Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
SINTERING OF THORIUM OXIDE COMPRISING MATERIALS
Document Type and Number:
WIPO Patent Application WO/2017/077131
Kind Code:
A1
Abstract:
A method for manufacturing a thorium oxide comprising material is described. The method comprising obtaining the thorium oxide comprising material, and sintering the thorium oxide comprising material, whereby at least during the sintering of the sintering process aluminum oxide is present as a sintering assisting additive. A resulting thorium oxide comprising material and a nuclear fuel comprising such material also are disclosed.

Inventors:
BINNEMANS KOEN (BE)
CARDINAELS THOMAS (BE)
VLEUGELS JOZEF (BE)
VERWERFT MARC (BE)
BAENA ANGELA (BE)
Application Number:
PCT/EP2016/076888
Publication Date:
May 11, 2017
Filing Date:
November 07, 2016
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
SCK CEN (BE)
UNIV LEUVEN KATH (BE)
International Classes:
G21C3/62
Foreign References:
DE10217138A12004-02-19
US4006096A1977-02-01
GB1370928A1974-10-16
Other References:
"Thoria-based nuclear fuels", 2013, SPRINGER
Attorney, Agent or Firm:
WAUTERS, Davy et al. (3190 Boortmeerbeek, BE)
Download PDF:
Claims:
Claims

1. - A method for manufacturing a thorium oxide comprising material, the method comprising

- obtaining the thorium oxide comprising material, and

- sintering the thorium oxide comprising material, whereby at least during the sintering of the sintering process aluminum oxide is present as a sintering assisting additive.

2. - A method according to claim 1, wherein said sintering is performed under an oxidizing or reducing atmosphere.

3.- A method according to any of the previous claims, wherein said thorium oxide comprising material is thorium oxide Th02.

4. - A method according to any of the previous claims, wherein said aluminum oxide is

5. - A method according to any of the previous claims, the method comprising, prior to said sintering, mixing the aluminum oxide material with the thorium oxide comprising material.

6. - A method according to claim 5, wherein said method further comprises, prior to said sintering, compacting the mixture of aluminum oxide material and the thorium oxide comprising material.

7.- A method according to any of the previous claims, wherein said aluminum oxide material is added in a weight concentration between 250 μg/g and 5000 μg/g.

8. - A method according to any of the previous claims, wherein said sintering is performed at a temperature of at least 1500 °C.

9. - A thorium oxide comprising material, the thorium oxide comprising material being a sintered thorium oxide comprising material sintered in the presence of aluminum oxide.

10. - A thorium oxide comprising material according to claim 9, wherein the thorium oxide comprising material is thorium oxide.

11. - A nuclear fuel comprising a thorium oxide comprising material according to any of claims 9 to 10.

12.- A nuclear installation comprising a nuclear fuel according to claim 11.

Description:
Sintering of thorium oxide comprising materials

Field of the invention

The present invention relates to thorium oxide comprising materials. More particularly, the present invention relates to methods and systems for producing thorium oxide comprising materials such as thoria based nuclear fuels, as well as to materials thus obtained.

Background of the invention

Thorium oxide comprising materials, such as e.g. thorium dioxide (thoria, Th0 2 ), have today regained worldwide interest as possible future nuclear fuel. The potential for thorium as alternative of uranium to become a source for nuclear energy applications has been recognized already since many decades. Thoria-based fuel manufacturing processes are conceptually identical to urania-based fuel manufacturing, i.e. oxide powders are pressed in cylindrical pellets and sintered to 95-96% of their theoretical density (TD). However, due to the very high melting point of Th0 2 (3651 ± 17 K), Th0 2 powders are much more difficult to sinter than U0 2 powders. This high melting point has obvious advantages for in-reactor performance, especially under accident conditions, but causes problems in fuel manufacturing. Sinterability may be improved by mechanical milling. Milling can easily be implemented for laboratory scale productions, but it is often undesired on industrial scale. Hence researchers have explored other means to improve the sinterability of Th0 2 powder, including various precipitation routes and calcination temperatures, the use of sintering aids and optimizing the sintering path.

Mukerjee et al. have recently reviewed these research activities on the sintering of Th0 2 , in "Thoria-based nuclear fuels" Eds. Springer, London (2013). The most commonly applied sintering aids for Th0 2 are CaO, MgO and Nb 2 Os, but also NiO, V2O5 and Ta 2 05 have been used. When Th0 2 is doped with ions with a valence state different from the 4+ valence of Th and which go into solid solution, defects are created on the anion lattice which reduces the sintering activation energy. Doping Th0 2 with a lower valence state ion (Ca 2+ , Mg 2+ and Ni 2+ ) creates oxygen vacancies and doping with higher valence state ions (N b 5+ , V 5+ and Ta 5+ ) creates oxygen interstitials. The effect is further enhanced by the sintering atmosphere: lower valency additives are chosen when sintering is performed under reducing atmospheres and vice versa. Sintering to 96 % - 98 % theoretical density has been reported at temperatures as low as 1150 °C. I n the context of nuclear fuel, it is important to consider not only the effect of the sintering additive during manufacturing, but also to consider the in-reactor performance of the doped fuel. While high diffusion rates are obviously an advantage in the fabrication phase, higher diffusion rates will also have an effect on the in-reactor creep, grain growth and fission product diffusion which often outweigh the advantages gained in manufacturing.

