Mechanisms Governing 90Sr Removal and Remobilisation in a VLLW Surface Disposal Concept

Flow-through columns were used to assess potential long-term trends in 90Sr biogeochemistry and transport in a Finnish near-surface very low-level waste (VLLW) repository concept. Experiments simulated the effects of water intrusion and flow through the repository barrier and backfill materials, examining impacts on 90Sr migration. Artificial rainwater containing 2.0 mg/L stable Sr (as a proxy for 90Sr) was pumped through column systems that had varying compositions from a matrix of rock flour (backfill material), bentonite (backfill/sealing material), and carbon steel (waste encapsulation material), for 295 days. Effluent geochemistry was monitored throughout. Sr retention behaviour in all column systems was broadly similar. Sr removal from influent rainwater was marked (~95% removed) at the beginning of the experiments, and this degree of removal was maintained for 20 days. Thereafter, Sr concentrations in the effluents began to rise, reaching ~2 mg/L by 295 days. Further, 56%–67% of added Sr was retained in the repository materials over the 295-day reaction period. Analysis of the effluents indicated that colloids did not form; as such, Sr output was likely to be aqueous Sr2+. Upon completion of the experiment, solid-associated Sr distribution and speciation in the columns were assessed through column sectioning and post-mortem analyses, which encompassed the following: total acid digests, sequential extractions, and XAS analysis. The total acid digests and sequential extractions showed that Sr was evenly distributed throughout the columns and that the majority (68%–87%) of solid-associated Sr was in the exchangeable fraction (MgCl2). This suggested that a major part of the solid-phase Sr was weakly bound to the column materials via outer-sphere sorption. Interestingly, a smaller amount of Sr (7%–23%) could only be extracted by aqua regia, suggesting that a proportion of Sr may bind more strongly to the barrier materials. XAS analysis of select samples confirmed that the dominant Sr phase was sorbed to the rock flour and bentonite, but not corroded carbon steel. Columns were also subject to remobilisation experiments using artificial rain- and seawater without added Sr. While rainwater remobilised Sr slowly, high-ionic strength seawater remobilised Sr at much higher rates in the systems containing bentonite. Interestingly, Sr was well retained in the rock flour-only system following rain and seawater intrusion. Overall, the results indicate that the column materials provide reactive surfaces for Sr removal should it be released from waste packages; however, the backfill and barrier materials have limited retention capacity, and the dominant sorption interaction is relatively weak. The safety case for the shallow disposal of radioactive waste should consider the possibility of seawater intrusion and that the bentonite-bound Sr was significantly more susceptible to remobilisation following seawater, despite retaining slightly more Sr during sorption experiments

Contribution of Energetically Reactive Surface Features to the Dissolution of CeO2 and ThO2 Analogues for Spent Nuclear Fuel Microstructures

In the safety case for the geological disposal of nuclear waste, the release of radioactivity from the repository is controlled by the dissolution of the spent fuel in groundwater. There remain several uncertainties associated with understanding spent fuel dissolution, including the contribution of energetically reactive surface sites to the dissolution rate. In this study, we investigate how surface features influence the dissolution rate of synthetic CeO2 and ThO2, spent nuclear fuel analogues that approximate as closely as possible the microstructure characteristics of fuel-grade UO2 but are not sensitive to changes in oxidation state of the cation. The morphology of grain boundaries (natural features) and surface facets (specimen preparation-induced features) was investigated during dissolution. The effects of surface polishing on dissolution rate were also investigated. We show that preferential dissolution occurs at grain boundaries, resulting in grain boundary decohesion and enhanced dissolution rates. A strong crystallographic control was exerted, with high misorientation angle grain boundaries retreating more rapidly than those with low misorientation angles, which may be due to the accommodation of defects in the grain boundary structure. The data from these simplified analogue systems support the hypothesis that grain boundaries play a role in the so-called “instant release fraction” of spent fuel, and should be carefully considered, in conjunction with other chemical effects, in safety performance assessements for the geological disposal of spent fuel. Surface facets formed during the sample annealing process also exhibited a strong crystallographic control and were found to dissolve rapidly on initial contact with dissolution medium. Defects and strain induced during sample polishing caused an overestimation of the dissolution rate, by up to 3 orders of magnitude

