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Synthesis of Thorium Dioxide Doped with Gadolinium Via a Wet Chemical Route

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Synthesis of Thorium Dioxide Doped with Gadolinium Via a Wet Chemical Route FACULTY OF SCIENCE Synthesis of thorium dioxide doped with gadolinium via a wet chemical route Peter Zsabka Promotor: Prof. Dr. Koen Binnemans Molecular Design and Synthesis,Department of Thesis presented in Chemistry, KU Leuven fulfilment of the requirements Co-promotor: for the degree of Master of Science Prof. Dr. Thomas Cardinaels in Chemistry Radiochemical Analysis and Processes, Institute for Nuclear Materials Science, SCK•CEN Academic year 2014-2015 © Copyright by KU Leuven Without writtenpermission of the promotors and the authors it is forbidden to reproduce or adapt in any form or by any means any part of this thesis. Requests for obtaining the right to reproduce or utilize parts of this publication should be addressed to KU Leuven, Faculteit Wetenschappen, Geel Huis, Kasteelpark Arenberg 11 bus 2100, 3001 Leuven (Heverlee), Tel.: +32 16 32 14 01. A written permission of the promotors is also required to use the methods, products, schematics and programs described in this work for industrial or commercial use, and for submitting this publication in scientific contests. I Abstract (English) Thorium gained interest as a source of 233U, an alternative to 235U-based nuclear fuel, since the onset of the commercial utilization of nuclear power. Nowadays enormous reserves of thorium(IV) nitrate and thoria (ThO2) have been built up as a side product of extraction of rare-earth elements from monazite ore. Without valorisation such reserves only count as highly radioactive waste that needs to be securely buried and monitored. From a technological point of view, the most straightforward utilization of thorium is its use as ThO2 ceramic pellets in pressurized water reactors. Fabrication of high density ThO2 pellets is more difficult compared to currently used UO2 pellets due to the very high melting point of thoria. Until know, pilot projects in which thoria pellets were used usually applied extensive mechanical treatment (milling or grinding) to improve the sinterability of pellet feed material. Wet chemical synthetic routes are generally considered as more safe compared to dry mechanical treatment, as hazardous dust formation can be avoided and powder properties can be engineered to some extent. Among others, the optimal process parameters of the oxalate precipitation route has been intensively investigated in the past as it is a cheap and simple wet chemical method and a candidate for industrial upscaling. Wet chemical synthesis routes were applied to investigate the feasibility of fabricating high- density thoria pellets without having recourse to any mechanical treatment of precursor powder feed material. Three synthetic routes were studied. On the basis of solid literature data, nano-sized thorium(IV) oxalate hexahydrate and derived thoria powder was obtained reproducibly under carefully selected reaction and heat-treatment parameters. As the powders proved to be highly prone to form agglomerates, the sintered pellets shown sign of heterogeneous sintering, and as a consequence reproducibility in terms of both pellet density and pellet integrity was very low. By adding trace amount of alumina sintering aid to the thoria powder, significant improvement in density has been attained. Further studies are needed to draw a definite conclusion whether the addition of sintering aid (either alumina or niobia) could ameliorate the abovementioned shortcomings of oxalate-route derived powder. The second method was homogenous precipitation route, in which diethyl oxalate was thermally decomposed to generate in situ oxalic acid. The latter type of thoria powder was fluffy and free flowing.Pellets produced via homogeneous precipitation route sintered to low densities, being unfavourable with respect to neutron economy during breeding, but more favourable for reprocessing due to ease in dissolution of irradiated pellets. In the third route tetrabutylammonium oxalate was used as precipitating reagent. In the latter case the precipitate could not be thermally decomposed and further processing of the material was abandoned. The second goal of the thesis was to study the effect of gadolinium doping on lattice parameter of thoria powder and sintered pellet samples.The Gd-dopant serves as a consumable neutron absorber, the use of which is promoted by the aim of obtaining higher neutron economy during the irradiation cycle.For the purpose of studying the effect of doping, the well-established oxalate precipitation route was applied. Lattice parameters of sintered Gd-doped thoria pellets fabricated from wet chemical route- derived powder are reported here for the first time. Evidence is given on the solid solution II formation during