To-date, comparative studies on in-reactor performa nce of doped Th0 2 have not been reported in the open literature.

Summary of the invention

It is an object of embodiments of the present invention to provide good methods and systems for producing high density thorium oxide comprising materials, such as for example thorium oxide nuclear fuels, as well as to provide resulting good high density thorium oxide comprising materials.

It is an advantage of embodiments of the present invention that sintering aids are provided that can assist for manufacturing thorium oxide comprising materials under oxidizing as well as reducing conditions.

It is an advantage of embodiments of the present invention that sintering aids are provided that can assist the sintering of coarse-grained thorium oxide comprising powders (with coarse-grained may be meant e.g. powder having a specific surface less than 5 m 2 g 1 ). It is to be noticed that for powders with higher specific surface, no adverse effects are noted.

The above objective is accomplished by a method and device according to the present invention.

The present invention relates to a method for manufacturing a thorium oxide comprising material, the method comprising

- obtaining the thorium oxide comprising material, and - sintering the thorium oxide comprising material, whereby at least during the sintering process aluminum oxide is present as a sintering assisting additive.

It is an advantage of embodiments of the present invention that the aluminum oxide is insoluble in the thorium oxide comprising material. It is an advantage of embodiments according to the present invention that a high density can be obtained, e.g. a density of at least 93%, advantageously at least 95%, even more advantageously 97% of the theoretical thickness.

Said sintering may be performed under an oxidizing or reducing atmosphere.

It is an advantage of embodiments of the present invention that the sintering additive is suitable for sintering in an oxidizing as well as in a reducing atmosphere.

The thorium oxide comprising material may be thorium oxide Th0 2 . It is an advantage of embodiments of the present invention that thorium oxide Th0 2 can be used as a nuclear fuel.

The aluminum oxide may be Al 2 03. Alternatively, the alumina may for example be introduced as alumino-silicates, embodiments of the present invention not being limited thereto.

The method may comprise, prior to said sintering, mixing the aluminum oxide material with the thorium oxide comprising material.

The method may further comprise, prior to said sintering, compacting the mixture of aluminum oxide material and the thorium oxide comprising material.

The aluminum oxide material may be added in a weight concentration between 250

The sintering may be performed at a temperature of at least 1500°C. The temperature may for example be at least 1600°C. The temperature may for example be in the range 1600°C to 1750°C.

The present invention also relates to thorium oxide comprising material, the thorium oxide comprising material being a sintered thorium oxide comprising material sintered in the presence of aluminum oxide.

The thorium oxide comprising material may be thorium oxide. The present invention also relates to a nuclear fuel comprising a thorium oxide comprising material as described above.

The present invention furthermore relates to a nuclear installation comprising a nuclear fuel as described above.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

Brief description of the drawings

FIG. 1 shows examples of synthesis methods for Th0 2 powder according to embodiments of the present invention. FIG. 1(a) illustrates a first route (Route 1), using as-received powder and FIG. 1(b) illustrates a second route (Route 2), using milled powder. The resulting pellets of both production paths were sintered in oxidizing and reducing atmospheres.

FIG. 2 shows Th0 2 powder (a,b) having a typical platelet morphology with platelet size of the order of 1-5 μιη , illustrating features of embodiments of the present invention. After intensive milling most of the platelets are broken down to sub-micron size grains which form irregular clusters (c, d). The AI2O3 powder is composed of submicron grains that form spherically shaped, soft agglomerates of approximately 30 μιη in diameter (e, f). As-received Th0 2 after mixing with AI2O3 shows some unaltered AI2O3 agglomerates and submicron Al 2 03crystals (g,h).

FIG. 3 shows SEM images of Th0 2 sintered at 1750 °C for 8 h without pre-milling, illustrating features of embodiments of the present invention. Left column: low magnification and right column higher magnification. Ceramics are thermally etched to reveal grain boundary contrast. (a,b) Thl-A: pure Th0 2 , air; (c,d) Thl-B: pure Th0 2 , reducing; (d,e) ThlAI-A: Al 2 0 3 doped, air; (g,h): ThlAI-B: Al 2 0 3 doped, reducing. FIG. 4 shows SEM images of Th0 2 sintered at 1750 °C for 8 h after milling the Th0 2 powder, illustrating features of embodiments of the present invention. Left column: low magnification and right column higher magnification. The ceramics are thermally etched to reveal grain boundary contrast. (a,b) Th2-A: pure Th0 2 , air; (c,d) Th2-B: pure Th02, reducing atmosphere; (d,e) Th2AI-A: AI2O3 doped, air; (g,h): Th2AI- B: AI2O3 doped, reducing atmosphere.

FIG. 5 shows grain size distribution (MLI) of the different pellets, illustrating features of embodiments of the present invention. For each of the compositions, the grain sizes of pellets sintered under oxidizing and reducing conditions are compared in one graph: (a) non-milled, pure Th0 2 ; (b) non-milled, ThO 2 -0.26 wt% AI2O3; (c) milled, pure Th0 2 ; (d) milled, ThO 2 -0.26 wt% Al 2 0 3 .

