microstructural evolution under irradiation Nuclear fuel bundle Extreme conditions Dislocation dynamics in alloys Predict strain localization and embrittlement in irradiated samples Experiment Tensile Test Simulation
Radiation Effects: Causes & Consequences Cause-displacements (dpa) and transmutation damage (specially He and H) coupling with thermal, stress and chemically driven processes Dimensional inestability creep (stress), swelling and growth Lower ductility and creep failure time
Irradiation Modes Fusion neutrons (what we expect), Fission neutrons (what we have), high energy protons (SPALLATION, what we can have) Defect / Gas production (in steels) Damage rate [dpa/year] Helium [appm/dpa] Hydrogen [appm/dpa] Fusion neutrons (3-4 GW reactor, first wall) Fission neutrons(bor 60 reactor) High energy protons (590 MeV) 20-30 ~ 20 ~ 10 10-15 1 ~ 130 40-50 10 ~ 800 WHAT APPEAR CRITICAL IS THE CONTENT OF IMPURITIES IN THE IRRADIATED SAMPLES
Radiation-induced damage Once we know what are the radiation environment in order to develop materials able to withstand them we have to know what are the effects of the radiation on materials properties? In other words: we have to have some knowledge about the so called radiation-induced damage
Depending on the scale materials, research can be focused on the: Sub-atomic: electrons and nuclei. Atomic: organization of atoms or molecules. Microscopic: groups of atoms that are normally agglomerated together. Macroscopic: viewable with the unaided eye. Keeping in mind that usually material behaviour is very much dictated by micro- nano-scopic features
Radiation Effects: Causes & Consequences Cause-displacements (dpa) and transmutation damage (specially He and H) coupling with thermal, stress and chemically driven processes Dimensional inestability creep (stress), swelling and growth Lower ductility and creep failure time
Deposition of energy As the metal is irradiated, the incoming particles loose their energy in the crystal through three types of processes: Inelastic interactions with target electrons, leading to ionization and/or excitations. Elastic collisions with the target (crystal) nuclei Nuclear reactions de dx de dx d de dx e de dx n Ullmaier
At energies over a few ev, the incoming particle will displace one or more atoms of the target lattice, creating a vacancy-interstitial pair: a Frenkel pair (FP). Increasing the number of projectiles (neutrons, ions ) will increase the number of FP s created. If the target is at finite temperatures, these defects will migrate. In a perfect crystal, it could be expected that after some time (annealing) they would recombine, restoring the crystal to its initial state.
Radiation damage to metals: A. Projectile mass M 1, energy E 1 (neutron, proton, heavy ion, ) A. Target Regular periodic array of atoms of mass M 2, at rest. -Initial collision is the primary collision -Struck atom is the primary knock-on atom (PKA) In the collision, energy T is transferred to the lattice atom If T>E d, the lattice atom is displaced, forming a vacancyinterstitial pair: a Frenkel pair
Energy of the primary event Taking into account energy and momentum in the center of mass (CM) system, the maximum transfer of energy is: T=T max sin 2 Θ/2 and the (relativistic) for T max is :
Some typical values: Metal E d [ev] Pb, Al 25 Ti, Cu 30 Fe, Zr, Co 40 Ni, Cr, Mn, V 40 Mo, Nb 60 Ta, W 90 For 1 MeV neutrons in non- relativistic approximation (E«M 1 c 2 ) T max 4M 1 M 2 E 2 2 1 2 M 1 M 2 M 2 1 4 M 1 M M 1 E 1 On a Fe target (M 2 =A=56, M 1 /M 2 56), T max 60 kev
Displacements produced by a PKA The number of defects produced by a PKA can be calculated using the Kinchin-Pease model, which is a linear displacement model based in the following assumptions: 1.The displacements are produced by a series of independent twobody collisions between knock-on ions and stationary (lattice) atoms, triggered by the PKA. 2.The energy transfer in the collision is given by the hard sphere, isotropic scattering model. 3.The sequence of collisions stops after n steps when: T/n <2E d
