THE THERMAL OXIDE REPROCESSING PLANT AT SELLAFIELD: Four Years of Successful Treatment of Irradiated Nuclear Fuel

The Thermal Oxide Reprocessing Plant at British Nuclear Fuels’ Sellafield site is designed to treat irradiated fuel from UK Advanced Gas-Cooled Reactors and European and Japanese Light Water Reactors, so as to separate recyclable uranium and plutonium from the 3-4% fission product waste. The plant started operating in March 1994 with the shearing and dissolution of its first irradiated fuel in the Head End Plant. In January 1995 the Chemical Separation Plant commenced processing this dissolved irradiated fuel feed solution and by late 1998 over 1800t of irradiated fuel had been reprocessed. The plant is now being steadily and successfully ramped up to its normal operating throughput. Following the completion of the baseload fuel Thorp will be able to reprocess fuel from worldwide nuclear reactors The performance of Thorp has been excellent, with both the mechanical Head End and the three-cycle Purex solvent extraction process and equipment operating better than predicted. In particular the uranium-plutonium separation stage, which received intensive development to deal with the effects of the fission product technetium, has given an overall separation performance well in excess of the minimum flowsheet requirement. Decontamination of the uranium and plutonium products from fission products has in general been better than flowsheet requirements and the solvent extraction equipment has operated stably under the automatic controls developed during the R&D programme. The uranium and plutonium products from Thorp have routinely met international specifications for the recycle of these materials as new nuclear fuel. Losses of uranium and plutonium into waste streams have been in line with expectation and each amount to about 0.2% of the feed at most. The “salt free” flowsheet design for Thorp allows all Head End and nearly all Chemical Plants Wastes to be vitrified or encapsulated in cement, and the operation of these processes with the Thorp liquid wastes has been shown to be both effective and efficient. The small amounts of liquid wastes that are discharged off site contain at most no more than 4% of the Sellafield Annual Discharge Limit and typically much less than 1% of this limit. The paper compares with flowsheet predictions certain key fission product and transuranic decontamination factors achieved in Thorp, shows how waste stream discharges are a small fraction of Sellafield Site discharge limits, demonstrates how uranium-plutonium separation performance has compared with expectation and summarises the overall performance of Thorp. The paper concludes that Thorp has met or exceeded its design expectations, and is a successful and environmentally responsible way of treating irradiated nuclear fuel.