A fuel element failure is a rupture in a nuclear reactor

Nuclear reactor

A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

's fuel cladding that allows the nuclear fuel or fission product

Fission product

Nuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos. The...

s in the form of dissolved radioisotopes or hot particle

Hot particle

A hot particle is a small, highly radioactive object, with significant content of radionuclides. Because radioactivity can be inherent to a substance or induced, and there are many initial sources of radioactivity, hot particles can originate from a multitude of sources.- Attributes :Hot particles...

s to enter the reactor coolant or storage water.

The de facto standard nuclear fuel

Nuclear fuel

Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...

 is uranium dioxide

Uranium dioxide

Uranium dioxide or uranium oxide , also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear reactors. A mixture of uranium and plutonium dioxides is used...

 or a mixed uranium/plutonium dioxide

Plutonium dioxide

Plutonium oxide is the chemical compound with the formula PuO2. This high melting point solid is a principal compound of plutonium. It can vary in color from yellow to olive green, depending on the particle size, temperature and method of production....

. This has a higher melting point than the actinide

Actinide

The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...

 metals. Uranium dioxide resists corrosion in water and provides a stable matrix for many of the fission products; however to prevent fission products (such as the noble gases) from leaving the uranium dioxide matrix and entering the coolant, the pellets of fuel are normally encased in tubes of a corrosion resistant metal alloy (normally Zircaloy

Zircaloy

Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance...

 for water-cooled reactors).

Those elements are then assembled into bundles to allow good handling and cooling. As the fuel fissions

Nuclear fission

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

, the radioactive fission product

Fission product

Nuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos. The...

s are also contained by the cladding, and the entire fuel element can then be disposed of as nuclear waste when the reactor is refueled.

If, however, the cladding is damaged, those fission products (which are not immobile in the uranium dioxide matrix) can enter the reactor coolant or storage water and can be carried out of the core, into the rest of the primary cooling circuit, increasing contamination levels there.

In the EU, some work has been done in which fuel is overheated in a special research reactor named PHEBUS. During these experiments the emissions of radioactivity from the fuel are measured and afterwards the fuel is subjected to Post Irradiation Examination (PIE) to discover more about what happened to it.

It can be seen that the fuel has failed mechanically and has formed a pool near the bottom of the bundle, it is interesting to note that the bottom of the bundle did not melt.