As an integral part of the investigations carried out at Sellafield, and to a lesser extent for the Dounreay boreholes, the cores from the boreholes were systematically examined by geologists and samples selected for detailed petrographic analysis. Thin sections were prepared from many of these samples. Some of the samples were then prepared for analysis using X-ray fluorescence, X-ray diffraction or scanning electron microscopy. Samples containing fluid inclusions were prepared as doubly polished fluid inclusion wafers. Thin sections were prepared from soil materials obtained from the investigations of the Quaternary deposits. Particular attention was given to samples of the mineralisation that was identified in the formations at Sellafield. In addition to the samples from the boreholes, there are also some samples obtained from surface exposures that were examined during the regional surveys. The samples and sections have been catalogued and incorporated into the national collection by the British Geological Survey. The ownership of NIREX (Nuclear Industry Radioactive Waste Executive) was transferred from the nuclear industry to the UK Government departments DEFRA and DTI in April 2005, and then to the UK's Nuclear Decommissioning Authority (NDA) in November 2006.

During the period from 1989 to 1997, Nirex undertook extensive geological investigations at sites near Sellafield, in Cumbria, and Dounreay, in Caithness, to examine whether or not they were suitable locations for a deep repository for radioactive waste. At Sellafield, thirty deep boreholes, and a further thirty-five shallow boreholes, were drilled. The majority of the deep boreholes were drilled to obtain continuous core and some 18.7km of rock core arose from the investigations. Two deep boreholes were drilled at Dounreay, totalling 2,286 metres of drilling. Again, much of the drilling was to obtain continuous core. In most of the boreholes a suite of data acquisition techniques were used, including: continuous coring, geophysical wireline logging, hydraulic testing, sampling and analysis of groundwaters. Mineralogical data were acquired by detailed core observation, petrographic analysis, fluid inclusion analysis and stable and radiogenic isotope studies. Testing on samples from the rock cores was undertaken to determine petrological, mineralogical, hydrogeological, geophysical and geotechnical parameters. The ownership of NIREX (Nuclear Industry Radioactive Waste Executive) was transferred from the nuclear industry to the UK Government departments DEFRA and DTI in April 2005, and then to the UK's Nuclear Decommissioning Authority (NDA) in November 2006.