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Appendix: Uranium Enrichment Appendix: Uranium Enrichment THE PRINCIPLES OF URANIUM ENRICHMENT Uranium 'enrichment' means the partial separation of the two isotopes, uranium-235 (U235) and uranium-238 (U238), found in natural uran­ ium, in order to obtain uranium with a higher concentration of the fissile isotope U235. 1 As explained in Chapter I, natural uranium, as found in uranium ore and concentrates, consists of only 0.7 per cent U235, the remaining 99.3 per cent being U238. To produce fuel for most thermal power reactors, natural uranium must be enriched to between 2 and 4 per cent U235. This is known as low-enriched uranium or LEU.2 Enrichment accounts for a small portion - about 5 per cent - of the total cost of nuclear-generated electricity although it amounts to between one-quarter and one-half of the relatively low fuel costs. If U235 is to be used for a nuclear weapon, natural uranium must be more enriched than this. Typically 90 per cent U235 is used for nuclear weapons although, as explained in Chapter I, it is possible to produce a nuclear weapon with uranium enriched as low as 10 per cent U235. For purposes of safeguards, uranium enriched to more than 20 per cent U235 is known as high-enriched uranium (HEU) or 'weapons-grade' uranium. The technology for the production of high-enriched uranium is broadly the same as that for the production of low-enriched uranium. Isotopes of chemical elements (in this case, uranium) are chemically identical, and differ only in their physical properties, these differences being due to slight differences in mass. Enrichment is therefore a physical rather than a chemical process, although it calls for considera­ ble skills in chemical engineering. The U235 and U238 isotopes differ in mass by only 1.3 per cent. A great deal of energy must therefore be expended in separating them. Whilst it is relatively easy scientifically to devise conceptually new enrichment (or 'separation') techniques/ devising new techniques which are cheaper than those already developed is very much harder. In general, the sophistication of enrichment technologies derives from the precision and materials engineering skills required for producing the separation elements and/or the high-speed 157 158 South Africa and Nuclear Proliferation moving parts.4 Enrichment processes are typically capital and energy intensive; and a nation proposing to establish a uranium enrichment capability faces a very large energy requirement and the need for an established industrial infrastructure. To date, only two technologies, gaseous diffusion and gas centrifuge, are used in commercial enrichment plants. Aerodynamic technology, conceived of in West Germany, is being developed in Brazil and South Africa. Theoretical work continues on other techniques, laser enrich­ ment being worthy of mention (at the end of this Appendix) because of its implications for weapons proliferation. The three technologies mentioned which are being or have been developed for commercial use all utilise a volatile compound of uranium: uranium hexafluoride (UF6), or 'hex', in the gaseous form. Although 'hex' has the advantage of being readily vaporised, it has the disadvantage of being highly toxic and corrosive. Each stage of the enrichment process will only separate or enrich the uranium (in the 'hex') by a very slight amount. The uranium hexafluoride feed must therefore be passed through many identical stages, typically connected in a cascade arrangement. An enrichment plant separates the natural uranium 'feed' into the enriched uranium 'product' and the depleted uranium 'tails'. Typical values for the feed, product and tail streams are shown in Figure A.I. The waste 'tails' consist of 'depleted' uranium: uranium with a U235 concentration less than that of the natural uranium feed. Typically the concentration ofU235 in the tails is 0.2 to 0.3 per cent. This is known as the 'tails assay'. In enrichment plants the tails assay can in practice be adjusted in response to changes in the relative costs of enrichment work (energy) and the natural uranium feed. Additional cycles of depleted uranium through the separation process will produce 3 per cent enriched product in greater quantity, and the assay of U235 in the tails will be further reduced; but this would require more work per unit of product, and so be more costly. Conversely, raising the tails assay from 0.2 to 0.3 per cent will result in a saving of20 per cent in enrichment work capacity (SWUs) but will raise the natural uranium feed requirements (per unit of product) by 20 per cent. The choice of tails assay thus has considerable implications for the cost of enrichment and nuclear fuel supply in general.s