Review of Canadian Experience With the Fabrication of Thoria-Based Fuels for Advanced Reactors and Fuel Cycles for Long-Term Nuclear Energy Sustainability and Security

Advanced nuclear fuels and fuel cycles for conventional reactors (such as Gen-III+) and other reactor types, including Gen-IV, small modular reactors (SMRs), microreactors, and other types of advanced reactors (AR's) are important for ensuring long-term nuclear energy security and sustainability. Thorium-based fuels are an example of an “advanced fuel”, which are of interest due to the large energy resource potential of thorium (thorium is nearly 3 to 4 times as abundant as uranium [1,2]). As a fertile fuel, thorium can be used to augment and extend uranium resources, and to help maximize nuclear fuel resource utilization through conversion by neutron absorption of fertile ²³²Th to fissile ²³³U.

Canadian Nuclear Laboratories (CNL), formerly Atomic Energy of Canada Limited (AECL), Chalk River Laboratories (CRL) has over 50 years of experience in fabrication and irradiation testing of various fuels made with thoria (thorium dioxide, ThO₂), which includes extensive experience in fuel pellet fabrication [3].

Thoria, as well as its solid solutions with uranium dioxide and plutonium dioxide (PuO₂), presents a fluorite crystal structure, similar to urania (uranium dioxide, UO₂). This structure has proven to have excellent resistance to neutron and fission fragment damage [4].

Pure ThO₂ pellets can be sintered in any atmosphere (oxidizing, reducing, or inert), as thorium has one main practical oxidation state (IV) and powder oxidation/reduction is not usually possible. It is known and recognized that thorium also has a +3 oxidation sate, although it results in a very narrow region of stability for hypo-stoichiometric thoria [5]. ThO₂ fuel pellet fabrication uses similar processes and equipment to that of UO₂ fuel fabrication. The production of high density ThO₂ pellets mainly depends on the particle size distribution, morphology, and sinterability of the starting powder.

Since thorium lacks a fissile isotope, most ThO₂ pellet fabrication processes include a step to add a fissile component, such as enriched UO₂, PuO₂, or ²³³U in the form of ²³³UO₂. Typically, there are two methods of combining ThO₂ with a fissile component: solution blending or mechanical mixing. Solution blending produces a uniform distribution and, if the fissile component can be added during the ThO₂ powder production stage, this method is preferred. Mechanical mixing can be achieved by a number of standard processes, including low intensity blending and high intensity mixing or comilling. Similar to the fabrication of Mixed Oxide ((U,Pu)O₂, MOX) fuel, the degree of dispersion of the fissile component into the ThO₂ matrix is a function of the intensity of the mixing method. For example, low-intensity methods produce heterogeneous powder mixtures and, hence, heterogeneous sintered pellets. This result is generally considered undesirable from a fuel performance perspective. In contrast, if the high intensity mixing method is adequate, it can produce a solid solution of ThO₂ and the fissile additive in the sintered pellet. Over the last 50 years, CNL, from now on referred to solely as CNL, has worked toward developing improved manufacturing and fabrication methods, processes, and procedures to make high-quality thoria pellets that could be used for advanced fuels and fuel cycles in current generation, and next-generation advanced reactors (ARs), small modular reactors (SMRs), and perhaps also microreactors.

As an alternative fertile nuclear fuel, thorium holds great energy potential. Thorium is three to five times as abundant as uranium [1,2], and as a fertile fuel, it can be used to breed fissile ²³³U through neutron capture (n + ²³²Th → ²³³Th (22.3-minute half-life) → ²³³Pa (26.9-day half-life) → ²³³U). While thorium-based fuels can be used in any reactor technology, the use of the (Th,²³³U) fuel cycle is special in that it can also be used to achieve high conversion ratios and even net breeding in optimized cores in a thermal spectrum reactor, due to the high neutron reproduction factor for the fissile isotope ²³³U (η∼2.29) in the thermal neutron energy spectrum. Thus, by conversion from fertile ²³²Th to fissile ²³³U, thorium-based fuels can be used to augment and extend uranium resources, which rely upon naturally-occurring fissile ²³⁵U (0.711 wt. % ²³⁵U/U) and fertile ²³⁸U (99.289 wt. % ²³⁸U/U). Thorium-based fuels can also be used to consume existing stockpiles of plutonium separated from spent light water reactor UO₂ fuel (LWR) in the form of (Th,Pu) fuel mixtures, such as (Th,Pu)O₂ [6,7]. This approach can also provide an effective pathway for the disposition/destruction of plutonium from other sources (such as weapons-grade plutonium), akin to the Parallex program between the USA, Russia, and Canada during the mid-1990 s [8].

Since fertile ²³²Th is 6 nucleons lighter than fertile ²³⁸U, the production of heavier radioactive minor actinides, such as isotopes of Americium (²⁴¹Am,²⁴³Am) and Curium (²⁴⁴Cm, ²⁴⁶Cm, ²⁴⁸Cm, and others) in thorium-based fuels is highly reduced, which is beneficial for minimizing potential long-term radioactive waste liability issues associated with high-burnup fuel. In the last 20 years, extensive research has been performed at CNL [9] to evaluate the potential of thorium-based fuels in pressure tube heavy water reactors (PT-HWRs) [10–20] and other reactor technologies, including fast spectrum systems [21], and various SMR technologies [22–24]. Regardless of the reactor technology or fuel cycle chosen, the fabrication of thorium-based fuels that can achieve high burnup levels (30 MWd/kg to 60 MWd/kg) while maintaining excellent fuel performance with fission product retention, good thermal conductivity, and structural integrity is a key issue, and one that CNL has investigated for more than 50 years.

This review paper provides an overview of Canada's contributions to the investigation of the fabrication of thoria-based fuels, primarily through work done at CNL. Importantly, a comprehensive discussion on the various methods for fabricating (Th,U)O₂ fuel is presented. The intent of this discussion is to highlight CNL's capabilities within the fuel fabrication discipline to fabricate, characterize, and assess advanced fuels for the potential use in different AR and SMR technologies of interest for Canada. The contents of this review will also reiterate CNL's ability to provide guidance and subject matter expertise to both governmental and international organizations on the merits and issues of different reactor technologies and their associated fuels and fuel cycles.

The focus of this review paper is on the fabrication and manufacturing of thoria-based fuels. For more information and details on CNL's previous experience with the irradiation-testing and irradiation performance characteristics of thorium-based fuels (such as irradiations in the National Research Universal (NRU) reactor), the reader is encouraged to see discussions in Refs. [25] and [26].

