Supplementary Material This manuscript is provided as a supplementary material for the review process only

INTRODUCTION

Uranium dioxide, UO₂, is the main form of nuclear fuel used in the present generation of nuclear reactors. The knowledge and understanding of its in-reactor behavior and its stability under subsequent storage and disposal conditions are of great technological importance.^1,2

The heat generated at nuclear power plants comes primarily from the slowing down of fission products with energies in the range 70 to 100 MeV. As a result, heat and radiation damage are produced inside the fuel pellets.^3,4,5

Fresh fuel has close to stoichiometric (UO_2.001) composition. However, under in-reactor irradiation the fuel might develop an increased degree of non-stoichiometry.¹ Solubility and dissolution of uranium oxide in aqueous environment strongly depends on uranium valence state, as U(VI) is more soluble than U(IV) by many orders of magnitude.⁶ Hence, the degree of non-stoichiometry in spent nuclear fuel has an important effect on its solubility and corrosion rate⁷ which governs the release rate of the majority of radionuclides.⁸

This work considers the explicit effect of radiation damage by fission fragments on non-stoichiometry in spent nuclear fuel and outlines a methodology developed for determining the degree of non-stoichiometry in UO_2+x. For this purpose, thin films of UO₂ on LSAT (lanthanum strontium aluminum tantalum oxide) and YSZ (yttria-stabilised zirconia) substrates were produced and irradiated with Xe and U ions, respectively. The irradiated and unirradiated films were analyzed by EDX (Energy Dispersive X-ray spectroscopy), XRD (X-Ray Diffraction), EBSD (Electron Backscatter Diffraction), SEM (Scanning Electron Microscopy) and XPS technique. The obtained results were compared.

Previous papers considered mechanisms of xenon transfer and its interaction with uranium in UO_2+x (ref 9) theoretically, as well as the influence of defects^10,11 and pressure^12,13 on the ionic composition of these oxides. Adsorption energies of water on differently oriented uranium dioxide planes were calculated.^14,15,16

The work on the study of the electronic structure of uranium and its alloys^17-21 and oxides UO_2+x (x≤0 and x≥0) employed theoretical calculation results,^22-34 photoelectron spectroscopy (PES)^35-41 and X-ray photoelectron spectroscopy (XPS) widely.^38,42-55 These techniques were used to study films on various substrates.^38-40,56-58

Determination of uranium oxidation state and sample’s ionic composition employs the U 4f doublet split due to the spin-orbit interaction by ∆E_sl (U4f)=10.8 eV.^17,50,59 The binding energy (BE) E_b(U 4f_7/2) of the U 4f_7/2 electrons in uranium and oxides grows as: ~377 eV (metallic U); ~380 eV (U⁴⁺); ~381 eV (U⁵⁺); ~382 eV (U⁶⁺).^{17,50,55,56,59,60,61,62} A special attention was paid to the study of mechanisms of structure formation, which leads to widening of main peaks and appearance of extra structure in the spectra.^50,59,63 The XPS spectra of some oxides exhibit typical shake-up satellites of about ~25% intensity of the basic peaks.^41,51,60,64 The calculated spectroscopic factors f_Anf reflecting the fractions of the basic peak XPS intensities with the deduction of shake-up satellite intensities for the U 4f and U 5f peaks are: f_U4f=0.83 and f_U5f=0.86.^65,66 Shake-up satellites are located from the basic peaks toward the higher BE by ∆E_sat(U4f): ~7 eV (U⁴⁺); ~8 eV (U⁵⁺); ~4 eV and ~10 eV (U⁶⁺).⁶⁰

Since the U 4f BE on the moving from UO₂ to UO₃ changes by ∆E_b~2 eV, one can reliably determine uranium oxidation states for individual uranium oxides on the basis of the U 4f_7/2 BE and positions (∆E_sat) and relative intensities I_sat (%) of the shake-up satellites.^56,60

The U 4f XPS structure is best resolved for the crystalline oxides. For complex amorphous oxides UO_2+x it is often difficult to segregate unambiguously the U 4f XPS peaks containing shake-up satellites into separate components for reliable quantitative information on uranium oxidation state and ionic composition of the studied oxide. Despite this, the XPS studies of uranium oxides by authors of refs 38,47,50,60,62 employed the U 4f XPS peak decomposition with shake-up satellite parameters in mind.

It is known that the traditional method of determination of the oxygen coefficient k_O=2+x for UO_2+x based on the U 4f/O 1s XPS intensity ratio (with photoionization cross-sections or experimental sensitivity factors in mind) does not give satisfactory results (see ref 49). It is due to the fact that the O 1s intensity grows due to the presence of complex oxides UO_2+x as well as impurity oxides containing excess oxygen on the sample surface.

The valence electron BE range (0 - ~35 eV) in XPS of oxides UO_2+x changes significantly as the oxygen coefficient grows.⁴⁵ Thus, on the moving from UO₂ to γ-UO₃ in the outer valence molecular orbitals (OVMO) with BE range (0 - ~15 eV) a sharp U 5f peak disappears, and in the inner valence molecular orbitals (IVMO) with BE range (~15 - ~35 eV) instead of a single atomic U 6p_3/2 peak two components appear.^42,61 Such a splitting is due to the IVMO formation in γ-UO₃.

The IVMO formation in uranium oxides due to interaction of the U 6p and O 2s atomic orbitals (AO) also results in formation of structure in other X-ray (emission, conversion, Auger) spectra of uranium oxides.^59,67,68,69 This structure parameters correlate with the uranium–oxygen interatomic distances in axial and equatorial directions in uranyl compounds and serve for quantitative determination of these compounds.^50,59

The peak of the U 5f electrons weakly participating in chemical bond in UO_2+x is observed in the photoelectron⁷⁰ and X-ray photoelectron^{52,53,54,68,69} spectra around zero BE. Spectra of compounds containing U(VI) do not exhibit this peak.⁶¹ Therefore, the present work for determination of the oxygen coefficient k_O=2+x, uranium oxidation state and ionic composition k(%) of the studied oxides UO_2+x on the surface of single-crystalline films used the technique developed on the basis of the dependence of the U 5f peak relative intensity I_5f (rel. units) on the oxygen coefficient k_O.^44,45,49