Helium behaviour in implanted boron carbide

Motte, Vianney; Gosset, Dominique; Miro, S.; Doriot, Sylvie; Surblé, Suzy; Moncoffre, N.

doi:10.1051/epjn/e2015-50007-5

Cited by 10 publications

(7 citation statements)

References 17 publications

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“…The goal was, however, mainly phase stability and not the properties of isolated defects, nor their role in mass transport kinetics. In spite of a number of experimental works devoted to helium bubbles [11,12,13,14,15,16,17,18,19,20,21] and the related question of helium kinetics [20,22,23,24], not much is known on self-diffusion.…”

Section: Introductionmentioning

confidence: 99%

“…In this work we focus on the influence of intrinsic vacancies created under irradiation on the diffusion of helium, which is produced by nuclear reactions in boron carbide submitted to a neutron flux. Such a picture can also be simulated by helium implantation [20], possibly coupled with ion irradiation. We investigate the possible mechanisms at the atomic scale by means of first principles calculations in the framework of Density Functional Theory (DFT).…”

Section: Introductionmentioning

confidence: 99%

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The influence of irradiation induced vacancies on the mobility of helium in boron carbide

Gillet

Roma

Crocombette

et al. 2018

Journal of Nuclear Materials

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Boron carbide, used as a neutron absorber, undergoes nuclear reactions producing relevant quantities of He. The understanding of He kinetics at the atomic scale in the material is still in its infancy, in spite of decades of experimental work devoted to the characterization of He containing, irradiated, boron carbide samples. The interplay of He itself with intrinsic defects created by irradiation on kinetics is still almost completely unknown. In this paper we present an exhaustive study of vacancies and substitutional helium impurities in boron carbide using Density Functional Theory. Analyzing the stability and mobility of these defects allows us to consider diffusion mechanisms other than the known interstitial mechanisms. We find that vacancies trap He interstitials, raising the activation energy of 2D diffusion to approximately 2 eV. The trapping mechanism is different according to the charge state of the vacancy : in p-type conditions, when vacancies are neutral or positive, He diffuses via a dissociative mechanisms and is trapped in a substitutional position; in n-type conditions, negative vacancies trap He atoms traveling in an adjacent {111} plane by a charge transfer driven distorsion. No favorable vacancy assisted diffusion mechanism was identified for substitutional He atoms, except the dissociative one previously mentioned. Other possible vacancy diffusion mechanisms, which we also analyzed, are hindered by the high activation energy of vacancy self-diffusion.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The influence of irradiation induced vacancies on the mobility of helium in boron carbide

Gillet

Roma

Crocombette

et al. 2018

Journal of Nuclear Materials

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“…This stage could however be related to lithium diffusion leading to unit cell reorganization [28]: in this temperature range, X-ray diffraction performed on a non-polished sample has shown extra-lines that could be attributed to lithium borates. But recent results [20] show that this temperature corresponds to a change in the intragranular helium bubbles from flat to more tridimensional.…”

Section: Annealingmentioning

confidence: 95%

“…But it is also worth considering the actual microstructure of the material. Studies in progress [20] have shown that annealing at 1500°C a helium implanted boron carbide sample leads to coalescence of the helium bubbles located into the grain boundaries, then a possible full opening of the latter: the resulting thermal barriers will certainly have strong effect on thermal conductivity despite a small volume change. This also means that the helium bubbles remain in the material up to the highest temperatures here attained: analyzing helium retention rate requires melting the material.…”

Section: Annealingmentioning

confidence: 99%

Evolution of thermo-physical properties and annealing of fast neutron irradiated boron carbide

Gosset

Kryger²,

Bonal

et al. 2018

Journal of Nuclear Materials

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Boron carbide is widely used as a neutron absorber in most nuclear reactors, in particular in fast neutron ones. The irradiation leads to a large helium production (up to 10 22 /cm 3) together with a strong decrease of the thermal conductivity. In this paper, we have performed thermal diffusivity measurements and X-ray diffraction analyses on boron carbide samples coming from control rods of the French Phenix LMFBR reactor. The burnups range from 10 21 to 8.10 21 /cm 3. We first confirm the strong decrease of the thermal conductivity at the low burnup, together with high microstructural modifications: swelling, large microstrains, high defects density, and disordered-like material conductivity. We observe the microstructural parameters are highly anisotropic, with high micro-strains and flattened coherent diffracting domains along the (00l) direction of the hexagonal structure. Performing heat treatments up to high temperature (2200°C) allows us to observe the material thermal conductivity and microstructure restoration. It then appears the thermal conductivity healing is correlated to the micro-strain relaxation. We then assume the defects responsible for most of the damage are the helium bubbles and the associated stress fields.

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“…B and C with energy up to a few MeV. Preliminary tests have been performed [20,21] aiming at reproducing the helium to damage ratio together with a significant helium concentration. We have irradiated a high density boron carbide sample with a dual beam on the Jannus-Saclay facility [22]:…”

Section: Applicationmentioning

confidence: 99%

Evaluation of damage in neutron irradiated boron carbide

Gosset

Herter

Motte

2018

Nuclear Instruments and Methods in Physics Research Section B:

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When irradiated in a reactor, neutron absorber materials undergo two different damages induced, first, by the elastic scattering of neutrons; second by the neutron absorption reactions. In boron carbide irradiated in a fast neutron flux, neutron scattering and the slowing-down of He and Li arising from the (n,α) absorption reaction both lead to the displacement of B and C atoms which energy ranges up to a few MeV. Simulating the neutron damage by ion irradiation requires the calculation of the damage produced in both cases. In this paper we propose an estimation of the actual defect production rate resulting from the fast neutron irradiation. For this, we first estimate the energy spectra of both the primary knocked-on atoms and atoms created by the absorption reactions, here in a Phenix-like neutron spectrum. We then deduce from SRIM calculations the actual energy distribution of all the atoms displaced along the displacements cascades induced by those primary projectiles. At last, we obtain an estimation of the number of displaced atoms per produced helium, about 305, most of them resulting from neutron scattering. This is far from a Kinchin-Pease or NRT estimation, of the order of several thousands, this arising from the fact that most of the energy is dissipated through electronic interactions. Such results are then used in order to perform ion irradiations aiming at a realistic simulation of synergetic effects of helium implantation and damage production.

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Helium behaviour in implanted boron carbide

Cited by 10 publications

References 17 publications

The influence of irradiation induced vacancies on the mobility of helium in boron carbide

The influence of irradiation induced vacancies on the mobility of helium in boron carbide

Evolution of thermo-physical properties and annealing of fast neutron irradiated boron carbide

Evaluation of damage in neutron irradiated boron carbide

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