Incredible Amounts of Energy

Naturally-occurring uranium, as extracted from uranium ore, contains only about 0.7% fissile uranium 235; the remaining 93.9% is non-fissile uranium 238. Uranium in this state is not suitable as fuel for light-water reactors and needs to be enriched to increase the percentage of uranium 235 to three to four percent (with the rest remaining as uranium 238). Once enriched, the uranium is pressed into pellets and placed in fuel rods, which are then welded shut. The fuel rods are then bundled to form fuel elements-ready for use in reactors.

How does Nuclear Fission Work?

The nuclei of uranium-235 are bombarded with neutrons. When a uranium-235 nucleus absorbs a bombarding neutron, the result is a highly unstable nucleus, uranium-236.

Within a fraction of a second this nucleus splits into two fission products. The resulting fragments fly apart at high speed but are quickly slowed down in the uranium's crystal lattice, with the result that the kinetic energy is converted into heat. Neutrons are also released in the fission process. These neutrons (known as 'fast' neutrons) have a very high energy level, too high, in fact, to induce a further fission of uranium-235 and hence start a chain reaction.

This is why, in LWR reactors, water is used not only as a coolant, but also as a moderator. Moderators effectively reduce the energy (speed) of the neutrons to a level which is suitable for inducing further nuclear fission.

Conclusion:

In the nuclear power plants operated by E.ON Kernkraft, self-sustaining fission reactions are not possible without water as a moderator.