Influence of Light and Temperature on the Extractability of Cerium(IV) as a Surrogate of Plutonium(IV) and its Effect on the Simulation of an Accidental Fire in the PUREX Process.

Abstract

Modeling of plutonium(IV) behavior during an accidental fire in a reprocessing plant was considered using various non-radioactive metallic surrogates. Among those elements, cerium(IV) was supposed to be a suitable candidate due to possible formation of a complex with TPB, but its extractability and stability have not been studied previously under representative plutonium uranium reduction extraction (PUREX) conditions. In this work, we investigated the chemical analogy between cerium(IV) and plutonium(IV) in this extractive process and combustion thereof. Distribution ratios are reported for acidities of 1–4 mol L–1 in equal volumes of nitric acid and a 30:70 mixture of tributylphosphate and hydrogenated tetrapropylene. The influences of light, temperature, and extraction time were studied by UV–vis spectroscopy. The results showed that cerium(IV) is extracted quantitatively but is reduced over time to cerium(III) in the organic mixture. Spectrophotometric investigations of this reaction kinetics revealed an apparent rate constant k of 0.021 ± 0.002 mol0.5 L0.5 min–1 at 298 K and an apparent fractional reaction order of 0.5. The activation energy of this reduction was found to be around 82 ± 2 kJ mol–1 by the Arrhenius plot method. The combustion of mono- and biphasic solutions prepared with a cerium(IV) concentration of 10 g L–1 revealed that the extracted complexes, Ce2O·6NO3·3TBP(org) or Ce4O4·8NO3·6TBP(org), are reduced during the combustion. Compositions of the resulting ashes and soot were analyzed and highlighted the presence of pyrophosphates and polycyclic aromatic hydrocarbons, with some traces of cerium. Ce(IV) is not suitable to represent Pu(IV) from a chemical point of view in HNO3/TBP–HTP solutions.