FIG. 6 shows elemental distribution maps (by EDS) of Al, illustrating features of embodiments of the present invention, (a) and (b) Finely distributed AI2O3 precipitates with sizes ranging from submicron to several microns, (c) Larger agglomerates which were not destroyed in the mixing process are sometimes observed as well.

The drawings are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.

Any reference signs in the claims shall not be construed as limiting the scope. In the different drawings, the same reference signs refer to the same or analogous elements.

Detailed description of illustrative embodiments

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

The terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

Where in embodiments of the present invention reference is made to aluminum oxide as a sintering aid, reference is made to any type of aluminum oxide that is insoluble in the thorium oxide comprising material to be sintered, including but not limited to AI2O3. It is to be noticed that the methods, systems and resulting products are not limited to the sole use of aluminum oxide as a sintering aid, but that this sintering aid may be used in combination with other sintering aids or other additive materials. The sintering aid aluminum oxide thus may, in the process, be combined with other additives.

Where in embodiments of the present invention reference is made to thorium oxide comprising materials, reference may be made to thorium oxide materials, but also to materials comprising thorium oxide but comprising for example also uranium oxide and/or plutonium oxide. In other words, reference may be made to materials comprising at least a fraction of thorium oxide. Furthermore, such materials may be doped with certain elements or materials, such doped materials still being referred to as thorium oxide comprising materials, or, where applicable, to thorium oxide materials.

In a first aspect, the present invention relates to a method for manufacturing a thorium oxide comprising material. The method may be especially suitable for manufacturing of a thorium oxide based nuclear fuel, although embodiments are not limited thereto. In some examples, the thorium oxide based material also may be suitable as a refractory material, in nuclear or non-nuclear applications. The method comprises obtaining the thorium oxide comprising material. The thorium oxide comprising material may be essentially thorium oxide, but alternatively may be thorium oxide mixed with another fuel material, such as for example a uranium oxide material or plutonium oxide material. Furthermore, advantageously, obtaining thorium oxide comprising material may also comprise a step of mixing the thorium oxide comprising material with an aluminum oxide additive, which may act as sintering aid and may comprise a step of compacting this material. According to embodiments of the present invention, the method also comprises sintering the thorium oxide comprising material, whereby at least during the sintering of the sintering process aluminum oxide is present as a sintering assisting additive. Suitable aluminum oxide concentrations that can be used, are below 1000 μg/g, e.g. between 100 μg/g and 750 μg/g, e.g. between 250 μg/g and 500 μg/g. The sintering may be performed in an oxidizing or reducing atmosphere. Examples of gas atmospheres that can be used are

Ar, H 2 , Ar-H 2 , Ar-H2-H 2 0, C0 2 -CO, vacuum, Air, Ar-0 2 , Typical temperature ranges for the sintering may be above 1500 °C, e.g. above 1600 °C, e.g. 1600 °C - 1750 °C. Further features and advantages of embodiments of the present invention may correspond with one or more features or advantages of illustrative examples shown below.

In a second aspect, the present invention relates to a thorium oxide comprising material. The thorium oxide comprising material according to embodiments of the present invention may be a sintered thorium oxide comprising material sintered in the presence of aluminum oxide. In embodiments of the present invention, the material may be prepared using a method as described in the first aspect. Further features and advantages may be one or more features or advantages as illustrated in the exemplary embodiments of the present invention described below. In one aspect, the present invention also relates to a nuclear fuel comprising the thorium oxide comprising material as described above.

In still another aspect, the present invention relates to a nuclear installation comprising a nuclear fuel as described above.

It is an advantage of embodiments of the present invention that the sintering aid used is an insoluble dopant in the fuel. In the following examples, embodiments of the present invention not being limited thereto, a comparison is made between coarse powder and finely milled powder with and without AI2O3 additions and sintered under both reducing and oxidizing conditions. Specific emphasis is paid to ascertain the non- solubility of AI2O3 in Th0 2 by performing detailed X-ray diffraction investigations of the undoped and doped systems and by microchemical analyses.

Example 1: Sample synthesis and preparation for microscopic and structural investigations

Natural Th0 2 with a purity of 99.99 % and a specific surface area 2.39 ± 0.06 m 2 g _1 , supplied by Materion (Arizona, USA), was used as precursor. As received granules of AI2O3 (grade SM8 from Baikowsky, Annecy, France) with a purity of 99.99 % and specific surface of 7.62 ± 0.11 m 2 g 1 was used as sintering aid. Several production paths were followed (see FIG. 1 for an overview). A first route was based on direct compaction or compaction after blending with AI2O3 of the as-received Th0 2 powder. A second route implemented pre-milling of the Th0 2 powder. For this purpose a Retsch MM400 mill was used, in combination with a closed Zr0 2 container of 25 mL volume with one Zr0 2 ball of 10 mm diameter. The Th0 2 was milled during 5 h at 30 Hz and achieved a specific surface area of 5.56 ± 0.07 m 2 g _1 . The as received and milled Th0 2 were dry mixed with 0.26 wt% AI2O3 in a WAB Turbula Schatz T2F, in a PET container of 50 mL during 5 h at 80 RPM. All samples were uniaxially pressed in an Atlas 8T press using an 11.07 mm diameter die with a pressure of 300 MPa. A saturated solution of stearic acid in acetone was used to lubricate die and punches.

