P. NETTER, AREVA, France

Abstract: This chapter introduces open and closed fuel cycles. There are discussions about reprocessing targets and constraints. The separation and purification of uranium and plutonium are described. A complete closed cycle for both uranium and plutonium is set out. Finally, the industrial-scale spent fuel reprocessing strategies for selected countries are given.

Key words spent nuclear fuel, uranium reprocessing, plutonium reprocessing, PUREX process.

16.1 Introduction: closed and open cycles

The need to reduce greenhouse gas emissions, the depletion of fossil natural resources and rising energy demand are all factors that support the continued development and deployment of nuclear energy. Additionally, there is the option to go further: by reprocessing the spent fuel, uranium and plutonium can be recovered for recycling. While this releases an otherwise unavailable energy resource, the decision to reprocess is, nevertheless, a political matter in which the policy varies from state to state. Thus we find that some countries that utilize nuclear power reject reprocessing/recycling. Further, the Fukushima accident (14 March 2011) has relaunched the debate over nuclear safety and reuse of reprocessed material (mixed oxide, MOX fuel) that seemed to be fading away with the memory of Chernobyl (26 April 1986).

It is helpful, at the outset, to distinguish between open and closed fuel cycles.

In the open cycle, used fuel is considered as waste, i. e. the owner takes the view that there is no value in the used fuel. Storage is the prevailing practice and it is considered that there is no merit in doing anything other than interim storage followed by disposal. Many countries (e. g. USA, Sweden, Finland) have adopted this approach and used fuel may be stored in pools or in dry storage systems at purpose-built sites. Storage solutions currently on the market enable spent fuel to be managed over a period of several decades or even longer. Permanent disposal of used fuel envisages burial in a deep geological formation where its long-term safety can be assured.

In the closed cycle, used fuel is recycled, Fig. 16.1, i. e. reprocessed to separate the useful materials — uranium and plutonium — from the minor actinides and fission products so that they can be incorporated into new fuel. The unwanted components go into a number of different waste streams. By far the greatest

proportion of the radioactivity (if not the volume), however, goes into high-level wastes (HLW), which are transformed into glass blocks. Countries that have chosen the closed fuel cycle option are France, United Kingdom, Japan, Russia, China and India (Table 16.1). Still in development are reprocessing strategies that aim to make use of the thorium cycle or use advanced techniques for partitioning and transmutation, including the use of dedicated ‘waste burning’ reactors in order to reduce the long-term radiotoxicity of the radionuclides in the waste. With reference to the example of pressurized-water reactors, it can be stated that for every 100 kg of fuel, and depending on burn-up and fuel type, 96-97 kg can be recycled of which 95-96 kg is residual uranium (recyclable as UOX, uranium oxide fuel) and 1 kg is plutonium (recyclable as MOX, mixed oxide U-Pu fuel). The remaining 3^ kg are fission products (high-level waste), which are then incorporated into a glass matrix or with long-lived intermediate waste from spent fuel assemblies (hulls and end-caps). Reprocessing and recycling are energy production: 1g plutonium or 100 g uranium is equivalent to 1 or 2 ton oil. 1000 ton of reprocessed and recycled spent fuel generate about as much energy as 25 million tons of oil. Additional to energy production is a reduction of the volume of the wastes (through compaction or incorporation into a glass).The volume of wastes can be reduced to the range of 0.5 m3/mtU. Overall, reprocessing used fuel to recover uranium and plutonium avoids the wastage of a valuable resource. Recycling can save up to 30% of the natural uranium that would otherwise be required.

Natural uranium

Chemistry

Mining

Recycled uranium

Enriched uranium

Plutonium

Waste final disposal

Recycling MOx fuel fabrication

Fuel fabrication

Reprocessing

16.1 The nuclear fuel cycle (Source: AREVA).

The current global nuclear capability of light water reactors generates about 7000 tons of used fuel per year. Stores of used fuel accumulated across the world amounted to around 172 000 tons U by 2007, of which 32 000 have been recycled.

Throughout the world, the standard method for the separation of uranium and plutonium use aqueous solution processes (wet routes) — liquid-liquid extraction. Alternative routes exist, especially those using pyrochemical processes and dry methods but these are small scale and/or in development.

All commercial reprocessing plants use the well-proven hydrometallurgical PUREX (plutonium uranium extraction) process. This involves dissolving the fuel elements in hot concentrated nitric acid. Chemical separation of uranium and plutonium is then undertaken by solvent extraction steps. The Pu and U can be sent to the input side of the fuel cycle — the uranium to the conversion plant prior to re-enrichment and the plutonium straight to MOX fuel fabrication.

Alternatively, some of the recovered uranium can be left with the plutonium, which is sent to a MOX plant, so that the plutonium is never fully separated out. This is known as the COEX (co-extraction of actinides) process, developed in France as a ‘Generation III’ process, but not yet in use. Japan’s Rokkasho plant uses a modified PUREX process to achieve a similar result by recombining some uranium before denitration, with the main product being 50/50 mixed oxides.

In either case, the remaining liquid after Pu and U has been removed is high-level waste, containing about 3% of the used fuel in the form of fission products and minor actinides (Np, Am, Cm). It is highly radioactive and continues to generate a lot of heat. It is conditioned by calcining and incorporating the dry material into compact, stable, insoluble borosilicate glass, then stored pending disposal.

One of the incentives for recycling spent fuel discharged from nuclear reactor cores is that it meets the dual requirements of a sustainable development policy’.

• Recovery and recycling of reusable materials, uranium and plutonium, so that demand for natural uranium is reduced

• Waste minimization. Reprocessing reduces waste toxicity (both waste volume and radiotoxicity), by conditioning it into stable canisters adapted to its level of activity and half-life for disposal.