Dissolution of spent nuclear fuel fragments at high alkaline conditions under H-2 overpressure

The long-term behavior of the UO2 fuel matrix under conditions of the Belgian "Supercontainer design" was investigated by dissolution tests of high burn-up spent nuclear fuel (SNF) in high alkaline solution under 40 bar of (Ar + 8% H-2) atmosphere. Four fragments of SNF, obtained from a pellet previously leached during two years, were exposed to young cement water with Ca (YCWCa) under 3.2 bar H-2 partial pressure in four single/independent autoclave experiments for a period of 59, 182, 252 and 341 days, respectively. After a decrease of the concentration of dissolved U-238, which is associated with a reduction of U(VI) to U(IV), the concentration of U-238 in solution is constant in the experiments running for 252 and 341 days. These observations indicate an inhibition of the matrix dissolution due to the presence of H-2. A slight increase in the concentration of Sr-90 and Cs-137 in the aqueous solution indicates that there is still dissolution of the grain boundaries. These findings are similar to those reported for spent nuclear fuel corrosion in synthetic near neutral pH solutions.