**5. Conclusions**

With the recent knowledge gained on nanoscience and nanomaterials, and the complex interaction that nanoparticles have in the environment, there is a new insight toward nanoparticles generated from the nuclear technology. It is a fact that long-term nuclear waste disposals and nuclear reactors are sources of UO2 and actinide- and lanthanide-doped UO2 nanoparticles. Therefore there is an effort to produce nanoparticles of these compositions to study not only their behavior in special physicochemical conditions but also their advantageous properties in the design of new fuel elements and processes.

There exist many ways to obtain nanoparticles of UO2, but until now all of them start from a solution of U6+ and reduces it to U4+. The way in which the nanoparticle is formed or the reduction is done differentiates one of the other processes. In the precipitation routes, the pH generates nanoparticles of U6+ salts that after intermediate- to high-temperature thermal treatments in reducing conditions convert to

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

micrometer agglomerates of UO2 nanoparticles of 80–120 nm and fcc crystal phase. Other chemical routes use U6+ organic salts in an organic solvent as dibenzyl ether, with amines and organic acids as stabilization agents, to induce the precipitation of non-agglomerated and highly crystalline UO2 nanoparticles of less than 5 nm during a low-temperature thermal treatment. In the sol-gel type syntheses, nanoparticles with U6+ are generated in the continuous solid phase, sometimes mediated by the addition of organic molecules. The gel is dried after and reduced to obtain micrometer-sized agglomerates of UO2 nanoparticles of around 90 nm crystallite size. In the electrochemical-assisted syntheses, electrons are directly supplied at the cathode to the uranyl solution to reduce the uranium ions to U4+, which precipitates as moderately agglomerated powders of 53 nm formed by 4–14 nm crystal size UO2 nanoparticles. The processes assisted by radiation consist in generating strongly reducing organic agents by irradiating a secondary alcohol with electrons or photons. These species reduce the U6+ to U4+ in the solution forming UO2, which aggregates in crystalline nanoparticles. In case of electron irradiation, small particles with a narrow size distribution (22–35 nm) were obtained, while for gamma irradiation 3.5–5 nm particles were formed. In case of X-rays photons, the product obtained are precursors of nanoparticles and need a subsequent intermediatetemperature thermal treatment to definitely form the UO2 nanoparticles with 3–15 nm and fcc crystal phase.
