In the absence of pore water movement the dominant transport mechanism for migrating radionuclides in seabed sediments is diffusion Apparent diffusion coefficients measured in sediments are sensitive to chemical (e g, redox) and physical (e g, porosity) conditions. The influence of pressure on diffusion, particularly compared with the effect of temperature, is reported here from laboratory experiments Hitherto the effect of in situ pressure has been overlooked or assumed to be negligible, and that of temperature to be dominant. This study attempts to clarify whether measurements made at atmospheric pressures are applicable to modeling radionuclide migration from radioactive waste canisters emplaced beneath the seabed Three isotopes, technetium (Tc-95m), neptunium (Np-237) and plutonium (Pu-238), and two experimental configurations were used a column method involving generation of a diffusion profile, and a half-cell technique involving measurement of the total activity transferred from a spiked to an unspiked side. Diffusion coefficients obtained suggest the absence of a pressure effect, and confirm temperature as the dominant control Arrhenius plots indicate a stronger temperature dependence for Np compared with Tc, apparently related to differences of sorptlon affinity.


Great Meteor East (GME) in the NE Atlantic is a study area for subseabed burial of high-level radioactive waste (Franas, 1984 Schuttenhelm et al., 1988). This contribution to feasibility assessment of the subseabed option provides measurements of radionuclide diffusion coefficients in GME sediments Experimental (apparent) diffusion coefficients are relevant in modelling radionuclide migration from emplaced waste canisters, applicable to estimating potential radiological impact. Ion mobilities in pore waters are sensitive to chemical and physical controls (e g, Hone et al, 1969, Manheim, 1970, Berner, 1980, Colley et al, 1984, Seyfried and Thornton, 1985, Colley and Thomson, 1985, Higgo et al, 1987, 1988a, b, Nakashima et at, 1988) This chapter reports the effect of hydrostatic (Fig. 1 is available in full paper) pressure on diffusion, particularly with respect to temperature, and aims to clarify whether diffusion coefficient measurements made at atmospheric pressure are valid Hitherto, the effect of pressure has been overlooked or assumed to be negligible, and that of temperature to be dominant.


Apparent diffusion coefficients, Da, can be derived from distribution coefficients, Kd (measured empirically as distribution ratios, Rd) obtained from batch sorption experiments (Higgo et al, 1987, 1988a) However, Rd measurements are susceptible to experimental conditions and are not necessarily lnterchangable with K, Consequently, some techniques for direct empirical measurement of Da in marine sediments have been developed (Higgo et al, 1987 1988a) These were adapted from previously established methods (e g, Schofield and Graham-Bryce, 1960, Brown et al 1964, Li and Gregory, 1974, Schreiner et al, 1982, Eriksen and Jacobsson, 1984.

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