For each of the pellet batches, two sintering atmospheres were applied: half of the pellets of each production route was sintered under reducing conditions and half under oxidizing conditions. Oxidizing sintering was performed under high purity synthetic air in a Carbolite TZF1800 three zone tube furnace equipped with molybdenum disilicide heating elements arranged vertically around an alumina tube of 6 cm internal diameter. Reducing atmosphere sintering was done under an Ar/H 2 - Ar/0 2 gas flow at an oxygen potential of -420 kJ mol 1 in a Linn HT 1800 Moly high temperature furnace with an alumina refractory matrix and molybdenum heating elements. Two sintering temperatures were applied for both sintering methods: 1625 ± 20 °C with a dwell time of 1 h and 1750 °C ± 20 °C with a dwell time of 8 h.

The heating and cooling rate for all heat treatments was 5 °C min _1 . It is important to underline that the samples were pressed and sintered without adding binders and/or lubricants to the powder. FIG. 1 summarizes the experimental paths followed to produce the Th0 2 and Th0 2 - 0.26 wt% Al 2 0 3 pellets.

For the optical microscopy, electron microscopy (Scanning Electron Microscopy, SEM), microanalytic (energy dispersive spectrometry, EDS and wavelength dispersive spectrometry, WDS) and X-ray diffraction (XRD) investigations, samples must be flat and smooth in such a way that the surface is highly reflective, free of scratches and free of deformation (i.e. without edge rounding). Remaining scratches would hinder the optical microscopic observations, excessive rounding has its impact on the XRD investigations and flatness is required for the WDS analyses. Samples were also ground to the appropriate height such that all analyses could be performed sequentially by all methods without additional sample preparation steps. Even though great care was taken with the sample preparation, artefacts such as grain pull-out could sometimes not be avoided.

Mounting was done in a chemically removable resin (Technovit 5071) which is a fast curing two-component liquid/powder resin with methyl methacrylate as base component. After adding the powder to the liquid, polymerization was complete after approximately 7 min. The resulting resin can be re-dissolved chemically in a suitable solvent such as acetone or thermally removed since it softens after 30 min at 150 °C. The samples had simple cylindrical geometries and were temperature resistant; hence they could easily be retrieved thermally before thermal etching, which was performed at a temperature of 1300 °C during 3 h in air. Grinding and polishing was carried out with a STRUERS TegraPol-11. The grinding steps were performed with silicon carbide water-resistant paper of successively finer grain sizes (P500 -> P800 -> P1200). To achieve planeness and edge retention, composite discs (Struers M D45 Allegro) were used for rough polishing with a Diamond Paste (DP) suspension of 15 μιη and 6 μιη. The final polishing steps were performed on a woven silk cloth (Struers DP-Dur) using a DP-suspension of 3 μιη and 1 μιη. Between each grinding or polishing step the samples were cleaned in an ultrasonic bath with demineralized water to avoid transfer of grinding or polishing particles from one step to the next. Example 2: Specific surface analysis

The Brunauer Emmett Teller (BET) surface area analysis was performed with a Micromeritics Tristar ® II 3020. This apparatus operates on the basis of nitrogen gas adsorption; the amount of adsorbed species on a powder sample can be measured in function of the relative pressure of the adsorbate. By applying the theory of Brunauer, Emmett and Teller (BET-theory) on the gathered data the specific surface area of a sample can be calculated. Powder samples of at least 2 g are loaded into dry valve vials then degassed under vacuum and dried at 180 °C during 2 h before analysis. Liquid nitrogen is used to cool the samples to about -196 °C during the measurement. Example 3: Optical microscopy

The general aspects of the sintered pellets were investigated by optical microscopy: homogeneity of the sintering process and pore distribution. The observations were done with a Leica M EF4M light microscope equipped with a Nikon's DN100 Digital Network Camera. The system was connected to a computer making it possible to analyze the images using the AxioVision LE software. In order to assess the general microstructural state of the pellets, they were analyzed across the diameter in three fields: edge, midway and center. Utilizing three objective lenses with magnification lx with a field of view of 13.76 χ 10.35 mm 2 , 10x with a field of view of 1.35 χ 1.01 mm 2 and with a field of view of 0.51 χ 0.68 mm 2 . A calibration of the magnification was performed following the ASTM E1951-14 as guideline. Images of a stage micrometre with a micrometric scale of 1000 μιη were taken for each microscope objective lens and the resulting calibrated magnification was implemented in the AxioVision LE software. Example 4: Density analysis

