Diffusion experiments of pertechnetate anion (95mTc0, w4-) were conducted at room temperature on some deep sea sediments and bentonite, in order to evaluate diffusion rates of mobile hazardous long-lived radionuclides in seabed sediments.

Measured apparent diffusion coefficients in both deep sea sediments (6–8 x 1010 M2/s) and bentonite (1–3 x 1010m2/s) have linear correlations on logarithmic scales with water contents and porosities of sediments These correlations are useful to estimate diffusion coefficients of mobile radionuclides in sediments

Retardation of pertechnetate was observed during the diffusion experiments on only one calcareous deep sea sediment sample containing black spots rich in pyrite Local accumulation of pyrite replacing diatoms can be an indicator of a microreducing-environment in deep sea sediments at least at the time of pyrite deposition.

Technetium fixation at these black spots, even after air oxidation of sediments and even under oxic experimental conditions, suggests the reduction of Tc oxides by some organic reducing agents still active The importance of these organic reducing agents in the radionuclide immobilization should be noted.

INTRODUCTION

An estimate of the radionuclide migration rate in deep sea sediments is an important basis for the safety evaluation of the seabed disposal of high-level radioactive wastes (Hollister et al, 1981) Technetium is considered to be one of the most mobile hazardous long-lived radionuclides in high-level waste. It migrates as pertechnetate anion (TcO4-) in groundwater without much sorption under oxic conditions (Allard et al, 1979), and is often called a ‘non-sorbing’ radionuclide (Skagius and Neretnieks, 1986)

When the pore water moves very slowly through the sediment, diffusion of radionuclides is the major transport process (Berner, 1980). Diffusivity of ‘non-sorbing’ ions such as pertechnetate anion in porous media has been considered to be related to the porosity of the media, especially for crystalline rocks (Skagius and Neretnieks, 1986) However, the relation between the diffusivity of ions and the porosity of the media has not yet been well established. In the present study, we try to determine this relationship for highly porous media

Several series of measurements of diffusion coefficients of the pertechnetate anion were made on selected samples of ocean floor sediments taken during the OECD/NEA's ESOPE expedition, together with a Na-bentonite sample (commercial clay) for comparison.

The retardation of radionuclide migration in deep sea sediments is expected for certain radionuclides because of the anoxic environment of sediments Technetium migrating as pertechnetate anion (TcO4-) is believed to precipitate as oxides of lower oxidation states such as TcO2-, under anoxic conditions (Allard et al, 1979) This pertechnetate reduction appears to be the man retardation mechanism of technetium migration (Bondietti and Francis, 1979).

Ferrous iron containing minerals and organic matter are the possible reducing agents in rocks and sediments (Allard et al, 1979, Nakashima and Nakamura, 1987) Although addition of metal iron to various sediments caused the retardation of technetium diffusion (Torstenfelt et al, 1982), this retardation has not yet been confirmed on natural sediment systems even under artificial anoxic conditions (Schreiner et al, 1982, Higgo, 1986) The geochemical importance of redox processes in the deep sea sediments at the GME study area has already been recognized especially for redox sensit

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