WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ

REPROCESSING AS A WASTE MANAGEMENT AND FUEL RECYCLING OPTION Experience at Sellafield in the UK

Chris Phillips and Andrew Milliken, Thorp Group

British Nuclear Fuels Plc, B570/4, Sellafield, CA20 1PG, UK ABSTRACT Irradiated nuclear fuel has been successfully reprocessed at the Sellafield site of British Nuclear Fuels in northwest England for almost 50 years. Three reprocessing plants have been used, all based on dissolution of the spent fuel into nitric acid followed by solvent extraction, and the Magnox uranium metal and Thorp uranium oxide plants are currently in operation. In excess of 40,000te of irradiated fuel has been processed in these facilities. The compact nature of the Sellafield site, and its coastal situation, has meant that particular emphasis has always been placed on the safe management of the wastes produced, and offsite discharges. The paper describes how this management has been improved progressively by construction and commissioning of downstream treatment plants in the case of Magnox reprocessing, and by modified flowsheet and equipment design to retain the wastes at source in the case of Thorp. The success of these measures is demonstrated in Thorp by the large proportion of all waste activity retained in glass or cement media (over 99.99%), the minimal impact of Thorp on Sellafield liquid waste discharges (generally 2% or less of Site Discharge limits) and the low losses of plutonium and uranium to all waste streams (less than 0.2% in each case). The flowsheet and equipment innovations that have allowed these highly favourable figures to be achieved are described, together with the operating experience of the plant over its first 2500te of throughput. With this successful experience in mind, the paper compares and contrasts reprocessing with direct storage/disposal of irradiated fuel, taking into account both the management of wastes and the potential for recycling plutonium and uranium as reactor fuel. The reduction in waste storage volume that is provided by reprocessing in Thorp is examined and weighed against the potential proliferation risk that may be posed by separating the plutonium. The relatively rapid activity decay in directly disposed irradiated fuel, rendering the plutonium in it progressively more accessible, is compared with the potential to burn separated plutonium in reactors. The recycle of reprocessed plutonium to thermal reactors as MOX fuel is examined, and the suitability of the plutonium product from Thorp for this purpose is described. MOX fuel is currently being produced at Sellafield for use in European and Japanese thermal reactors. The intrinsic merit of such recycle, and the extra energy extracted from nuclear fuel as a result, is discussed. The paper shows that the high quality of the Thorp uranium product, with its freedom from any significant fission product or plutonium contamination, makes it fully suitable for recycle.

WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ

INTRODUCTION British Nuclear Fuels Plc has provided uranium purification and enrichment, new fuel, nuclear material transport and reprocessing services to United Kingdom gas-cooled nuclear reactors for more than 40 years. It has progressively extended these services to European and Japanese Light Water Reactors over the last 20years. The company is based in the UK at sites near Chester (enrichment), near Preston (fuel manufacture), near Warrington (Head Office and transport), at Sellafield in Cumbria (reprocessing and electricity generation), and in South West Scotland (electricity generation). During the 1970’s and 1980’s, BNFL used reprocessing expertise obtained from its Magnox uranium metal reprocessing plants to develop and build the Thermal Oxide Reprocessing Plant (Thorp) in which is reprocessed uranium oxide fuel from Light Water Reactors in Europe and Japan and the UK Advanced Gas-Cooled Reactors. Thorp has an advance order book of some 7000te of fuel from Japanese, European and UK based nuclear power utilities, and has a capacity of 5te of fuel per day. It commenced active operation in 1994 and has so far reprocessed over 2500 te of irradiated fuel. A detailed description of Thorp is given elsewhere (1,2). In parallel with Thorp, a range of waste treatment and clean up plants using ion exchange, floc, vitrification, precipitation and ultra-filtration technologies have been developed, designed and built on the Sellafield site so as to reduce discharges to the environment of radioactive materials and allow for safe long term storage of radioactive waste. The development of these plants, and work on the decommissioning of the older Sellafield facilities, has provided BNFL with extensive experience in the safe management of radioactive materials. It was recognised that this expertise and experience would be of value to nuclear sites in other countries and in 1990 BNFL Inc, a USA-based subsidiary, was set up. BNFL Inc has now won some $2 billion in contracts from the US Department of Energy’s Sites at Hanford, Idaho Falls, Oak Ridge, Rocky Flats and Savannah River together with reactor and diffusion plant decommissioning sites at Charlevoix, Michigan and Tennessee respectively. In addition, at the Hanford site, BNFL Inc and its partners have contracts for the immobilisation of stored liquid waste that potentially will be worth some $6.9 billion over the next 20 years. BNFL Inc has also won contracts with utilities in Canada, and the BNFL Group has contracts with European, Russian, Japanese, and Korean utilities. BNFL’s 1997 integration with the UK Magnox Electric Company allowed it to become a major electricity generator, operating 8 Magnox nuclear power stations around the UK with 3 others being decommissioned. In 1998 these reactors provided some 8% of the UK electricity requirement and saved the emission of around 22 million tonnes of CO2, compared with equivalent coal-fired generation. They also saved the emission of approximately 150,000 tonnes of sulphur dioxide and 45,000 tonnes of nitrogen oxides. In early 1999 BNFL, in partnership with the US-based Morrison Knudsen, completed an agreement to purchase the Westinghouse global nuclear business from the CBS Corporation. In late 1999, BNFL entered into an agreement to purchase the nuclear power businesses of ABB. With these acquisitions, BNFL has become a global nuclear business with extensive and balanced experience and expertise across the whole nuclear fuel cycle for all types of reactor systems.

WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ

REPROCESSING OF NUCLEAR FUEL AT SELLAFIELD History of Reprocessing at Sellafield The Butex Plant The first Sellafield reprocessing plant started work in 1952 using a solvent extraction process with di-butyl carbitol (“Butex”) as the solvent, to separate uranium and plutonium from natural uranium metal fuel contained in aluminium cans irradiated in the original Windscale air-cooled “Piles”. After a period of cooling in the storage pond to allow the more intense radiation emitting species to decay, the aluminium cans were removed mechanically from the fuel, and the irradiated uranium dissolved in nitric acid. This dissolver solution was reprocessed by the Butex solvent extraction process using unpulsed columns packed with raschig rings to aid phase contact. Because of the relatively inefficient mass transfer performance of these columns, they needed to be some 30m in height, and located in a large concrete cell about 60m in height. This process had in initial throughput of about 1te irradiated uranium and about 280g plutonium per day. All process equipment within the concrete cell was constructed from all-welded stainless steel, and no equipment with moving parts was used. Thus the process equipment that was exposed to high radioactivity was expected to need no maintenance throughout the life of the plant, and this expectation has very largely been realised in practice. This no-maintenance philosophy has influenced the design of all subsequent chemical processing plants at the Sellafield Site and contrasts with the “canyon” philosophy of removable equipment developed in the USA. The Butex plant is now shut down and out of use and the Piles are being decommissioned. The Magnox Plant In 1956 the Calder Hall nuclear power station was opened, using “Magnox” natural uranium metal fuel of larger dimensions than fuel for the Butex plant and clad in magnesium-aluminium alloy. This was followed by the construction of a series of Magnox reactors, producing electricity for UK national use. The increased number of Magnox reactors required that a new reprocessing plant with increased capacity should be constructed. Development work at the Hanford Nuclear Site in the USA in the 1950’s had shown that, for the solvent extraction processes, a much better solvent was tri-butyl phosphate (TBP) diluted with kerosene (“odourless” kerosene or OK). TBP/OK is relatively cheap, gives separation factors from fission products well excess of 107 and limits losses of plutonium to waste streams to less than 0.2%. It is also quite stable to radiation, so degradation products are formed relatively slowly. Development and design work for the Magnox Reprocessing Plant commenced in the late 1950’s and it was commissioned and brought into full use by 1964. The design base was for a plant capable of reprocessing 1500te of fuel per year with a burn-up of up to 3,000MWD/te. The plant continues to operate successfully to the present day and fuel up to 7,000MWD/te has been

WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ

processed at throughputs up to 6 te/day. The total amount of fuel now processed in the Magnox Plant is in excess of 40,000te. The plant consists of the following unit processes (Figure 1): • Mechanical decanning of the fuel. This is preferred to chemical dissolution because of the

larger volume of high active waste produced by the latter, which cannot be evaporated to small volume. The original under-water storage of the cut-off fuel can pieces has now been superseded by a cement encapsulation process.

• Fuel dissolution into 8M nitric acid in a continuously operating dissolver, with off-gas treatment to remove volatile fission products and oxides of nitrogen. The rate of feed of the decanned uranium fuel rods is adjusted to produce a continuous output of uranyl nitrate solution at 250gU/l.

