Spent nuclear fuel

Used nuclear fuel is a complex mixture of the fission products, uranium, plutonium and the transplutonium metals. In fuel which has been used at high temperature in power reactors it is common for the fuel to be heterogeneous; often the fuel will contain nanoparticles of platinum group metals such as palladium. Also the fuel may well have cracked, swelled and been used close to its melting point. Despite the fact that the used fuel can be cracked, it is very insoluble in water, and is able to retain the vast majority of the actinides and fission products within the uranium dioxide crystal lattice.
[t] Oxide fuel under accident conditions
Main article: Nuclear fuel response to reactor accidents

Two main modes of release exist, the fission products can be vaporised or small particles of the fuel can be dispersed.

Fuel behavior and post irradiation examination (PIE)

Materials in a high radiation environment (such as a reactor) can undergo unique behaviors such as swelling [4] and non-thermal creep. If there are nuclear reactions within the material (such as what happens in the fuel), the stoichiometry will also change slowly over time. These behaviors can lead to new material properties, cracking, and fission gas release:

    * Fission gas release
          o As the fuel is degraded or heated the more volatile fission products which are trapped within the uranium dioxide may become free. For example see J.Y. Colle, J.P. Hiernaut, D. Papaioannou, C. Ronchi, A. Sasahara, Journal of Nuclear Materials, 2006, 348, 229.

    * Fuel cracking
          o As the fuel expands on heating, the core of the pellet expands more than the rim which may lead to cracking. Because of the thermal stress thus formed the fuel cracks, the cracks tend to go from the center to the edge in a star-shaped pattern.

In order to better understand and control these changes in materials, these behaviors are studied. A common experiment to do this is post irradiation examination, in which fuel will be examined after it is put through reactor-like conditions [5][6] [7] [8]. Due to the intensely radioactive nature of the used fuel this is done in a hot cell. A combination of nondestructive and destructive methods of PIE are common.

The PIE is used to check that the fuel is both safe and effective. After major accidents, the core is normally subject to PIE in order to find out what happened. One site where PIE is done is the ITU which is the EU center for the study of highly radioactive materials.

In addition to the effects of radiation and the fission products on materials, scientists also need to consider the temperature of materials in a reactor, and in particular, the fuel. Too high a fuel temperature can compromise the fuel, and therefore it is important to control the temperature in order to control the fission chain reaction.

The temperature of the fuel varies as a function of the distance from the center to the rim. At distance x from the center the temperature (Tx) is described by the equation where ρ is the power density (W m−3) and Kf is the thermal conductivity.

    Tx = TRim + ρ (rpellet2 - x2) (4 Kf)-1

To explain this for a series of fuel pellets being used with a rim temperature of 200 °C (typical for a BWR) with different diameters and power densities of 250 MW·m−3 have been modeled using the above equation. Note that these fuel pellets are rather large; it is normal to use oxide pellets which are about 10 mm in diameter.

Radioisotope battery

The terms atomic battery, nuclear battery and radioisotope battery are used interchangely to describe a device which uses the radioactive decay to generate electricity. These systems use radioisotopes that produce low energy beta particles or sometimes alpha particles of varying energies. Low energy beta particles are needed to prevent the production of high energy penetrating Bremsstrahlung radiation that would require heavy shielding. Radioisotopes such as tritium, nickel-63, promethium-147, and technetium-99 have been tested. Plutonium-238, curium-242, curium-244 and strontium-90 have been used.

There are two main categories of atomic batteries: thermal and non-thermal. The non-thermal atomic batteries, which have many different designs, exploit charged alpha and beta particles. These designs include the direct charging generators, betavoltaics, the optoelectric nuclear battery, and the radioisotope piezoelectric generator. The thermal atomic batteries on the other hand, convert the heat from the radioactive decay to electricity. These designs include thermionic converter, thermophotovoltaic cells, alkali-metal thermal to electric converter, and the most common design, the radioisotope thermoelectric generator.

Radioisotope thermoelectric generators

A radioisotope thermoelectric generator (RTG) is a simple electrical generator which converts heat into electricity from a radioisotope using an array of thermocouples.

238
Pu has become the most widely used fuel for RTGs. In the form of plutonium dioxide it has a half-life of 87.7 years, reasonable energy density and exceptionally low gamma and neutron radiation levels. Some Russian terrestrial RTGs have used 90
Sr; this isotope has a shorter half-life and a much lower energy density, but is cheaper. Early RTGs, first built in 1958 by the U.S. Atomic Energy Commission, have used 210
Po. This fuel provides phenomenally huge energy density, (a single gram of polonium-210 generates 140 watts thermal) but has limited use because of its very short half-life and gamma production and has been phased out of use in this application.

Radioisotope heater units (RHU)

Radioisotope heater units normally provide about 1 watt of heat each, derived from the decay of a few grams of plutonium-238. This heat is given off continuously for several decades.

Their function is to provide highly localised heating of sensitive equipment (such as electronics in outer space). The Cassini-Huygens orbiter to Saturn contains 82 of these units (in addition to its 3 main RTG's for power generation). The Huygens probe to Titan contains 35 devices.

Fusion fuels

Fusion fuels include tritium (3H) and deuterium (2H) as well as helium-3 (3He). Many other elements can be fused together, but the larger electrical charge of their nuclei means that much higher temperatures are required. Only the fusion of the lightest elements is seriously considered as a future energy source. Although the energy density of fusion fuel is even higher than fission fuel, and fusion reactions sustained for a few minutes have been achieved, utilizing fusion fuel as a net energy source remains a theoretical possibility

First generation fusion fuel

Deuterium and tritium are both considered first-generation fusion fuels; they are the easiest to fuse, because the electrical charge on their nuclei is the lowest of all elements. The three most commonly cited nuclear reactions that could be used to generate energy are:

    2H + 3H \rightarrow n (14.07 MeV) + 4He (3.52 MeV)

    2H + 2H \rightarrow n (2.45 MeV) + 3He (0.82 MeV)

    2H + 2H \rightarrow p (3.02 MeV) + 3H (1.01 MeV)