Laboratory scale advection-matrix diffusion experiment in Olkiluoto veined gneiss using H-3 and Cl-36 as tracers
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Laboratory scale advection-matrix diffusion experiment in Olkiluoto veined gneiss using H-3 and Cl-36 as tracers Mikko Voutilainen1, Pekka Kekäläinen1, Jukka Kuva1,2, Marja Siitari-Kauppi1, Maarit YliKaila3and Lasse Koskinen3 1
Department of Chemistry, University of Helsinki, 00014, Helsinki, Finland. Department of Physics, University of Jyväskylä, 40014, Jyväskylä, Finland. 3 Posiva oy, Olkiluoto, 27160, Eurajoki, Finland. 2
ABSTRACT Spent nuclear fuel from Finnish power plants is planned to be deposited deep in the crystalline bedrock. The bedrock needs to be well characterized to properly assess the risks inherent to the long term safety of the site. In the bedrock the possibly released radionuclides are assumed to be mainly transported by water conducting fractures and their transport is retarded by matrix diffusion and sorption. In this work transport properties of an 80 centimeters long veined gneiss drill core sample from Olkiluoto was studied using an advection-matrix diffusion experiment, which was developed to demonstrate the effect of rock matrix for transport of radionuclides in an advective fracture. The experiment was performed using H-3 and Cl-36 as tracers, and effective diffusion coefficients (De) of (1.7 ± 0.7) × 10-13 m2/s and (1.4 ± 1.0) × 10-14 m2/s and porosities of 1.1 ± 0.3 % and 0.23 ± 0.10 % were determined, respectively. A lower porosity and De for Cl-36 than for H-3 indicates an effect of anion exclusion and the results were found to be in agreement with previous laboratory experiments. However, the comparison to results from a similar in-situ experiment showed that the transport of H-3 and Cl-36 is retarded more in laboratory than in in-situ conditions by matrix diffusion. INTRODUCTION Solute exchange between flowing fracture groundwater and matrix pore water of the rock is important for the safety evaluation of nuclear waste repositories. Transport of radionuclides in water conducting fractures is retarded by matrix diffusion and sorption. A project called “rock matrix REtention PROperties” (REPRO) at ONKALO, the underground rock characterization facility in Olkiluoto, Finland, consists of an extensive series of in-situ sorption and diffusion experiments that are supplemented by laboratory work aiming to estimate the relevance of the retention phenomena in the transport of radionuclides [1]. So far only a few similar studies aimed at providing data on radionuclide retention in in-situ conditions have been performed in crystalline rock formations [2-4]. In this work a measurement setup for studying the retarding effect of matrix diffusion for radionuclides in a well-controlled fracture flow was constructed. In this advection-matrix diffusion experiment a short concentrated pulse of H-3 and Cl-36 was injected into a water flow through an artificial fracture. The artificial fracture was formed on the circumference of a 0.8 m long plastic tube by placing a drillcore sample into center of the tube. The sample was taken from a drillhole in which similar experiments were performed in in-situ
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