Nuclear materials are an incredibly interesting field of research into materials processes. There are a number of multiscale studies done to try to capture important phenomena, and a lot of information to draw on.
Nuclear fuel is a materials scientist's delight
With potential temperatures ranging from 300 to almost 2700 C in less than a centimeter, a soup of a large portion of the periodic Table of Elements (Table of Nuclides actually), and persistent radiation fields, nuclear fuels is a playground for less common material processes. Some of the neat things are:
- Gas is produced inside grains and moves as bubbles to the grain boundaries forming labyrinthine tunnels until a tunnel reaches a free surface (like a crack) and then the gas can vent.
- If there is a large enough bubble (from cracking for example), the hot side evaporates and condenses on the cold side and moves the pore towards the hot central fuel region. The moving pore bulldozes everything in its way and leaves long columnar grains in its wake. At the centre, bubble accumulate into a central void.
- High temperatures and temperature gradients produce thermomigration (aka: The Soret effect); diffusion driven by temperature gradients which can move oxygen and plutonium around the fuel.
Predictions of centreline melting in operational fuel
Normally, nuclear fuel is separated from the water coolant by a metallic sheath. Sometimes defects can occur in the sheath, allowing water to contact, and oxidise the fuel which reduces the conductivity and melting temperature. This work considers how high the power would have to be to melt the oxidised fuel and shows that the melting is self-limiting, which is an additional safety factor.
The image to the left shows a segment where the sheath failed and melting may have occurred at the centreline. Superimposed is the predicted temperature and oxidation, with a white ring around the molten zone.
Laser flash melting of advanced fuels
Solid nuclear fuels, the common of which is Uranium dioxide, melt at such high temperatures (over 2800 C!) that it is hard to even measure the melting temperature. When other materials like plutonium, thorium, and oxygen are added, measurements are even harder since the constituents diffuse, vaporise, and react with their surroundings. Laser flash melting experiments are short (subsecond) but the observations can be complicated by transient effects and non-congruent melting / freezing. Phase-field models can be used to help interpret this observations.
The image to the right shows the observed surface temperature, predicted oxygen concentration and molten pool over time.