The stability of bubble lattices in irradiated materials


In the intense neutron irradiation environment of a reactor core, it is well known that inert gases can be produced, particularly in the fuel rod cladding materials. As the concentration of inert gas increases, a high density of nanoscale bubbles can form. These bubbles have a serious impact on the mechanical and physical properties of the material, especially when they form on grain boundaries, where they can lead to life-limiting embrittlement.
Under certain conditions, these nanoscale bubbles have been observed to self-assemble into a regular, periodic lattice – a bubble lattice, imitating the cubic lattice structure of bcc and fcc material 2 dimensionally while only exhibiting 1-dimention symmetry in hcp materials (where bubbles become aligned in a direction perpendicular to that of the basal plane direction).

Bubble lattices only seem to form for certain specific values of the governing parameters, temperature and dose rate. Whilst there has been significant effort to unravel the fundamental mechanisms leading to bubble lattice formation in the past, there still is much debate as to their nature. However, currently the most favoured mechanism for formation is the anisotropic diffusion of host metal self-interstitial atoms (SIAs).

In order to understand the mechanism in-situ irradiation experiments are being conducted to allow for the formation of these bubble lattices to be examined dynamically under a variety of conditions, specifically temperature and dose. Using a range of ion species and host materials for implantation experiments it is hoped that the mechanism for bubble lattice formation will be better understood.