Geochemistry and physics of plutonium migration

Abstract

Similarities in the distribution pattern of the worldwide Pu-239 fallout and in the filtered industrial release of the same nuclide were demonstrated. This analogy led to the assumption that predictions concerning the subsequent fate of the minute quantities of the HEPA-filtered plutonium released by industrial operations may be drawn by studying the environmental transfer of the worldwide fallout plutonium. The history of Pu-239, 240 concentration in the ground-level air and its deposition known from previous investigations for the vicinity of Heidelberg was brought up to date by new values. After subtracting the wet deposition from the total amount of deposition found in the soil a dry deposition velocity for PuO 2-particles of 0.05 cm/s was obtained under the meteorological conditions of the area studied. Differences were found in the residence time of the worldwide fallout in the troposphera and in the filtered airborne PuO 2 discharge in the ground-level air. Nevertheless, the analogous distribution patterns of the PuO 2 particles and the natural aerosols suggest that in the case of dry deposition velocity, the results obtained can be applied for PuO 2 stack discharge too. The decrease of PuO 2 contamination in the biologically active top soil was estimated by computer simulation. The Pu-239, 240 fallout deposition was used as the input function and the measured Pu-239, 240 depth distribution as the result of the combination of irregular input and continous dissipation. The downward migration found out of a 2-cm-thick layer of Rhine valley loess loam was proceeding at a rate of approximately 50% contamination per 3.5 years. By comparing this migration with the movement of an HTO-tracer a water-transport response as small as 10 −3 was estimated. Unlike the stable PuO 2, hydrolytic changes can be expected for Pu(NO 3) 4 upon contact with an aqueous environment after release. The equilibrium concentration of all valence states of plutonium was calculated for the whole range of natural waters. The sum of all potentially dissolved plutonium oxidation states may reach up to 1 mol/l under extreme but naturally occurring outdoor conditions.