The number of Frenkel pairs due to a PKA of energy T is: N d = 0 if T < E d N d = 1 if E d T 2E d N d = T/2E d if T > 2E d First correction: lower energy transfer are preferred so: N d (T) = T/2E d 0.8 Second correction: at higher energies (T > A [kev]), larger part of the energy is lost by transfer to electrons, so that the damage energy should be written: E D = T Q Where Q are the inelastic losses
Deposition of energy II A number of particles are available to be used in irradiations: electrons, protons, neutrons and ions We can expect a difference in behavior: they are not only very different in mass but we can also expect different types of interactions. For the charged particles is a Coulomb interaction, while the interaction of neutrons is well approximated by a hard collision model (beyond possible nuclear reactions). We define then a primary recoil spectra for a given energetic particle, that refers to the relative number of collisions in which an energy between T and T+dT is transferred from the primary recoil atom to other target atoms
The function 1 T d ( E, T) P( E, T) dt N dt T d provides the fractional number of recoils between T d and T. dσ/dt is the displacement cross section or the probability that a particle of energy E transfers a recoil energy T per unit dose and energy interval dt For light ions dσ/dt α (ET 2 ) -1 (Rutheford c.s.) where small energy transfers are favored. For neutrons dσ/dt is constant initially dσ/dt [barn/ev]
Weighted average recoil spectra 1 T d ( E, T ) W ( E, T ) dt ED( T ) E ( E) T d dt D E D is the damage energy created by a recoil of energy T (E D =T-Q) and E D ( E) T T d max dt d ( E, T ) E dt D ( T ) T max d dt d dt coul HS 4( M1M ( M M dt dt 1 M 2 2 1 dt A E ) ) 2 E ( Z1Z E 2 e 2 ) 2 dt 2 T
Typical weighted recoil spectra If we take the recoil energy up to which half of the displacements are produced T 1/2, it is 60 ev (~2E d ) for 5MeV e - and 60keV (2000E d ) for reactor n
Displacement Damage Magnitudes, and Damage Functions In general: Damage Magnitude = N x x t x < d > N = Atomic concentration = Neutron Flux t = Irradiation time < d > = Damage cross section d 0 ( E d 0 n ( E ) ( E n n ) de ) de n n
Displacement Damage Magnitudes, and Damage Functions The index i corresponds to each nuclear channel. T of Damage Function per recoil n T from E recoils of Probability neutron nuclear cross section ) ( i where : and L(T) ), ( ) ( ), ( ) ( ) ( ) ( ) (,, n i n i n i i d i n i d n d E T F dt T L E T F E E E E n E
Displacement Damage Magnitudes, and Damage Functions The Damage Functions could be DAMAGE ENERGY: L (T) = f 1. T Absorbed energy by the material to produce displacement, and f 1 acts as a Partition Function. DISPLACEMENTS: Quantify the number of displacements during the collisional phase, appearing the concept of the Threshold Displacement Energy. Specific function L (T) = (T).
Displacement Damage Magnitudes, and Total Displacements Total displacement rate: R = N < d > Total Number of Displacements: Including irradiation time t: N < d > t Displacement per atom (dpa): dividing by N: total dpa = < d > t
Displacement Damage Functions Simple Interpretation: Damage Function (E) as number of displacements per PKA can be formulated as... (T) = (T+B) / (C+B) with T = Recoil energy; B = Binding energy lost by each atom ejected from lattice; C = energy for no more displacements. KINCHIN-PEASE : B=0;C = 2E d (T) = T / 2E d NEUFELD-SNYDER B=E d ; C=2E d (T) = (T+E d )/3E d SNYDER - NEUFELD: B=C=T (T) = (T+E d ) / 2E d
Displacement Damage Functions Lindhard-Robinson Including Lindhard model of electronic losses through partition function ( E) ( E) E 2E d with (E) = displacement efficiency
Displacement Damage Functions Norgett-Robinson-Torrens (NRT) Avoiding recoil energy dependence of the displacement efficiency, (E) = cte. (0.8), and considering DAMAGE ENERGY E D = E - Q; E D =E / (1+k L g( )) g( ) = +0.40244 3/4 +3.4008 1/6 0 T < E d (T) = { 1 E d < T < 2.5 E d 0.8T / 2 E d, T > 2.5 E d
WE HAVE RADIATION-INDUCED DAMAGE IN TERMS OF: Thermo-mechanical effects Atomistic effects Nuclear reactions
THERMO-MECHANICAL EFFECTS Stress, strain: cracks and other deleterious effects In terms of thermo-mechanical response both melting and vaporization are undesired effects The integrity of any component based on materials that melt or vaporize is seriously compromised Depending on the material and the resulting temperature gradients, other effects such as cracking or roughening appear These effects may appear at temperatures much lower than the melting point
