Investigation of ThO2 as a structural analogue for spent nuclear fuel dissolution under repository conditions

Currently, the preferred option for the long-term disposal of spent nuclear fuel (SNF) and potentially for future thorium-based fuels in Finland and Sweden is disposal in a geological repository. In deep bedrock, the release of Th, U and other radionuclides through man-made barriers and the geo- and biosphere will be controlled by the dissolution of the fuel by groundwater. Thorium dioxide is isostructural to uranium dioxide, sharing the same fluorite structure (space group Fm3m) and making it a useful analogue material for nuclear fuel, which mainly consists of UO2 (>95%). This thesis aimed to investigate the dissolution of ThO2, which was synthesised to approximate as closely as possible the microstructure of UO2 in a nuclear fuel matrix. The investigation consists of dissolution studies conducted using pellets, fragments (2 to 4 mm) and particles (80 to 160 µm) of 232ThO2. The evolution of dissolving surfaces and grain boundaries were examined by combining different microscope imaging techniques (scanning electron microscopy (SEM), atomic force microscopy (AFM), SEM with electron backscattering diffraction detector (SEM-EBSD) and profilometer imaging). Part of the dissolution experiments were conducted in the presence of a 229Th tracer to gain additional data on the dissolution and precipitation by following the change in isotopic ratio 229Th/232Th. Furthermore, the pellets from these experiments were measured with direct alpha spectrometry to estimate the contents and thickness of the 229Th-rich layer formed on the pellet surface. The results of all dissolution studies showed a relatively fast release of thorium during the early stage of the experiment followed by a slow decrease in the thorium concentration and suppression of the dissolution rate as the experiments continued over 100 days. Microscopic studies revealed that the grain boundaries play a significant role in the initial release of thorium. It was also observed that the different surfaces of thorium dioxide particles behave differently either dissolving, precipitating or showing an almost inert nature, most likely due to the different surface energies of the heterogeneous material. High-resolution (sector field) inductively coupled plasma mass spectrometry (SF/HR-ICP-MS) was used for the thorium isotope analyses of leached solutions. The developed method was powerful for analysing thorium isotopes. In addition, alpha spectrometry was used for thorium analysis for comparative purposes. The alpha spectrometry yielded a lower detection limit for 229Th and a higher detection limit for 232Th than SF-ICP-MS, which was 1 x 10-12 mol for both isotopes. Thus, these methods provided comparable results for the analysed 229Th concentration. However, the chemical separation needed before alpha spectrometry is very time consuming compared to the sample preparation necessary for HR-ICP-MS. When combined with simulations, direct alpha analysis confirmed that during leaching a new layer, with a maximum thickness of 0.1 µm, formed on the surface of ThO2 pellets. Alpha spectrometry also provided interesting insight into the dissolution and co-precipitation behaviour of 229Th and 232Th decay series daughter nuclides. The surface layer contained not only 229Th and its daughters, but also an elevated concentration of daughters from the 232Th decay series, indicating that they were first released from the bulk during the leaching experiment, subsequently co-precipitating or adsorbing onto the surface of the pellet.

Grantor dc:publisher

VTT

Year dc:date.issued

2017

Author dc:creator

• Myllykylä, Emmi

Statement dc:rights

• Julkaisu on tekijänoikeussäännösten alainen. Teosta voi lukea ja tulostaa henkilökohtaista käyttöä varten. Käyttö kaupallisiin tarkoituksiin on kielletty.
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• Publikationen är skyddad av upphovsrätten. Den får läsas och skrivas ut för personligt bruk. Användning i kommersiellt syfte är förbjuden.

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/10138/187229

OAI record oai:identifier

https://helda.helsinki.fi/server/oai/request?verb=GetRecord&metadataPrefix=dim&identifier=oai%3Ahelda.helsinki.fi%3A10138%2F187229