In a recent study, used 20% enriched uranium dispersed in a range of different matrices was examined to determine the physical locations of different isotopes and chemical elements.
* A solid solution of urania in yttria-stabilized zirconia (YSZ) (Y:Zr atom ratio of 1:4).
* Urania particles in an inert matrix formed by a mixture of YSZ and spinel (MgAlO).
* Urania particles dispersed in the inert matrix formed by a mixture of YSZ and alumina.
The fuels varied in their ability to retain the fission xenon; the first of the three fuels retained 97% of the Xe, the second retained 94% while the last fuel only retained 76% of this xenon isotope. The Xe is a long-lived radioactive isotope which can diffuse slowly out of the pellet before being neutron activated to form Cs. The more short-lived Xe was less able to leach out of the pellets; 99%, 98% and 95% of the Xe was retained within the pellets. It was also found that the Cs concentration in the core of the pellet was much lower than the concentration in the rim of the pellet, while the less volatile Ru was spread more evenly throughout the pellets.
The following fuel is particles of solid solution of urania in yttria-stabilized zirconia dispersed in alumina which had burnt up to 105 GW-days per cubic meter. The scanning electron microscope (SEM) is of the interface between the alumina and a fuel particle. It can be seen that the fission products are well confined to within the fuel, little of the fission products have entered the alumina matrix. The neodymium is spread throughout the fuel in a uniform manner, while the caesium is almost homogenously spread out throughout the fuel. The caesium concentration is slightly higher at two points where xenon bubbles are present. Much of the xenon is present in bubbles, while almost all of the ruthenium is present in the form of nanoparticles. The ruthenium nanoparticles are not always colocated with the xenon bubbles.