Stability of advanced nuclear fuel  "Th/Pu MOX" under repository conditions
Creators: name: Rodriguez-Villagra, Nieves
Publisher: name: Zenodo
Year: 2015
Subjects: subject: ThO2
subject: Plutonium
subject: CeO2

description: The document addresses the challenges of irradiated nuclear fuel and explores the potential of new fuel materials to replace those that have been used in commercial nuclear reactors for over 40 years. These new materials offer significant advancements in areas such as non-proliferation, matrix stability for final disposal, and minimization of radioactive waste management. Furthermore, these candidate materials for nuclear fuel are versatile, as they can be used in both current light water reactors (LWRs) and next-generation reactors (GIII+ and GIV).

The initial design of this doctoral thesis and the results presented are the outcome of an R&D collaboration agreement between the Institute for Transuranium Elements (ITU) of the EU-JRC and CIEMAT. The research focuses on studying the behavior of Th-based fuels under simulated storage conditions. These materials can only be handled in nuclear or radioactive facilities. Consequently, CIEMAT's radioactive facilities were essential for unirradiated material, while ITU's hot cells were necessary for experiments involving irradiated nuclear fuel. The fuel irradiations were conducted in a PWR reactor in Obrigheim (KWO), Germany, using ThO₂ fuel doped with 3% Pu and achieving a burnup of 38.8 MWd/kg HM.

The experimental work was designed to analyze the stability and behavior of irradiated THOMOX matrices (mixed Pu and Th oxides) under conditions consistent with the final disposal of radioactive waste. These materials serve a dual purpose: first, to burn the growing global inventories of Pu, and second, to harness energy from it. Th-based matrices, which are alternatives to MOX fuels (U-based), have been proposed by the international community (e.g., the IAEA) as candidates for energy production. These matrices avoid increasing the long-term radiological impact of waste (as they do not generate additional Pu), and in the short term, they can help reduce the growing stockpiles of reprocessed Pu. Additionally, developing the Th fuel cycle—starting with the implementation of Th/Pu MOX fuels in thermal reactors—provides valuable experience with Th technologies (manufacturing, irradiation, and reprocessing) and serves as a transition towards their use in advanced reactors in the future.

Notably, the thesis includes highly relevant results, given that it deals with an innovative material and fuel type that has not been burned before. The main topics addressed include:



The SOLGEL fabrication process for ThO₂-based fuels with an optimal Pu content for burning in a reactor.

Characterization of all starting materials, including surface analysis of ThO₂ reactive sites and other relevant oxides.

Characterization of irradiated nuclear fuel, including chemical analysis of fission and activation products generated during irradiation.

Long-term stability studies of Th/Pu MOX irradiated matrices under repository conditions, along with result modeling.


Based on the above, the thesis consists of two main components: an experimental study of these materials and the implementation of an Alteration Model applied to Th-based matrices. Experimental leaching studies of ThO₂ matrices in various aqueous environments simulating repository conditions demonstrate that these matrices are highly effective in retaining radionuclides. Due to their insolubility, the release of dissolved radionuclides (fission and activation products) occurs at rates lower than those of UO₂-based fuels. The proposed alteration model (a kinetic model) derived from experimental and empirical data enables extrapolation of matrix behavior during repository storage.

Finally, it is worth highlighting the international relevance of both the empirical results obtained and the proposed matrix alteration model. These findings have been incorporated into international EU-funded projects (e.g., REDUPP, LWR-DEPUTY, OMICO, Thorium Cycle) and presented at key conferences (e.g., Spent Fuel Workshop, MRS). Countries interested in the resurgence of nuclear energy have expressed significant interest, underscoring the importance and impact of this work. Additionally, several results included in the thesis have already been accepted, presented, and published in high-impact international journals in the field.