ALAMBIC, a simulation tool to assess the red-oils hazards in reprocessing facilities

Abstract

International audienceIn the French nuclear industry, the reprocessing of spent nuclear fuel is based on the PUREX aqueous process (“Plutonium Uranium Refining by EXtraction”). The main involved compounds are an organic mixture of hydrocarbon diluted tri-n-butyl phosphate (TBP) as well as a nitric acid solution used to dissolve the spent fuel. Even if the neat TBP is thermally stable, an extended contact with an acid solution (nitric acid and extractable heavy metal nitrates) forms a mixture called red-oils due to reaction at elevated temperature between involved species. These complex reactions and the formation of unstable by-products can lead to a thermal runaway in the plants’ evaporators potentially followed by an explosion, a containment failure and a radioactive release into the environment [1]. In the past, these phenomena have been responsible of several significant accidents such as, two accidents in the United States at Savannah River plant (1953 and 1975) and more recently one accident in Russia at Tomsk (1993). This raises the question of exothermicity of reactions involving extractant TBP, its diluent (hydrogenated tetra-propylene, HTP) and their by-products and calls for a better knowledge of underlying thermochemical phenomena. In the context of safety reviews regarding the French reprocessing plants at La Hague, one of the important risks concerning the evaporators is the explosion risk due to the red-oils compounds formation. Therefore, in order to simulate the behaviour of reactant solutions in evaporators during an accidental transient due to a thermal runaway, a simulation tool, ALAMBIC, is being developed at IRSN to support, in the next future, safety analyses. Its aim will be to simulate the thermal degradation of TBP in contact with nitric acid/uranyl nitrate solution and thus investigate the conditions leading to violent thermal runaways. This tool is built from modules of the ASTEC software package (Accident Source Term Evaluation Code) [2], one able to simulate the thermal-hydraulic behaviour coupled with another one to calculate the chemical evolution. For the latter, several key data are mandatory as the thermodynamic properties of species in aqueous and organic liquids or the kinetics rate laws of TBP and by-products degradations. Before addressing the chemical kinetics, a study based on quantum chemical methods has been started to fill the lack of knowledge regarding the standard heat of formation of n-butyl phosphate species [3]. Similarly, efforts have been recently continued towards the uranyl nitrate complex because the potential by-products are not as evident as TBP-nitric acid system. We will first introduce the safety controls useful for safety reviews applied to evaporators in reprocessing facilities. Then, a highlight will be made on the recent theoretical results obtained about the reaction pathways of uranyl nitrate complexes. Finally the state of progress of ALAMBIC simulation tool will be presented.References1. V.N. Usachev et al., Radiochemistry 45, 1 (2003).2. L. Chailan et al., Overview of ASTEC code and models for Evaluation of Severe Accidents in Water CooledReactors, IAEA Technical Meeting, Vienna (Austria), October, 9-12, 20173. M. Saab et al., J. Chem. Phys. 146, 244312 (2017