AERIAL AND LIQUID EFFLUENT TREATMENT IN BNFL'S THERMAL OXIDE REPROCESSING PLANT (THORP)

摘要

British Nuclear Fuels plc (BNFL) completed construction of its Thermal Oxide Reprocessing Plant (THORP) at Sellafield in 1992, at a cost of £1,850M. After Government and Regulatory approval, active commissioning was initiated in January 1994. Since then, the whole of the plant has been progressively commissioned and moved towards full operational status.rnFrom the outset, the need to protect the workforce, the public and the environment in general from the plant's discharges was clearly recognised. The design intent was to limit radiation exposure of members of the general public to " As Low as Reasonably Practicable" (ALARP). Furthermore no member of the most highly exposed (critical) group should receive an annual dose exceeding 50 microsieverts from either the aerial or marine discharge routes.rnThis paper describes how the design intent has been met, concentrating mainly on aerial discharges. It describes the sub-division of the plant's ventilation system into a number of separate systems, according to the volume and source of the arising and the complexity of the treatment process. The dissolver off-gas, central off-gas, cell and building ventilation systems are described, together with the development programme which was undertaken to address the more demanding aspects of the performance specification. This ranged from small-scale experiments with irradiated fuel to inactive pilot plant trials and full-scale plant measurements. In addition wind tunnel tests were employed to assist dispersion modelling of the gases as they are discharged from the THORP stack. All the resulting information was then used, with the aid of mathematical models, in the design of an off-gas treatment system which could achieve the overall goal.rnAs far as liquid discharges are concerned, the paper describes the overall philosophy behind the THORP flowsheets which allows the environmental impact target to be met. The treatment of some waste streams is undertaken in dedicated equipment within the THORP building. Other streams are mixed with effluents from elsewhere on the site and treated in existing site facilities. After treatment and sentencing the effluents containing low levels of radioactivity are discharged down a 3 km pipeline into the Irish Sea.rnTaken together, the aerial and liquid discharges from THORP are predicted to fall well within the original design intent of 50μSv. Aerial discharges are predicted to give a maximum critical group dose of 22μSv and marine discharges will give a dose of 36μSv to a separate critical group. Moreover the discharges are also predicted to fall comfortably within the limits on individual radionuclides which have been imposed as part of the Government's Aerial and Liquid Discharge Authorisations.rnThe detailed attention given to the development and design phases of the effluent treatment system should ensure that these predictions are mirrored by actual performance as the plant becomes fully operational.