Example: material loss Exfoliation of metal He plasma induced surface modification in W M. Rubel, trans. On fus. Sci. and tech. 53 (2008) 459 S. Takamura et al. Plasma and Fusion Res., 1, (2006) 051
ATOMISTIC EFFECTS Radiation-induced damage by atomistic effects is the consequence of interaction between the projectile particles of the irradiation and the atoms of the material: Ion irradiation Atomic displacements Frenkel pairs Neutron irradiation: defects and vacancies Nuclear reactions creating foreign elements transmutation reactions producing He and H
Ion irradiation: Stopping power Nuclear stopping power & Electronic stopping power
Nuclear stopping power (Elastic collision) Dominant when E kev amu Produces Frenkel pairs resulting in displacements per ion Well known mechanism Frenkel pair production
Maximum transferred kinetic energy 4mM Ek,max E 2 i ( m M ) If this value is larger than the displacement energy a recoiled atom is produced In turn, the recoil may produce subsequent displacements (cascades) if its energy exceeds the displacement energy Displacement cascade damage from movement of silicon atom after primary collision
Electronic stopping power (inelastic collision) Dominant when E MeV amu Produces excitations and ionizations Above certain threshold S th produces modification of properties: crystalline phase, conductivity, refractive index, density Not well understood, there exist several phenomenological models Swift heavy ions from direct target explosions may induce huge electronic damage on dielectrics (lenses)
Amorphous tracks on LiNbO 3
Neutron reactions Elastic scattering Conservation of momentum and energy No charge: hard sphere collision model Neutrons are very penetrating, i.e., little interaction However, when they interact, they generate dense cascades
Neutron reactions Inelastic scattering Result of interaction is the emission of a neutron less energy than the incident The lost energy is used to excite the nucleus Typically the excited nucleus returns to the ground state emitting a gamma photon
Neutron reactions Activation and transmutation Neutron irradiation, apart of producing damage by direct collisions, produces transmutation reactions As a result, a significant amount of nuclei are radioactive with very different decay times This is the origin of undesired nuclear waste that can be controlled by appropriate selection of materials In addition, transmutation reactions produces light species (H, He) that result detrimental for the host materials
Radiation-induced changes in microscopic material properties:
Ion Irradiation: e.g.w limitations 1. W brittleness at T 400ºC, bellow the DBTT, (due to the high activation energy of screw dislocation glide) limits the application of pure W to the temperature window in between DBTT and recrystallization (~1300ºC). T. J. Renk, et al. Fusion Engineering and Design 65 (2003) 399. 2. Surface modification at T< 3400 ºC (below the melting point). Cyclic e-beam heat loads experiments (H=50 MW/m 2, t=30 s) T s =~1300 ºC. S.Tamura et al. JNM 307 311 (2002) 735. Schematic diagram of the relation of surface modifications to fluence and peak temperature after He irradiation. K. Tokunaga et al. JNM 329 (2004) 757. He irradiation (E He =50 kev) T s =~1700 ºC W. Sakaguchi, et al. Proceedings of ITC 18 (2008). Surface modification by particle (He and H) and electron beam heating is completely different
3. Light species retention blistering and material ejection Single beam (D. Nishijima et al. JNM 329 (2004) 1029) Mutliple beams synergetic effects (K. Tokunaga et al. JNM 390 391 (2009) 916.) H T s =1127ºC F = 7MW/m 2 1x10 24 m -2 He T s =1258ºC Bubbles and holes are formed after He beam irradiation The formation of bubbles mainly depends on: Sample microstructure Irradiation conditions (flux, fluence, temperature and particle beam) SYNERGETIC EFFECTS ARE RELEVANT H+He T s =1165ºC Surface modification due to the mixture and single beam irradiation is different. Holes with a diameter of a few 100 nm are observed for He -irradiated samples. Smooth surface for samples irradiated with double beam.
