RADIATION DAMAGES IN MATERIALS – PART II Dr. Celine Cabet CEA, DEN, DMN Service de Recherches de Métallurgie Physique JANNUS laboratory +33 1 69 08 16 15 celine.cabet@cea.fr
Outline 1. Background on alloys and radiation effects 2. Radiation hardening: example of PWR pressure vessel steel 3. Radiation swelling: example of fast reactor cladding 4. Creep irradiation: example of fast reactor cladding… cont’d 5. Radiation growth: example of LWR Zr-alloy cladding 6. Conclusions | PAGE 2
1. Basics of crystalline structure Structure of metals Crystalline structure = precise pattern of atoms following a unit cell that is periodically reproduced Steels • Steels = Iron + Carbon + alloying major and minor elements (Cr, Ni, etc.) that participate to the global mechanical and chemical properties and to the radiation resistance Ni eq Ni eq • Steels can have different crystalline structures depending on Austénite Austénite 15/15 - the type and quantity of alloying elements Austenite Austénite Austénite - temperature A+M A+M + Ferrite + ferrite + ferrite - fabrication route (thermo-mechanical 316 treatments) Martensite Martensite Martensite Ferrite EM10 Cr eq Cr eq F17 C. Cabet | Radiation damages in materials | PAGE 3
1. Basics of crystalline structure Structure of metals Crystalline structure = precise pattern of atoms following a unit cell that is periodically reproduced Steels • Two important steel types: - Austenitic (gamma) Ferritic and martensitic (alpha and alpha ’) - Zirconium • Hexagonal close pack structure (hcp) C. Cabet | Radiation damages in materials | PAGE 4
1. Basics of crystalline structure Structure of metals Crystalline structure = precise pattern of atoms following a unit cell that is periodically reproduced This crystal is not perfect ! - Extra atoms (interstitials) or lack of atoms (vacancies) = point defects - Staking fault = dislocations - grain boundaries, interfaces C. Cabet | Radiation damages in materials | PAGE 5
1. Basics of crystalline structure under irradiation Radiation effects in metals • Neutrons dissipate energy in the matter by colliding atoms • Primary damage: atoms are expelled from their equilibrium site and collide other atoms • Atomic displacement cascade : interstitials + vacancies (Frenkel pair) reorganization / atomic diffusion (thermally activate): some atoms go back to their initial site others remains in the crystalline network as interstitials and vacancies C. Cabet | Radiation damages in materials | PAGE 6
1. Basics of crystalline structure under irradiation Radiation effects in metals • Computer simulation of displacement cascades (DM) • Fast recombination Few surviving defects in the crystalline structure: • 0,28 ps Point defects: interstitials and vacancies • I and V clusters formed directly in the cascade PKA 0,4 ps 15 keV 0,02 ps 0,8 ps 0,1 ps Material damage is quantified in displacement per atom = dpa C. Cabet | Radiation damages in materials | PAGE 7
1. Basics of crystalline structure under irradiation Radiation effects in metals • Defects rapidly evolve with time. Depending on the dose, temperature, material characteristics… these defects - Recombine together - Annihilate along dislocations and grain boundaries that acts as sinks for defects Driving force for interstitial annihilation at dislocation is (slightly) higher than for vacancies bias - Group to form clusters Intersticial clustering -> faulted loop (Frank) Vacancy clustering -> cavities C. Cabet | Radiation damages in materials | PAGE 8
1. Basics of crystalline structure under irradiation Radiation effects in metals • Defects can migrate and interact with the microstructure and dislocation network - Vacancies can be attracted to cavities - Frank loops can unfault into other types of loops or as a dislocation line - Defects can drag solutes (coupling). This can accelerate or modify precipitation Precipitation/dissolution • Extended defects: dislocations, cavities, precipitates change in the microstructure Segregation • with a direct impact on the material properties dislocation lines cavities extended defects Frank loop (~60 nm) C. Cabet | Radiation damages in materials | PAGE 9
1. Basics of crystalline structure under irradiation Radiation effects in metals Segregation Precipitation/ cavities dissolution extended defects Vacancy clusters C. Cabet | Radiation damages in materials | PAGE 10
