Effect of external gamma irradiation on dissolution of the spent UO2 fuel matrix

Leaching experiments were performed on UO₂ pellets doped with alpha-emitters (^238/239Pu) and on spent fuel, in the presence of an external gamma irradiation source (A⁶⁰Co = 260 Ci,  Gy h⁻¹). The effects of α, β, γ radiation, the fuel chemistry and the nature of the cover gas (aerated or Ar + 4%H₂) on water radiolysis and on oxidizing dissolution of the UO₂ matrix are quantified and discussed. For the doped UO₂ pellets, the nature of the cover gas clearly has a major role in the effect of gamma radiolysis. The uranium dissolution rate in an aerated medium is 83 mg m⁻² d⁻¹ compared with only 6 mg m⁻² d⁻¹ in Ar + 4%H₂. The rate drop is accompanied by a reduction of about four orders of magnitude in the hydrogen peroxide concentrations in the homogeneous solution. The uranium dissolution rates also underestimate the matrix alteration rate because of major precipitation phenomena at the UO₂ pellet surface. The presence of studtite in particular was demonstrated in aerated media; this is consistent with the measured H₂O₂ concentrations (1.2 × 10⁻⁴ mol L⁻¹). For spent fuel, the presence of fission products (Cs and Sr), matrix alteration tracers, allowed us to determine the alteration rates under external gamma irradiation. The fission product release rates were higher by a factor of 5-10 than those of the actinides (80-90% of the actinides precipitated on the surface of the fragments) and also depended to a large extent on the nature of the cover gas. No significant effect of the fuel chemistry compared with UO₂ was observed on uranium dissolution and H₂O₂ production in the presence of the ⁶⁰Co source in aerated conditions. Conversely, in Ar + 4%H₂ the fuel self-irradiation field cannot be disregarded since the H₂O₂ concentrations drop by only three orders of magnitude compared with UO₂.