• Solvent extraction using 20% TBP/OK. The flowsheet is a 4-cycle “late split” type, with separation of the uranium and plutonium taking place in the second cycle of solvent extraction. There were originally two subsequent purification cycles each for the uranium and plutonium streams. Separation of uranium and plutonium is achieved by chemically reducing the latter using ferrous sulphamate. This is very effective, but does produce a medium active waste stream containing ferric ions that cannot be evaporated to small volume.

• The solvent extraction contactors are mixer settlers (Figure 2a). These were developed by Sellafield R&D Department and feature purely gravity flow from one stage of the unit to the next, making use of the differing densities of the 2 phases. There is no need for interstage pumping. These mixer-settlers have proved to be very stable to operate and easy to control and require much less shielded cell space than the unpulsed columns used in the Butex Plant.

• Uranium trioxide production by a thermal denitration process, and plutonium dioxide production by precipitation with oxalic acid and calcination, complete the process.

U

Pu

LA

MA

HA

U & Pu Products

Original Storage Pond

New Fuel Handling

Plant

Cement Encapsulation of Fuel Cans

Fuel Rod Dissolution and

Solvent Extraction

High Level Wastes Plant

Actinide Removal Plant

(EARP)

Thermal Denitration & Acid Recovery

Plutonium Dioxide

Production

Low Active Effluent Neutralisation

Uranium Trioxide Product

Plutonium Dioxide Product

Vitrification Sea Discharge

Stack Off Gas Treatment

Receipt, Storage & Decanning

Magnox Plant

Figure 1: The Magnox Plant

WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ

The Thermal Oxide Reprocessing Plant (Thorp) It was recognised in the early 1970’s that the requirement for oxide fuel reprocessing from the UK Advanced Gas-Cooled Reactors, and from world Light Water Reactors, could not be met by existing Sellafield facilities. Plans for a purpose-designed oxide fuel reprocessing facility were therefore drawn up and the plant’s future customers agreed to provide much of the funding. Development work for Thorp started in the mid 1970’s, and the start of construction followed in 1985. Construction was completed to time and budget in 1992 (2). Commissioning of the plant commenced in late 1991, and the first active fuel was sheared and dissolved in early 1994. Active operation of the separation processes followed in early 1995, and the plant was granted its full operating licence by the UK Nuclear Installations Inspectorate in mid 1997. Thorp is an integrated plant with all aspects of reprocessing, from receipt and storage of the irradiated fuel through to production and storage of the plutonium and uranium products, in one building. The plant is designed to process 5te fuel per day, with an overall throughput of up to 900te/year. Fuel burn-ups of up to 40GWD/te and minimum out-of-reactor cooling times of 5 years can be handled, with future developments allowing up to 60GWD/te fuel to be processed. At a burn-up of 40GWD/te, the plutonium throughput of Thorp is about 50kg/day. Thorp consists of the following unit processes (Figure 3): • Receipt and Storage where fuel assemblies are received, removed from their storage

containers, monitored to check irradiation levels and fissile content and transferred remotely to the dry shear cave.

Figure 2. Solvent Extraction Contactors (a) Mixer-settler (part bank); (b) Pulsed column

(a) (b)

WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ

• Mechanical Head End where the fuel assemblies are fed incrementally to a shear with a vertically moving blade which chops the fuel into 50 mm lengths.

• Head End Chemical Plant where the chopped fuel pins fall into one of three batch dissolvers for dissolution into hot (100°C), 7.5M nitric acid. Off-gases from this process are treated to remove acid vapour, oxides of nitrogen, carbon-14 and iodine-129.

• Dissolver Solution Clarification where remaining undissolved solids are removed before the dissolver solution is fed to the Head End Accountancy Tanks and then onto a series of buffer storage tanks between Head End and Chemical Separation.

• Solvent extraction using the Purex process and both 20% and 30% TBP/OK. The flowsheet is a 3-cycle “early split” type, with uranium/plutonium separation in the first solvent extraction cycle, followed by single cycles of uranium and plutonium purification. The uranium /plutonium separation is achieved using uranium IV to reduce the plutonium and this “salt-free” reagent means that the waste streams do not contain extraneous ions and can be evaporated to small volume and vitrified.

• Uranium and plutonium finishing processes where thermal denitration and oxalic acid precipitation/calcination are used as in the Magnox Plant.

