THE EFFECT OF IONIZING RADIATION ON U6+-PHASES

U⁶⁺-minerals commonly form during the alteration of uraninite and spent nuclear fuel under oxidizing conditions. By the incorporation of actinides and fissiogenic elements into their structures, U⁶⁺-minerals maybe important in retarding the migration of radionuclides released during corrosion of spent nuclear fuel. Thus, the stability and the structural transformation of the U⁶⁺-minerals in radiation fields are of great interest. In this study, in order to examine the effects of ionizing radiation on U⁶⁺-minerals, electron irradiation (~8-33x10¹⁷ (e-/cm²/sec) at the room temperature) experiments were completed on: boltwoodite, K[(UO₂)(SiO₃OH)](H₂O)_1.5 kasolite, Pb[(UO₂)(SiO₄)](H₂O) saléeite, Mg[(UO₂)(PO₄)]₂(H₂O)₁₀ carnotite, K₂(UO₂)₂(V₂O₈)(H₂O)₃ liebigite, Ca₂[(UO₂)(CO₃)₃](H₂O)₁₁ schoepite, [(UO₂)₈O₂(OH)₁₂](H₂O)₁₂. During the irradiation, boltwoodite, saléeite, and carnotite became amorphous at doses of 1-4x10¹⁰ Gray (Gy), while the amorphization dose (D_c) of kasolite, 50x10¹⁰ Gy, was about an order of magnitude higher than that of boltwoodite. This high D_c for kasolite is consistent with the hypothesis that the D_c increases as the mass of the inter-layer cation increases. Only amorphization, rather than chemical decomposition, occurred in boltwoodite, saléeite and carnotite, even at doses as high as 80x10¹⁰ Gy. In contrast, uraninite nanocrystallites began to form with a random orientation at ~43x10¹⁰ Gy in liebigite which had already become amorphous prior to irradiation in the vacuum of the TEM. For schoepite, the D_c was only 0.51x10¹⁰ Gy, and randomly oriented uraninite nanocrystallites formed at 7.8x10¹⁰ Gy which is approximately the same dose as compared with the D_cs for the other U⁶⁺-phases. Because the predicted cumulative dose by the ionizing radiation in spent nuclear fuel is ~10⁷-10⁸ Gy during the first 10^2-3 years after discharge, an additional contribution of ionizing radiation dose by α-decay events is needed in order to induce amorphization in these U⁶⁺-phases.