Density was measured after compaction of the cylindrical pellets (green density) and after sintering (geometric or immersion density). Geometric density was determined by weighing the pellets with an analytical balance Mettler AG 204 placed in a fume hood, the pellet dimensions were measured with a ca lliper. The diameter was measured at top, center and bottom of the pellet. The pellet height was measured in two positions at 0° and 90° angle. An average of these measurements was taken for the density calculation. Error propagation was done taking into account the uncertainty of the measurements. The relative uncertainty of the geometrical density method is 1%. I mmersion density measurements were performed with a Sartorius Density Determination kit and used the difference of a sample weight in air and in water. Water was allowed to thermally stabilise before starting the test. In order to avoid adhesion of water to the sample holder wire, to reduce the surface tension and to avoid air bubble formation three drops of a surfactant were added to the water. The temperature of the water was measured before and after the test with a thermocouple and the average was used to determine the water density. Measurements of a reference aluminium cylinder were made before and after the sample measurements. The relative uncertainty of the immersion density method is 0.1 %. Given its higher accuracy, the immersion density is preferred over the geometrical density analysis when the porosity is fully closed. However, when samples have a substantial fraction of open porosity, the immersion density can no longer be used as the liquid will partially penetrate the sample.

Example 5: X-ray diffraction analysis

XRD analysis was performed with a Philips X'Pert Pro diffractometer provided with a Cu LFF tube (CUKCQ = 1.54059 A) and operated in Bragg-Brentano parafocusing geometry and Θ-Θ configuration. A position sensitive Real Time Multiple Strip (RTMS) detector (X'Celerator) was used. The detector has a n active length of 2.122° and is operated in scanning mode. Thanks to the RTMS technology, high quality diffraction patterns could be acquired in 2 h. The incident beam configuration had a fixed divergence slit with an aperture of 0.5°, 0.02 rad axial Soller slits and a copper beam mask of 10 mm. The diffracted beam path was equipped with a nickel filter and 0.02 rad axial Soller slits. Diffraction patterns were acquired over the range 20-140° 2Θ. The step size was 0.004° in all cases. The goniometer was aligned and calibrated using a high purity sintered silicon disc. Validation of the calibration was performed using a sintered AI203 disc (NIST Standard Reference Material 1976b). The samples were thermally stabilized during at least 3 h inside the diffractometer and the temperature was recorded before and after the measurements. Typically, the temperature was around 24 °C at the start of the measurement and around 25 °C at the end; the temperature difference never exceeded 2 °C. The average temperature was used to correct for thermal expansion. The diffraction patterns were analyzed with the least squares minimization package implemented in the PANalytical X'Pert HighScore Plus software suite taking only CUKCQ reflections into account for the calculation. The as-measured lattice parameter aT at temperature T is recalculated to its value at 20 °C using the linear thermal expansion coefficient cc= 9.04 χ 10-6 °C _1 .

Example 6: Scanning Electron Microscopy and Electron Probe Micro Analysis

SEM analysis and energy dispersive spectrometry (EDS) analyses were performed using a Jeol JSM 7100FA SEM field-emission microscope equipped with a secondary and backscattered electron detector and a Bruker XFLash ® 6/10 high resolution Silicon Drift Detector (SDD) EDS system operating under the Q.UANTAX 400 software. The high count rates that can be achieved with this type of detector (up to 106 cps) allow fast, high quality X-ray mapping. Sintered, polished pellets were coated with a 5 nm platinum layer to avoid electron charging during the measurements. The accelerating voltage for microstructural observation was 5 to 10 kV, the beam current was 0.2 nA, the spot size was determined according to the desired observation. Elemental distribution mapping (1500 χ 1125 pixels) was done by EDS using an accelerating voltage of 20 kV and a beam current of 1 nA and 1.1 ms dwell time.

Micro-chemical analysis by wavelength dispersion spectroscopy (WDS) was performed to analyze the Al matrix content. Elemental concentrations of Th and Al were analyzed with a CAMECA SX100 microprobe using the Thivia and ΑΙκα lines, respectively. Care was taken to avoid interference of the second order Thivia line with the ΑΙκα signal. An accelerating voltage of 20 kV and a beam current of 200 nA were applied. Measurements were taken across the sample diameter (typically 100 points) in a spot mode with a dwell time of 120 s (Pk) and 120 s (Bg), yielding an Al detection limit of 50 ug/g.

Example 7 : Results on Powder characteristics

The as-received Th0 2 powder had a specific surface of 2.4 m 2 /g, and a platelet morphology with both coarse and fine platelets (see FIG. 2 a, b). These platelets are agglomerates of crystallites of 90 nm as derived from a Rietveld refinement of the powder diffractogram and assuming that peak broadening is solely due to crystal size. After high-impact milling, the specific surface increased to 5.6 m 2 /g and the powder contained a large fraction of submicron size particles and formed irregular clusters (see FIG. 2 c, d). The AI2O3 had a high specific surface (7.2 m 2 /g) and presented spherical- shaped agglomerates composed of 0.3 μιη crystals. When the soft AI2O3 agglomerates fall apart, the fine crystals are easily dispersed (see FIG. 2 e, f). After adding AI2O3 to the as-received Th02 powder, they were gently mixed in order not to alter the Th02 powder morphology. The Th02-Al203 blend showed still some intact AI2O3 agglomerates, and the Th02 kept its platelet morphology unaltered (see FIG. 2 g, h). Gentle mixing of AI2O3 with Th0 2 was also applied for the milled Th0 2 to allow comparison of the two processes.

