Simulation of c-component dislocation loops by ions and protons irradiation. A tool to understand the breakaway growth of recrystallized Zr-based alloys

During normal in-service operation, the PWR fuel assemblies exhibit a macroscopic elongation. This dimensional change is the result of three different phenomena: thermal creep, irradiation-creep and irradiation growth which occurs in the absence of any applied stress. This last phenomenon, referred as stress-free growth, is related to irradiation-induced microstructural changes taking place in the hexagonal close packed grains and corresponding to an elongation along the basal plane and a shortening along the c-axis, the volume remaining constant (Fig 1). Under neutron irradiation, recrystallized Zr-based alloys undergo stress-free growth which accelerates for high irradiation doses (Fig 2). This acceleration is correlated to specific c-component vacancy dislocation loops.

Since these defects are responsible for the breakaway growth, which influences the performance of the fuel assembly, it is important to know the effect of various factors on their microstructure.The industrial feedback observed on some PWR fuel assemblies suggests that a macroscopic stress applied in the early life could affect the c-loop microstructure and therefore impact the subsequent stress-free growth.In addition, some feedbacks show that in-service hydrogen pick-up could also influence the fuel assembly radiation-induced deformation.In order to have a better understanding of this phenomenon, thorough analytical studies of c-loops nucleation and growth has been undertaken using charged particles irradiation.Protons and ions irradiation experiences have been carried out on various worldwide facilities at several temperatures combined with TEM exams [1,2].
Two complementary studies were carried out to simulate neutron irradiation: in-situ tensile tests under 1MeV Kr 2+ ion irradiation and bending experiments under 600keV Zr + ion irradiation in order to investigate the impact of a macroscopic stress applied under irradiation on the evolution of c-loop microstructure [3].In-situ Kr 2+ ion irradiation was performed at high dose (up to 25dpa) on RXA Zircaloy-4 at 573K with the stress applied from the beginning of the irradiation.We observe in few grains a clear effect of stress on the c-loop linear density (Fig 3).
Zr+ ion irradiations were performed at 573K on RXA Zircaloy-4 and M5® and two loading histories were considered, using a four point bending device specifically designed to apply a tensile or a compressive stress under irradiation.During the first loading experiment, the external stress was applied at an early stage of irradiation.Transmission Electron Microscopy (TEM) observations showed that the initial applied stress has no effect on the incubation dose of c-loops.For the second loading experiment, the macroscopic stress was applied when c-loops are already created.Same trend, that the one observed for in-situ experience, but with a lower effect of stress on the c-loop linear density was observed on RXA Zircaloy-4 and M5® due to dispersion from grain to grain and a much larger statistic for this experiment (Fig 4) [1].Nevertheless, qualitative agreement between two experiments is noticed.Indeed, the c-loop linear density is lower when a tensile stress is applied parallel to the c-axis, which is in good agreement with the Stress Induced Preferential Absorption (SIPA) mechanism.Based on these results under stress level below the transverse yield strength and at 573K, the quantitative statistical study conducted on a large number of Zircaloy-4 and M5® grains with different orientations of the c-axis with respect to the loading direction, shows that an externally applied stress has a slight effect on the c-loop microstructure.A trend for the evolution of their densities with the component along the c-axis of the deviatoric stress tensor was obtained and is consistent with the SIPA mechanism, but seems to be not very significant considering the dispersion from grain to grain.The average loop size is weakly affected by the applied stress and the coupling between the axial irradiation creep and the accelerated irradiation growth appears to be low at the grain scale.
[2] L. Tournadre, F. Onimus, T. Jourdan, J.-P.Mardon, X. Feaugeas, "Impact of hydrogen pick-up and applied stress on c-component loops: toward a better understanding of the radiation induced growth of recrystallized zirconium alloys", ASTM STP 1453 (2014), pp.853-893.This document and any information it contains shall not be used for any other purpose than the one for which they were provided.Legal action may be taken against any infringer and/or any person breaching the aforementioned obligations.