This paper is structured as follows: the production methods of (Th,U)O₂ fuel based on solution blending are discussed in Sec. 2. Section 3 discusses various mechanical mixing techniques used to prepare (Th,U)O₂ material. Section 4 outlines recent fuel fabrication and pre-irradiation characterization efforts undertaken at CNL to investigate how fuel fabrication methods impact fuel quality, which can subsequently have an effect on fuel performance, including defective fuel behavior. It is well-recognized and understood that fuel fabrication methods can affect the grain size of ceramic (thoria) fuel particles and other fuel pellet design characteristics, which will subsequent impact the irradiation performance of such fuels, as discussed in related previous research by CNL scientists [26].

The Solution-Gelation (Sol-Gel) process, also called hydrothermal denitration and based on a method developed at Oak Ridge National Laboratory (ORNL) [27], was used in the mid-1970 s at AECL's Whiteshell Laboratories (WL) to produce (Th,U)O₂ fuel in two forms, microspheres and extrusion clays. It was also used at AECL-CRL in the early to mid-1980 s to produce pure ThO₂. A Sol is a liquid containing solid particles that are evenly distributed and stably suspended in the liquid. The Sol may be transformed to a Gel by the removal of interparticle repulsive forces [28].

Figure 1 depicts the Sol-Gel process used to prepare dense (Th,U)O₂ fuel [28–30]. In the first process step, the Sol was prepared by exposing a thorium nitrate solution (Th(NO₃)₄·4H₂O) to steam at various temperatures in a rotary calciner. The thorium nitrate undergoes an endothermic reaction in steam and decomposes to submicron ThO₂ powder and nitric acid (HNO₃), which is required for the Sol-Gel process as it results in the ThO₂ being readily dispersed in dilute HNO₃. From here, the Sol was prepared by blending ThO₂ in sufficient uranyl nitrate (UO₂(NO₃)₂) solution to give the desired U/Th ratio. At this stage, the UO₂(NO₃)₂ solution contains excess nitric acid to stabilize the ThO₂ particles as a Sol. The pH was adjusted to 3.9 using ammonium hydroxide (NH₄OH), to convert the uranium to species that adsorb on the ThO₂ surface, producing a ThO₂-UO₃ Sol. The ThO₂-UO₃ Sol can then be used to produce either fuel microspheres or clays.

One of the most important variables of the Sol-Gel process is the viscosity of the Sol, which is a function of the uranium concentration, pH, density, and aging time (as with time, continued chemical reactions lead to increased crosslinking during gelation, which can increase the modulus and viscosity of the gel). For example, increases in pH and temperature of U(VI) solutions enhance adsorption of uranium on ThO₂ through hydrolysis of U(VI) [31]. At a pH of about 3.0, the Sols behave as Newtonian fluids, whilst at higher pH values, concentrated Sols transform into thixotropic Gels. Overall, the adsorption of uranium by ThO₂ increases when the viscosity of the ThO₂-UO₃ sols increases.

Once the ThO₂-UO₃ Sol is prepared, it can then be used to prepare fuel microspheres [32–34]. Microspheres were formed by forcing the ThO₂-UO₃ Sol through a nozzle into an organic liquid (2-ethyl hexanol). Under specific conditions, the surface tension of the aqueous Sol in the immiscible organic liquid forms microspheres. Droplets are formed by pumping the ThO₂-UO₃ Sol through vibrating nozzles of various diameters. Droplet size strongly depended on the nozzle diameter. Table 1 shows the effect of the nozzle diameter on the sintered microsphere size.

A mix of dry Argon (>99 at% Ar, less than 1 at% O₂, N₂, or H₂O) and steam at 220 °C removed water from the Sol producing the Gel. Gelled microspheres of (Th,U)O₂ were later dried in a pure, dry Argon atmosphere, and sintered to high density at about 1300 °C. Sintered (Th,U)O₂ microspheres presented densities higher than 9.9 g/cm³ and were subsequently vibration-packed into fuel cladding to produce fuel called Spherepac [33].

The ThO₂-UO₃ Sol can also be used to prepare clay for fuel extrusion [29,30,33]. After the ThO₂-UO₃ Sol was prepared, it was mixed with an organic binder (a phenolic resin) and filtered by dry suction. The resulting filter cake (paste) containing from 15 to 24% moisture was then mixed to eliminate local variations in moisture content and extruded in a laboratory-scale hydraulic ram press. The extruded wet slugs were then transferred to a roll rack, cut, dried at 100 °C, and sintered in a reducing atmosphere at 1600 °C. Fuel rods with a density higher than 9.5 g/cm³ were successfully produced during efforts conducted at the AECL-WL. Figure 2 depicts an image of the fuel pellets produced with this method. Figure 3 shows micrographs of the sintered pellets. Note: diameter of fuel pellets is ∼1.21 cm.

Co-precipitation results in a completely homogeneous mixture of thorium and uranium, since the mixing occurs at an atomic scale. The development of the coprecipitation technique took place during 1979-1981 at AECL-CRL. In this method, a (Th+U) nitrate solution was prepared by mixing uranyl and thorium nitrate solutions [35–38]. After reducing the uranium to U(IV), (Th+U) salts were coprecipitated by adding solid oxalic acid ((CO₂H)₂) to the solution. The precipitated oxalate was then filtered, washed with distilled water, dried, granulated and calcined. The scanning electron microscope (SEM) characterization showed that the (Th+U) coprecipitated particles presented a square platelet geometry, typical of oxalate-derived ThO₂, with the mass fraction mean particle size being ∼1.34 μm. This as-produced powder however, exhibited high friction on pressing and severe pellet end-capping was observed. In order to overcome this problem, the powder was agglomerated by wet grinding in a stirred-ball mill.

Another technique used by CNL to incorporate the fissile components into ThO₂ pellets was solution impregnation. In this technique, pressed green (nonsintered) pellets of pure ThO₂ were impregnated by immersing the pellets in enriched UO₂(NO₃)₂ solution, resulting in the green pellet absorbing the UO₂(NO₃)₂ solution [33,39,40]. Following impregnation, the pellets were rinsed and dried with the UO₂(NO₃)₂ remaining in the green pellet. Standard sintering methods resulted in ThO₂ pellets having the uranium in solid solution. However, it was found that the uranium was not uniformly distributed within the pellet. Figure 4 depicts images of longitudinal cross section of green ThO₂ pellets impregnated for different times. Analysis showed that after 30-40 min of impregnation time, essentially pure water reached the pellet center, since most of the dissolved UO₂(NO₃)₂ was deposited in the outer regions of the ThO₂ fuel pellet.

This work indicated that uranium (in oxide form) preferentially bonds with the ThO₂ particles on the surface of the fuel pellet, resulting in the retention of the uranium on the periphery of the pellet while pure liquid is drawn to the center through the pores, and interstitial gaps between ThO₂ grains. Therefore, this method resulted in fuel pellets being uranium rich in the periphery and pure ThO₂ in the center.

