Key Challenge 1
Evolution of Irradiation Damage
Leads: Chris Race (Modelling) and Michael Preuss (Experimental)
The Zr alloys used in light water reactor (LWR) fuel assemblies exhibit a range of complex microstructural changes under irradiation. The significant structural and elastic anisotropy of the hcp metal, coupled with the strong texture of the processed alloys, leads to the development of a highly anisotropic population of dislocation loops and a volume-preserving macroscopic shape change in components, known as irradiation-induced growth (IIG). Buckling of fuel assemblies under IIG is a life-limiting factor, but a full understanding of the process of loop formation has so far proved elusive.
In fuel assemblies, Zr is only lightly alloyed, but the alloying elements have low solubility in the metal matrix and are present in a variety of second phase particles (SPPs). These particles exhibit amorphisation and dissolution under irradiation, and the alloying elements thereby released have a strong effect on corrosion, hydrogen pick-up (KC-3) and irradiation hardening (KC-2). A detailed understanding of the effect of irradiation on SPPs is a necessary part of understanding the processes under investigation in the other Key Challenges. The scarcity, operational complexity and cost associated with neutron-irradiated samples mean that experimental ion irradiation will remain an important tool in alloy development and in understanding fuel performance. The complexity of microstructural evolution in Zr alloys and its dependence on atomic-scale processes with a wide range of kinetic barriers means that the different dose rates and primary damage mechanisms under alternative irradiation sources must be fully taken into account when designing and interpreting experiments. Robust protocols, derived from a detailed mechanistic understanding of irradiation damage in zirconium alloys, are required.
Key Challenge 1 will address the following scientific objectives:
Determine the process and mechanisms in the formation of dislocation loops responsible for IIG;
Understand and predict the effect of irradiation on SPPs, and understand the effect of alloying elements on the evolution of the irradiated microstructure (in particular Nb, Fe and Sn);
Understand the effect of temperature and dose-rate on primary irradiation damage under different irradiation types, and develop robust experimental protocols.
The broad range of neutron-irradiated material available to the MIDAS programme affords us a rare opportunity to fully tackle the above challenges.
Work Packages
WP 1.1: Establishing irradiation-induced defect properties
The sensitivity of the nature and extent of irradiation damage in zirconium alloys to details of the alloy chemistry, material microstructure and irradiation conditions makes clear the need for mechanistic understanding to be founded on accurate data concerning the defects involved. This will entail a comprehensive investigation of the structures, energies and migration barriers of all relevant defects using Density Functional Theory (DFT), as the only modelling tool able to capture the required scale of the system under study, and to deliver results with sufficient accuracy.
Previous DFT studies by the MIDAS team and by others have demonstrated the ability of DFT to yield mechanistic insight into irradiation damage processes. What we now require is a comprehensive modelling-based investigation to support new mechanistic understanding. All of the objectives across the four KCs have their origins at the atomistic scale and so the output of this WP will underpin activity across the programme. We will undertake DFT calculations of point defects (native Zr and alloying elements), point defect clusters, dislocation line-segments and small dislocation loops, and SPP phases.
Particular challenges that we will address are:
The properties and migration behaviour of point defects are sensitive to the local elastic strain state. We will need to characterise point defect behaviour as a function of local strain and their elastic properties, requiring large numbers of calculations;
Correct modelling of clusters of defects and dislocation loops requires DFT accuracy, but on a large (1000 atom) scale. The elastic properties in particular require highly accurate calculations;
Addressing the role of alloy chemistry in microstructural evolution will require treatment of each alloying element individually and in interaction with one another and with native defects.
WP 1.1 Key Deliverables
Accurate data on defect structures, formation energies and migration barriers.
Key data on the properties of alloying species, to facilitate understanding in other WPs.
WP 1.2: Predicting primary irradiation damage
Whilst the DFT simulations in WP 1.1 will be crucial to a quantitatively accurate understanding of microstructural evolution under irradiation, many of the key processes that require investigation involve length and time scales beyond the reach of quantum mechanical methods. To investigate these phenomena, we will employ existing empirical potentials for Zr and its alloys and perform large-scale molecular dynamics and statics calculations. These simulations will also be used to inform microstructural models to track the evolution of the SPP population. Microstructural simulations will require extending the initial framework we have established to more physically capture irradiation damage effects on SPPs. Using such simulations, we will:
Establish the initial defect populations resulting from interactions with different irradiating particles. Collision cascade simulations will achieve statistical validity and explore the effects of irradiation temperature and spectrum on the initial point defect production and clustering;
Investigate the interaction of populations of dislocation loops in order to understand their interaction and determine their favoured arrangements;
Simulate x-ray diffraction profiles of populations of irradiation-induced defects to further refine experimental approaches to quantifying and characterising defect content;
Directly simulate the evolution of rafts of a-type loops to identify possible mechanisms for the direct formation of the large c-type vacancy loops associated with break-away growth;
Provide mean-field predictions of excess defect content to inform microstructural models based on the classical Kampmann Wagner Numerical (KWN) framework to track irradiation-induced amorphisation, dissolution, and coarsening of SPPs.
The above simulations will require significant computational resource. It will also be necessary to validate the empirical potentials that we use to ensure that they correctly reproduce defect formation energies, the elastic properties of Zr and collision dynamics at energies in the keV per atom range. The analysis of neutron-irradiated material in WP 1.4 will complement the modelling of primary damage. Work underway at by team members at Imperial College London, and to be developed further as part of Key Challenge 3, to develop tight-binding models of the Zr alloy system, will extend the range of calculations here and in WP 1.1.
