Thorium oxide dissolution in HNO3-HF mixture: kinetics and mechanism

This paper is an attempt to find out thorium oxide dissolution mechanism in HNO3-HF mixture. In a previous paper, several parameters effects on thorium oxide dissolution have been described, with specific focus on hydrofluoric acid effect, which can lead to an increase of the dissolution rate if present in small amount, but precipitates as ThF4 at higher content. Based on this previous study, experimental data were fitted using several dissolution models in order to find out the best one. Finally, a revisited model based on literature and considering the ThF4 formation was proposed. It describes the main steps of dissolution and is able to fit the experimental data for a wide range of solution compositions. This point is crucial since it allows considering an extrapolation of the established model to not-yet-studied conditions.

Conceptual potential of a pyroelectrochemical technology for the thorium engagement in the fast neutron fuel cycle

The use of thorium in combination with plutonium in nuclear power generation offers a solution to the problem of reducing the accumulation of long-lived transplutonium nuclides. Along with this, the existing uranium fuel cycle (UFC) has such disadvantage as the vulnerability to unauthorized use of nuclear materials. The thorium fuel cycle (TFC) is devoid of these drawbacks. The engagement of thorium in nuclear power is possible provided the availability of an appropriate technology for reprocessing irradiated thorium. A fuel cycle based on thorium oxide may not differ in principle from the already developed pyrochemical fuel cycle involving uranium and plutonium oxides. Thorium oxide is most commonly obtained in compact state by electrolysis of molten salts from thorium-containing electrolytes. The most thorough studies of physical and chemical and electrochemical behavior of thorium in molten haloids of alkali and alkaline-earth metals were conducted in the 1960ies and the 1970ies. Since extensive experimental material has been accumulated by now for justification of the use of pyroelectrochemical and chemical processes for regeneration of fuel in molten salts, then it has also been proposed that technologies for fuel reprocessing in molten chlorides of alkali metals should be applied resulting in a crystalline product that can be used for the fuel element fabrication. Unlike uranium and plutonium, thorium behavior in molten salt environments is less complex. In molten salts, thorium exists predominantly in the form of Th4+, and the mixture of uranium and thorium dioxides with ThO2 content reaching up to 50 % can be obtained by electrolysis of molten salts. Therefore, the existing amount of knowledge about the chemistry of thorium allows regarding the use of pyrochemical processes in production of thorium oxide as highly promising, and the available data on the physical and chemical properties of thorium and its compounds in high-temperature molten salts makes it possible to state that the pyroelectrochemical technology can be potentially used in production and reprocessing of thorium fuel.