1. Basics of crystalline structure under irradiation Radiation effects in metals Cavity Alloying element Dislocation loop Vacancy (substitution site) Screw dislocation (vacancies) Impact on the material properties ? Alloying element Dislocation loop (interstitial site) Interstitial (interstitials) Precipitate C. Cabet | Radiation damages in materials | PAGE 11
1. Basics of crystalline structure under irradiation Radiation effects in metals Impact on the material properties ? C. Cabet | Radiation damages in materials | PAGE 12
Radiation embrittlement LWR vessel steel CEA | 7 juin 2012 | PAGE 13
2. Radiation embrittlement – LWR vessel steel bcc bainitic steel with Mn, Ni, Mo… Ductile-Brittle Transition Temperature • Measured with Charpy impact test and fracture toughness DUCTILE Fully • Loss of ductility can lead to Ductile loss of toughness or even failure in some alloys Δ USE ENERGY DUCTILITY OR FRACTURE Δ T DBTT APPEARANCE 75% Ductility BRITTLE 30% Ductility from G. Was Brittle Fracture TEMPERATURE
2. Radiation embrittlement – LWR vessel steel DBTT of vessel steel before and after irradiation, 290°C Embrittlement due to hardening • 3.58 10 18 n/cm² DBTT of irradiated steel Δ T DBTT - Higher strength increases the 2.22 10 19 n/cm² probability of failure by cleavage, leading to higher transition temperature 7.05 10 18 n/cm² - DBTT increases with fluence - At high dose: occurrence of brittle Mn, S, Ni enriched phases (late blooming Data from the French surveillance program phases) • The trend is not linear and saturates (?) J. Rist, EDF Δ T DBTT C. Cabet | Radiation damages in materials | PAGE 15
2. Radiation embrittlement – LWR vessel steel Effect of steel purity on hardening and embrittlement • Effect of chemical composition through a large body of analytical studies - P, S segregate at grain boundaries - Cu, Ni clusters inside the grains • Cu content was shown to have a strong impact from G. Was C. Cabet | Radiation damages in materials | PAGE 16
2. Radiation embrittlement – LWR vessel steel Microstructure origins of embrittlement • Formation of nanoscale precipitates rich in Cu, Ni, Si, P, Mn • Composition and size don’t seem to change with dose • Number increases with dose and Cu content 20 keV Neutron radiation produces an from G. Was extremely high number density of nanoscale copper-, manganese-, nickel-, silicon-, 40 keV and phosphorus-enriched precipitates. 10 keV Combination of experimental, modeling, and microstructural studies leads to advances in predictive Fe Cu Ni Mn Si P atoms capability. C. Cabet | Radiation damages in materials | PAGE 17
2. Radiation embrittlement – LWR vessel steel Recommended values for DBTT shift calculation • Several empirical estimates have been developed to account for the shift in DBTT with dose and chemical composition F: fast fluence • NUREG (Nuclear Regulation Board, USA) 1 2 𝐺 ∆𝑆𝑈 𝑂𝐸𝑈 = 22 + 556 %𝐷𝑣 − 0.08 + 2778 (𝑄 − 0.008) 10 19 • concentration in weigth % EDF Framatome CEA ∆𝑆𝑈 𝑂𝐸𝑈 0.35 = 8 + 24 + 238 %𝐷𝑣 − 0.08 + 1537 𝑄 − 0.008 + 192 %𝑂𝑗 2 %𝐷𝑣 𝐺 10 19 maximum mean | PAGE 18 C. Cabet | Radiation damages in materials
Swelling SFR cladding CEA | 7 juin 2012 | PAGE 19
3. Swelling – SFR cladding tubes Swelling is a critical consequence of irradiation for austenitic steels … 15/15Ti before irradiation (Phénix) 15/15Ti after irradiation (Rapsodie) … and leads to steel embrittlement [FISSOLO, ASTM-STP 1046, 1988] Elongation (%) • Diameter increase • Elongation Embrittlement at D V/V>6% • Swelling (%) « twist » along the spacer wire C. Cabet | Radiation damages in materials | PAGE 20
3. Swelling – SFR cladding tubes Swelling is principally due to a bias… • Irradiation Frenkel pairs are created = 1 interstitial + 1 vacancy • Defects evolve - Formations of clusters, small loops - Recombination/annihilation of defects HOWEVER Preferential absorption of interstitials at sinks (dislocations) 100 nm Vacancies are in supersaturation 304 SS Nucleation and growth of cavities SFR 450°C This mechanism was observed in the early SFR reactors Main consequences of swelling : • Changes in dimensions (elongation, loop deformation, arching/bending) • Build-up of internal stresses due to inhomogeneous swelling (under dose gradient, temperature gradient) creep is favored • Increase in the fuel pellet/clad gap local heating and promotion of the oxide/clad interaction (internal corrosion) • Embrittlement due to porosity at high dose swelling must be controlled in SFR cladding tube C. Cabet | Radiation damages in materials | PAGE 21
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