Breaking the bottleneck in radiation materials science with transient grating spectroscopy

Abstract

Nuclear power applications are characterized by harsh mechanical, chemical, thermal, and irradiation environments that present a challenge for the materials engineer. Nuclear materials research and development is a subject of managing constraints: a component must be proven to retain its integrity in the reactor environment for the entirety its operating lifetime, and the material must not impede the delicate neutronics balance that makes a reactor work. It is not surprising, then, that materials often represent the major engineering hurdle in moving a new reactor concept closer to reality, especially since many advanced reactor concepts utilize higher temperature regimes, larger radiation fluxes, and more corrosive coolants. However, if nuclear materials research is the bridge between academic concept and commercial reality, it is frequently a long and expensive bridge to cross. In order to validate a new material for use in a specific reactor environment, one must test the material in representative conditions, or test the material in a sucient number of conditions that the material's response to an arbitrary reactor environment can be accurately predicted. Transient grating spectroscopy (TGS), long used in the materials science field to characterize the properties of thin films, is adapted for use as a method of characterizing radiation-damaged samples. TGS has the ability to simultaneously measure elastic, thermal, and acoustic material properties. It is also non-contact and non-destructive, and relatively inexpensive to build and adapt for dierent uses. This means it is an ideal candidate for moving the field of nuclear materials closer to the goal of having the ability to fully characterize the radiation-induced property changes in samples in situ and in real-time while they are irradiated. This thesis demonstrates, via a TGS setup built in the MIT Mesoscale Nuclear Materials laboratory, that TGS will be a valid method for quantifying radiation damage by using it to characterize (1) cold-worked irradiated samples, (2) samples with high concentrations of constitutional vacancies, and (3) samples irradiated for 14 years in the EBR-II reactor. In (1), it is shown that TGS is a viable method for measuring thermal diffusivity changes due to radiation damage at low doses in cold-worked single crystal niobium. In particular, an initial decrease in thermal diffusivity at very low doses is measured, which is attributed to electron scattering by point defects, followed by an increase and saturation of thermal diffusivity as dose increases, which is attributed to less ecient electron scattering as point defects cluster into mesoscale defects. In (2), the impact of vacancies on the TGS signal is considered by using a material with a high concentration of constitutional vacancies that are stable at room temperature. Molecular dynamics simulations showed that increasing vacancies led to a softening material, but the opposite eect was observed in experiments. This study underlines the importance of having better methods of measuring radiation damage in situ, in real time, because ex situ experiments are not capable of capturing defect populations that are produced during irradiation but which anneal out when the irradiation source is removed. In (3), we observe a similar increase in thermal diffusivity with irradiation as was observed in (1), but in this case, the eect is due to radiation-induced segregation removing minor alloying elements. Study (3) also demonstrates the utility of using TGS on real nuclear materials, as the TGS results are consistent with the extensive characterization carried out on these samples by previous researchers. These three studies illustrate the utility of TGS for characterizing radiation damage in nuclear materials in a cost-eective, time-ecient manner.