The solution impregnation fuel concept is attractive from an irradiation performance standpoint. In this fissile distribution, the initial concentration of fissile material is in the periphery of the pellet and closest to the coolant. Therefore, it can be expected to reduce fuel operating temperature in a similar manner to duplex-type (a heterogeneous dual-annulus fuel pellet in which the inner fuel pellet annulus core has a different fissile content than the outer fuel pellet annulus shell) fuel.

The other attractive feature of this fabrication method, particularly for plutonium addition, is the fact that green pellet fabrication can take place without the use of a glove box, for more convenient manipulation, since the green pellet has low radiotoxicity, and other protocols and procedures can be implemented to ensure adequate radiation safety protection. Subsequent pellet processing operations will need take place in a glove box (since they involve Pu or ²³³U). This approach greatly reduces the size and complexity of the containment (glove boxes) required for pellet fabrication compared to traditional methods for fissile material addition.

Two key steps of the ThO₂-based fuel pellet fabrication process are the addition of the fissile component and the sintering of fuel compacts. Since the 1960s, CNL has established research programs to investigate ThO₂-based fuel fabrication processes. Most ThO₂ pellet fabrication performed at CNL has involved some type of treatment to the as-received powder to improve its sinterability and/or to add a fissile component. The following is a list of mechanical mixing techniques examined by CNL, as discussed in Ref. [39]

• Wet attrition milling;

• Homogenizer (wet);

• Twin shell blender with an intensifier bar (a high speed rotor) followed by two passes through a high speed blade mill;

• Tubular mixer;

• High intensity mixer; and

• Dry vibration milling (several variations).

Attrition milling was used in the 1970s to produce (Th,U)O₂ pellets for a great number of experiments. The attrition mill consisted of a vertically-oriented pot for mechanically reducing solid particle size by intense agitation of the powder material being milled along with coarse milling media. The powder material can be added, as a slurry (wet), or directly, as dry powder, to the media with CNL pursuing efforts associated with the attrition milling of ThO₂ and UO₂ in a wet state [38]. This method proved to have a fast throughput and be a very effective method for breaking up UO₂ agglomerates. After milling, the milled slurry was poured into a pan and the water was allowed to evaporate. This drying method resulted in the production of very hard dried cakes, which were broken up by forcing them through a standard sieve screen. The resulting granules made a free flowing, final press feed, and green pellets of adequate strength for handling were produced. Sintered densities of about 9.6 g/cm³ were fabricated. Microscopic examination of cut and polished pellet surfaces revealed granular-like structures, indicating that some of the cake granules had not been completely broken up during final pressing and their structure remained in the pellets after sintering. Various granular-like pellet structures were created and investigated using the wet attrition milling method. Figure 5 depicts two examples of granular-like microstructures observed in ThO₂-based fuel pellets. Figure 5(a) shows a granular-like microstructure with a very dense and homogeneous structure within the granules, whilst porous (low density) regions were observed between granules. Figure 5(b) shows a granular-like microstructure with dense and homogeneous granules surrounded by very large open pores. Porous regions provide an easy path for fission gasses to escape to the element free volume and simultaneously act as a heat conduction barrier.

(Th,U)O₂ pellets were also produced at CNL by wet mixing highly enriched uranium dioxide (HEU-UO₂) and ThO₂ using a homogenizer. The homogenizer consisted of slotted rings, rotating at very high speed against static rings. The gap between the rings generated intense sheer forces that broke up the powder agglomerates. The HEU-UO₂ powder was then added to de-ionized water and dispersed, followed by the addition of ThO₂ powder. Following homogenization, a long chain water-soluble flocculant was added. The flocculant functional groups bonded with the powder particles, resulting in rapid powder settling. The bulk of the water above the settled powder was decanted, the remaining sludge was vacuum filtered, and the cakes were oven dried. Softer dried cakes were produced with this method than with the attrition-milling/pan-drying method. The dried cakes were granulated using a screen and the product was fed directly to the final press to produce green pellets. Despite the fact that a high-intensity mixer was used to mix the powder, HEU agglomerates were observed in the green and sintered pellets amongst an otherwise uniform homogeneous microstructure.

The fabrication process of ThO₂-based fuel pellets is similar to the fabrication process of standard UO₂ and MOX fuel pellets. The essential steps are milling/blending, granulation and binder addition, cold pelletization, and sintering. These fabrication steps are well understood and the associated process equipment is readily available.

The experience of CNL with ThO₂-based pellet fabrication has shown that the sinterability and physical properties, such as particle size distribution and surface area, of the starting powder are some of the most important factors for a successful ThO₂-based fuel fabrication process. Although good quality ThO₂ powders are readily commercially available, powders from different lots or suppliers do not behave consistently during fuel pellet fabrication. This problem occurs mainly because ceramic grade ThO₂ powder is not in high demand and ThO₂ powder specifications and production processes have not been developed sufficiently. Consequently, most of the ThO₂ powders have required pretreatment in order to achieve the specified high sintered densities.

ThO₂ and UO₂ or PuO₂ are miscible in all proportions. However, solid solutions of these powders are difficult to achieve during the sintering process. Since sintering is a solid-state diffusion process, metal atoms/ions diffuse at a much slower rate than nonmetal atoms/ions [41,42]; therefore, the distance that metal atoms/ions diffuse is limited to a few microns. As a result, the mixing method must be able to produce a well dispersed powder mix, typically at the level of individual micrometer sized particles [39]. Figure 6 depicts a sample flowchart of a ThO₂-based fuel pellet fabrication process, including three mixing methods, adapted from Refs. Dimayuga [43] and Dimayuga et al. [44].

The microstructure and the fissile component (U or Pu) distribution in ThO₂-based fuels strongly depend on the method selected to mix the ThO₂ and the fissile component (UO₂ or PuO₂) powders. CNL has investigated the effect of the dry mixing methods on the quality of the sintered pellets, in terms of the density, microstructure, and the fissile component distribution. In particular, development work has been performed using three different methods and their combinations (Fig. 6).

Low intensity blending methods are usually combined with a high intensity method to improve fissile component distribution. This method typically produces pellet microstructures containing nearly pure fissile component (UO₂ or PuO₂) regions in a ThO₂ matrix. In the early to mid-1960 s, (Th,U)O₂ pellets were fabricated using a two-stage mixing process to blend the powders. The first stage consisted of a twin shell blender with an intensifier bar (a high speed rotor). This twin shell blender produced a homogeneous powder mixture after 2 h. The second stage consisted of a high-speed blade mill, which was used to break up UO₂ agglomerates and increased powder homogeneity. After powder blending, some UO₂ agglomerates remained and the pellets produced with this method presented granular-like microstructures [35].

