JEFF WILKS, URENCO UK Limited, UK

Abstract: Most nuclear power stations use fuel that has concentrations of the uranium-235 isotope that are higher than found in natural uranium. The process of increasing the concentration of uranium-235 above natural is called enrichment and this chapter describes the main enrichment technologies that are in use or have been investigated. The most important enrichment technologies utilise uranium hexafluoride as a feed material and so this chapter also summarises its properties, describes processes used to manufacture it and considers a number of important issues associated with its use.

Key words: uranium hexafluoride, uranium conversion, uranium enrichment.

7.1 Introduction

The precursors to the development of nuclear power were nuclear programmes carried out for military purposes. The history of these military programmes has been documented on many occasions and is not the subject of this chapter. What is of relevance is that early atomic weapons were based on uranium and in particular, the potential for uranium to sustain a nuclear chain reaction releasing huge amounts of energy. When released in large amounts over a very short time period then this energy release forms the basis of a nuclear weapon. When the energy release is controlled, collected and used to generate electricity then it forms the basis of nuclear power generation.

Natural uranium contains three isotopes. Uranium-238 (238U) is the bulk isotope comprising over 99% of the total with uranium-235 (235U) present at around 0.71% by weight and uranium-234 (234U) at 0.0053%. Of the three isotopes, only 235U is fissile and capable of sustaining a nuclear chain reaction. The proportion of 235U in natural uranium is sufficient to sustain such a reaction under very specialised conditions, for example when using heavy water as a moderator. It is not sufficient, under any circumstances, to sustain a reaction of sufficient intensity for use in nuclear weapons. The weapons programme therefore required that technology be developed that would allow the proportion of 235U in uranium to be increased to very high levels, typically greater than 90%. The process of increasing the proportion of 235U to levels above that found naturally is known as enrichment and its application to civil nuclear power generation is the subject of this chapter.

Enriched uranium is not necessary for nuclear power generation. The first reactor to generate electricity on an industrial scale, Calder Hall in the UK, used natural uranium and successful commercial designs such as the British Magnox reactor and the Canadian CANDU reactor have done so since. The technology to enrich uranium was already established, however, and as nuclear power reactor designs were developed around the world, many of those designs chose to make use of low enriched, rather than natural uranium. Low enriched uranium allows higher power densities than can be achieved in reactors using natural uranium and generates less spent fuel, a waste requiring careful management. It also allows normal water to be used as the moderator and heat transfer medium, rather than the heavy water used in the CANDU and the graphite moderated, carbon dioxide cooled combination in Magnox reactors. Over 90% of nuclear power is now generated from low enriched uranium, typically containing 235U at levels of between 3% and 5% by weight.

The process of mining and refining uranium produces a uranium ore concentrate often referred to as yellowcake, the main component being triuranium octaoxide (U3O8). This material is impure and not suitable for enrichment and therefore needs to be converted into another chemical form. For reasons that are explained later, most enrichment technologies and specifically the two main technologies of gaseous diffusion and gas centrifuge require that the yellowcake be converted into uranium hexafluoride (UF6 , or ‘hex’) to allow enrichment to take place. This chapter describes the enrichment process and the main enrichment technologies. It also considers the properties of UF6 that have led to it being used for enrichment and the processes used to manufacture UF, from yellowcake. Finally, some relevant side issues, such as transport, sampling and analysis and tails deconversion are considered.