Hi,ENGINEERZ

Nuclear fuel

Nuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. Nuclear fuels are the most dense sources of energy available to humans. Nuclear fuel in a nuclear fuel cycle can refer to the material or to physical objects (for example fuel bundles composed of fuel rods) composed of the fuel material, perhaps mixed with structural, neutron moderating, or neutron reflecting materials.

The most common type of nuclear fuel contains heavy fissile elements that can be made to undergo nuclear fission chain reactions in a nuclear fission reactor. The most common fissile nuclear fuels are 235U and 239Pu. The actions of mining, refining, purifying, using, and ultimately disposing of these elements together make up the nuclear fuel cycle, which is important for its relevance to nuclear power generation and nuclear weapons.

Not all nuclear fuels are used in fission chain reactions. Plutonium-238 and some other elements are used to produce small amounts of nuclear power by radioactive decay in radioisotope thermoelectric generators and other atomic batteries. Light nuclides such as 3H (tritium) are used as fuel for nuclear fusion

The thermal conductivity of uranium dioxide is low; it is affected by porosity and burn-up. The burn-up results in fission products being dissolved in the lattice (such as lanthanides), the precipitation of fission products such as palladium, the formation of fission gas bubbles due to fission products such as xenon and krypton and radiation damage of the lattice. The low thermal conductivity can lead to overheating of the center part of the pellets during use. The porosity results in a decrease in both the thermal conductivity of the fuel and the swelling which occurs during use.

According to the International Nuclear Safety Center [1] the thermal conductivity of uranium dioxide can be predicted under different conditions by a series of equations.

The bulk density of the fuel can be related to the thermal conductivity

Where ρ is the bulk density of the fuel and ρtd is the theoretical density of the uranium dioxide.

Then the thermal conductivity of the porous phase (Kf)is related to the conductivity of the perfect phase (Ko, no porosity) by the following equation. Note that s is a term for the shape factor of the holes.

Kf = Ko.(1-p/1+(s-1)p)
Rather than measuring the thermal conductivity using the traditional methods in physics such as lees's disk, the Forbes' method or Searle's bar it is common to use a laser flash method where a small disc of fuel is placed in a furnace. After being heated to the required temperature one side of the disc is illuminated with a laser pulse, the time required for the heat wave to flow through the disc, the density of the disc, and the thickness of the disk can then be used to calculated to give the thermal conductivity.

λ = ρCpα
λ thermal conductivity
ρ density
Cp heat capacity
α thermal diffusivity
If t1/2 is defined as the time required for the non illuminated surface to experience half its final temperature rise then.

α = 0.1388 L2 / t1/2

This must be becomig very difficult for you to understand so lets close this topic by only telling you in the end about the types of Oxide Fuel which are:

UOX (Uraniumm Oxide)
MOX (Mixed Oxide)