1.1.1 Overview

Many kinds of nuclear reactors have been developed since the discovery of uranium’s nuclear decay in 1938. These can be divided into generations, in the order in which they were developed, as follows:

— Generation I

The initial prototypes built between 1957 and 1963.

— Generation II

Commercially viable reactors built from the mid 1960s onwards.

— Generation III

Advanced reactors, generating much more power and with much more concern about safety, built since the early 1980s.

— Generation III+

The next generation of reactors, with structural safeguards against meltdown and/or passive safety features.

— Generation IV

The reactors of the future, highly efficient, with advanced safety features and producing little spent nuclear fuel, but not expected to come on stream until 2030 at the earliest.

(Remark: At the international level, Generation III is often classed as part of Generation II, so Generation III+ is referred to as Generation III.)

Of the types of nuclear reactor that have been developed, there are only a few that can be used in commercial operation. The different types can be broken down by the following aspects:

— Fuel

e. g. natural uranium, enriched uranium, plutonium, thorium; whether they use clad or unclad solid fuels (cladding materials are zirconium, aluminium, magnesium or magnesium oxide — Magnox); fuel elements may be rods, plates, tubes or pellets

— Neutron energy

thermal reactors (moderated neutrons, using moderators such as graphite, light water H2O or heavy water D2O) and fast reactors (without moderating the neutrons)

— Coolant

light water H2O, heavy water D2O, gas (air, but mainly carbon dioxide and helium).

The first basic distinction here is between thermal and fast reactors. Fast reactors are better known as fast breeders, because when they are operating they ‘breed’ more fissionable plutonium from the uranium than they use, which means that they can get around twice as much energy out of the uranium. Fast breeders have failed to establish themselves, however, for a number of reasons (political reasons in Germany).

Amongst the thermal reactors, there are a number of combinations of moderators and coolants which have been developed successfully for commercial use (Table 2.1). The two main families involved here are gas-cooled reactors (Magnox reactors), advanced gas-cooled reactors and high-temperature reactors and water reactors (light and heavy water reactors).

The most important of these are the light water reactors, as they are also operated in Germany at present. They have proved themselves worldwide, and are the reactors of choice not least because of their safety aspects. Apart from a few exceptions, light water reactors are the only ones that have been designed and built worldwide for some years now.