Irradiation Induced Phenomena in Nuclear Glass Contributors

In France, non-recoverable radioactive wastes coming from spent nuclear fuels are stored in alumino borosilicate glassy matrices. It is envisaged to store them in a deep geological repository to isolate them from the rest of the environment. These glasses will be subjected to internal irradiations, and after some hundreds of years, to water corrosion due to the arrival of the groundwater in contact with them. Studies are carried out to quantify the radiation and water corrosion effects to characterize the long-term waste package behavior. Experimental and numerical approaches have been considered in order to study the variations of the structural and mechanical properties under irradiation.


Introduction
In France, non-recoverable radioactive wastes coming from spent nuclear fuels are stored in alumino borosilicate glassy matrices.It is envisaged to store them in a deep geological repository to isolate them from the rest of the environment.These glasses will be subjected to internal irradiations, and after some hundreds of years, to water corrosion due to the arrival of the groundwater in contact with them.Studies are carried out to quantify the radiation and water corrosion effects to characterize the long-term waste package behavior.Experimental and numerical approaches have been considered in order to study the variations of the structural and mechanical properties under irradiation.
Experimentally, it has been shown, on industrial and simplified glasses doped with short lived actinides or irradiated externally by heavy ions, that the elastic effects lead to different structural (decrease of the polymerization glass networks, increase of the disorder) and mechanical modifications (swelling, decrease of the hardness, increase of the fracture toughness, increase of the internal energy).A computational approach using classical molecular dynamics has been developed in parallel to clarify the correlations between the structural and mechanical property changes.This presentation will expose the main results from our simulations.

Computational method
An empirical pair potential of the Buckingham form has been developed recently to model atomic interactions [1].Hence the interaction between any (i,j) pair of constituent atoms is modelled using the following relation:


The adjustable parameters have been determined in order to reproduce structural and elastic properties of a set of SiO 2 -B 2 O 3 -Na 2 O glasses.A dependence between the ionic charges and the glassy compositions has been introduced in order to reproduce the Boron anomaly, i.e. the nonlinear evolution of the B coordination versus the R(= ) ratios.These potentials were then used to study fracture mechanisms and nanoindentation in pristine and irradiated simplified nuclear glasses.

Results
The three simulated glass compositions are the following: SBN12 glass: 59.6%SiO 2 -28.2%B 2 O 3 -12.2%Na 2 O SBN14 glass: 67.8%SiO 2 -18.0%B 2 O 3 -14.2%Na 2 O, this glass is characterized by the same molar ratios as the real nuclear glass SBN55 glass: 55.3%SiO 2 -14.7%B 2 O 3 -30.0%Na 2 O For each glassy composition, pristine structures (quenched from the liquid state with a rate equal to 5x10 12 K/s) and disordered structures have been prepared.The disordered structures have been obtained after the heating of the pristine ones and their faster quench (with a rate equal to 10 14 K/s) in order to reproduce the main radiation effects, i.e. depolymerization and increase of the internal The fracture simulations have made possible to identify the different steps leading to the cracking of the materials [2].Under the application of an external tensile stress, nano cavities are firstly formed.Then they begin to coalesce as the external stress increases forming increasingly large voids until the complete opening of the structure.During the fracture process, two different behaviors have been noticed: the Si and 4-coordinated B atoms on one hand accumulating stresses in their local environments, and, on the other hand, the 3-coordinated B and Na atoms adapting themselves much more easily to the local stress changes and then facilitating the viscous flow.This observation has made possible to propose an explanation of the origin of the fracture toughness increase in the simplified SBN14 glass.The radiation effects induce a decrease of the average B coordination, with a rise of the 3-coordinated B concentration.In consequence, the plasticity of the material increases, and the energy cost needed for the sample breakage is larger, hence the fracture toughness increase.
Concerning the nanoindentation simulations [3], a Vickers indenter in diamond has been pushed then removed from a simulation box and, using the loadingunloading curves, the hardness values have been quantified (Fig. 1).With the increase of the disorder, a systematic hardness decrease is observed [4] in the three glassy compositions that has been correlated to an increase of the 3coordinated B and non-bridging oxygen concentrations.Taking into account the disordering and depolymerization effects, this study using the classical molecular dynamics method has therefore allowed reproducing changes of the mechanical properties analogous to what has been observed in simplified nuclear glasses irradiated externally by heavy ions.Thus, the structural origins of these changes have been better understood.
R7T7 glass: the French nuclear glass to confine the high level radioactive wastes Borosilicate R7T7 glass R7T7 glass composition: more than 30 components The nuclear glasses will be subjected to internal irradiations (α disintegrations from minor actinides and / radiations from fission products)   Use of classical molecular dynamics to investigate the consequences of ballistic effects Depolymerization %B [3]  %B [4]  Q  Increase of the [3] B relative to the [4] B  it explains why the pastic phase increases in the disordered glass because the [3]  Correlations with the % [3] B and %NBO Hardness decreases with the % [3] B Hardness decreases with the %NBO Pristine glasses Disordered glasses Irradiation: increase of [3] B concentration and %NBO + increase of free volume → hardness decrease