WP 1.2 Key Deliverables
Characterisation of the defect populations produced during early irradiation.
Atomic scale insight into the mechanisms of formation of c-type dislocation loops.
Microstructural models for SPP evolution informed by atomistic simulation of defect populations.
WP 1.3: Diffusion Elastic Modelling
The strong elastic anisotropy of Zr leads to anisotropy in the behaviour of irradiation-induced defects. The strong texture in fuel tubes means that many macroscale fuel failure phenomena also exhibit a strong anisotropy. The energies, mobilities and interactions of point defects, precipitates and dislocation loops are strongly affected by these anisotropies. Hence, a correct treatment of defect behaviour in an elastically anisotropic material is required if we are to truly understand the behaviour of Zr components under irradiation. This WP will develop a methodology for treating the evolution of defect populations in an elastically anisotropic matrix, building on work at UKAEA and Imperial. This diffusion-elastic approach will make use of the properties calculated in WP 1.1 and will benefit from the recent analysis of elastic properties of defects and dislocations to extend our insight into damage evolution to the microstructural and component scale. We will:
Use the results of WP 1.1 to evaluate interactions between point defects, defect clusters and dislocation loops; and between the defects and external strain fields;
Incorporate the details of elastic defect interactions and strain-dependent diffusion barriers (from WP 1.1 into a meso-scale diffusion-elastic modelling formalism;
Use this formalism to simulate the evolution of the irradiation-induced ensembles of defects to understand the formation of loops of different types, the arrangements, their annealing (shrinkage) or coarsening (growth) at different temperatures and elasticity-mediated self-consistent symmetry breaking effects;
Relate the strain fields due to the evolving loop populations in individual grains, via consideration of the alloy texture, to the extent of macroscale IIG.
WP 1.3 Key Deliverables
Mechanistic understanding of irradiation-induced defects at the mesoscale and the role of long-ranged elastic interactions.
Definitive understanding of the mechanisms underlying IIG from atomic scale to its manifestation at the macroscale.
WP 1.4: Advanced characterisation of irradiation damage
The state-of-the-art materials characterisation facilities available at MIDAS institutions, complemented by synchrotron x-ray analysis, and coupled with rare access to neutron irradiated material, will allow us to make a large step forward in understanding the nature of microstructural evolution in Zr alloys, and to develop robust protocols for the use of surrogate irradiations in future experiments. Whilst proton irradiation experiments can reproduce the phenomenology of damage under neutron irradiation, we currently lack a sound basis for claiming equivalence with respect to any particular defect evolution process. Experiments at the University of Manchester have shown that temperature-dependent microstructural evolution in irradiated Zr is complex and the characteristic temperature for rapid loop evolution (likely via coarsening) is very close to reactor and experimental irradiation temperatures. To untangle these various effects, we need additional, carefully controlled proton irradiation experiments on model alloys to be analysed in conjunction with the new neutron-irradiated material. These experiments will also be used to understand the irradiation-induced effects that control SPP evolution. Further, in-situ annealing experiments using, for example, synchrotron xray diffraction can reveal a great deal about loop stability. Previous work at Manchester has revealed that SPP damage can include amorphisation, composition change due to selective sputtering (leading to core shell structures), and irradiation-induced dissolution and re-precipitation. What is lacking, and will be addressed as part of MIDAS, is a quantitative understanding of how these phenomena are related to the nature and severity of irradiation damage. Finally, we wish to explore with UKAEA colleagues the challenges when Zr is exposed to an energy spectrum that will cause transmutation and He bubble formation. We will:
Apply the full range of analytical experimental techniques (STEM, ultra-high resolution EDX, XRD, 3DAP) to characterise the dose and composition dependence of the damage in the neutron-irradiated samples;
Undertake proton irradiation experiments in pure Zr across a range of doses, dose-rates and temperatures, and conduct post-irradiation in-situ heat treatments of samples using synchrotron x-ray diffraction for line profile analysis and localised conductivity measurements to quantify the interaction of the rates of damage production and defect migration and understand the effect of time-at-temperature on the microstructure;
Undertake dual-beam irradiation experiments (He-implantation) to understand potential He bubble formation if Zr was exposed to a fusion environment;
Use irradiation experiments and post-irradiation in-situ studies on model alloys to disentangle trends observed in the behaviour of the neutron-irradiated alloys in the BOR-60 sample set;
Determine quantitative relationships between irradiation damage and SPP evolution for proton- and neutron-irradiated material and relate it to the growth data..
In conjunction with the simulations of primary damage in WP KC-1.2 we will develop the capabilities necessary to experimentally quantify damage to the irradiated microstructure. This will include further refinement of the XRD quantitative characterisation of loop populations developed at Manchester, and improved methods of automated defect detection in TEM micrographs (building on UKAEA work). MIDAS will also facilitate the development of a capability for positron annihilation lifetime spectroscopy (PALS) at Dalton Cumbrian Facility for the detection of defects, such as point defects and small clusters, below the threshold of visibility in TEM and APT; work which will be supported by colleagues in Finland.
WP 1.4 Key Deliverables
New measurements of the nature and extent of damage under neutron irradiation.
Robust experimental protocols for the use of proton and heavy-ion irradiation to simulate neutron damage on a process-by-process basis.
New capabilities for the quantitative characterisation of irradiation damage in Zr alloys.