Coast erosion threat to a nuclear waste shaft at Dounreay, Scotland

The risk from coast erosion to a shaft containing nuclear waste at Dounreay, Caithness is assessed. The physical setting of the shaft, data on the rates of erosion in the Devonian Flagstones and the drift capping of the local cliffs and future influences on erosion rates and flooding are summarized.

A two-layer model of superficials and flagstones is used to address the immediate concern of lateral erosion towards the shaft. Superficials' erosion is assessed from a comparison of successive cliff surveys. Flagstones' erosion proceeds chiefly by the deepening of slots in the weaker interbeds, with associated cantilever failures and block removal through wave action. The rate of slot development is estimated from the growth rate of tafoni within them. Whether erosion proceeds by superficials' or flagstones' erosion alone or in combination depends on the level of rockhead in the cliffs in relation to tide and wave levels. These threshold rockhead levels vary with the morphology and degree of exposure of the cliffs. They will also rise with time relative to Ordnance Datum in response to the general rise in sea level.

Estimates of the periods remaining before the shaft is exposed by coast erosion are made through the above model, both planimetrically and sectionally. They range from about 160 to over 240 years. Flooding by the sea is likely to commence about 400 years hence. The UKAEA plans to remove the nuclear waste from the shaft within the next two decades, well before the above factors would have an impact.