Fig
Fig. 1: Changes taking place in the hexagonal close packed grains [1]

RXAoElementIn5StressQ1FUELoo
n/m² (E > 0.82 MeV) F. Garzarolli and al., ASTM STP 1023, 1989 Breakaway growth FUEL Objectives MINOS November, 2015-CONFIDENTIAL-AREVA AREVA NP-All rights reserved -p.3 Prediction of the macroscopic elongation of fuel assemblies under neutron irradiation Q1: Coupling under irradiation between 'stress-free' growth and axial irradiation creep ?macroscopic stress applied under irradiation Hypothesis: c-loops are responsible for the growth breakaway 1. In-situ tensile tests under 1 MeV Kr 2+ ion irradiation 2continue with dose increasing MINOS November, 2015-CONFIDENTIAL-AREVA AREVA NP-All rights reserved -p.3 FUEL Evolution of <c> loops density vs. irradiation protons dose in Zircaloy-4 and M5® alloys Large and faulted <c> loops in consistent densities with increasing dose Fewer <c> loops in M5 ® than in RXA Zy-4 P.Bossis et al, 15 th ASTM, 2009 [2] D.Gilbon et al, 10 th ASTM, Samples taken in the (TD, RD) plane of an intermediate product: TREX MINOS November, 2015-CONFIDENTIAL-AREVA AREVA NP-All rights reserved -p.3 Tube of ~11 mm thick o is a trademark of AREVA NP registered in the USA and in other countries external stress on the c-loops MINOS November, 2015-CONFIDENTIAL-AREVA AREVA NP-All rights reserved -p.3 FUEL Specific and appropriate samples Sampling for both experiments allowed a large variability of c-axis orientation with respect to the direction of the applied loading MINOS November, 2015-CONFIDENTIAL-AREVA AREVA NP-All rights reserved -p.3The entire thickness of the sample is irradiated MINOS November, 2015-CONFIDENTIAL-AREVA AREVA NP-All rights reserved -pat the beginning of the irradiation Stretching of the sample until dislocation glide occurs Final dose: 25-27dpa (SRIM calculation with "full damage cascades" option) MINOS November, 2015-CONFIDENTIAL-AREVA AREVA NP-All rights reserved -p.3 Numerous <c>-loops in grains with <c>axis far from the tensile direction Few <c>-loops in grains with <c>-axis close to the tensile direction FUEL Impact of tensile stress on <c>-loop linear density and diameter MINOS November, 2015-CONFIDENTIAL-AREVA AREVA NP-All rights reserved -pWithin each sample but for few grains, the <c>-loop density shows a significant decrease with increasing deviatoric stress tensor component in the c-direction  Combining the data from all samples, the slope remains the same but dispersion is observed  The <c> loop average diameter is not dependent on the deviatoric stress tensor component Damage created on a thin layer of 300 nm depth  Irradiation campaign in two steps  Two mechanical loading MINOS November, 2015-CONFIDENTIAL-AREVA AREVA NP-All rights reserved -p.3Not significant effect of stress on <c> loops microstructure and density  In agreement with SIPA mechanism  <c>-loop size distribution quite homogeneous in each grain Zy-4:   = −,  ×     <> + ,  ×   (m -2 ) M5 ® :   = −,  ×     <> + ,  ×   (m -2 )  Tensile stress ∥ <c> : low density  Tensile stress ⊥ <c> : slightly higher density  Compressive stress ⊥<c> : low density  Compressive stress ∥ <c> : slightly higher density MINOS November, 2015-CONFIDENTIAL-AREVA AREVA NP-All rights reserved -p.3 FUEL Conclusions on stress impact MINOS November, 2015-CONFIDENTIAL-AREVA AREVA NP-All rights reserved -p.3 produce relevant data in much shorter time than neutron (hours/days or weeks / years)  create c-component dislocation loops  representative of neutron damage (mainly protons) An excellent tool :  to understand the breakaway growth of recrystallized Zr-based alloys  to model FA irradiation growth  Other applications (corrosion, ..)? MINOS November, 2015-CONFIDENTIAL-AREVA AREVA NP-All rights reserved -p.3 Thank you for your attention FUEL Back-up MINOS November, 2015-CONFIDENTIAL-AREVA AREVA NP-All rights reserved -p.3 Influence of the applied stress on the elastic interaction energy between the point defects and the dislocation loops  Modification of the capture efficiency of Self-Interstitial Atom (SIA) by a dislocation loop  For a vacancy <c>-loop located in the basal plane MINOS November, 2015-CONFIDENTIAL-AREVA AREVA NP-All rights reserved -p.3 Tensile stress applied perpendicular to the caxis W.G. Wolfer, Journal of Nuclear Materials, 90 (1980) 175 Tensile stress applied along the caxisAny reproduction, alteration, transmission to any third party or publication in whole or in part of this document and/or its content is prohibited unless Company Name has provided its prior and written consent.