Radiological exposure from shore sediments containing thorium 229

Exposure from thorium-229 entering the aquatic recipient from a hypothetical fuel repository for high-level radioactive waste and contaminating the shore has been investigated. The aim of the present study is to carry out realistic calculations of the external and internal exposure of thorium-229, considering both daughter products and β-radiation. The external dose has been assessed under the assumptions that: - thorium-229 in the shore sediment is in steady state with its daughter products - the activity is homogenously distribute in the shore sediment - radiation absorption occurs in air. The calculations have been performed for the heights 1.0 and 0.1 m above the ground. To determine the horizontal and vertical distribution naturally occurring thorium in shore sediments, field mesurements were performed. A homogenous distribution of thorium can be considered relevant for the further dose calculations. The external α-dose rate from 1 μCi of thorium deposited on the shore was calculated to 5 x 10-6 rem h-1. The contribution from β-radiation will give a dose commitment of the same order of magnitude. As skin dose the β-dose rate is equivalent to 1 x 10-4 rem h-1. The dose commitment to the skeleton for an intake of 1 μCi of thorium-229 is 90 rem. (Author)