During the 1980s, (Th,Pu)O₂ pellets were also produced at AECL-CRL using a Turbula blender to mix ThO₂ and PuO₂ powders [45]. The Turbula is a low-intensity blending technology that consists of a cylindrical vessel tumbling in modified end over end fashion, similar to a paint shaker. Fuel pellets contained 86.05 wt. % Th and 1.53 wt. % Pu in (Th,Pu)O₂.

The produced sintered (Th,Pu)O₂ pellets presented an average pellet density of 9.47 g/cm³. Average grain size of sintered pellets was 3–4 μm. Figure 7 depicts an alpha autoradiograph of a sintered pellet. Distinct PuO₂ particles were (seen as dark spots in Fig. 7) were found to survive the blending stage and appeared in the sintered microstructure [45].

The fissile plutonium concentration in the fuel (the dark spots) was macroscopically uniform or relatively homogeneous, although at a more microscopic level (over the length scale of ∼1 mm), it was more heterogeneous. Qualitatively, from a practical perspective on the length scale of the MOX fuel pellet (∼1.2 cm), the PuO₂ is mixed relatively well with the ThO₂. It is expected that very fine mixing of the PuO₂ and ThO₂ grains to create a more homogeneous, fine-grain structure in the fuel pellet will help reduce fission gas release (FGR) under irradiation, as discussed by Barry in Ref. [26].

Another method used at CNL involved the use of two types of high intensity mixers to blend ThO₂ and the fissile component (UO₂ or PuO₂) powders. First, a high-intensity vibratory mill was used to comill the powders and prepare a high-fissile additive concentration (about 30 wt. %) Master Mix. This method reduced the size of any large powder particles and broke up any agglomerated fissile component powder due to the high intensity milling. The final desired fissile component concentration was achieved by blending an appropriate amount of Master Mix with ThO₂ powder using either a high or a low intensity blender. The high intensity blender consisted of a cylindrical stainless steel vessel rotating perpendicularly to the cylindrical axis. Intensifier blades rotating at high speed protrude into the mixing vessel through the axis of rotation, breaking up the agglomerated powder upon impact. This kind of powder blending typically results in a sintered pellet microstructure in which areas of a homogenous distribution of PuO₂ particles within the Master Mix are contained in the ThO₂ matrix. This result occurs because the high-intensity blending method employed to combine the Master Mix with the ThO₂ powder may not break up the agglomerated hard Master Mix.

A large part of the ThO₂-based fuel development work at CNL has focused on comilling ThO₂ powder with the fissile additive (UO₂ or PuO₂). Milling is an extremely high-energy process that is capable of breaking up agglomerated powder, as well as large individual powder particles. In comilling, both ThO₂ and fissile powders are added simultaneously to the vessel containing a milling media. The milling-media/powder mixture is intensely agitated by a high-speed electric motor connected to an off-center weight that creates high-frequency, low-amplitude vibrations. Colliding milling media breaks up powder particles and agglomerated powder and generates a homogenous powder mix.

(Th,U)O₂ pellets of various compositions were fabricated at AECL-CRL in the 1960s using a two stage milling process [46]. The powders were dry blended with Sterotex powder and then dry ball-milled for 2 h in porcelain pots with porcelain balls. The resulting powder mix was then forced to pass twice through a laboratory swing hammer mill. The uniformity of mixing was confirmed by gamma (γ)-spectrometry of powder aliquots. The milled powder mixtures were prepressed, granulated, and pressed into green pellets at a pressure of about ∼310 MPa. Green pellets were sintered in a reducing atmosphere of nitrogen-hydrogen (N₂-H₂) gas for approximately 3 h. The results showed that the average density of sintered pellets decreased from 9.61 to 9.36 g/cm³, as a function of the uranium concentration. The average grain size for all compositions lay in the range of 13 to 15 μm.

In the early to mid-1980 s, (Th,Pu,)O₂ pellets were fabricated at CNL using a dry vibration comilling method composed of a hardened tool steel vessel containing two tungsten carbide balls as the milling media [47]. Compaction was in a single-ended steel die at a pressure of 2250 kg/cm². Typical green pellet density was of the order of 7 g/cm³. Sintering was conducted at high temperature in a reducing atmosphere of argon-hydrogen (Ar-H₂) gas for approximately 2 h. Average sintered pellet density was about 9.64 g/cm³. Although a solid solution was not observed, an even distribution of PuO₂ particles in a ThO₂ matrix was achieved.

Recently, (Th,U)O₂ pellets were produced by dry comilling ThO₂ and HEU UO₂ powders using a vibratory mill [48]. The milled product was fed directly to a final press to produce green pellets that were sintered at high temperature. Sintered pellets presented a very high theoretical density (about 98.7%). Figure 8 depicts a micrograph of a typical sintered pellet microstructure. Ceramography (the examination and evaluation of ceramic microstructures) of sintered pellets revealed uniform grain-like microstructures with minimal porosity and no residual granules, as well as an even uranium distribution in the ThO₂ matrix. The average grain size was 6–8 μm.

Most recently, CNL's Thoria Roadmap Project conducted a review of science and technology (S&T) requirements to support water-cooled reactor designs incorporating cylindrically-clad ceramic fuel pellets.

As part of this project, CNL conducted a number of ThO₂-based pellet fabrication trials using dry vibration milling of pure ThO₂, ThO₂-UO₂ and ThO₂-PuO₂ powders. Thorium-based simulated high-burnup nuclear FUEL (SIMFUEL) fabrication experiments were conducted to estimate thermal properties. Thorium-based SIMFUEL was fabricated using UO₂ with varying uranium content. Thermal conductivity determination of the SIMFUEL pellets was conducted at both the Institute for Transuranium Elements (ITU) in Karlsruhe, Germany (now Joint Research Center (JRC) -Karlsruhe)) and CNL [49].

Fabrication experiments of pure ThO₂ and ThO₂-PuO₂ fuel pellets were also conducted. The fabrication trials sought to produce a high pellet density and optimal microstructure in terms of grain size and uranium or plutonium homogeneity. The following subsections present two recent experiments performed to evaluate the effect of fabrication parameters on the quality of the fuel.

A series of fabrication experiments were performed to evaluate the effect of the morphology and particle size distribution of ThO₂ powder on the pellet density and microstructure. ThO₂ powder with 99.99% purity was acquired from Rone-Poulenc Inc. The ThO₂ powder was milled in a vibratory mill using zirconia (ZrO₂) cylinders as milling media. Milled ThO₂ powder samples with different particle size distributions were produced using 1-hour, 2-hour, 3-hour and 4-hour milling times.

