Fission products oxidation-reduction potentials

Table 9.7 Formal oxidation-reduction potentials for actinides and fission products in acid solutions ...

For the rapid determination of Tc in a mixture of uranium fission products. Love and Greendale have used the method of amalgam polarography. It consists in a selective reduction of technetium at a dropping mercury electrode at a potential of —1.55 V vs. SCE in a medium of 1 M sodium citrate and 0.1 M NaOH. Under these conditions, technetium is reduced to an oxidation state which is soluble in mercury. The amalgam is removed from the solution of fission fragments and the amount of Tc determined in nitric acid solution of the amalgam by a y count. For Tc the measurement accuracy is within 1 %, and the decontamination factor from other fission products 10 . 

Diffusion of Tc and other fission products such as Cs and -Eu as well as the actinides Np, Am and natural uranium was studied in a sample of a sediment from an enclosed brackish water bay of the Baltic Sea. Under oxidizing conditions TcOq did not interact with the sediment to any large degree. Deeper laying sediments were depleted of oxygen and showed negative redox potentials. In this case the apparent diffusivity of Tc 0 =5-10 m -s was low compared to compacted clay with >a=8 -10 " m- s", indicating a reduction of TcOq [30]. 

The release behavior of the less volatile fission products from overheated fuels was studied in detail in the Sascha experiments (Albrecht, 1987 a). These results demonstrated that the low-volatility elements (e. g. barium, ruthenium, cerium) were essentially released at the highest temperatures realized in the tests, i. e. from the liquid melt. In Fig. 7.11. some of the measured release rates for low-volatility elements are shown and compared with the assumptions made in the NUREG-0772 report. With the exception of barium (and potentially strontium), the release rates of these elements apparently depend only little on the amount of steam supplied. Barium release rates in a pure steam atmosphere have proved to be lower by a factor of nearly 100 than in an Ar + 5% steam atmosphere or in an Ar -H 5% H2 atmosphere. This behavior was assumed to be due to a reduction of the BaO initially present by still non-oxidized Zircaloy, which converted it into the more volatile elemental form. 

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