The Thermal Oxide Reprocessing Plant, or THORP, is a nuclear fuel reprocessing
plant at Sellafield in Cumbria, England. THORP is owned by the Nuclear
Decommissioning Authority and operated by Sellafield Ltd. Spent nuclear fuel
from nuclear reactors is reprocessed to separate the 96% uranium and the 1%
plutonium, which can be reused in mixed oxide fuel, from the 3% radioactive
wastes, which are treated and stored at the plant. The uranium is then made
available for customers to be manufactured into new fuel.
THORP is due to close in 2018 once all existing reprocessing contracts have
been fulfilled. History
Construction of THORP started in the 1970s, and was completed in 1994. The
plant went into operation in August 1997.
Between 1977 and 1978 an inquiry was held into an application by British
Nuclear Fuels plc for outline planning permission to build a new plant to
reprocess irradiated oxide nuclear fuel from both UK and foreign reactors. The
inquiry was to answer three questions: "1. Should oxide fuel from United
Kingdom reactors be reprocessed in this country at all; whether at Windscale or
elsewhere? 2. If yes, should such reprocessing be carried on at Windscale?
3. If yes, should the reprocessing plant be about double the estimated site
required to handle United Kingdom oxide fuels and be used as to the spare
capacity, for reprocessing foreign fuels?". The result of the inquiry was
that the new plant, the Thermal Oxide Reprocessing Plant, was given the
go-ahead in 1978, although it was not completed until the 1990s at a cost of
£1.8 billion. In 1998/99, the plant faced severe
economic difficulties when it failed to reach its reprocessing targets.
Shut-downs for six months in the first half of 1998 and for several further
months from December 1998, due to leakages, resulted in a failure to
achieve the target of reprocessing 900 tonnes of fuel over that period. Most of
the reprocessing contracts were with Germany and Japan.
Design features The chemical flowsheet for THORP is
designed to add less non-volatile matter to the first cycle PUREX raffinate, one
way in which this is done is by avoiding the use of ferrous compounds as
plutonium reducing agents. In this plant the reduction is done using either
hydrazine or HAN. The plant releases gaseous emissions of Krypton-85, a
radioactive Beta-emitter with a half-life of 10.7 years. The
Radiological Protection Institute of Ireland commenced 24 hour atmospheric
monitoring for Krypton-85 in 1993, prior to the plant's commissioning.
The cooled oxide fuel is chopped up in the Shear Cell and the fuel dissolved in
Nitric Acid. It is chemically conditioned before passing to the
chemical separation plant. Pulsed columns are used to initially separate
the majority of the Uranium and Plutonium from the fission products by
transferring them into the solvent phase, which comprises Tri-butyl
phosphate in odourless kerosene. 2 further pulsed columns and a
mixer/settler assembly then separate the Uranium and Plutonium into separate
streams. Pulsed columns then purify the
Plutonium, removing the troublesome fission products that remain. Uranium
purification is achieved using mixer settlers similar to those in use on the
existing Magnox reprocessing plant. Evaporation of the 2 product streams
then occurs before further processing is undertaken.
Pulsed columns were chosen to avoid the risk of a criticality incident occurring
within the plant. This can happen if sufficient fissile material comes
together to start an uncontrolled chain reaction, producing a large release of
neutrons. The risks and mechanisms are well understood and the plant design is
arranged to prevent its occurrence, i.e. intrinsically safe.
2005 leak On 9 May 2005 it was announced that
THORP suffered a large leak of a highly radioactive solution, which first
started in July 2004. British Nuclear Group's board of inquiry determined that
a design error led to the leak, while a complacent culture at the plant delayed
detection for nine months. Operations staff did not discover the leak until
safeguards staff reported major fluid accountancy discrepancies.
Altogether 83 cubic metres of nitric acid solution leaked from a small
fractured feedpipe, which was discovered when a remote camera was sent in to
examine THORP's Feed Clarification Cell on 19 April 2005. All the fluids
collected under gravity into the secondary containment, which is a
stainless steel tub embedded in 2 metre thick reinforced concrete, capable of
holding 250 cubic metres of fluids. The solution from the spill was
estimated to contain 20 metric tons of uranium and 160 kilograms of plutonium.
The leaked solution was safely recovered into primary containment using
originally installed steam ejectors. Radiation levels in the cell preclude
entry of humans and robotic repair of the fractured pipe is expected to be
difficult. Officials are considering bypassing the faulty tank to resume
operations. The pipe fractured due to lateral motion
of an accountancy tank, which measures volume by weight and moves horizontally
and vertically in the process. The tank's original design had restraint
blocks to prevent lateral motion, but these were later removed from the design
for seismic uncoupling. However it appears this design change was not
evaluated for fatigue, and it is inconceivable a proper review would have
permitted this change. The incident was classified as Level 3
out of 7 on the International Nuclear Event Scale, a "serious incident", due
to the amount of radioactive inventory that leaked from primary to secondary
containment without discovery over a number of months. This was initially
considered by BNFL to be surprisingly high, but the specifications of the
scale required it. No radioactive material leaked to the
environment and no one was injured. Return to service will be dependent on
appropriate modification proposals and any further regulatory requirements
resulting from the ongoing investigations. As of March 2006 various
restart and closure options were still being considered.
The British Nuclear Group was convicted for breaches of health and safety
regulations following the accident, and fined £500,000.
Production eventually restarted at the plant in early 2008 but almost
immediately had to be put on hold again, for the repair of an underwater lift
that moves fuel for reprocessing. See also
Nuclear fuel cycle Reprocessed uranium
Red oil PUREX
Pyrometalurgical Processing Other reprocessing sites
COGEMA La Hague site Rokkasho Reprocessing Plant
West Valley Reprocessing Plant Mayak
Tōkai, Ibaraki References
External links Nuclear Decommissioning Authority
Sellafield Limited Bellona Foundation - Excellent Site for
Detailed Info on Sellafield and associated environmental issues.
Nuclear unit closed after checks - BBC, 23 April 2005
Huge radioactive leak closes Thorp nuclear plant - The Guardian, 9 May 2005
THORP Fractured Pipe – Board of Inquiry Report, BNFL, 26 May 2005
Thorp officials work to restore UK nuke reprocessing facility - Bellona
Foundation, 7 July 2005 Thorp board of enquiry report released -
Nuclear Engineering International, 27 July 2005
Nuclear staff suspended over leak - BBC, 4 August 2005
Workload forces delay over Thorp reopening - News & Star, 11 November
2005 Leak of Radioactive Liquor in the Feed
Clarification Cell at BNG THORP Sellafield, Review of the Management and
Technical Aspects of the Failure and its Implications for the Future of THORP,
John Large, 13 April 2006 - includes diagrams
Thorp restart approved - Nuclear Engineering International, 17 January
2007 Culture clubbed - Nuclear Engineering
International, 25 April 2007 HSE report into the THORP leak
The sentencing of British Nuclear Group over the accident at THORP, Mr Justice
Openshaw, 16 October 2006