The as-received ThO₂ fuel pellets were produced following a conventional fuel fabrication process of pre-pressing, granulating, final pressing, and sintering. Fuel pellets produced with milled ThO₂ powder were fabricated using only final pressing and sintering.

Figure 9 shows SEM images of as-received (a) and 3-hour milled (b) powder samples, along with their semilog plots of the particle size distribution and the corresponding fitting to a series of Gaussian functions. A plate-like morphology was observed in the as-received ThO₂ powder (Fig. 9(a)).

Powder properties, such as average particle size, specific surface area, and grain size, have been shown to influence the sinterability of powder [50]. Dynamic Light Scattering (DLS) analysis shows a typical log-normal particle size distribution with an average particle size of 9.07 μm and standard deviation of 5.54 μm. A Gaussian peak separation fit shows three overlapped distributions centered at 3.5, 8.8 and 15.2 μm with areas of 4, 34 and 62%, , respectively. The 3-hour milled powder (Fig. 9(b)) shows a log-normal bi-modal particle size distribution typical of one-stage milling processes, with a submicron and micron distribution. Gaussian fittings of the 3-hour milled powder particle analysis reveal that the peak corresponding to the micron size distribution can be further decomposed in two overlapped distributions. Table 2 shows the average and standard deviation of the particle size distributions determined by DLS along with the Gaussian fitting peak parameters. No significant changes to the particle size distribution were observed between 1-hour and 2-hour milling times. The micron peak became less sharp and wider as a function of milling time. The centers of the overlapped peaks show that the micron particle distribution is composed of two distributions with averages of about 2 and 4 μm. While the average of the second overlapped peak remained constant at 2 μm, the average of the third peak increased to about 8 and 9 μm, for 3 h and 4 h of milling time, respectively. This increase might be due to the agglomeration of fine particles. Fine powders tend to form agglomerated masses due to van der Waals and electrostatic forces. Figure 9(b) shows that the 3-hour milled powder sample presents particle agglomerates larger than 10 μm. Although the size of individual particles cannot be easily identified by SEM, a number of particles in the micrograph scale range can be identified as the agglomerates. This result suggests that the ultrasonic time or intensity used to disperse the powder for DLS might be insufficient to disperse these agglomerates during the analysis.

The average submicron particle size was about 0.3 μm for the four different milling times. The percent under the curve of the submicron particle distribution remained almost constant (about 18%) for 1-hour, 2-hour and 3-hour milling times and then increased to about 33% for 4 h of milling time. This result brings to question the effect of the milling time on the integrity of the milling media. As longer milling times favor the removal of surface material of the milling media by self-abrasion, this might eventually provide erroneous results on the particle size distribution measurements.

Figure 10 shows the powder average particle size and the pellet green density and immersion density as a function of the powder milling time. Figure 11 depicts the open, closed, and total porosity as a function of the powder milling time. In these figures, values at zero hours correspond to the values obtained for pellets produced with as-received powder. Error bars were evaluated as ± 1 standard error.

The highest sintered densities were achieved when 1-hour and 2-hour milled powders were used. Sintered densities slightly decreased when 3-hour and 4-hour milled powders were used. The lowest average sintered density (about 9.07 g/cm³) was produced when as-received powder was used as starting material. These low sintered density values may be associated with the morphology (shape and particle size distribution) as well as low surface area of the as-received powder. Irregularities of plate-like particles prevent particle mobility, producing gaps between particles and creating large open voids during pellet pressing in the as-received powder (Fig. 12). When milled powder was used, however, compacts with relatively higher green densities (compared to those achieved with the as-received material) were produced. Pellets produced with 1-hour, 2-hour and 3-hour milled powder presented green densities higher than 6.60 g/cm³ while the green density of pellet produced with as-received powder was only 6.42 g/cm³.

Figure 12 depicts a ceramographic image of an unetched pellet produced with as-received powder. A typical granular-like microstructure can be observed, showing a number of residual granules that have not homogenized during the final pressing and sintering steps. Significant porosity was observed throughout the pellet. The large amount of interconnected pores may be associated with the number of granular-like regions observed on the pellet micrograph, as well as potentially associated with the plate-like shape of the particles. Particle irregularities hinder particle mobility and the ability to attain maximum compaction (as seen with the granules highlighted in Fig. 12). In this case, it is thought that the plate-like shape (null-sphericity) of particles produce gaps between grains and create large open voids during pellet pressing. Random arrangements of anisometric particles typically have higher porosity and a wider range of pore sizes [51].

Pellets produced with 1-hour and 2-hour milled powder also presented higher green densities than pellets produced with 3-hour and 4-hour milled powder. Milled powder produced with 1-hour and 2-hour milling time exhibited log-normal bi-modal particle size distributions, with well-defined submicron and micron peaks, when analyzed with DLS (also referred to as Particle Size Analysis (PSA). Milled powder produced with 3-hour and 4-hour milling times exhibited log-normal bi-modal particle size distributions, with a well-defined submicron peak but a wider micron peak. It is well known that compacts produced with powders having two distinct particle size distributions exhibit higher compaction factors than compacts produced with powders having only one particle size distribution due to smaller particles being distributed and introduced into the interstices of larger particles, thereby reducing the pore size and the overall porosity of the compacts [51]. Higher compaction factors (Volume before compaction/Volume after compaction), with better particle-to-particle physical contact will help improve pellet densification during the sintering process. The open porosity of pellets produced with milled powder remained almost constant for the four different milling times (Fig. 11). A lower amount of closed porosity was observed in pellets produced with powder having well-defined submicron and micron peaks (1-hour and 2-hour milling time) compared to those produced with milled powder having a wider micron peak (3-hour and 4-hour milling time). This result confirms that both the particle shape and size distribution, play key roles during the densification and grain growth processes of the green pellets.

Resintering experiments were performed on (Th,²³³U)O₂ fuel pellets to evaluate the densification behavior of sintered pellets, in terms of pellet density, porosity, dimensions, and grain size distribution. The resintering of manufactured (Th,²³³U)O₂ fuel could potentially occur during reactor operations when fuel pellets, particular those in high-power density fuel elements near the center of the reactor core, are at elevated fuel temperatures. The fresh (Th,²³³U)O₂ pellets were sintered previously at ∼1,700 °C (1,973 K), although they were not sintered long enough to achieve the maximum densification. Typical resintering densification tests of nuclear fuel are performed at or slightly above 1,700 °C (1,973 K) for 24 h, under reducing conditions. These tests are carried out to verify that there will not be excessive in-pile densification of nuclear fuel. Some nuclear fuels exhibit densification as a result of irradiation due to the elimination of small pores in the fuel pellets.