Example 8 : Results on Density, grain size and porosity analysis in sintered pellets The variables in the pellet processing include mechanical processing of the Th0 2 powder (as received vs. pre-milled), addition or not of 0.26 wt% AI2O3, sintering atmosphere (air vs. reducing) and dwell temperature and time (1625 °C, 1 h and 1750 °C, 8 h) (see description above and FIG. 1). The results of the density analysis of the various batches are given in Table 1. The densities are given in absolute values (g/cm 3 ) and as fraction of the theoretical density (TD). The theoretical density of the mixtures was calculated taking 10.00 g/cm 3 as theoretical density for Th0 2 and

The as-received Th0 2 powder had low sinterability. At 1625 °C, the densities were barely reaching 80% TD under both reducing and oxidizing conditions. At 1750 °C, the density reached 88% TD under oxidizing and 91% TD under reducing conditions. The addition of AI 2 03to the as-received powders increased the sinterability substantially. At 1625 °C, the density was already 86% TD under oxidizing conditions and reached 90% TD under reducing conditions. At 1750 °C, final densities of 95% were achieved under both conditions (Table 1).

As expected, milling of the Th0 2 powder yielded well-sinterable starting material: batches produced with milled powder achieved final densities in excess of 98% TD when sintered at 1750 °C, irrespective of the sintering atmosphere. Densities in excess of 96% TD were already achieved at 1625 °C, and also at this lower temperature, the sintering atmosphere did not have an effect on the achieved density. The addition of AI2O3 to milled Th0 2 powder had a slight, but measurable adverse effect on the final density, which still reached densities above 97% TD.

Also at lower sintering temperatures, the density was slightly lower for powder with AI2O3 added as compared to the pure Th0 2 material.

The Scanning Electron Microscopy (SEM) observations were in excellent agreement with the density results. The observations on the material prepared from as-received powder are presented in FIG. 3 and the results on ceramics prepared from milled powders are given in FIG. 4. All observations were made on polished and thermally etched samples. The grain size analysis was determined using the linear intercept method and the grain size distributions are represented in FIG. 5 . The binning is chosen according to the MLI intercepts for the successive ASTM E-112 grain size classes 19 (0.4 μιη intercept) to 8 (20 μιη intercept). Average values of the grain size are also reported in Table 2. Ceramics sintered with as-received Th0 2 powder still had considerable open porosity (FIG. 3 a-d). Pellets sintered under reducing conditions showed a slightly lower porosity and substantially larger grains (2.4 μιη) than pellets sintered under oxidizing conditions (0.9 μιη) (FIG. 5 a). When AI2O3 was added to the as-received powder, the sintering proceeded much further and much less porosity and substantially larger grains were observed (FIG. 3 e-h and FIG. 5 b). In contrast to the case of pellets produced with pure, as-received Th0 2 , there was no effect of sintering atmosphere on the final density or on grain size for the doped pellets (11 ± 2 μιη under reducing conditions and 9 ± 2 μιη under oxidizing conditions). For both atmospheres, the remaining porosity was largely intragranular. In FIG. 3 e, one observes a second phase with a slightly different shade and surrounded by porosity. Similar observations were made for all AI2O3 doped samples and as will be shown below, these second phases are large AI2O3 agglomerates.

With milled powder, the sintering readily proceeded to almost complete density for the undoped Th0 2 (Table 1), and this was confirmed by the SEM observations (FIG. 4 a-d and FIG 5 c). Especially for the Th0 2 sintered in air, the remaining porosity was very low (FIG. 4 a,b) and was in majority located on grain boundaries. Ceramics sintered with milled powder under reducing conditions had a slightly higher porosity and mainly the intragranular fraction was higher. The grain size for both oxidizing and reducing conditions was of the order of 10-11 μιη.

When AI2O3 was added to milled Th0 2 powder, the porosity was slightly higher and coarser than for the undoped milled Th0 2 regardless of the sintering atmosphere (FIG. 4 e-h and FIG. 5 d). The average value of the grain size slightly differs between the milled, doped systems sintered under reducing (7 ± 2 μιη) versus oxidizing (11 ± 2 μιη) atmosphere. From the grain size distribution (FIG. 5 d) the difference is also apparent. Under reducing conditions, grains are slightly smaller and the grain size distribution is more peaked (70 % falls in the class 5.0 μιη < M LI < 7.1 μιη) than under oxidizing conditions.

Example 9 : Results on Lattice parameter of Th0 2 and AI2O3 distribution

A careful assessment was made of the lattice parameter of Th02 to examine whether Al would enter into solid solution. The lattice parameter of the as-received Th0 2 powder was determined as 559.75 ± 0.01 pm, in excellent accordance with the reference value for Th0 2 (559.7 ± 0.1 pm). The 422 reflection had a full width at half maximum of 0.15 °2Θ. Unit cell parameter and peak widths of the 422 reflection for all sintered samples are given in Table 2. For all sintered samples, the peak widths were significantly smaller than that of the starting powder (FWHM of 0.08-0.09 °2Θ for all sintered samples vs. 0.15 °2Θ for the powder) due to crystal growth. Within the experimental uncertainty, the unit cell parameter of all samples was identical (559.73 ± 0.01 pm to 559.74 ± 0.01 pm), regardless of sintering atmosphere or dopant addition. The absence of lattice contraction or dilation with addition of AI2O3 indicates that the solubility of Al in Th0 2 is negligible at room temperature. In Table 1 the sample identification reflects the production path (†): "Th" stands for Th0 2 , the number indicates the powder processing ("1" represents as-received Th02 and "2" milled powder), "Al" stands for AI2O3 addition and the final character indicates the sintering atmosphere: "A" for oxidizing and "B" for reducing conditions. Densities are obtained by immersion, except for low-density samples, indicated by (*), where the geometrical density is used.