• Wastes Treatment processes: The Head End processes generate mainly Intermediate Level solid wastes and these are encapsulated in cement for long term storage. The Chemical Plants generate High Level and Intermediate Level liquid wastes that, because of the Salt-Free flowsheet, can both be vitrified. The small amount of Low Level liquid waste that is produced is floc-treated in the Enhanced Actinide Removal Plant (EARP) as necessary to

Conversion to PuO2

Uranium Product

Plutonium Product

Receipt & Storage

Fuel Receipt

Stack

Pond Storage

Off-Gas Treatment

Shearing Dissolution

Solid Wastes�

Centrifuge

Buffer Storage

Vitrification

Solvent Extraction

Process

Conversion to UO3

Head End Chemical Plants

Cement Encapsulation

High Level Wastes Plant

HA

MA Evaporation

EARP & Sea Discharge

MA

LA

Figure 3: Thorp Unit Processes

WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ

remove residual activity before discharge to sea. The floc so formed is also encapsulated in cement.

Thorp Design Principles The design principles for Thorp differ in a number of ways from the Magnox Plant: Head End • Fuel shearing rather than decanning is used because of the range of designs of oxide fuel, and the

multi-pin assemblies used, do not allow the stripping of cladding from individual fuel pins. The vertically acting shear blade together with series of different “gags” or clamps allow the full worldwide range of oxide fuel assembly geometries to be held securely during shearing.

• Batch rather than continuous dissolution of the fuel is employed, using three dissolvers on a rotating basis. This allows for full monitoring and fissile material accountancy of the dissolver liquor before it is fed to the Chemical Plant, and provides for each batch of fuel cladding “hulls” to be monitored for undissolved material prior to encapsulation, and returned for re-dissolution if required.

• Removal of undissolved solids after dissolution is necessary because the higher irradiation of oxide fuel gives rise to a greater amount of these solids than in Magnox fuel. This is done using one of two suspended bowl centrifuges, with the motors and gearboxes situated outside the radiation shielding for ease of maintenance. Recovery of the solids from the centrifuge bowls is accomplished batch-wise by water jet and the resulting slurry sent direct to cement encapsulation.

Chemical Plants • In order to minimise the amount of activity discharged in liquid wastes, all the chemical reagents

fed to the process are “salt-free” so that they do not restrict the ability to concentrate and encapsulate waste streams.

• It was required to minimise the extent, size and complexity of the process equipment in contact with radioactive material, so as to minimise operator radiation dose, reduce capital and operating costs, and reduce the number of waste streams. This was achieved by using the “early-split” flowsheet. Careful flowsheet design, development and proving also allowed single cycles only to be used for the subsequent uranium and plutonium purification stages. This simplification of the flowsheet reduces at source the number of waste streams produced.

• Pulsed, perforated plate columns are used as the solvent extraction contactors in the Highly Active (HA) and Plutonium Purification (PP) cycles (Figure 2b) to allow critically safe operation with the higher plutonium concentrations of oxide fuel. Their relatively short aqueous-solvent contact time also minimises solvent degradation from the increased amounts of fission products in oxide fuel.

General • Head End equipment is designed with no or low maintenance requirements where possible but

provision for remote maintenance is made for the shear and related equipment. All Chemical

WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ

Plant equipment in contact with radioactive material is designed for minimum or no maintenance throughout the life of the plant. In the highly active parts of the Chemical Plant, no moving mechanical parts are used, and fluidic and compressed air devices are employed for liquid pumping and instrumentation.

• Thorp is built on an existing nuclear site alongside the Magnox Plant. A major design target for Thorp was that it should contribute no more than 10% at most to the activity in discharged liquid wastes from the Sellafield site as a whole. In practice this target has been readily achieved, with total alpha and beta activity discharges at 2 to 4%, and 0.3 to 0.6% of the Site Limit respectively

• From the outset Thorp was designed as a civil reprocessing plant with full accountancy and safeguards provisions designed in. These provisions were designed in consultation with Euratom and also, where designation was anticipated, the IAEA.

