Effects on Fuel Due to Swelling and Core Burnup:

One of the needs of a good fuel is to be resistant to radiation damage which could lead to dimensional modification (for instance, through swelling, cracking, or creep). Near the beginning reactors and a few older gas-cooled reactors used unalloyed uranium as the fuel. While unalloyed uranium is irradiated, dimensional changes occur that present disadvantages to its use as a fuel. An effects are of two categories: 1) dimensional instability without appreciable modification in density experiential at temperatures below about 450ºC, and 2) swelling, accompanied by a decrease in density, which becomes important above 450ºC. Other reactors use ceramic fuels, along with uranium dioxide being the most general; have the benefits of high-temperature stability and adequate resistance to radiation. Uranium dioxide (UO₂) has the ability to retain a huge proportion of the fission gases, given the temperature does not exceed about 1000º C. Other oxide fuels have same qualities.

Even by fission product swelling is less along with oxide fuels, this irradiation-induced volume rise has been observed in UO2 and mixed-oxide fuels for a number of years. That swelling of the fuel has commonly been attributed to both gaseous fission-product bubble establishment and the accumulation of solid fission products. Swelling could cause excessive pressure on the cladding that could lead to fuel element cladding failure. A Swelling also becomes a consideration on the lifetime of the fuel element through helping to determine the physical and mechanical changes resulting from irradiation and high temperature within the fuel and the cladding. The Fuel components life or core burnup, that denotes the meaningful lifetime of the fuel in a reactor, is also determined through the decrease in reactivity because of the decrease in fissile material and the accumulation of fission- product poisons. Below operating conditions, fuel pellets undergo marked structural modifications as a result of the high internal temperatures and the large temperature gradients. Thermal stresses lead to radial cracks and grain structure changes. These structural modifies tend to increase along with the specific power and burnup of the fuel.