Table 1 : Powder characteristics, synthesis conditions, green density and density after sintering at 1625 °C (1 h) and 1750 °C (8 h) Grain size

Sample ID

Cptn) Lattice parameter

(pm, ± 0.01 ) l- HM 422 reflection (°2Θ)

T €>2 powder 0.09 * 559.74 0.15

Thl-A 0.9 ± 0.2 559,74 0.08

Thl-B 2.4 ± 0.6 559.73 0.08

Till A I- A 9 + 2 559.73 0.09

ThlAI-B 1 1 ± 2 559.73 0.08

Th2-A 1 1 ± 2 559.73 0.08

Th2-B 10 ± 3 559.73 0.09

Th2Al-A 1 1 ± 2 559.74 0.08

Th2Al-B 7 + 2 559.74 0.09

Table 2 : Grain size, lattice parameter and peak width of the 422 reflection of the samples sintered at 1750 °C (8 h)

In Table 2 the grain size is derived by MLI on sintered and thermally etched samples ( ) except for the Th0 2 powder, indicated by (*), where the grain size is derived from Rietveld refinement of the XRD pattern. The local chemical analyses performed by Energy Dispersive and Wavelength Dispersive X-ray Spectrometry (EDS and WDS) further confirmed that the alumina did not dissolve in the thoria matrix and remained present as AI2O3 precipitates. Elemental mapping by EDS showed the presence of AI2O3 as precipitates and agglomerates. In all samples, some large (~30 μιη), AI2O3 agglomerates were found (FIG. 6 c), in accordance with the SEM observations of the powder (FIG. 2 g, h) which evidenced that the gentle mixing process does not destroy all the AI2O3 agglomerates. The AI2O3 was also present as finely distributed submicron particles (FIG. 6 a and b). Zones with higher densities and zones with lesser densities of AI2O3 precipitates were observed, indicating that homogeneous mixing was not achieved. The matrix concentration was analyzed by WDS analysis, but it could only be ascertained that the Al concentration in solid solution with the Th02 matrix remained below the detection limit of 50 μg/g. Discussion

The as-received Th0 2 powder had a coarse, platelet morphology. As expected, it was readily established that at 1625 °C and even at 1750 °C, as-received Th0 2 sintered incompletely both under air and under reducing atmosphere. At 1625 °C, there was no significant difference between the densities obtained for different atmospheres (TD≡ 80%), whereas at 1725 °C, a slight but significant sintering atmosphere effect was observed with a higher density and larger grain size under reducing conditions than under oxidizing conditions. With milled Th0 2 powder, sintering is known to be highly improved, and this was also confirmed in the present study, where densities above 96% TD were obtained at 1625 °C and almost 99% TD at 1750 °C. Unlike sintering with as- received powder, identical results were obtained under reducing or oxidizing sintering atmosphere, both in terms of final density and in grain size.

While sintering of Th0 2 is not expected to be highly influenced by variations in sintering atmosphere (vacuum, inert, oxidizing or reducing) given the stability of Th in its tetravalent state, the present observations can be understood in terms of the creation of defects on the anion sublattice at elevated temperatures. At temperatures above 1400 °C, a sub-stoichiometric domain may set in under highly reducing conditions. In the Th-Th0 2 phase diagram, a maximum deviation from stoichiometry Th0 2 - , with x = 0.13 was reported at 2730 °C. The stability of Th0 2 is, however, much higher than that of U0 2 , and the development of macroscopically measurable deviations from stoichiometry requires highly reducing conditions. In their assessment of thermodynamic properties in the Th-0 system at elevated temperatures, Ackerman and Tetenbaum have experimentally analyzed the oxygen potential above substoichiometric Th0 2 - . From their analyses, expressions for the partial molar enthalpy (ΔΗ02) and partial molar entropy (ASo 2 ) of diatomic oxygen solution in substoichiometric Th0 2 - were obtained as a function of departure from stoichiometry. The expected deviation from stoichiometry can then be calculated for any given oxygen potential and temperature. Under the currently applied conditions (po2 = -420 kJ mol "1 ), only very small deviations from stoichiometry are expected (x≡ lO "7 at 1750 °C and x≡ 10 "9 at 1625 °C). The present observations indicate that for the well-sinterable milled powder, the small effect induced by the sintering atmosphere goes unnoticed, but for powders which have poor cation mobility, the effect of even moderately reducing conditions on the cation mobility is noticeable. While milling is an effective means to improve the sinterability of Th0 2 , the handling of very fine powders which are susceptible to electrostatic charging may cause operational concerns, and suitable doping may offer a more appropriate solution.

