I submitted this thesis back in September 2019 while I was working in the Imperial College London Centre for Nuclear Engineering under the supervision of Professor Robin Grimes and Dr. Mark Wenman.
Some useful things you can get out of this thesis:
- Lots of information about the oxide layer formed on Zr-based nuclear fuel claddings. Both the inner and outer oxide layer are covered, but far more was known about the outer layer at the time.
- Information about pellet-cladding interaction (PCI), particularly chemical interactions.
- Crystallography and atomic positions of ZrO2. The main focus is on the monoclinic, tetragonal and cubic crystal structures. Orthorhombic and cotunnite structures are covered briefly.
- A primer on density functional theory, electronic orbital pseudopotentials and the plane-wave approach (used throughout the thesis). CASTEP was the DFT code used for this work.
- Fission product yields and decay chains involving iodine.
- Radioparagenesis and fission product implantation in the internal oxide layer.
The most fun idea explored is that implantation of the fission products tellurium and iodine in the internal oxide layer might reduce corrosion resistance.
The internal oxide layer is formed during the manufacturing process because it is exposed to oxygen in air. During operation, there is negligible oxygen availability inside the cladding for it to grow further. The implantation of fission products (Te, I, Xe, Cs) into the oxide accumulate internal stresses in the crystal lattice due to the lattice mismatch, especially large atoms such as Xe. The mismatch is made worse however when an atom is occupying an unfavourable site in the lattice. For example, Te atoms (which are chemically similar to oxygen) will preferentially occupy oxygen sites in the ZrO2 crystal lattice. However, Te nuclei generated from fission events are neutron-rich and will typically decay by beta minus decay with half-lives ranging from a few minutes to several hours. These Te nuclei will decay to I and subsequently Xe and Cs. Over time, the oxide layer accumulates these highly mismatched dopant atoms, increasing stresses and promoting the formation of new surfaces (crack initiation). Once a path to bare Zr metal is opened, iodine can proceed unimpeded by the oxide to corrode the cladding.
To all you nuclear/materials scientists in the future who know what actually happens; it's probably not the whole story, but you gotta admit, it's a fun one considering the information I had at the time!