Thorp Performance Thorp was commissioned and set to work without significant difficulty, reflecting the considerable R&D programme that underpinned its design. Over 2500te of fuel has now been processed. Performance in routine operation has been good, and in the solvent extraction plant it has been excellent. The solvent extraction equipment has operated stably with minimal operator intervention. Decontamination of fission products from the uranium and plutonium products has ranged from the expected flowsheet value up to an order of magnitude better than flowsheet. Decontamination of plutonium from the uranium product and uranium from the plutonium product has also been up to an order of magnitude higher than the minimum flowsheet requirement. Losses of uranium and plutonium to waste streams from the solvent extraction plant amounted to no more than 0.013% and 0.049% respectively of the feed. Even from Thorp as a whole, when losses in undissolved fuel must be taken into account, the figures for uranium and plutonium are only 0.19% and 0.22% respectively. WASTE MANAGEMENT AT SELLAFIELD Classification of Wastes At Sellafield liquid wastes are classified into four groups (Figure 4): • Highly Active: The Highly Active Liquid Waste contains the bulk (>98%) of the fission products

from the reprocessed fuel. This waste has, since the start of reprocessing at Sellafield, been evaporated to small volume (about 200l/te of oxide fuel) and stored in stainless steel, cooled and agitated tanks. Vitrification of this waste is now in progress and this brings about a further 3-4 fold volume reduction.

• Salt-Free Medium Active Wastes: These wastes arise typically from the uranium and plutonium purification cycles and because they are salt-free they are evaporated to small volume, nitric acid recovered from them, and the concentrate sent to the Highly Active waste route.

• Salt-Bearing Medium Active Wastes: These wastes arise in Thorp only from the use of reagents to wash the TBP/OK solvent free of impurities and degradation products so that it can be re-used. In the older reprocessing plants salt-bearing wastes also arose from the U/Pu separation step and so contained more alpha activity. In all cases this waste is now treated by floc precipitation to

WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ

remove residual activity so that the supernate can be discharged as Low Active Waste. The floc is encapsulated into cement for long term storage.

• Low Active Wastes: These are typically evaporator overheads, steam ejector condensates, supernates from precipitation processes, and overheads from acid recovery processes. They are very low in activity and, after sampling and monitoring, are discharged to sea.

Solid Wastes fall into 3 Groups: • Vitrified HA Waste, stored in stainless steel flasks within an engineered, cooled vault. • Medium Active Wastes: These are typically the stripped Magnox fuel cans, the oxide fuel hulls

after dissolution of the fuel, the solids “cake” removed from oxide fuel dissolver product by centrifuge, and the floc product from medium active liquid waste activity removal processes. In the past the Magnox fuel cans were temporarily stored under water in concrete silos. Now all medium active solid wastes are encapsulated in cement, and the backlog of silo-stored wastes is being retrieved and similarly encapsulated.

• Low Level Waste: This is typically laboratory and plant trash that has been in contact with radioactive material. It is compacted, sealed into purpose-made flasks, overpacked using standard containers and stored in purpose-built concrete vaults at a site close to Sellafield.

Waste Management Plants Over the period from the mid 1980’s to the present, a range of waste treatment and storage plants have been designed, constructed, commissioned and set to work on the Sellafield Site. The main examples of these plants are shown in Table 1. These plants allow all wastes generated from current reprocessing to be appropriately treated and are also allowing the backlog of “historic” wastes on the Site to be progressively conditioned for long-term storage.

Figure 4: Liquid and Solid Waste Treatment Processes

Overpacking�

HA Liquid Wastes

Salt-Free MA Liquid Wastes

Salt-Bearing MA/LA

Liquid Wastes

LA Liquid Wastes

MA Solid Wastes

Low Level Solid�Wastes

HA Evaporation�

Vitrification Vitrified HA Waste

Storage

MA Evaporation�

Nitric Acid

Recovery

Conditioning &

Evaporation

Neutralisation� Monitoring�

Sea Discharge Floc

Treatment�

Cement Encapsulation

& Storage

Compaction Vault Storage

Acid Recycle�

WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ

These plants have transformed waste treatment and handling at Sellafield. Over the period from 1973 to the present, alpha active liquid discharges have been reduced by 99.91% and beta active liquid discharges by 98.17% (3).