Fuel pellets made of (Th,²³³U)O₂ were manufactured at AECL-CRL in 1986/87 using a mix of ²³³UO₂ and ThO₂ powders. The particle size of both powders was below 325 mesh screen (less than 44 μm). The (Th,²³³U)O₂ pellets were fabricated following a conventional fabrication process of blending, prepressing, granulating, final pressing, sintering and grinding. The pellet density was about 9.6 g/cm³, whilst the grain size was approximately 2-3 μm.

Sintered fuel pellets of (Th,²³³U)O₂ were subjected to four resintering cycles of 6 h at a soak temperature of about 1,740 °C (2,013 K). The resintering parameters (sintering gas, heating and cooling rates, and soak temperature and time) used in the four resintering tests remained constant across all runs.

Figure 13 shows the average pellet immersion density and volume shrinkage as a function of resintering time. Error bars were evaluated as ± 1 standard error. The volume shrinkage was evaluated as the percent of the ratio between the pellet volume after each resintering run and the pellet volume prior to resintering. Based on these results, it is clear that the largest shrinkage (the highest densification) occurred during the first 6 h of resintering. After the first resintering run, the average pellet density increased about 1.55%. After the second resintering run, the increase in the average pellet density was negligible (approximately 0.10%). After 24 h of resintering, the average pellet density increased from 9.67 to 9.83 g/cm³ (96.5% to 98.1% Theoretical Density (TD)).

A microstructure characterization study was conducted on selected (Th,²³³U)O₂ fuel pellets. Figure 14 depicts ceramographic images of unetched (Th,²³³U)O₂ pellets taken from different resintering tests. The prior to resintering unetched pellet image, shown in Fig. 14(a), shows a spongy-like pellet surface. A large number of small pores can be seen evenly distributed throughout the pellet, which may be the result of an incomplete grain growth process. Figures 14(b)–14(d) show that, as result of the resintering tests, the amount of small pores decreases when compared to the prior to resintering pellet ceramographic image (Fig. 14(a)).

Figure 15 depicts ceramographic images of etched sintered pellets. Figure 15(a) shows an image of the prior to resintering pellet with Figs. 15(b)–15(d) showing images of pellets resintered at different sintering times. A well-developed grain-like microstructure can be observed in all images. Table 3 shows the average pellet density and grain size measurements, for the different resintering tests. As seen in Table 3, as resintering time is increased, there is a slight increase in the pellet density, increasing from 9.67 g/cm³ (prior to resintering) to 9.83 g/cm³ (after 24 h). There are some apparent inconsistencies in the trend of the grain size with resintering, where there is a gradual decrease in the grain size from 6 h (42 μm) to 18 h (27 μm), but then the grain size increases up to 49 μm after 24 h. It is noted that the statistical uncertainty in the average grain size is relatively large, ranging from ±23 μm (at 6 h) to ±16 μm (at 24 h). Thus, further measurements to establish the average grain size and to reduce the relative statistical uncertainty (to less than ±10%) may be necessary. It is recognized that the average grain size will be important with respect to fuel performance behavior, since a larger initial grain size will impact the fission gas release under irradiation conditions. It is also noted that the degree of heterogeneity in the sintered (Th,²³³U)O₂ fuel pellet is not completely represented by the metrics shown in Table 3.

Table 4 shows pore size distribution, in terms of overall, intergranular (existing or occurring between the grains or granules) and intragranular (existing within a grain or granule) porosity, which were determined from conducting the linear intercept method on micrographic images.

As expected, the densification results showed that only a small increase in pellet density was achieved during the resintering tests as the prior to resintering pellets were already of relatively high density (see Fig. 13). After the first 6 h of resintering, pellet density attained a plateau regardless of the resintering time.

Since the sintering process was virtually complete in the prior to resintering pellets and the starting grain size was very small, the grain growth processes dominated over pellet densification during resintering tests. During the grain growth process, small grains are consumed by large grains and pores are swept by expanding grain boundaries and coalesce, growing in size until they pin the grain boundaries (at which point grain growth ceases).

For example, during resintering #1 (first 6 h), the average grain size of the pellet increased significantly. It has been suggested that at the final stage of green pellet sintering, pores are present mostly at grain boundaries [53]. The rapid shrinkage of the fraction of fine pores, controls the densification rate and grain growth while large pores remain unchanged or eventually increase their size at triple-point grain junctions due to the coalescence of fine pores [54].

As densification proceeds in resintering #2, 3 and 4, a very small increase on the pellet density is achieved (Fig. 13) while a decrease in the standard deviation of the grain size distribution was observed (Table 3). The decrease in the standard deviation of the grain size distribution suggests that a more homogenous grain size distribution is obtained after resintering #2. Table 4 shows that pores at triple-point grain junctions (intergranular porosity) increase their size, as the resintering process continues. This is mainly due to the coalescence of the remaining small pores. Simultaneously, pores trapped within grains cannot be further eliminated by sintering, remaining as intragranular porosity (Table 4) and inhibiting further pellet densification.

In the late 2010s, CNL prepared a series of thorium-uranium oxide pellets with the goal of conducting thermal conductivity measurements on homogeneous and heterogeneous ThO₂-UO₂ fuels with different UO₂ concentrations from room temperature to ∼1400 °C (1,673 K) [55], as well examining the oxidation behavior and resistance of thoria-urania fuel under off-normal reactor conditions. The focus on oxidation behavior was pursued to improve upon the understanding of postdefect (Th,U)O₂ fuel behavior, with attention to fission gas release, the diffusivity of fission products, grain growth, and thermal conductivity. Table 5 summarizes the thorium-uranium compositions prepared for these studies, where the number in the sample ID correlates to the weight percentage of UO₂ in ThO₂ (ex. U1.5 represents 1.5 wt. % of UO₂ in ThO₂).

The Invincia vibratory mill, with zirconia cylinders as milling media, was used to simultaneously mix and mill the thorium-uranium powders (U1.5-U60) for a total of 3 h with any adhered powder being removed from the walls after every hour. Fifteen minute and 1-hour mixing intervals were also investigated for 1.5 wt. % of UO₂ in ThO₂, but as these mixing times were not further pursued for the rest of the (Th,U)O₂ compositions, they will not be discussed further within this paper.

