Highlights

Damage Correlation between In-situ Ion Irradiation and Neutron Irradiation

Scientific Achievement

Significant advances in computational materials science offer new opportunities to advance the fundamental understanding of irradiation damage processes and to develop radiation-resistant advanced materials for next-generation nuclear energy systems.  Molecular dynamic (MD) atomic simulations are used to model the primary damage of displacement cascades; kinetic Monte Carlo (kMC) simulations are performed to examine the kinetics of defect evolution.  The coupled MD/kMC simulations can provide data on visible defect clusters comparable with experimental measurements.  The nature of neutron irradiation experiments has limited their use in providing direct experimental evidence for these computational models.  In situ TEM ion irradiation experiments allow real-time observation of defect formation and evolution under well-controlled irradiation conditions, and can be performed in an efficient and inexpensive manner.  This in situ experimentation can be a powerful tool to guide and validate modeling efforts.  It is important though to understand the different aspects of damage produced by neutron irradiation and in situ ion irradiation.

A direct comparison of defect microstructure produced by neutron irradiation and in situ ion irradiation was made by irradiating exactly the same material under equivalent irradiation conditions. Pure molybdenum was neutron-irradiated at reactor ambient (~80°C) in the high flux isotope reactor at Oak Ridge National Laboratory to doses between 0.001 and 0.3 dpa.  The material was also irradiated at the IVEM-Tandem facility at Argonne National Laboratory by 1 MeV Kr⁺⁺ at 80°C to the dose range equivalent to that in the neutron irradiation experiments and at dose rates over three orders of magnitude.  These two sets of irradiation experiments eliminated variables of material purity and pre-irradiation microstructure.  In situ ion irradiation was also carried out at 20 and 300°C to study the temperature dependence.  Quantitative analysis was made to determine the number density and distribution of defect clusters as a function of specimen thickness, dose, dose rate and irradiation temperature using the weak beam dark field imaging technique.  The results showed that a simple comparison based on equivalent dose (dpa) is inadequate.  More specific parameters are proposed to allow detailed damage correlations by taking into account of the surface sink effect, displacement damage rate, and damage profile in in situ ion-irradiated specimens.  The experimental findings are also compared with computational data. Future work is planned to examine the depth distribution of defect clusters by the tomographic imaging technique in in situ ion-irradiated specimens.

Significance

The irradiation damage study of Mo serves as an example to show the important role of the in situ TEM ion irradiation technique in advancing the fundamental understanding of irradiation damage by a coordinated approach of computational modeling and experimental validation.  The experiments are intended to show an emerging new research direction in using the IVEM-Tandem facility to benchmark computational modeling and simulate neutron irradiation damage.

Performers