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

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.Tällä hetkellä geologinen loppusijoitus on todennäköisin vaihtoehto käytetylle ydinpolttoaineelle niin Suomessa kuin Ruotsissakin. Sama konsepti on potentiaalinen myös mahdollisille tulevaisuuden torium- pohjaisille polttoaineille. Polttoaineen liukoisuus pohjaveteen määrää uraanin, toriumin ja muiden radionuklidien vapautumista eri vapautumisesteiden ja geosfäärin läpi aina biosfääriin asti. Toriumdioksidilla (ThO2) on samanlainen fluoriittityypin kiderakenne kuin uraanidioksidilla. Tämän takia ThO2:ia voidaan käyttää analogimateriaalina myös tutkimuksissa, jotka koskevat pääosin uraanioksidista (UO2 > 95 %) koostuvaa polttoainetta. Tämän väitöskirjan päämääränä oli tehdä liukoisuustutkimuksia sellaisella syntetisoidulla toriumoksidilla, jonka mikrorakenne muistuttaa mahdollisimman tarkasti käytetyn polttoaineen UO2:n mikrorakennetta. Tutkimus sisältää ThO2-pelleteillä, -murskeilla (2 - 4 mm) ja -partikkeleilla (80 - 160 µm) tehtyjä liukenemiskokeita. Liukenevien pintojen ja raerajojen kehittymistä/muuntumista liuotuskokeissa seurattiin erilaisin mikroskooppisin kuvantamistekniikoin (elektronimikroskopia (SEM), atomivoimamikroskopia (AFM), SEM-mikroskopia varustettuna elektronien takaisinsirontadiffraktiodetektorilla (SEM-EBSD) ja profilometrinen kuvantaminen). Osa liukenemiskokeista tehtiin siten, että liuosfaasiin oli lisätty 229Th- merkkiainetta, jotta saataisiin lisätietoa liukenemisesta ja saostumisesta seuraamalla 229Th/232Th -isotooppisuhdetta. Merkkiainekokeista saatujen pellettien pinnan koostumusta ja muodostuneen pintakerroksen paksuutta arvioitiin mittaamalla ja mallintamalla alfaspektri suoraan pellettien pinnasta. Kaikki liukoisuuskokeet osoittivat, että torium liukenee suhteellisen nopeasti kokeiden alkuvaiheessa. Alkua seuraa toriumkonsentraatioiden hidas lasku liuoksessa ja liukoisuuden väheneminen, kun kokeita jatkettiin yli 100 päivää. Mikroskopialla tehdyt tutkimukset paljastivat, että raerajoilla oli merkittävä rooli alussa tapahtuvassa toriumin vapautumisessa liuokseen. Pintatutkimukset osoittivat myös, että samoissa koeolosuhteissa eri partikkelit ja pellettien/palasten? pinnat voivat käyttäytyä eri tavoin: joko liueta, saostua tai pysyä muuttumattomina. Tämä johtuu hyvin todennäköisesti niiden erilaisista pintaenergioista ja materiaalin heterogeenisestä luonteesta. Toriumin isotooppien pitoisuuksien määrittämiseen koeliuoksista käytettiin korkean erotuskyvyn induktiokytketty plasma -massaspektrometriaa (HR-ICP-MS). Kehitetty menetelmä oli tehokas toriumisotooppien määrittämiseen. Isotooppien pitoisuuksien vertailevana määritysmenetelmänä käytettiin alfaspektrometriaa, jolla on matalampi määritysraja 229Th-isotoopille ja korkeampi määritysraja 232Th-isotoopille kuin massaspektrometrilla. Massaspektrometrin määritysraja on tässä tapauksessa molemmille toriumisotoopeille 1 x 10-12mol/L. Mittausmenetelmät antoivat yhteneviä tuloksia 229Th-konsentraatiolle. Alfaspektrometrian edellyttämä kemiallinen erotus on kuitenkin vaivalloista ja aikaa vievää verrattuna ICP-näytteiden valmistukseen. Toriumoksidipellettien alfaspektrometrinen määritys yhdistettynä Monte Carlo -simulaatioihin osoitti, että liukenemiskokeiden aikana 232ThO2-pellettien pinnalle syntyi uusi pintakerros, jonka paksuus oli maksimissaan 0.1 µm. Alfaspektrometriatuloksista voitiin myös nähdä, että 229Th:n ja 232Th:n hajoamissarjojen tytärnuklidit liukenevat ja kerasaostuvat liuotuskokeissa: uusi pintakerros sisälsi nimittäin 229Th:n ja sen hajoamissarjan tytärnuklidien lisäksi myös kohonneita pitoisuuksia 232Th-sarjan tytärnuklideja. Tämä tarkoittaa, että 232Th:n tyttärien on ensin täytynyt vapautua pelletin sisältä liuotuskokeen aikana, minkä jälkeen ne ovat saostuneet tai adsorboituneet 232ThO2-pelletin pintaan