Green pellets were pressed to a green density of 60–70% TD with pressing pressures of 3000 psia (∼20.4 MPa) using a die with a diameter of 13.19 mm. No prepressing was required. Sintering took place at 1750 °C in a reducing atmosphere of pure hydrogen for 6 h before being cooled. Resulting densities are presented in Table 5. Heterogeneous thorium-uranium samples U79 and U93 (79 and 93 wt. % UO₂ in ThO₂, respectively) were prepared at JRC-Karlsruhe using Spark Plasma Sintering (SPS). SPS uses the simultaneous application of pressure and temperature to rapidly sinter powder materials into fully dense solids for a range of materials, including UO₂ and ThO₂ [56–63]. Using a 6-mm diameter graphite die, 300 mg of the two UO₂ and ThO₂ compositions were sintered at a maximum temperature of 1,300 °C (1,573 K) for 5 min with a pressure of 35 MPa. X-ray Diffraction (XRD) using a Cu Kα radiation with a Bragg Brentano geometry revealed that the lattice parameter varied linearly with the concentration for 0, 3, 8, 30, 60 and 100 wt. % UO₂ in ThO₂ (see Fig. 16) with the XRD results for the SPS prepared pellets indicating that both samples prepared at JRC-Karlsruhe were biphasic. If the samples were not bi-phasic, then there would have been a nonlinear variation of the lattice parameter with UO₂ content. For example, at 0% UO₂, the lattice parameter is ∼5.6 Å, while at 100 wt. % UO₂, the lattice parameter is ∼5.47 Å. Using these two bounding values and assuming biphasic material, then a simple linear interpolation for 30 wt. % UO₂ gives ∼0.70 × 5.6 + 0.3 × 5.47 = 5.56 Å, which corresponds directly with the lattice parameter (∼ 5.56 Å) shown in Fig. 16 at 30 wt. % UO₂.

Overall, the thermal diffusivity measurements collected at CNL (LFA) and JRC-Karlsruhe (formerly ITU) (LAF-1) were within the uncertainty of the flash method (5%) with the exception of the lower temperatures for the semitransparent samples (Fig. 17(a)); ThO₂ and (Th,U)O₂ with 1.5, 3, 8 and 13 wt. % UO₂ were found to be semitransparent to the infrared wavelength of the laser and were subsequently coated with graphite for the thermal diffusivity measurements. The thermal diffusivity decreased rapidly with increasing uranium content from U1.5 up to U60, with sample U60, U79, and U93 being close to that of UO₂ (Fig. 17(b)). Similar to the thermal diffusivity, for all temperatures, the thermal conductivity of (Th_1 − x,U_x)O₂ decreases with increasing UO₂, as shown in Fig. 18 for CNL measurements. It is noted that for Industrial Standard Toolset (IST) computer codes that are used in the Canadian nuclear industry for the computational modeling and simulation of nuclear fuel, and associated performance and safety analyses [64], the thermal conductivity data is more useful and relevant.

A subset of the samples used for thermal diffusivity and conductivity trails were examined for oxidation behavior, which are presented in Table 6 [65]. Oxidation conditions were intended to simulate defective fuel behavior under normal operating conditions.

To probe the various depths of the oxidized (Th,U)O₂ samples, different techniques were used: Neutron Powder Diffraction Analysis (NDA), X-ray Absorption Spectroscopy (XAS), and X-ray Photo-electron Spectroscopy (XPS). XRD and NDA revealed that the as-fabricated UO₂, ThO₂, and (Th,U)O₂ samples were comprised of a single cubic phase with a fluorite structure. Examination of the local structure around the uranium atoms using XAS indicated that the Th and U are randomly distributed in the solid solutions, confirming the XRD and NDA findings. Oxidized MOX samples showed a higher degree of oxidation on the surface (as determined with XPS) compared to the subsurface of the samples (probed by XAS). The bulk of the samples, which were inter-rogated through the NDA technique, showed that with high UO₂ content, two cubic structures co-exist (UO_2.06 and (Th,U)O₂) with the weight fraction of the UO_2.06 decreasing with decreasing UO₂ content [65].

Pellet fabrication of ThO₂ and (Th,Pu)O₂ were pursued at CNL's Recycle Fuel Fabrication Laboratories (RFFL) with the aim to improve upon the homogeneity of mixed oxide fuels, thereby improving fuel performance, particularly at high burnups [66]. In particular, developing a fabrication methodology that would eliminate residual granules, as well as produce pellets with grain sizes >5 μm, for a range of Pu concentrations, was desired.

To determine the impact of various blending methods, first vibratory milling (or comilling as it was referred to in Sec. 3.2.3 (Powder Co-Milling) of ThO₂ and PuO₂ powders was conducted to reduce and homogenize the particle size of the PuO₂ in the ThO₂ matrix. The second blending method used a two stage method to produce higher PuO₂ concentration mixtures where first ∼30 wt. % PuO₂ was vibratory milled with a subtotal of the necessary ThO₂, which was subsequently followed with low intensity blending of additional ThO₂ powder to achieve the desired Pu concentration. This two stage blending method produced “islands” of homogenous PuO₂-ThO₂ within a ThO₂ matrix (identified as Master Mix, as discussed in Sec. 3.2.2 (High Intensity Blending with a Master Mix) [66]. The experimental matrix, which includes blend number, Pu concentration, blending method, sintering parameters, and immersion density results, are presented in Table 7. Precompaction, granulation, lubrication and final compaction were completed before sintering in a 10% hydrogen and 90% nitrogen environment. It is recognized that the range of sintering temperatures shown in Table 7 is rather large, with the low range between 1600 °C and 1800 °C, and the high range between 1800 °C and 2000 °C, as bounding values. In actuality, the specific sintering temperatures used in the sintering furnace were exact values and were controlled relatively well. Specific values for sintering temperatures are not shown due to proprietary and intellectual property issues.

A key finding of the study conducted by Leeder et al. [66] was that (Th,Pu)O₂ sintered for 8-10 h at high temperature produced fuel higher densities and larger grain sizes. Pellets produced using the Master Mix method (Sintering “Islands”) required longer sintering times than compared to those produced using comilling (Homogeneous Blending). Importantly, (Th,Pu)O₂ fuel microstructure was found to be dependent on both blending method and sintering time. For example, pellets manufactured by comilling, a sintering time greater than 8 h, and a high sintering temperature, showed a homogeneous microstructure with an average grain size of ∼9 μm and an average sintered density of 9.76 g/cm³. MOX fuel with these specifications are expected to have improved irradiation performance, especially at burnups greater than 40 MWd/KgHE. In contrast, pellets manufactured using the Master Mix method, a sintering time of 16 h, and a high sintering temperature, resulted in fuel with an overall average grain size of 4 μm (∼8 μm in areas containing Pu) and an average sintered density of 9.78 g/cm³. Blending using the Master Mix method also produced a slightly granular microstructure with areas of both large and small grains, suggestive of a bi-modal grain size distribution. The higher Pu concentration within the thoria matrix causes increased grain growth during sintering, and while it is expected to have improved fuel performance, the bi-modal grain size distribution may result in slightly higher fission gas release than (Th,Pu)O₂ fuel with a homogeneous Pu and grain size distribution [66].

