**3.1 Precursor formation: U6+ to U4+ reduction and polymerization**

Regardless of the type of radiation used, U6+ is in the form of an uranyl nitrate UO2(NO3)2 acidic aqueous solution and the uranyl concentrations used range from 10 to 50 mM. The chemical reduction reactions, which were presented by Rath et al., are described next [34]. The irradiation with electrons or photons produces water photolysis:

$$\mathrm{H\_2O} \xrightarrow{\mathrm{e^-, hv}} \mathrm{H^+} , \mathrm{OH^+} , \mathrm{OH\_2^-} , \mathrm{H\_2O\_2} , \mathrm{H\_3O^+} \tag{12}$$

$$\text{e}^-\_{\text{aq}} + \text{H} \to \text{H}^\bullet \tag{13}$$

where H• is a reducing agent and OH• is an oxidizing radical. By adding an organic species, which commonly is a secondary alcohol, these species are scavenged, and a strongly reducing agent is produced (reactions Eqs. (14) and (15)). Propan-2-ol, for example, reacts with both H• and OH• forming a strongly reducing 1-hydroxy 2-propyl radical H3C▬C•OH CH3:

$$\text{CH}\_3\text{C}-\text{CHOH}-\text{CH}\_3 + \text{H}\bullet \rightarrow \text{H}\_3\text{C}-\text{C}\bullet \text{OH}-\text{CH}\_3 + \text{H}\_2\tag{14}$$

$$\text{CH}\_3\text{C}-\text{CHOH}-\text{CH}\_3 + \text{OH}\bullet \rightarrow \text{H}\_3\text{C}-\text{C}\bullet \text{OH}\text{CH}\bullet + \text{H}\_2\text{O} \tag{15}$$

Thus, in that milieu, the following reducing and polymerization reactions are possible:

$$\text{UO}\_2^{2+} + \text{e}\_{\text{aq}}^{-} \rightarrow \text{UO}\_2^{1+} \tag{16}$$

$$\text{C}\_2\text{CO}\_2^{2+} + \text{H}\_3\text{C}-\text{C}\bullet\text{OH}-\text{CH}\_3 \rightarrow \text{UO}\_2^+ + \text{H}\_3\text{C}-\text{CHOH}-\text{CH}\_3 + \text{H}^+ \tag{17}$$

$$\text{U}\text{O}\_2^{1+} + \text{H}\_3\text{C}-\text{C}\bullet\text{OH}-\text{CH}\_3 \rightarrow \text{UO}\_2 + \text{H}\_3\text{C}-\text{CO}-\text{CH}\_3 + \text{H}^+\tag{18}$$

$$\mathrm{UO}\_{2}^{+} + \mathrm{nUO}\_{2} \rightarrow \mathrm{(UO}\_{2}) \text{ nanopparticle} \tag{19}$$

The following reaction (Eq. (20)) is also possible, which retires hydrated electrons from the solution, though NO3 �<sup>2</sup> anions finally convert to NO� 3 :

$$\rm{NO\_3^- + e\_{aq}^- \to NO\_3^{-2}}\tag{20}$$

During irradiation, there is UO2 2+ consumption to form the UO2° nanoparticles. The UV-visible absorption spectra of uranyl nitrate exhibit maxima at 427, 477, and 495 nm; the maxima gradually disappear during irradiation, due to the precipitation of the precursor. This is a usual way to follow the nanoparticle precursor formation kinetics during irradiation.

According to Rath et al., an induction time of 135 min after irradiation was necessary for the nanoparticles to form in the presence of 1% volume fraction of propan-2-ol and 50 kGy of absorbed dose [34]. The same work shows that this value depends on the scavenger concentration and the viscosity of the uranyl solution. Induction time also increased with the ethylene glycol concentration, which was used to obtain higher viscosity values.
