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Fuel elements

Fuel elements contain the nuclear fuel of a reactor in their fuel rods. The fissile materials contained in nuclear fuel (such as uranium-235) can be fissioned in a controlled, physical process. This releases thermal energy which is subsequently converted in the nuclear power plant into electrical power by means of turbine and generator.

Uranium and MOX fuel elements

Depending on whether the nuclear fuel of a fuel element contains just fissile uranium dioxide or a mixture of uranium dioxide and plutonium oxide (some percent), one differentiates between

  • Uranium fuel elements and
  • Mixed-oxide (MOX) fuel elements.

Since plutonium always generates anew from the uranium-238 during reactor operation, pure uranium fuel elements contain plutonium over time, too. This is also fissioned and thus contributes to energy production.

In German nuclear power plants, MOX fuel elements are always used together with uranium fuel elements.

In a reactor loaded with MOX fuel elements, the core contains about two- to five-times as much plutonium as a reactor core loaded with uranium fuel that has been in operation for a longer period of time. Besides, after a certain period of operation, the MOX core has clearly higher contents of so-called transuranium elements such as neptunium, americium and curium.

Behaviour in the event of a core melt

Correspondingly to the higher contents of plutonium, neptunium, americium and curium in the MOX fuel elements, larger amounts of these materials are released in the event of a core melt and may be discharged into the environment. In that case, plutonium, neptunium, americium and curium are available as dust particles or are bound to dust particles.

To a substantial extent, all these materials are only released from the melt at temperatures clearly above ca. 2,000˚C. In the event of a core melt accident, the effects on the environment are determined by the composition of all the radioactive materials in the nuclear reactor at the time of accident (inventory) and by the share being released. In most accident courses, the differences between a uranium core and a MOX core play only a minor role in terms of effects on the environment.

State of 2018.02.09

© Federal Office for the Safety of Nuclear Waste Management