REDUPP

Reducing Uncertainty in Performance Prediction

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 Obiettivo del progetto (Objective)

'At the core of the safety case for long-term geologic disposal for spent nuclear fuel lie the issues regarding spent fuel dissolution. We have today significant knowledge regarding dissolution of uranium oxide in the laboratory. However, uncertainties remain regarding spent fuel dissolution under realistic conditions. Two main questions stand out as necessary to address: First, the synthetic groundwater used in the experiments to date does not contain all of the chemical elements that occur in natural groundwaters. Some of the trace elements may produce radiolysis products that are more aggressive than those produced in the synthetic groundwaters. Second, the fragments used in laboratory experiments contain sharp edges and defects generated by the crushing process. These sites contain atoms that have fewer bonds than the bulk material and constitute high-energy sites. As dissolution occurs, these sites dissolve faster than lower energy sites, such as plane surface sites. As the high-energy sites disappear through dissolution, the dissolution rate decreases. We can approach the long-term dissolution rate in the laboratory, but we cannot at this time estimate how far away we are from it. The two items mentioned above lead to uncertainty concerning the dissolution rate. The objectives of the work proposed here are to reduce the uncertainty in the dissolution rate to be used in the safety case and thereby increase the confidence that can be placed in our ability to demonstrate that the geologic repository will function as designed. A second objective of this work is to provide for the training of young research workers who can continue to support the research needed in the future concerning radioactive waste disposal.'

Introduzione (Teaser)

When it comes to handling spent nuclear fuel, direct geological disposal is the preferred method in some countries. EU-funded scientists developed realistic models of how dissolution affects the surface of the spent nuclear fuel that should decrease uncertainties and increase the safety of deep underground repositories.

Descrizione progetto (Article)

Much research has been conducted and significant knowledge has been amassed on the complex solid-fluid interaction of radioactive material and natural groundwater entering the spent fuel canister. However, there are still open questions. Among these is how results of laboratory experiments can be translated to the real repository environment.

Scientists initiated the EU-funded project 'Reducing uncertainty in performance prediction' (REDUPP) to answer two different aspects of this question. The team investigated how results of relatively short-term dissolution experiments can be extrapolated to the very long times of the process in the repository. In addition, they explored the effect of trace elements found in natural groundwater.

Spent nuclear fuel is mainly composed of uranium oxide (UO2), which has a fluorite structure. REDUPP research focused on a series of materials with similar fluorite structure, including cerium oxide and calcium fluoride. Extensive laboratory dissolution experiments were performed using fragmented or powdered samples that contained sharp edges and defects. During dissolution, the fragments became more rounded.

Among the project goals was to determine how this gradual change of the sample surface affects the dissolution rate. By integrating their experimental results with computational modelling, REDUPP scientists developed a theoretical model of how the solid surface structure interacts with water during dissolution of spent nuclear fuel in groundwater.

The effect of the presence of trace elements was studied using real groundwater in dissolution experiments with UO2. Previous experiments used synthetic groundwater having a different chemical composition than natural groundwater. The lack of all the chemical elements that appear in natural waters introduced a non-trivial uncertainty in past modelling efforts needing to be reduced.

Extremely low solubility of some of the materials used required careful and precise analyses of the solutions. Highly specific analytical data was obtained by means of high-resolution inductively coupled plasma mass spectroscopy. The improvement of databases for spent fuel dissolution was another outcome of the REDUPP project.

The results of REDUPP research are of interest to both research departments throughout the world and stakeholders in the nuclear waste management industry. Collaborations such as this are expected to have major impact not only on the fundamental issue of how solid surface structure affects dissolution, but also in reducing uncertainties in disposal safety assessments.