Understanding Radionuclide Migration From the D1225 Shaft, Dounreay, Caithness, UK
- PDF / 2,528,433 Bytes
- 8 Pages / 595 x 841 pts Page_size
- 4 Downloads / 156 Views
Understanding Radionuclide Migration From the D1225 Shaft, Dounreay, Caithness, UK David Savage1, Claire Watson1, James Wilson1, Alex Bond1, Warren Jones2, Richard Metcalfe1, Tony Milodowski3, Colin Munro2, James Penfold1 and Sarah Watson1 1 Quintessa Limited, The Hub, 14 Station Road, Henley-on-Thames, RG9 1AY, UK 2 Dounreay Site Restoration Limited, Dounreay, Thurso, Caithness, KW14 7TZ, UK 3 British Geological Survey, Keyworth, Nottinghamshire, NG12 5GG, UK
ABSTRACT A 65 m vertical shaft was sunk at Dounreay in the 1950s to build a tunnel for the offshore discharge of radioactive effluent from the various nuclear facilities then under construction. In 1959, the Shaft was licensed as a disposal facility for radioactive wastes and was routinely used for the disposal of ILW until 1970. Despite the operation of a hydraulic containment scheme, some radioactivity is known to have leaked into the surrounding rocks. Detailed logging, together with mineralogical and radiochemical analysis of drillcore has revealed four distinct bedding-parallel zones of contamination. The data show that Sr-90 dominates the bulk beta/gamma contamination signal, whereas Cs-137 and Pu-248/249 are found only to be weakly mobile, leading to very low activities and distinct clustering around the Shaft. The data also suggest that all uranium seen in the geosphere is natural in origin. At the smaller scale, contamination adjacent to fracture surfaces is present within a zone of enhanced porosity created by the dissolution of carbonate cements from the Caithness flagstones during long-term rockwater interactions. Quantitative modelling of radionuclide migration, using the multiphysics computer code QPAC shows the importance of different sorption mechanisms and different mineralogical substrates in the Caithnesss flagstones in controlling radionuclide migration.
INTRODUCTION A 65 m vertical shaft was sunk at Dounreay in the 1950s to build a tunnel for the offshore discharge of radioactive effluent (‘LEDT’) from the various nuclear facilities then under construction. In 1959, the Shaft was licensed as a disposal facility for radioactive wastes and was routinely used for the disposal of ILW until 1970. All consignments to the Shaft ceased in 1977 following an explosion in the head-space above the waste column. In 1998, it was decided to retrieve the waste for treatment and surface storage. The first phase of decommissioning is hydraulic isolation, creating a containment barrier between the waste and the groundwater that flows through the surrounding rock. This has involved grouting 400 boreholes in a 10 m wide band of rock around the Shaft. Despite the operation of a hydraulic containment scheme, some radioactivity is known to have leaked into the surrounding rocks. The Shaft is unlined so that wastes are currently in direct contact with shallow-dipping micaceous siltstones and sandstones (flagstones) of the Middle Devonian Dounreay Shore Formation. Logging of the macroscopic mineralogical features, and detailed mineralogical analysis of the drillc
Data Loading...