The results obtained with AI2O3 sintering of as-received powder showed a significant improvement over the undoped sintering. At 1625 °C, the pellets reached densities between 86% TD (oxidizing atmosphere) and 90% TD (reducing atmosphere) and at 1750 °C, the density was 95% TD under both conditions. The average value of the grain size also increased from 1-2 μιη for the undoped sintering to 9-11 μιη with AI2O3 addition. This grain size is identical to the range of grain sizes that were obtained after intensive milling of the starting powder. Densities close to 95% TD and grain size in the order of 10 μιη are generally aimed at in nuclear fuels for Light Water Reactors.

Doping of milled powder with AI2O3 does not have an additional effect on grain growth or density as compared to results obtained with milled, undoped Th0 2 . Both at 1625 °C and at 1750 °C, the densities achieved with milled and doped powder are slightly but systematically lower than without AI2O3 doping. At 1625 °C, the densities achieved with milled, AI2O3 doped powder are identical both under reducing and oxidizing atmospheres and reached 95% TD which is around 1.5% lower than achieved without doping. At 1750 °C, the densities were also identical for both atmospheres and were slightly above 97% TD, which is around 1.5% lower than for the undoped systems. In all cases, it was mainly the intragranular porosity which was higher for the milled, doped system compared to the milled only case, indicating that 1750 °C was a slightly too high sintering temperature for the milled, doped system resulting in porosity trapping. The grain sizes for the milled + doped system are not affected under oxidizing sintering conditions and are slightly reduced under reducing conditions: there are no indications for undesirable, excessive grain growth. A careful assessment was made of the lattice parameter of Th0 2 to examine whether Al would enter into solid solution. Given the large difference in ionic radius (Al 3+ has an ionic radius of 53 pm, which is half of the Th 4+ ionic radius: 106 pm), one expects low solubility of Al in Th0 2 , but at the same time, the large difference in ionic radius would cause significant lattice distortions even when only a small amount of Al would go into solution. The reference value for the Th0 2 lattice parameter is 559.7 ± 0.1 nm as reported in the JCPDS database. Other values have been reported as well, ranging from 559.2 pm to 556.0 pm. Our analyses showed highly precise and very consistent values for the Th0 2 lattice parameter, regardless of the sample processing (doping, milling or sintering atmosphere). It was found that the unit cell parameters were all identical within experimental uncertainty. The X-ray diffraction results evidenced the non- solubility of Al in the Th0 2 matrix at room temperature. The amount of Al in solution in the Th0 2 matrix is also below the detection limit of the WDS analysis (50 μg/g).

While the current experiments indicate negligible solubility of Al in Th0 2 after cooling to room temperature, one might expect that in analogy to the AI2O3 - Zr0 2 system, solubility increases with temperature. The solubility limit of AI2O3 in Zr0 2 is around 5 mol% at 2133 K, it reduces to around 1-2 mol% at the tetragonal-monoclinic phase transition at 1423 K and reduces further to vanishingly small values at low temperatures. The improved sintering of Zr0 2 by small AI2O3 additions is attributed to reduced volume diffusion activation energy when AI2O3 dissolves in the Zr0 2 matrix at elevated temperatures. Upon cooling, the excess Al 3+ ions first segregate to the Zr0 2 grain boundaries, where they form a solid solution space charge layer of ~5 nm and at elevated dopant concentrations above 0.25 wt% AI2O3, discrete AI2O3 precipitates are observed. A similar mechanism with a finite solubility of AI2O3 in Th0 2 at elevated temperatures could explain the current observations of enhanced sintering and the precipitation of AI2O3 after cooling to room temperature.

It is an advantage of embodiments of the present invention that AI2O3 substantially increases the sinterability of coarse-grained (2.4 m 2 /g), as-calcined Th0 2 . Compared to the undoped sintering behavior, a gain of 7% was obtained both at 1625 °C and 1750 °C under oxidizing conditions. Under reducing conditions, the AI2O3 effect was more pronounced at lower temperatures, with a 10% density increase at 1625 °C. Because undoped Th0 2 benefits from increased sintering rates under reducing conditions at temperatures exceeding 1625 °C, the AI2O3 effect at 1750 °C was slightly lower (4% density increase).

Nearly identical final densities (95% TD) were achieved for AI2O3 doped, coarse grained Th02 compacts at 1750 °C regardless of the sintering atmosphere. The addition of AI2O3 to milled Th02 powder (5.6 m 2 /g) slightly reduced its sinterability by 1.5%, but both under oxidizing and reducing conditions, the final densities at 1750 °C were still well above 97% TD. The increased sintering rate of pure Th02 under reducing atmosphere at elevated temperatures is attributed to the creation of intrinsic anion defects.

A high precision unit cell analysis was performed to assess the solubility of Al in the Th02 matrix. For all samples, the unit cell parameter was found to be identical and showed a very narrow spread between all samples (559.74 ± 0.01 pm). The values agree with the currently adopted unit cell value for Th02 (559.7 ± 0.1 pm according to the JCPDS) and have a more narrow spread. X-ray diffraction results thus indicate that AI2O3 does not dissolve in the Th02 matrix at room temperature, and also the microprobe analysis could not detect any solid solubility of Al in Th02.