The interaction of uranium(VI) with silicic acid was investigated by solvent extraction of 232 U-tracer amounts with thenoyltrifluoroacetone (ΗΤΤΑ). Experiments were carried out with silicic acid concentrations of 0.01, 0.03 and 0.067 M in 0.2 M NaC10 4 at pH 3.3-4.5. The fraction of polymeric silicic acid in these solutions was studied spectrophotometrically as a function of pH and ageing time by kinetic investigations of the ammoniumheptamolybdate reaction. An equilibrium constant of Κ, = (6.2 ±0.3) · IO" 3 [log Κ, = -(2.21 ±0.02)] was obtained for the reaction:
U02 2+ + Si(0H)4 -U020Si(0H)3 + + H + . (1)For 0.067 M silicic acid solutions, which contained significant amounts of polymeric silicic acid, a second equilibrium constant of K 2 * = (1.4 ±0.2) · 10" 6 [log K 2 * = -(5.85 ±0.06)] was found for the formation of a neutral uranium(VI) complex according to :U02 2+ + (=SiOH)j -(=SiOH)j_2(=Si0)2U02 + 2 H + .(2)
Alpha-spectrometric Pu determinations in aerosol samples collected after the Chernobyl accident in Austria show a 238pu/239 (24~ ratio in the range of 0.33 0.76 with a most reliable value of 0.47. From 241Am measurements in old Pu preparations and air filter analyses also 241pu activities have been calculated. The ratio 241pu/239 (24~ during the main contamination period is 74.6- [-5.7, when maximum 241pu concentrations in air achieved 4.5~.0 mBq/m 3.
241Am and 238,239 (24o)p u concentrations have been determined in soil samples from the Atolls of Mururoa and Fagataufa as well as from Palomares for comparison. Mean activity ratios of 241Am/239 (24~ for Mururoa, Fangataufa and Palomares with 0.017+0.005, 0.050!-_0.012 and 0.25+0.07 respectively, show, that also in the most unfavorable case the 24LAm contamination remains less than about one third of the 239 (240)pu level. The estimated isotopic composition of the different types of weapons-grade Pu based on these data suggests generally a much lower 241pu/239(24~ ratio for Pu used in the first fisson bombs than for the material taken later on for construction of thermonuclear devices. 241pu/239(24~ activity ratios of 0.9-+0.2 have been found at the safety shot area of the Colette motu and 8.4+0.8 at Palomares, with some hints to a lower one, viz, 5.7+1.5 for at least one of the accidentally involved bombs.
Mean annual sedimentation rates over the last 20-30 years were determined in the pre-alpine Mondsee (Upper Austria) using Cs-137 and Pb(Po)-210 profiles for sediment core dating and two natural sediment markers . Lower sedimentation rates of about 2-3 mm yr -1 were observed in the central part of the lake near the shore at 18-20 m and in the southern part at 30 m depth . Higher sedimentation rates of 4-7 mm yr -1 were found in the central part of the lake at 47 and 65 m and in the northern bay at 18 and 41 m depth . At both these sites the Pb-210 profiles were strongly disturbed in the upper zone of the sediment cores, whereas the Cs-137 pattern remained intact. The higher annual sediment accumulation rates can be explained only partly by deposition of allochthonous material discharged by the streams, enhanced eutrophication in these parts of the lake, erosion and sediment focusing by turbidity currents being also probable .
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