Thorium-based fuels are potentially attractive for advanced fuels and fuel cycles in ARs and various SMR technologies to help conserve uranium resources, to help reduce long-term radioactive waste liabilities, and to ensure long-term nuclear energy sustainability and security.

CNL has invested significant effort over the past 50 years in ThO₂, (Th,U)O₂, and (Th,Pu)O₂ fuel development resulting in valuable data and experience. Most ThO₂ pellet fabrication processes include a step to add a fissile component (U or Pu), as thorium lacks a fissile isotope. Typical fissile components are enriched uranium dioxide (UO₂), plutonium dioxide (PuO₂), as well as ²³³U in the form of ²³³UO₂. This review paper has focused on the effects of the different ThO₂/fissile-component mixing methods used at CNL on the sintered pellet density, microstructure and fissile component distribution and homogeneity, as well as thermal diffusivity and oxidation behavior. ThO₂/fissile-component mixing methods were classified as solution blending and mechanical mixing. Solution blending methods included Sol-Gel, Co-Precipitation and Solution Impregnation processes. If the fissile component is added during the ThO₂ powder production stage, solution blending methods have typically produced a uniform distribution of the fissile component within the ThO₂ matrix.

Some general observations regarding ThO₂-based fuel pellet fabrication are:

• Selection of the powder mixing method. Special attention should be taken during the selection and operation of the ThO₂-fissile component mixing method to avoid granular-like microstructures in the sintered pellets.

• Starting ThO₂ powder. The sinterability and the properties of the powder, including particle morphology, size distribution and surface area, are among the most important characteristic of the starting ThO₂ powder. These will strongly influence the final quality of the fuel in terms of density, microstructure (granular- or grain-like), grain size distribution and fissile component distribution and homogeneity.

• Blending of PuO₂ and ThO₂ powders: blending methodology has an impact on final sintered density and grain size of (Th,Pu)O₂ pellets. For the same sintering temperature and sintering time, co-milling was resulted in a higher sintered density than blending using the Master Mix method, which produced Pu “islands”. However, it is advised that blending, pressing, and sintering methods are selected in tandem as all process steps have a significant impact on final pellet characteristics. The use of co-milling helps reduce the grain size and ensure a more uniform distribution of fuel grains of different oxides within the pressed and sintered oxide fuel pellet.

• ThO₂ powder pressing. ThO₂ is harder and more abrasive than UO₂. Caution should be exercised during the selection of powder binders and lubricants during green pellet production. Green pellets tend to end cap (delamination of the ends of the pellet) as ThO₂ powders are not as amenable to the cold pellet pressing process. Use of pre-pressing of powders and final pressing of pellets with pressures in the range 20 to 35 MPa helps to improve the homogenization and densification of the sintered pellets by breaking up larger granules and displacing interstitial voids.

• Green pellet sintering. Since surface energy reduction is the driving force in sintering, high specific surface area fine powders are required. The sintering operating parameters must be selected in conjunction with the powder mixing method, as discussed above, as the properties of the powder strongly influence the final quality of the sintered pellet. Extended periods of sintering (24 hours or more) at 1,700 °C or higher allows a reduction in the number of small pores, as they are filled with diffusing fuel granules, while the gases initially trapped in the voids are able to escape from the fuel pellet.

• In-reactor performance. Re-sintering trials on (Th,²³³U)O₂ fuel pellets revealed that greater than 97% TD can be achieved, thereby verifying that there will not be excessive in-pile densification of (Th,²³³U)O₂ fuel.

• Characterization of fuel during fabrication. Characterization of fuel at different stages of the fabrication process must be thorough. Examination of sintered pellets must always include a microstructure analysis of cut and polished pellets to identify the type of microstructure produced (granular- or grain-like) and determine grain size distribution, fissile component distribution, and homogeneity.

• Thermal conductivity of (Th,U)O₂. Increasing UO₂ content and temperature caused a decrease in thermal conductivity; UO₂ contents of 60% and higher indicated that the thermal conductivity of the thorium-uranium oxide fuel was close to that of UO₂. With higher UO₂ content, the effective thermal conductivity can be improved with a biphasic microstructure.

Going forward into the future, with its experience and expertise, CNL is well positioned to support the next generation of ARs and SMRs using advanced fuels and fuel cycles, including the use of thorium-based fuels.

The authors recognize the help and assistance, guidance, and oversight provided by the following staff at CNL: Ali Siddiqui, Cathy Thiriet, Gordon Burton, Tina Wilson, Daniel Cluff, Mouna Saoudi, Lin Xiao, Lori St. Cyr, Jeffrey Baschuk, the scientific and operations staff of the Recycled Fuel Fabrication Laboratory (RFFL) at CNL and other CNL staff not named here.

• Atomic Energy of Canada Limited (AECL), under the auspices of the Federal Nuclear Science and Technology (FNST) Work Plan (Award No. FST-51120.0.A049; Funder ID: 10.13039/501100004953).

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.

Greek Symbols

α =

alpha

γ =

gamma

Subscripts or Superscripts

²³³U =

uranium isotope 233

²³²Th =

thorium isotope 232

Acronyms and Abbreviations

AECL =

atomic energy of Canada limited

AR =

advanced reactor

CANDU =

Canada deuterium uranium

CLS =

Canadian light source

CNL =

Canadian nuclear laboratories

CRL =

chalk river laboratories

CT =

coulometric titration

Cu =

copper

DLS =

dynamic light scattering

FE-SEM =

field emission-scanning electron microscope

HEU =

high enriched uranium

ITU =

Institute for Transuranium Elements

JRC =

joint research center

K =

Kelvin

LFA =

laser flash analysis

MOX =

mixed oxide

NDA =

neutron powder diffraction analysis

NRU =

national research universal

O/M =

oxygen to metal ratio

ORNL =

oak ridge national laboratory

PSA =

particle size analysis

PT-HWR =

pressure tube heavy water reactor

PuO₂ =

plutonium dioxide

RFFL =

recycle fuel fabrication laboratories

SIMFUEL =

simulated high-burnup nuclear fuel

SEM =

scanning electron microscopy

SMR =

small modular reactor

Sol-Gel =

solution-gelation

SPS =

spark plasma sintering

S&T =

science and technology

TD =

theoretical density

ThO₂ =

thorium dioxide

UO₂ =

uranium dioxide

WL =

Whiteshell laboratories

XAS =

X-ray absorption spectroscopy

XRD =

X-ray diffraction

XPS =

X-ray photoelectron spectroscopy

ZrO₂ =

zirconium dioxide