Irradiation damage and recovery of zirconium-based alloys

Zirconium-based alloys are used for structural components in Light Water Reactor (LWR) nuclear fuel. Improved understanding of irradiation-induced changes, as well as potential recovery from these changes, are central in the continued development of fuel rod performance modelling. Most of the irradiation damage is caused by neutrons. The energy of neutrons produced by fission ranges up to about 10 MeV. The amount of energy required to displace an atom of Zr from its normal lattice position is about 40 eV, and any fast neutron (E>1 MeV) can easily transfer enough energy to displace a Zr atom (the primary knock-on), which in turn can displace additional atoms. The immediate results are small clusters of vacancies, individual vacancies, individual interstitials, and after a short time, vacancy and interstitial loops. Collectively this is termed irradiation damage.
Recovery from irradiation damage, and softening of irradiation hardening, may occur due to short-term temperature excursions during in-reactor operation or due to slightly elevated temperatures during long-term storage of discharged fuel. A proper understanding of irradiation damage as well as the potential recovery is essential for further refining the modelling of fuel rod performance during and after in-reactor operation.
To properly understand the mechanisms behind irradiation damage, and the associated irradiation induced growth, in zirconium alloys we need a better understanding of the formation and evolution of populations of dislocation loops. Plausible hypotheses for mechanisms exist, but we currently lack some of the detailed experimental data that will enable us to test these hypotheses. In particular we need to develop techniques that can quantitatively measure the dislocation loop populations in statistically representative volumes of irradiated material.
Over the last four years, the Zirconium Group at the University of Manchester has been developing the use of x-ray diffraction line profile analysis for the quantification of dislocation loops. This has required a concerted effort employing experiment, simulation and theory and we are now approaching the point where we have a set of characterisation tools ready to deploy in systematic studies.
This PhD studentship aims to assist in taking these tools through the transition from the development stage to one in which they can be disseminated widely for use by the community in general. Again, this will require a combination of simulation, theoretical and experimental work to benchmark and validate the techniques that we have developed. In the course of achieving this, the student will also study the evolution of loop populations as a function of temperature, in order to understand the development of irradiation damage structures in zirconium as well as the mechanisms involved in temperature induced recovery from irradiation damage, both in short-term temperature transients and long-term storage. This will allow us to test the hypothesised growth mechanisms and to disentangle the effects of time, temperature, dose and dose rate on irradiation damage and recovery. In particular the student will:
- Undertake carefully designed irradiation and annealing experiments, to study the response of loop populations to time and temperature;
- Create atomistic models of representative loop populations and generate simulated x-ray diffraction line profiles from them;
- Combine the results of the above to further develop the CMWP line profile analysis software for the purpose of quantifying loop populations.
Aims:
- Improve our understanding of the effects of time-at-temperature on irradiation damage, both in the context of defect annealing and the production of damage. Develop an improved understanding of the interplay of irradiation temperature and dose rate.
- Provide benchmark data for calibration of the use of x-ray diffraction line profile analysis to count loops, based on irradiation experiments experiments