Nanostructured materials: Properties Light species get pinned at grain boundaries Self-healing behavior Frenkel pair annihilation Delay the pressurized bubble formation D. Nishijima2003 ECA Vol. 27A, 2.163
Structural materials requirements Structural materials are going to be affected mostly by neutrons Selection criteria for structural materials are mainly based on: High thermo-mechanical High radiation resistance High thermal conductivity Low activation Low corrosion properties Good compatibility with coolant
Fe-Cr radiation damage depends on Cr concentration Swelling Displaced atoms create voids causing significant dimensional changes Hardening Embrittlement 400 C Porollo et al. JNM, 256 (1998) 247 Interstitials and vacancies agglomerate into dislocation loops affecting impact properties and fracture toughness He embrittlement and leading to embrittlement and hardening akweb.iae.kyoto-u.ac.jp/ duet/ Kayano JNM 155 (1988) 978 Phase separation is at the origin of hardening and embrittlement in FeCr steels
Degradation of Impact Properties under neutron irradiation ~32 dpa, 332 C, ARBOR 1 irradiation Irradiation effects Unirradiated ~ 200 K -30% Ductile-Brittle Transition Irradiated EUROFER 97 C. Petersen, FZK Concerns: i) ΔDBTT > 200 K ii) Effect of Helium? TBM design window Operational window
Backup Viewgraphs
Steels are one way or another protected against ions and X-rays However, they suffer neutron irradiation accompanied by transmutation reactions that give rise to H and He Typically 10 appm He / dpa This produces swelling, fragilization, loss of mechanical properties, enhancement of corrosion The best steels can nowadays resist at most 18 months in a fusion power plant New materials include ferritic martensitic steels with high Cr content presenting low swelling and low corrosion However, fragilization is an issue that will require much experimental and modeling effort
Structural materials: History Because of their high swelling resistant and good thermal properties, Cr Mo ferritic/martensitic steel, were the best candidates 25 years ago Reduced Activation Ferritic/Martensitic (RAFM) steels were started to be considered 20 years ago.
Structural materials Pros: high radiation resistant up to relatively high doses of the order of ~200 dpa. But: its operating temperature is limited to 550 C, since at higher temperatures they suffer a significant loss of resistance (strength). This fact is very important since it prevents increase the efficiency of a possible power plant by increasing the operating temperature
First proposed solution: Increase the RAFM operation temperature by tuning the chemical composition (selective alloying). After some trials this road has been demonstrated to be extremely complicated among other factors because of the restricted number of elements which can be used preserving the reduced-activation properties. Actual solution (alternative): nanostructured, oxide dispersionstrengthened RAFM steels (ODS-RAFM).
Final lenses of a laser fusion reactor Industrial laser mirror http://www.scitech.ac.uk /(similar dimensions of final lenses)
Final lenses Ions must be deflected to avoid melting. Final lens is irradiated with X-rays and Neutrons
Power (MW/m 2 ) Ions and X-rays 6.0x10 4 He fusion product Range ~ 6 μm 4.0x10 4 2.0x10 4 0.0 48 MJ direct drive target, R=8 m T debris D debris Range ~ 1 μm 0.0 1.0x10-6 2.0x10-6 3.0x10-6 4.0x10-6 Time (s) Ions 25% Energy = 0.015 MJ/m 2 on lenses Severe thermo-mechanical effects X-rays 1 % Energy
Colour centres HiPER 4a 5% absorption 6000 pulses = HiPER 4a Operation at 800 K saturation Lens lifetime comparable to facility lifetime Rivera et al. Proc. SPIE 7916 (2011)
Thermo-mechanical effects Steady state in Prototype (50 MJ @ 10 Hz) Stresses lower than yield strength (48 MPa) silica lenses can withstand the radiation-induced mechanical stresses Thermal loads mainly due to X- rays but too low to induce fatigue failure In DEMO (>100 MJ) temperature limit exceeded. Lens must be situated further way!!!! Garoz et al. Nucl. Fusion 53 (2013) 013010
Colour centres Steady state in Prototype (50 MJ @ 10 Hz) The high steady state temperature maintains the optical absorption low Problems for reactor startup procedure Garoz et al. Nucl. Fusion 53 (2013) 013010
Thermomechanical effects Steady state in Demo (150 MJ @ 10 Hz) Even assuming full ion mitigation the silica temperature approaches the melting point To keep Steady temperature ~ 800 K Lens must be located at 16 m Paramo et al. (2013) in preparation
The universe is not so different, we only have to seek deeper NGG