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nd Int.Workshop Irradiation of Nuclear Materials: Flux and Dose Effects November 4-6, 2015, CEA -INSTN Cadarache, France 2 Centre of Excellence for Nuclear Materials disorder.The disordered glasses are considered as models of irradiated ones and their mechanical properties have been compared to those of the pristine glasses.
It's important to understand the radiation effects on the macrocopic properties to certify the long term behavior of the glass SiO 2 , B 2 O 3 and Na 2 O are the major components Swelling under irradiation in R7T7 glass PAGE 4 R7T7 glass has been irradiated internally by 244 Cm with different doses and dose rates Final swelling is around 0.6% for each dose rate S. Peuget et al., J. Nucl.Mater.444 (2014) 76 Saturation of the swelling after a critical dose (4 10 18 α/g) Fracture toughness increase -Hardness decrease PAGE 5 R7T7 glass has been irradiated externally by heavy ions (Kr, Au) Fracture toughness increases by around 25% in the R7T7 glass irradiated by Au ions Microindentation is used to measure the critical charge P c for which the fracture probability is equal to 50% → the fracture toughness can be deduced et al.J. Nucl.Mater.444 (2014) 76 R7T7 glass has been doped by short lived actinides ( 244 Cm) or irradiated externally by heavy ions (Kr, Au, Si) Hardness is measured by Vickers micro indentation Hardness decreases by around 30%

Irradiation
Fracture toughness and hardness have been measured in the three simplified glasses (SBN14 = CJ1, CJ3, CJ7) subjected to heavy ion or neutron irradiations The fracture toughness and hardness changes are qualitatively the same as in the R7T7 glass Anology between R7T7 glass and simplified glasses: fracture toughness and hardness J. deBonfils et al., J. Non-Cryst.Solids, 356 (2010) 388 Decrease of the hardness after irradiation by heavy ions Objective: To understand the atomistic origin of the swelling, fracture toughness increase and hardness decrease under irradiation Empirical interatomic potentials are used fitted on structural data (Boron coordination, structure factors) and macroscopic properties (density, elastic moduli): H. Kieu et al., J. Non-Cryst.Solids 357 (2011) 3313Series of displacement cascades simulated by classical molecular dynamicsSwelling under ballistic effectsDecrease of the bulk modulusExperimental swelling in SBN14 irradiated by heavy ions: ~4.0%Saturation dose: 5 10 20 keV/cm3   4.0 10 20 keV/cm 3 2 10 18 α/g Equivalence Bulk modulus decreases from 85GPa to 61GPa (-28%)(the decrease of the elastic moduli in the real glass is equal to -30%)

FormationStress
rectangular parallelepiped box (10 5 atoms) of 250 x 50 x 100 Å 3 3D initial notch : 30Å deep (X direction), 20Å high (Z direction), Ly (Y direction) 2 layers of frozen atoms (top and bottom) H. Kieu et al., J. Non-Cryst.Solids 358 (2012) 3268 The four steps of fracture mechanisms Nucleation / Growth / Coalescence / Decohesion (SBN14 glass) and disordered SBN14 glass Decrease of the Young modulus from 74.0GPa to 51.6GPa (-30%) Decrease of the elastic limit Increase of the plasticity region (the non linear part of the stressstrain curve): duration from 10ps to 13ps 24ps : Stretching of the Si-O and 4 B-O distances 24 to 40ps : Relaxation of the Si-O and 4 B-O distances RDF [4] B-O RDFs versus time in the pristine glass: [3] B-O and Na-O The cations behave differently depending on their local coordination 3 B and Na  « Soft » elements PAGE 19 RDF [3] B-O No stretching of 3 B-O or Na-O distances RDF Na-O Origin of the fracture toughness increase in the disordered glasses In the disordered (i.e.irradiated) glasses: B atoms enhance the plastic processes The enhancement of the plastic processes consumes a larger energy  it explains the fracture toughness increase under irradiation x 25nm (<>2 10 6 atoms) Indentation speed : 10m/s Step of indentation : 0.1Å Temperature : 300K Indentor : Vickers diamond tip (136° apex angle) At full loading, a 50ps relaxation period is applied by keeping the indenter still D. Kilymis et al., J. Non-Cryst.Solids 382 (2013) 87 Simulation of hardness: indentation profiles and experimental values Hardness in silica doesn't change much between pristine and disordered structures Experimental hardness is better reproduced when the Na 2 O concentration increases Hardness decreases in the disordered (i.e.irradiated) SBN14 glass (experimental behavior is reproduced) , J.-M.Delaye, J. Non-Cryst.Solids 401 (2014) 147 Origin of the hardness decrease PAGE 25 Increase of the hardness with the %SiO 2 Decrease of the hardness in the disordered glasses the complex and simplified nuclear glasses subjected to ballistic effects: density decreases, hardness decreases, fracture toughness increases Classical molecular dynamics has been used … … to simulate displacement cascades in SBN14 glass swelling is associated with depolymerization and increase of disorder … to simulate fracture behavior the increase of the [3] B concentration favors plasticity → origin of the fracture toughness increase … to simulate nano indentation the increase of the [3] B and NBO concentrations facilitates indenter penetration → origin of the hardness decrease Thanks PAGE 27 B. Beuneu (LLB), A. Kerrache (CEA Marcoule), L. Cormier (IMPMC): Neutron spectroscopy M. Barlet (CEA Saclay: DSM / IRAMIS), R. Caraballo, M. Gennisson (CEA Marcoule): Hardness measurements O. Bouty (CEA Marcoule): WAXS spectroscopy

Anology between R7T7 glass and simplified glasses: density
Three simplified glasses have been prepared:The swelling is qualitatively the same as in the R7T7 glass S. Peuget et al.Nucl.Instr.Meth.Phys.Res.B 246 (2006) 379 J. deBonfils, PhD, Université de Lyon I, 2007