**Abstract**

Nanostructured actinide materials have gained the attention of the nuclear community after the discovery of enhanced properties in fuels that undergo high burn up. On these conditions, the UO2 grains experimented recrystallization and formed a new rim of UO2 nanoparticles, called high burn up structures (HBS). The pellets with HBS showed closed porosity with better fission gas retention and radiation tolerance, ameliorated mechanical properties, and less detriment of the thermal conductivity upon use. In this chapter, we will review different ways to obtain uranium nanoparticles, with emphasis on their synthesis and characterization. On the one hand, we will comment on radiation chemical syntheses, organic precursor-assisted syntheses, denitration processes, and biologically mediated syntheses. On the other hand, we will include for each of them a reference to the appropriate tools of the materials science that are used to fully characterize physical and chemical properties of these actinide nanoparticles.

**Keywords:** UO2, nanoparticles, grain sizes, synthesis, characterization

## **1. Introduction**

Nanomaterials, which are present naturally in the environment and also as a result of anthropogenic activities (incidental or engineered), gain the attention of scientist and technologists due to their promising applications. The surface-tovolume ratio, grain size, morphology, composition and elemental distribution affect nanoparticle's physicochemical and electrical properties, surface reactivity, material growth, or dissolution rates [1]. These characteristics can be thus engineered to take advantage of the nanoparticles over their macroscopic equivalents, for example, to favor faster catalysis of reactions, high loading of medicines or absorption of toxins from polluted zones.

In the nuclear material's field, actinide oxides nanoparticles became under systematic study after the detection of two main issues:

First, the discovery of a rim structure in UO2 pellets that had have a burn up of 40–67 GWd/tM (also called high burn up structures or HBS [2]). The pellet, initially formed by micrometer-sized grains recrystallized in a ring of nanoparticles at the rim. The pellets with HBS presented better fission gas retention, ameliorated radiation tolerance and mechanical properties as the plasticity [3]. The direct consequence of this observation was an increment in the number of publications dealing with different synthesis of UO2 nanoparticles to form pellets mimicking from the beginning the HBS structure [3–7].

Second, the fact that actinides tend to form colloids of aggregated nanoparticles [8, 9]. Indeed, in contact with water, metallic U corrosion is known to form fine UO2 particulates [10, 11]. This material has different properties than micrometer particulated material, affecting, for example, the expected behavior in spent nuclear fuels, radioactive wastes, and contaminated places, due to their differences in mobility, solubility, surface reactivity, complexation, speciation, weathering, eco-toxicity, and biological uptake. In particular, because their small size, nanoparticles may have a toxic effect on living organisms that is not present with micrometer-sized particles. Thus, there is a need for expanding the actual knowledge on actinide nanoparticles with emphasis in their physicochemical properties, grain sizes, crystal phases, elemental distribution and reactivity, for predicting and controlling their behavior under different conditions. This knowledge will also serve to redesigning long-term nuclear waste disposals and mobility barriers.

Both former topics request well-characterized actinide nanoparticles, especially those composed of UO2. That, added to the scientific motivation per se, is represented in the increased number of publications in the past 25 years dealing with different synthesis and characterization of UO2 nanoparticles. In the next sections we resume and discuss different methods to obtain particles of uranium dioxide with grain sizes in the sub-micrometer range. We divided the methods by the type of synthesis. On one side, there are those which follow a wet chemical route, subdivided in processes that use a wet denitration step and processes which need an organic precursor, such as variation of sol-gel or Pechini syntheses. On the other side, we explain those methods which use irradiation with particles or photons to induce UO2 particle formation. In addition, we describe biologically assisted syntheses, which make use of cells and bacteria to precipitate UO2 nanoparticles.

It is worth to mention at this point that many of the published syntheses in articles or patents were focused to the production of UO2 for its use in nuclear reactors. This application requires a powder with good fluency and compressibility to further handling for pellet fabrication. Thus, fractions of particles with submicrometer diameter, which sometimes are referred as "very fine powder," were separated from the bulk and discarded. In addition, very often nanoparticles aggregate in micrometer-sized particles. Only with high-resolution microscopy techniques, or indirectly through BET surface area measurements, for example, it is possible to detect the nanometric structure of the material. Therefore, in more than one publication, nanoparticles are wrongly classified as micrometer-sized particles. Here we attract the attention on this fact in some of the reported works.

## **2. Chemical and electrochemical routes**

#### **2.1 Syntheses from inorganic uranyl salts**

In the group of the wet chemical syntheses, one of the most common practices to obtain UO2 to manufacture nuclear fuel pellets is the physicochemical precipitation, followed by calcination and reduction [12]. The ammonium di-uranate (ADU) and the ammonium uranyl carbonate (AUC) routes are two well-known examples. Both start from an inorganic uranium salt such as the uranyl nitrate hexahydrate (UNH), involve thermal treatments in different atmospheres and, at intermediate to high temperatures, obtain the fluorite fcc UO2 phase.

