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Nuclear Energy

Article excerpt

ABSTRACT

The technical principles and practices of the civil nuclear industry are described with particular reference to fission and its products, natural and artificial radioactivity, elements principally concerned and their relationships, main types of reactor, safety issues, the fuel cycle, waste management, issues related to weapon proliferation, environmental considerations and possible future developments.

Keywords: nuclear structure, fission, criticality, radioactivity, reactors, fuel cycle, actinides, transmutation, hazards, environmental issues, future prospects

Introduction

With rising concern about climatic effects attributed in part to the use of fossil carbon-based fuels, and about possible difficulties in their future supply, nuclear energy promises to become increasingly important in coming decades. This follows a period in which it fell out of political favour for various reasons, to some extent based on misunderstandings of the science or technology involved. A great deal of argument is sure to surround any concrete proposal to increase capacity or replace that taken out of service, and it is important that the discussion should be well informed. Much of it will rightly be about complex ethical, economic, aesthetic and social issues, but at least the technical aspects are not too difficult to understand at a level sufficient for the purpose.

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Basic principles

Atomic and nuclear structure

Atoms comprise three types of particle: electrons, protons and neutrons, with some sub-structure that need not be considered here. A proton has a unit of positive electrical charge and very slightly more than one unit of mass on the scales used; a neutron has a similar mass but no charge; while an electron has unit negative charge but only an eighteen-hundredth of a mass unit.

An atom has a small nucleus containing one to over a hundred protons, with the same number of electrons forming a cloud around it. (How a single electron can form a cloud is related to the impossibility of defining its actual position.) Since electrical charges of the same sign repel each other, a nucleus with more than one proton, i.e. any but hydrogen, must be held together by a similar or larger number of neutrons.

The possibility of nuclear energy depends on the curious fact that protons and neutrons (collectively "nucleons"), whether taken individually or bound together in hundreds, are very slightly more massive than in clusters of intermediate size (Figure 1). Forming relatively small clusters, either by combining individual nucleons (fusion) or by splitting very large combinations (fission), thus releases a small amount of mass that in accordance with Einstein's equation E = [mc.sup.2] appears as a substantial amount of energy. Fusion as a useful source would be highly desirable, but for half a century the prospect of a technical demonstration has receded like the end of the rainbow, while industrial application is still more problematic, so for the present and foreseeable future, nuclear energy means fission.

Being almost 2,000 times as massive as an electron, protons and neutrons provide over 99.9% of the whole atomic mass. On the other hand, although atoms themselves are small with effective diameters about a hundred-millionth of a centimetre, nuclei are 10,000 times smaller still. Thus the electrons occupy virtually all of the volume. Individual elements, for instance in a flame, emit light of characteristic wavelengths, a fact interpreted as showing that their electrons are allowed only certain discrete energy levels, each as it turns out with limited occupancy and imposing a particular spatial configuration. The readily-observable properties of atoms in bulk depend on interactions between their outer electrons, so their identity--whether for instance iron, carbon or chlorine--depends ultimately on the number of protons, known as the atomic number of the element in question. …