Radiogeological Assessment of Candidate Sites for Nuclear Waste Repositories, Exemplified by Studies of the Stripa Pluto
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RADIOGEOLOGICAL ASSESSMENT OF CANDIDATE SITES FOR NUCLEAR WASTE REPOSITORIES, EXEMPLIFIED BY STUDIES OF THE STRIPA PLUTON, SWEDEN
H. A. WOLLENBERG, S. FLEXSER AND L. ANDERSSON Lawrence Berkeley Laboratory, University of California, Berkeley,
CA
94720
ABSTRACT Investigation of candidate sites for nuclear waste isolation will require an assessment of their radiogeologic settings. Studies at the Stripa research facility in granitic rock of central Sweden incorporated the distribution and abundance of naturally occurring radioelements in rocks encompassing the underground experiments and in the accompanying fracture-controlled groundwater system. These studies showed that besides defining the natural radioactivity baseline upon which the effects of radioactive waste will be superimposed, radioelement distributions can be used to determine the apparent age of the groundwater and its flow paths. In crystalline rocks, where the groundwater systems are confined to the joints and fractures, the uranium daughter element, radon-222 in the water serves as a natural tracer to locate fractures along which significant flow is occurring and to measure the flow rates. The heat production from radioactive decay of uranium-238, thorium-232 and potassium-40 in the rock, combined with measurements of regional and local geothermal heat flow, permit calculation of the apparent size of the rock mass that will encompass the repository. This method is especially useful in terranes such as at Stripa where the contacts between plutons and older rocks are concealed.
INTRODUCTION To properly characterize candidate sites for radioactive waste isolation, it will be necessary to obtain a good understanding of their radiogeologic set2tings. 38 2 3 2The distribution and abundance of the naturally-occurring radioelements, U, Th, their daughters and 40K in the rock mass encompassing the repository and in neighboring rocks, comprise the baseline upon which the effects of the radioactive waste are superimposed. The distribution of these radioelements is also a good indicator of the geochemical homogeneity of the rock mass. At the Stripa experimental facility in an inactive iron mine in central Sweden (1), radiogeologic studies included gamma-spectrometric surveys on the surface and underground of the U, Th and K contents of the quartz monzonite pluton encompassing the experiments, high-grade metamorphic rocks surrounding the pluton, and neighboring larger granitic plutons (2). A geological cross section through the experimental workings comprises Figure 1. DISTRIBUTION OF RADIOELEMENTS The abundance of the radioelements, K, U and Th, was measured, both on the surface and underground by a portable gamma-ray spectrometer to obtain a preliminary indication of the geochemical homogeneity of the Stripa pluton and to calculate its radiogenic heat production. Gamma readings were made, at both
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