Most elements on Earth are stable, and the structure of their atomic nuclei is constant. A few of them decompose radioactively, however: that is to say, their atomic nuclei turn into those of other elements by emitting radiation or particles.

In a nuclear reactor, or a reactor at a nuclear power plant, nuclear fission is induced deliberately and the resulting radioactive decay used. Atomic nuclei are split by bombarding them with neutrons.

Design and Construction of Nuclear Power Plants. First Edition.

Rudiger Meiswinkel, Julian Meyer, Jurgen Schnell.

© 2013 Ernst & Sohn GmbH & Co. KG. Published 2013 by Ernst & Sohn GmbH & Co. KG.

Plutonium 239,

Other transuramc e ements

Fig. 2.2 The nuclear fission process

The process of nuclear fission is shown in Figure 2.2. In the reactor, uranium U-235 nuclei are bombarded with neutrons, causing them to fission and emit radiation, known as ‘nuclear radiation’ (cf. Section 2.3). The products of decay are usually two fission products, such as krypton or barium, and two or three neutrons. The neutrons that are emitted can in turn split other atomic nuclei, setting off a chain reaction in which energy is released.

The fission products that arise when atomic nuclei split are unstable: they give off radioactive radiation, turning into stable end products, releasing more energy in the process. This post-decay heat keeps on being generated even after a nuclear reactor has been shut down, and requires special post-cooling systems (Figure 2.3).

A constant steady chain reaction needs a certain minimum mass of fissionable material, also known as the ‘critical mass’. Critical mass exists if the number of secondary fissions (second generation neutrons) is equal to the number of primary fissions (first generation neutrons).

Uranium U-235 is the only element occurring in nature that can maintain fission via a chain reaction. U-235 accounts for just 0.72% of the total mass of uranium occurring

naturally, so it does not provide the critical mass required: this has to be increased, i. e. the uranium has to be enriched. This can be done using diffusion, gas centrifuges or separation nozzles.

The critical mass of U-235 required is less if the neutrons that are released when its nuclei split can be slowed down to lower, thermal speeds (moderated). This can be done using what is known as a moderator. Apart from carbon in graphite form and heavy water (deuterium oxide, or D2O), this is best done using light water, or H2O. The water molecules slow the neutrons down very effectively, thus maintaining the chain reaction; and the water absorbs the energy from nuclear fission, which heats it up considerably, making it ideal for generating electricity. When using H2O as moderator, the natural uranium has to be enriched to around 3.5% U-235.