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 (which is the site licensee company). 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.
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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.
The chemical flowsheet for THORP is designed to add less involatile 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 (hydroxylamine nitrate).
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 (18,250 imperial gallons) 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 (INES), a "serious incident", due to the amount of radioactive inventory that leaked from primary to secondary containment without discovery over a number of months.[1] This was initially considered by BNFL to be surprisingly high, but the specifications of the scale required it.
No radiation 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.[2]
The British Nuclear Group was convicted for breaches of health and safety regulations following the accident, and fined £500,000.[3]
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.[4]