Although the ADU synthesis originally was not tuned to produce nanoparticles, first studies describe that depending on pH and synthesis conditions, a fine powder with sub-micrometer structure and a grain size of 370 nm was observed [13]. Some years ago, Soldati et al. took advantage of characterization methods from the

*Uranium Dioxide Nanoparticulated Materials DOI: http://dx.doi.org/10.5772/intechopen.91017*

nanoscience and demonstrated that the UO2 particles obtained by the ADU route in the standard conditions described elsewhere (i.e., pH 9 and 60°C thermal bath) were indeed agglomerates of rounded, but irregular, nanoparticles of homogeneous composition, fcc Fm-3 m crystal phase, and 80–120 nm crystal sizes [4]. In that experience, to obtain about 100 g UO2 nanoparticles with those characteristics by the ADU method requires a filtrating step, produces about 2 L ammonia water waste, needs 12–16 hours thermal treatments at intermediate to high temperatures, and consumes air and a reducing atmosphere such as H2:Ar (10:90) [4, 13].

In these syntheses, UO2, and some mixed oxides with Gd or Pu, can be obtained from a solution of the actinides (as nitrates or oxides) in 1 M HNO3, concentrations of 50–400 g/L, 60°C, and pH between 4 and 9 [4, 13–16]. The precipitation of ADU is favored by mixing the mother solution with a basic 13 M (NH4OH) solution [14, 15, 17] or bubbling NH3 gas [4, 13] (Eqs. (1)–(3)).

For example, for the case of ADU, the involved reactions are:

$$\text{NH}\_3\text{g}^\text{g} + \text{H}\_2\text{O} \rightarrow \text{NH}\_4^+ + \text{OH}^- \tag{1}$$

$$\text{2U}\text{O}\_2^{+2} + \text{6OH}^- \rightarrow \text{U}\_2\text{O}\_7^{-2} + \text{3H}\_2\text{O} \tag{2}$$

$$\text{U}\_2\text{O}\_7^{-2} + 2\text{NH}\_4^+ \rightarrow \text{U}\_2\text{O}\_7(\text{NH}\_4)\_2\downarrow\_{(\text{ADU})}\tag{3}$$

Once that the precipitated phase is completely formed, the solution is stirred for 1 hour and vacuum filtrated, washed with milliQ water, and dried between 80 and 120°C for 24 hours. After that, the ADU is converted to U3O8 by calcination at 800°C in air for 6–8 hours (Eqs. (4) and (5)).

$$\text{U}\_2\text{O}\_7(\text{NH}\_4)\_2 \xrightarrow{400^\circ \text{C}, \text{air}} 2\text{UO}\_3 + 2\text{NH}\_3\uparrow + \text{H}\_2\text{O}\uparrow\tag{4}$$

$$\textbf{3UO}\_3 \xrightarrow{400^\circ \text{C to } 800^\circ \text{C, air}} \textbf{U}\_3 \textbf{O}\_8 + 1\prime\_2 \textbf{O}\_2 \tag{5}$$

Finally, the U3O8 is reduced to UO2 by thermal treatment between 650 and 700°C for 7 hours in pure H2 or mixtures of H2 and Ar or N2 in proportions of 8–10% (Eq. (6)).

$$\text{U}\_3\text{O}\_8 + 2\text{H}\_2 \xrightarrow{650^\circ \text{C}, \quad (10.90)} \text{H}\_2\text{Ar} \\ \text{H}\_2 + 2\text{H}\_2\text{O} \\ \uparrow \tag{6}$$

On the other side, the AUC, for example, is precipitated from the UNH-HNO3 solution with (NH4)2CO3 [14] and converted to UO2 at 650°C in a water vapor/ hydrogen atmosphere. However, to the best of our knowledge, only micrometric particle sizes were reported by AUC syntheses.

#### **2.2 Syntheses from organic uranyl salts**

An alternative way for precipitating UO2 nanoparticles from the inorganic salt uranyl nitrate are the synthesis from the organic salts uranyl acetylacetonate (UAA) or acetate (UA), mediated by organic solvents and temperature. Wu et al., for example, obtained 3–8-nm-large cubic UO2 nanocrystals by decomposition at 295°C, under Ar, of UAA in a mixture of oleic acid (OA), oleylamine (OAm), and octadecene (ODE) [18]. Non-agglomerated and highly crystalline UO2 particles were obtained in a similar synthesis by Hudry et al. at temperatures of 280°C [19]. These nanoparticles were isotropic faceted nanodots of 3.6 � 0.4 nm diameter. Moreover, Hu et al. used UA dissolved in oleylamine (OAm) and oleic acid (OA) which after heating in an oil bath, centrifuging, washing with ethanol, and

dispersing in cyclohexane resulted in two-dimensional nanoribbons of U3O8 with dimensions of about 4 � 100 nm. Higher autogenous pressure, in an autoclave, was useful for obtaining wider nanoribbons. With the addition of octadecene (ODE) or toluene, U3O7 nanowires were obtained whose width is about 1 nm and length varied in the range of 50–500 nm depending on the temperature-time conditions of the process [20]. In addition, sphere-shaped UO2 nanoparticles with an average diameter of 100 nm, which consisted in 15 nm nanocrystal subunits, were obtained by Wang et al. from a 0.5 mM UA aqueous solution mixed with ethylenediamine, autoclaved, and heated at 160°C for 48 h [21]. On the other hand, Tyrpekl et al. obtained 5–11 nm UO2 nanoparticles by annealing a dry precipitate of (N2H5)2U2(C2O4)5 � nH2O at 600°C in Ar [22].
