An investigation of the interactions of Eu3+ and Am3+ with uranyl minerals: implications for the storage of spent nuclear fuel.

Abstract

The reaction of a number of uranyl minerals of the (oxy)hydroxide, phosphate and carbonate types with Eu( III ), as a surrogate for Am( III ), have been investigated. A photoluminescence study shows that Eu( III ) can interact with the uranyl minerals Ca[(UO 2 ) 6 (O) 4 (OH) 6 ]·8H 2 O (becquerelite) and A[UO 2 (CO 3 ) 3 ]· x H 2 O (A/ x =K 3 Na/1, grimselite; CaNa 2 /6, andersonite; and Ca 2 /11, liebigite). For the minerals [(UO 2 ) 8 (O) 2 (OH) 12 ]·12H 2 O (schoepite), K 2 [(UO 2 ) 6 (O) 4 (OH) 6 ]·7H 2 O (compreignacite), A[(UO 2 ) 2 (PO 4 ) 2 ]·8H 2 O (A = Ca, meta -autunite; Cu, meta -torbernite) and Cu[(UO 2 ) 2 (SiO 3 OH) 2 ]·6H 2 O (cuprosklodowskite) no Eu( III ) emission was observed, indicating no incorporation into, or sorption onto the structure. In the examples with Eu 3+ incorporation, sensitized emission is seen and the lifetimes, hydration numbers and quantum yields have been determined. Time Resolved Laser Induced Fluroescence Spectroscpoy (TRLFS) at 10 K have also been measured and the resolution enhancements at these temperatures allow further information to be derived on the sites of Eu( III ) incorporation. Infrared and Raman spectra are recorded, and SEM analysis show signi fi cant morphology changes and the substitution of particularly Ca 2+ by Eu 3+ ions. Therefore, Eu 3+ can substitute Ca 2+ in the interlayers of becquerelite and liebigite and in the struc- ture of andersonite, whilst in grimselite only sodium is exchanged. These results have guided an investi- gation into the reactions with 241 Am on a tracer scale and results from gamma-spectrometry show that becquerelite, andersonite, grimselite, liebigite and compreignacite can include americium in the structure. Shifts in the U v O and C – O Raman active bands are similar to that observed in the Eu( III ) analogues and Am( III ) photoluminescence measurements are also reported on these phases; the Am 3+ ion quenches the emission from the uranyl ion