the sintering process. Lattice parameter values of powder samples were determined after calcination at 700 °C and 1000 °C for 1 h. The X-ray diffractogram of these samples showed the presence of two cubic phases. Lattice parameter values were subsequently determined on pellets that were sintered at 1750 °C for 8 h. Sintering resulted in one single cubic phase for all applied dopant concentration indicating solid solution formation. Besides the decrease of lattice parameter, gadolinium doping on thoria also caused a progressive decrease in the sintered pellet densities with higher dopant concentration. Although wet chemical routes are considered to provide highly homogeneous distribution of constituting elements, evidence was found, that due to crystallization processes and thermal decomposition patterns during calcination of precursor powders leave back inclusions with composition different from that of the bulk matrix. The amount of inhomogeneity is nonetheless lower than the limit of detection attainable with XRD method, and was only possible to capture by EDS. III Abstract (Nederlands) Thorium wekte belangstelling als een bron van 233U, een alternatief van 235U-gebaseerde nucleaire brandstof, sinds het begin van commercieel gebruik van kernenergie. In deze tijd zijn enorme reserves thorium(IV) nitraat en thorium dixoxide (ThO2) ontstaan als een bijproduct van extractie van zeldzame aardelementen uit monaziet erts. Zonder valorisatie worden zodanige reserves alleen als hoogradioactief afval geacht dat veilig moet begraven en gecontroleerd worden. Ten opzichte van technologie is de eenvoudigste benuttiging van thorium als ThO2 keramische pellets in drukwaterreactors worden gebruikt. Fabricage van ThO2 pellets met hoge dichtheid is meer ingewikkeld in vergelijking met de tegenwoordig gebruikte UO2 pellets vanwege het heel hoge smeltpunt van thorium dioxide. Tot nu toe hebben piloot projecten waarbij thorium dioxide pellets werden gebruikt gewoonlijk extensieve mechanische behandeling (malen of vermalen) toegepast om de sinterabiliteit van pellet voedingsmateriaal te verbeteren. Natte chemische synthetische route is in het algemeen meer veilig geacht dan droge mechanische behandeling, want gevaarlijke stofvorming kan vermeden worden en poedereigenschappen kunnen tot op zekere hoogte gepland worden. Onder anderen werden de optimale procesparameters van oxalaat precipitatie weg in het verleden intensief onderzocht, omdat deze een goedkope en eenvoudige natte scheikundige methode is en een kandidaat voor industriele uitbreiding is. Natte chemische synthetische route werden aangewend om de uitvoerbaarheid van productie van thorium dixoxide pellets met hoge dichtheid te onderzoeken zonder toepassing van enige mechanische behandeling van precursor poeder voedingsmateriaal. Drie synthetische routes werden bestudeerd. Op grond van vaste literatuurgegevens werden thorium(IV) oxalaat hexahydraat en afkomstig thorium dixoxide poeder in nano-maat reproduceerbaar met zorgvuldig geselecteerde reactie en warmtebehandeling parameters verkregen. Aangezien dat gebleken is dat de poeders zeer vatbaar voor de vorming van agglomeraten zijn, tonen de gesinteerde pellets het teken van heterogene sinteren, en als gevolg de reproducibiliteit ten aanzien van zowel pellet dichtheid als pellet integriteit zeer laag was. Door toevoeging van spoor hoeveelheid aluminiumoxide sinterhulpmiddel aan thorium dixoxide poeder, werd aanzienlijke verbetering betreffende dichtheid bereikt. Verdere studies zijn nodig voor het trekken van definitieve conclusie of de toevoeging van sinterhulpmiddel (hetzij aluminiumoxide hetzij niobium oxide) de bovenvermelde tekortkomingen van uit oxalaat-route komende poeder zou kunnen verbeteren. De tweede methode was de homogene precipitatie route, waarbij diethyl oxalaat thermisch ontbonden was om in situ oxaalzuur voort te brengen. Het laatstgenoemde type van thorium dioxide poeder was donzig en vrij vloeibaar. Pellets geproduceerd via homogene precipitatie route werd tot lage dichtheid gesinteerd, welk ongunstig aangaande neutron economie gedurende voortbrenging is, maar gunstiger is voor reprocessing ten gevolge van vergemakkelijking in het oplossen van bestraalde pellets. Bij de derde route werd tetrabutylammonium oxalaat als precipitatie reagens gebruikt. In voormeld geval kon het precipitaat thermisch niet ontbonden worden en verdere verwerking van het materiaal was gestopt. De tweede doelstelling van het thesis (proefschrift) was om de invloed van de doping van gadolinium op rooster parameter van thorium dioxide poeder en gesinteerde pellet monsters IV te bestuderen. Gd-dopant (Gd-doping agens) dient als een verbruikbare neutron absorber. Het gebruik ervan wordt met de bedoeling bijgestaan om hogere neutron economie tijdens de bestralingscyclus te krijgen. Om de invloed van doping te bestuderen, werd de goed
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