|Fig. 1: Nuclear fuel pellets. (Source: Wikimedia Commons)|
Nuclear fuel is one of the most energy dense materials, making it an attractive energy source, but it can also be one of the most dangerous because of its radioactivity. In the event of an accident, it is imperative that the release of radioactive materials during fission be minimized or well-contained. Thus, it is important to understand the behavior of nuclear fuel both during use and under accidental conditions.
The most commonly used sources for nuclear fuel are U-235 and Pu-239. Typically, plants use oxide forms of uranium such as uranium oxide and mixed oxide (MOX) because oxides have higher melting and thermal conductivities. Uranium ore is mined and crushed to form "yellow cake" (U3O8), which has a uranium concentration of almost 80%. The yellow cake is then converted to UF6, which is then solidified and shipped to an enrichment facility. Uranium oxide is formed by heating UF6 to a gaseous form and then chemically processing it to form UO2 powder. It is then processed into pellets, as shown in Fig. 1. Mixed oxide (MOX) is a blend of plutonium and uranium used as an alternative to low- enriched uranium (LEU) fuel, UO2. The fuel is structured in pellet form, sintered and then sealed into zirconium alloy cladding to prevent the release of fission products to the coolant. 
During use, the fuel must be capable of withstanding extremely high temperatures and swelling from fission gas formation. One of the biggest problems is stresses and strains produced by high concentrations of fission products. These products can cause the fuel to swell during use, or exert pressure on the zirconium cladding when released as noble gases.  For example, one of the fission products released is Iodine, which is known to cause stress corrosion cracking in zirconium alloys, posing potential problems for the cladding.  These problems are anticipated, and can be counteracted through preventative measures.
In the event of an accident, the largest problem is core-melting, which occurs when the plant loses its core cooling capabilities. In such cases, it is possible to stop the fission of uranium by inserting neutron-absorbing fuel rods in the fuel but this does not stop the heat being released through radioactive decay. This heat can cause the reactor core to melt, releasing large amounts of radioactive gases and materials. One example was the Three Mile Accident in Pennsylvania, where a water reactor core was partially melted due to cooling failure, leading to the release of fission product gases. Luckily, the gases were able to be contained within the facility. This was not the case at the Fukushima Accident. The plant was able to shut down properly in response to the earthquake but when the tsunami hit, the plant lost power, leading to cooling problems and eventual melting of the three active reactor cores. The zirconium alloy cladding also reacted with the incoming water at high temperatures, producing hydrogen gas, which accumulated and led to explosions in four of the cores. 
Even as we have increased safety regulations over time to anticipate accidents and reactions, there is still need for more research in better understanding the reaction of the zirconium cladding and nuclear fuel (UO2) with water and the release, distribution and reactivity of radioactive fission products. One group at Argonne National Laboratories is working on determining how UO2 changes as it melts. There are currently many melt models but little experimental structure measurements, and the models themselves vary in details (bond length estimates for example). They found that oxygen's valence number in uranium dioxide changes from eight to a mixture of six and seven while melting.  This can drastically change the way the material reacts with other materials, which could have a huge impact on safety considerations at nuclear power plants.
© Prastuti Singh. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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