Structural modeling of possible contaminant pathways below nuclear installations

Abstract

Dounreay Nuclear Power station is situated in northern Caithness, Scotland on complex normally faulted Devonian sedimentary rocks with a thin, intermittent cover of superficial deposits comprising predominantly glacial tills. The principal aim of this work is to gain an understanding of processes and controls on fluid flow pathways within such complex geological terrains. The boundary between the bedrock and superficial deposits, and fracture networks within the bedrock, can have a considerable impact on the transmissivity of any possible radionuclide particles. An understanding of the bedrock-superficial boundary and the nature of how fractures and faults influence and control the transport of fluids is of key concern. Initial work has involved the interpretation of eight reflection seismic survey lines which were collected for the United Kingdom Atomic Energy Authority (UKAEA) from within the Dounreay power station site. Each seismic survey line was imported into SMT Kingdom software and the initial stratigraphic picks for shallow bedrock strata were produced on seismic survey line UKAEA-HR-8 guided by the gamma ray log of the Nirex 1 borehole. Four main reflectors were identified and corresponding horizons were produced for each of these reflectors. A series of normal faults were identified using the seismic surveys and interpreted surface faults. Fault surfaces and polygons were produced and the data gridded to take into account the presence of these fault surfaces and produce a basic structural model. This model with additional seismic and borehole data will form the framework for the bedrock geological model. The covering superficial deposits will be incorporated into the geological model to produce a unified model to which geological and physical properties of the subsurface may be applied. Surface excavations at the site demonstrate that the boundary between the bedrock and superficial deposits is gradational over a zone comprising regolith at the top of the bedrock, to glacial till with considerable rock fragments at the base of the superficial deposits. Attempts will be made to model a volume to represent this gradational boundary with variable attributes in order to fully incorporate its effects on contaminant pathways. The bedrock-superficial boundary interacts with a number of major faults (that penetrate to rockhead within the site) and a related complex fracture network. This fracture network is exceedingly well constrained in one small area of the site by a dense grid of closely spaced boreholes. It provides an opportunity to develop a well constrained discrete fracture model. This model will be used to determine relationships between fracture trends, larger scale faulting and flow pathways in order to develop and constrain stochastic techniques that can be used to incorporate the effects of fracturing on possible contaminant pathways elsewhere on the site