The energy required to separate the U235 and U238 isotopes is measured in separative work units (SWUs). The production of Ikg of3 per cent enriched uranium from natural uranium feed, in a plant Appendix: Uranium Enrichment 159 ,---+ Product 1 kg of enriched uranium 13 per cent U235) Feed 5.5 kg of natural uranium 10.7 per cent U2351 ----+( Tails 4.5 kg of depicted uranium 10.2 per cent U2351 Figure A. I Typical Relationship between Feed, Tail and Product Streams in an Enrichment Plant operating a 0.3 per cent tails assay, requires 3.42SWU, for example. The same quantity of separative work could also be used, in the same plant (with the same tails assay), to produce 2kg of 2 per cent enriched uranium or 0.65kg of 4 per cent product. Thus, for example, the SWU can be used to compare plant capacities or enrichment demand regardless of the U235 concentrations involved. It should be noted that the separative work unit has the dimensions of mass. References in the literature to 'metric tons of separative work' when describing the size of an enrichment plant are thus to thousands of separative work units (and not the quantities of feed or product). With a tails assay of 0.2 per cent, a 1000MW(e) light water power reactor needs 366t of natural uranium feed and 257 OOOSWU to produce the 85t of 2.4 per cent enriched uranium required for its initial fuel load. Thereafter between one-quarter and one-third of the reactor fuel core is removed and replaced on an annual basis. Each annual reload requires 33.9t of 3.15 per cent enriched uranium which is equivalent to 196t natural uranium feed and 157200SWU at the same 0.2 per cent tails assay. With a 0.3 per cent tails assay, however, the natural uranium requirements increase to 235t, whereas the separative work requirements fall to 125 OOOSWU. 6 This information is summarised in Table A.l. (In practice less feed and separative work capacity is required annually because reactors typically operate only at load factors of about two-thirds.) In principle, it is possible to convert an enrichment plant used for 160 South Africa and Nuclear Proliferation producing low-enriched uranium for power reactors into one for producing weapons-grade high-enriched uranium, either by batch recycling of the product or by adding more separation stages in series (possibly by rearranging cascades previously operating in parallel). Once a nation has acquired an enrichment plant for civil purposes, in connection with a nuclear power programme, it is therefore relatively Table A.I SWU and Natural Uranium Requirement for Enriched Uranium Products Product Natural uranium Separative Tails assay feed work per cent U235 tonnes U SWU First fuel load for 1000MW(e) Pressurised water reactor (PWR): 85t of 2.4 per cent U235 0.2 370 260000 0.3 440 n.a. Annual fuel reload for 1000MW(e) PWR: (i) Assuming 100 per cent load factor: 34t of 3.1 per cent U235 0.2 200 150000 0.3 240 120000 (ii) Assuming 65 per cent load factor: 23t of 3.1 per cent U235 0.2 130 100000 0.3 160 80000 Fissile material for small nuclear explosive device: 15kg of 90 per cent U235 0.2 2.7 3400 0.3 3.4 3000 25kg of 90 per cent U235 0.2 4.5 5700 0.3 5.6 4800 50kg of 90 per cent U235 0.2 9.0 11400 0.3 11.2 9700 n.a. not available Sources: Adapted or computed from: Ted Greenwood, George W. Rathjens and Jack Ruina, Nuclear Power and Weapons Proliferation, Adelphi Papers no. 130 (London: IISS, 1976) p.43; International Nuclear Fuel Cycle Evalua­ tion, Enrichment Availability, Report of INFCE Working Group 2 (Vienna: IAEA, 1980) pp.73, 78; The Uranium Institute, The Uranium Equation: The Balance of Supply and Demand 1980-1995 (London: Mining Journal Books, 1981) p.41. Appendix: Uranium Enrichment 161 easy to adapt the plant in order to produce weapons-grade uranium. Producing high-enriched uranium from a given amount of natural uranium feed would of course be more expensive than producing low­ enriched uranium; but if one compares the SWUs needed to produce a given quantity of90 per cent enriched uranium from 3 per cent enriched uranium with that required to produce the 90 per cent enriched uranium from natural uranium, it is found that about three times less separative work is required along with a five-fold reduction in the amount of feed material required. 7 The minimum amount of high-enriched uranium required to produce a nuclear weapon is 15-25 kg. 8 Such a weapon could give an explosion yielding the same as the explosion of20 kilotons of TNT. The Hiroshima bomb, the first enriched uranium explosive device, contained 60 kg HEU. Table A.I shows that the minimum separative work requirements to produce explosive devices of these sizes would be in the range 3000- 12000SWU.
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