Table 1: Waste Management Plants at Sellafield Plant Purpose Process Operational

Date Enhanced Actinide Recovery Plant (EARP)

Removal of actinides from MA wastes

Precipitation of an iron floc that adsorbs the activity, followed by ultrafiltration. Cement encapsulation of the floc

1994

Windscale Vitrification Plant (WVP)

Vitrification of high level wastes from reprocessing

Evaporation and calcination of the liquid waste, mixing with borosilicate glass frit, melting and pouring to form vitrified product in stainless steel flasks

Line1&2 1990 Line3 early 2001

Site Ion Exchange Plant (SIXEP)

Clean up of fuel storage pond water

Ion Exchange process using the zeolite clinoptilolite to remove Cs and Sr. Spent ion exchange material is encapsulated in cement

1985

Magnox Encapsulation Plant (MEP)

To encapsulate stripped Magnox cans for long term storage.

Compaction, placement into SS flasks and encapsulation into cement of Magnox cans

1990

Waste Encapsulation Plant (WEP)

To encapsulate solid wastes from Thorp for long term storage

Placement into flasks and cement encapsulation of fuel hulls, centrifuge “cake”, barium carbonate from C14 removal, pond clean up solids and general solid waste

1994

Segregated Effluent Treatment Plant (SETP)

Monitoring and conditioning of low level wastes prior to sea discharge

Solids removal and neutralisation of acidic and alkaline low active wastes.

1994

Waste Packaging and Encapsulation Plant (WPEP)

Conditioning of solid wastes for long term storage

Placement into flasks and cement encapsulation of floc solids from EARP, solids from SETP and general maintenance wastes

1994

Wastes Compaction Plant (WAMAC)

Compaction of low level solid wastes

Mechanical compaction of low level solid wastes (trash), packaging and consignment to concrete vault storage

1996

Solvent Treatment Plant (STP)

Incineration of spent TBP/OK solvent

Alkaline hydrolysis of the solvent, then incineration of the combustible material in a vortex incinerator. Low level liquid product sent to EARP or SETP as appropriate

mid 2000

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Volumes and Activities of Wastes Table 2 presents the volumes and activities of the liquid and solid wastes arising from Thorp reprocessing.

Table 2: Volumes and Activities of Thorp Wastes Waste Stream Final

Conditioned Volume m3/t (U)

Alpha Activity GBq/t (U)

Beta Activity GBq/t (U)

Gamma Activity GBq/t (U)

% of Total Activity

Vitrified high/intermediate liquid waste

0.09 5.3 e 4 6.2 e 6 98.42

Cement encapsulated Head End intermediate level waste

0.55 99 1.0 e 5 1.58

Cement encapsulated floc from treating low active salt-containing wastes

0.06 3.3 172 0.003

Discharged low level liquid waste

-- 0.05 2.43 Nil <4 E -5

It can be seen that in excess of 98.4% of the activity in the waste is concentrated into the very small volume of the vitrified product, which is straightforward and safe to store indefinitely. Nearly all the remaining activity is contained in the cement encapsulate which occupies about 0.6m3 per tonne uranium reprocessed and is also satisfactory for long term storage. The discharged low level waste activity amounts to less than 4 e -5 % of the total activity in the reprocessed fuel. For the calendar years 1997,1998 and 1999, Table 3 shows the main constituents of these activity discharges as a percentage of the Sellafield Site yearly limits, and it can be seen that they form a small fraction of them and are, in general, well within the design target of less than 10%. The cobalt-60 discharges in 1997 and 1998 were over this target; this was due to a particular pond storage issue that was resolved by 1999. REPROCESSING AS AN ALTERNATIVE TO DIRECT DISPOSAL The once-through cycle with the direct disposal of the irradiated nuclear fuel after one pass through the reactor achieves a utilisation of only a small fraction of the energy potential of the mined uranium. One te of spent fuel still contains the energy equivalent of 10,000 te of oil. There is no experience of the construction or long term operation of the repositories that would be necessary for this option, so costs are uncertain. Recycling of the plutonium and unused uranium by reprocessing uses approximately 20% less uranium than once-through, thus considerably reducing the need to mine fresh uranium and thereby minimising the environmental issues

WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ

associated with waste arisings from uranium mining and milling. There is also long term successful operating experience of this option so costs are well proven. These clear advantages of recycling are usually weighed against some further advantages and perceived drawbacks. These issues include: waste handling and environmental impact, potential proliferation risk from separated plutonium, possibilities for partition and transmutation of the waste, use of MOX fuel in thermal reactors, recycle of uranium to thermal reactors. Waste Handling and Environmental Impact It has been shown in the previous Sections of this paper that reprocessing in a modern plant such as Thorp, supported by a range of latest technology waste treatment plants, produces low volumes of waste in forms that are suitable for long term storage, together with only exceedingly small activity discharges to the environment. One tonne of irradiated oxide fuel within a typical 20-element skip has a volume of some 2.5m3. From Table 2 it can be seen that following reprocessing in Thorp, the waste from this fuel will occupy a volume of about 0.7m3. This is a worthwhile volume reduction and furthermore the storage arrangements for this waste are already engineered and in routine successful use. The same cannot be said about the arrangements for direct disposal of spent fuel. Thus, based on current successful technology, reprocessing of spent fuel through a modern reprocessing plant has only minimal environmental impact and is an established technology in reliable routine use.

