Dissolution of LnYSiAlO glass (Ln=La or Ce) in aqueous media. II. Influence of heavy ion irradiation on mechanisms and rate of hydrolysis

LnYSiAlO glasses (Ln=La or Ce) are considered as potential matrices for the storage of specific minor actinides. Collision cascades induced by heavy recoil atoms emitted during α-disintegration of radioelements can cause important modifications in the glass structure and thus influence its chemical durability. To simulate these effects, glass samples were irradiated with bismuth ions accelerated to 200 keV using an implanter (integrate dose ≈ 1.7 × 1010 Gy, corresponding to 1016 at/cm2). RBS spectrometry performed on glasses after irradiation indicated an accumulation of Y and RE elements in the first 20 nm near the surface. Monoliths were then leached in static bidistilled water (pH ≈ 5.5) for one month in an autoclave heated at 90 °C. Normalized releases as well as pH free evolution were measured as a function of time. For the cerium containing glass, the oxidation of Ce(III) during leaching was the dominant parameter and few effects linked to irradiation were observed. Concerning LaYSiAlO glass, the presence of bismuth ions implanted beneath the glass, played a more important role in the dissolution process. The likely oxidation of Bi(I) into Bi(III) or Bi(V) during leaching could partly explain the strong increase of the pH that stabilized around pHf=10 after a few days (pHf ≈ 6 for non-irradiated sample). Solution analysis also showed a lower release of Y and RE elements after irradiation, probably because of the low solubility of Y and RE hydroxide around pH=10. Moreover, the hypothesis of a segregation of yttrium under irradiation was made, leading to an increased mean size of localized and Y-enriched deposits at the surface of the leached glasses.