Energy and Environment

Uranium Reactor Fuel

The fuel processes describe here are a summary of those described on the BNFL briefing note “Manufacturing Nuclear Fuel” [4] and from the BNFL website.

As described in the foregoing, natural Uranium consists of only 0.3% fissile U235 and 99.3% fertile U238 which may be used in the production of fissile Pu239. Uranium ore, as mined, consists of a number of oxides and other compounds of uranium. At the mine head, the ore is milled and leached in acid and alkaline solutions close to produce “yellowcake” (largely triuranium octoxide, U3O8) and exported to the fuel processing organisations (BNFL in the UK), where it is undergoes a number of chemical processes to produce first uranium tetrafluoride (UF4) and then uranium hexafluoride (UF6). At this stage it is sent for enrichment using cascaded gas centrifuges. In the UK this process is preformed by Urenco at Capenhurst.

The now enriched UF6 is returned to BNFL and converted to uranium dioxide (UO2) powder. This is achieved by mixing it with steam and hydrogen in a kiln. The UO2, now in powder form, is pressed, heated in a furnace and ground to produce the fuel pellets of the required dimensions. In the UK, the fuel pellets are then sent to Springfields (managed by Westinghouse) to be loaded inside zirconium alloy (or sometimes, stainless steel) tubes which are pressurised, sealed and combined to make complete fuel assemblies before despatch for use in a reactor.

Nuclear Fuel Recycling

The processes described here are a summary of those described by British Nuclear Fuels Ltd (now Sellafield Ltd which operates on behalf of the Nuclear Decommissioning Authority) [4].

Nuclear fuel normally spends some four to five years inside a reactor, producing power. The spent fuel is then removed from the reactor and placed in fuel storage ponds at the power station to allow it to cool and the radiation levels to reduce. This period may last for a further 2 – 3 years before the fuel is loaded into secure containers, known as flasks, which may then be transported to Sellafield either by road, rail or by sea. Upon arrival at Sellafield, the fuel is received into a storage area and again stored in ponds to allow radiation levels to decrease further. Next it is chopped up and sent to the Thermal Oxide Reprocessing Plant (Thorp) where it is dissolved in nitric acid and subsequently chemically separated into uranium (~96%), plutonium (~1%) and fission products (~3%). The uranium and plutonium is reused to make new fuel while the fission products are transferred to a vitrification plant to be mixed with glass and stored as high level waste.

The reprocessed uranium and plutonium is then sent to the associated Mixed Oxide (MOX) Plant fuel to be transformed into uranium dioxide and plutonium dioxide powder, mixed together with other agents and tumbled to make granules; the granules are fed into pellet presses to make MOX pellets by hardening in a sintering furnace and grinding to the required dimensions. The fuel pellets can now be fed back to the fuel fabrication process and then reused in a reactor.

This MOX process was initially developed for the fast breeder programme and could be adapted for this purpose to produce MOX fuel with a higher concentration of plutonium.

Do we have sufficient plutonium to launch a Fast Breeder Programme?

It is theoretically possible to start a fast breeder using only uranium fuel and then gradually moving to plutonium but this would cause complications in the core design so the desired route is start with MOX fuel. This requires the use of plutonium from a preceding thermal reactor programme. The question to be addressed is do we have adequate supplies of plutonium in the country to start a major FBR programme?

No MOX fuel has been used in a British reactor other than the Prototype Fast Reactor so it must be assumed that virtually all the all plutonium produced in the UK remains available to use. When the author worked for the National Nuclear Corporation in the mid 1970s it was thought that there was sufficient plutonium in the UK to fuel two fast breeder reactors of ~1.2GW(e) each. The core of an FBR is much smaller that that of a PWR and the fuel is more highly enriched but the total fissile material is approximately the same. So, if we assume that 25% of the fuel from a Magnox Reactor, AGR or PWR is refuelled every year and that this yields 1% of its mass as plutonium, it would require approximately 20 reactor years for a thermal reactor to generated sufficient plutonium for a FBR of similar output; A reasonable assumption would be 25 years, allowing for wastage.

In the 29 years since 1980 we have had Sizewell B, the 7 AGR stations working most of the time plus the remaining Magnox stations, principally Oldbury and Wylfa. On that basis, we should have adequate supplies of supplies of plutonium to start up programme of ten to twelve 1.2GW(e) FBRs and, if we were to build, say, 25 new PWRs, a further FBR could be fuelled every year or better since both the EPR and AP1000 have favourable breeding ratios. The doubling time for an FBR (the time taken to breed enough surplus plutonium fuel another reactor of the same size) depends upon both the design and the efficacy of the fuel reprocessing activities. A conservative estimate of this doubling time is about 20 years. In practice, it would take some twenty years to develop a commercial FBR and, on the basis of the foregoing assumptions, it would seem that there are adequate supplies of plutonium for a programme of 2 or 3 built FBRs every year. Once launched, such a programme would be self sustaining. The critical path is likely to run through fuel reprocessing and/or FBR design.

Nuclear Waste Disposal

The Nuclear Decommissioning Authority [5] has estimated the waste from the currently existing civil nuclear programme (assuming no more reactors built) plus the military submarine and weapons programmes. This is given in Table 1, column 2.

Westinghouse has estimated that the total waste form an AP1000 Reactor, when it is decommissioned after a 60 year life, will be about 12,800 tonnes [6]. Most of this is concrete with about 10% steel so, at an average density of 3 gm/cm3, gives a volume of about 4,500 m3.  Of this 90% will be low level waste and perhaps 10% will be intermediate level waste. The FBR has a smaller core than a PWR of the same output but the fissile inventory is similar so it is reasonable to assume that the figures for a FBR would be similar to that of the PWR. It is further assumed that the spent nuclear fuel from both types is recycled and reused as MOX. Thus, extrapolating for a 100 GW(e) programme, about 400,000 m3 low level waste and about 50,000 m3 intermediate level waste will be created at end of life after 50 to 60 years of operation.

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