Table 3: Liquid Effluent Discharges from Thorp Head End and Chemical Plant in 1997 - 1999

Contaminant Activity Discharged as Percentage of Site Limit

1997 1998 1999*

Uranium 3.27 2.10 0.52 Total alpha activity 3.75 2.26 1.66 Total beta activity 0.54 0.59 0.28

Technetium 99 0.08 0.03 0.01 Ruthenium 106 0.35 0.31 0.36 Caesium 134 0.64 0.57 0.35 Caesium 137 1.12 1.25 1.08 Cerium 144 0.81 1.77 0.84

Americium 241 8.77 3.71 1.62 Carbon 14 3.75 1.53 0.66 Cobalt 60 10.8 17.10 5.20

*January to November figures scaled up to full year

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Proliferation Risks It is sometimes claimed that the separation of plutonium from irradiated fuel, and its subsequent use in MOX fuel, increases the risk of plutonium being diverted into clandestine use. There are a number of counter arguments to this including: • Long term storage of one-pass spent fuel does not provide indefinite protection of the

plutonium within it by virtue of the associated highly active fission products. These will decay to low levels within some 100 years and the plutonium is then rendered accessible, and in reasonably concentrated form. This has been recognised internationally, including in the USA (4). In contrast, recycling by reprocessing reduces the plutonium in the ultimate waste by a factor of approximately 100.

• Plutonium fabricated with uranium into Mixed Oxide fuel is kept within industrial control throughout the cycle of fuel fabrication, use within a reactor and reprocessing. The plutonium in MOX fuel is intimately mixed with uranium and extraction of plutonium from it would require the theft of the fuel assemblies in their heavy transport flask, mechanical breakdown and dissolution of the fuel followed by chemical processing.

• As the proportion of MOX fuel in the reactor core is raised above about 30%, there is a net destruction of plutonium in the overall cycle.

• The processing of fissile materials is subject to the most stringent safeguards and security measures, regulated internationally by the International Atomic Energy Agency (IAEA) and the European agency Euratom. Many studies (5) conducted by the IAEA with direct participation of recognised experts from the USA, Euratom and the main nuclear nations, have concluded that large scale reprocessing plants, including the transport of materials to and from them, can be effectively safeguarded through a combination of existing techniques, the choice of which are largely plant specific. The recommendations were widely accepted and implemented in the design and operation of modern reprocessing and MOX fuel fabrication facilities.

• The reactor grade plutonium in irradiated nuclear fuel from civil power reactors has a relatively low fissionable Pu239 content and also contains plutonium isotopes that are neutron absorbers. Although it is possible to make a crude nuclear device using reactor grade plutonium it is not attractive for this purpose and requires particular expertise and skills to do so.

Partition and Transmutation Irrespective of whether irradiated fuel is reprocessed or disposed of directly, the present concept is that the minor actinides (americium, neptunium and curium with very small amounts of heavier elements) are consigned with the bulk of the fission products to high-level waste. Some of these long lived nuclides have half lives of 107 years or more and so will still be present in significant amounts at a time when it may be assumed that containment, and even records of storage, are no longer effective. An alternative is to remove all such long lived nuclides from the waste before it is consigned to storage, so that they can be transformed by fission or neutron absorption into stable or shorter-lived species that would cease to pose any risk within the reasonably assured lifetime of engineered containment. This is Partition and Transmutation (P&T).

WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ

In an accompanying paper (6) it is argued that P&T, though attractive in principle, has significant technical and practical problems which make it unlikely to be realisable within current technical knowledge. Nevertheless it is concluded that a future technical advance may change this view and that research in this field continues to be justified. To realise P&T it is almost certain that aqueous reprocessing technology will be required. If P&T is to be pursued in the future it is therefore necessary to ensure that expertise and knowledge of reprocessing technologies is kept alive and current. Operation of reprocessing facilities such as Thorp ensures that this is achieved. Use of MOX Fuel in Thermal Reactors In the current absence of commercial Fast Reactors, Mixed Oxide Fuel (MOX) for Light Water Reactors, where some 4–7 wt% recycled plutonium is mixed with natural or depleted uranium, allows the energy resources of the plutonium to be utilised. One gram of plutonium, recycled as MOX fuel, will provide the energy equivalent of about 1te of oil or 2 te of coal (7). The plutonium product from Thorp is fully suitable for incorporation into MOX, and some key specification parameters are summarised in Table 4.

Table 4: Quality of Thorp Plutonium Dioxide Product Contaminant Plutonium Dioxide Composition Typical measured

Value BNFL Specification

ISO Specification TC85/SC5WG3 Rev 2, 1996

Uranium microg/gPu 12 < 1000 Report Fission Products Bq/gPu 650 < 3 e 5 Report Non Volatile Oxides

microg/gPu 170 < 5000 < 5000

Already initial amounts of plutonium have been used in Mixed Oxide (MOX) fuel in a demonstration facility at Sellafield, and a full production facility for producing MOX fuel is in an advanced stage of commissioning. Recycle of Uranium to Thermal Reactors The uranium in unirradiated LWR fuel is typically enriched up to 4%. Following irradiation this is reduced to about 1 – 1.6% which still represents a valuable separative work excess in comparison with natural uranium. It is possible to re-use reprocessed uranium as reactor fuel either directly without any re-enrichment or with a re-enrichment step to return it to 3 –4%. The quality of the uranium product form Thorp is summarised in Table 5 which indicates that it easily meets International Specifications for recycle as reactor fuel.

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Because of their neutron efficiency, the Canadian CANDU reactors are particularly suitable for using reprocessed uranium directly as new fuel, and currently Thorp’s uranium product is being considered for this use. In addition studies have been completed to demonstrate the suitability, after re-enrichment, of the Thorp uranium product for producing new fuel for the UK Advanced Gas-Cooled reactors. A pilot trial of such AGR fuel is now in the planning stage. CONCLUSIONS Thorp is a modern reprocessing facility that uses well established technology in an innovative way to minimise the number of processing steps to meet product quality specifications, and to minimise the number of waste streams produced. The plant produces only three types of liquid waste stream and nearly all the radioactivity is concentrated into the small volume vitrified waste material. Activity discharges from Thorp are a small fraction of the overall discharge limits for the Sellafield site. Product qualities are excellent and allow ready recycle into fresh reactor fuel, preserving separative work and allowing considerably more energy to be extracted from the original fuel. Reprocessing is thus shown to be a proven and environmentally responsible method of treating spent irradiated fuel from the World’s nuclear reactors. Reprocessing has the additional advantages of allowing Partition and Transmutation of long lived nuclides, and of facilitating the recycle of uranium and plutonium into new reactor fuel, thus allowing significantly more energy to be extracted from the original uranium. REFERENCES 1. Phillips, C, The Thermal Oxide Reprocessing Plant at Sellafield, Waste Management ‘99,

Tucson, USA, February 27 – March 2, 1999 2. Harrop, G, Phillips, C, Spectrum ’92, Boise, USA, 1992 3. Annual Report on Discharges and Monitoring of the Environment, BNFL, 1998, pp 28 4. Renkes, Gregg D, ANS/ENS International meeting, Washington, November 10-14, 1996 5. Website www.iaea.org/worldatom

Table 5: Quality of Thorp Uranium Trioxide Product

Contaminant Uranium Trioxide Composition Typical Measured Value ASTM Specification for UF6:

C 787-96* Trans-U alpha activity: Pu+Np Bq/gU

4.0 < 25

Non-U gamma activity: Bq/gU

35.0 < 524#

Technetium microg/gU 0.03 < 0.5 * Same as BNFL specification # Derived from ASTM specification of < 1.1 x 105 MeV Bq/kgU on “worst case” basis of all activity being due to Ru-106

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6. Wilson, P.D., Partition and Transmutation as a Waste Management Option: An Appraisal, Waste Management 2000 , Tucson, USA, February 27 to March 2, 2000

7. Website www.nuke-energy.com sponsored by Washington Nuclear Corporation.