The stability of β-Cs2U2O7 and SrZrO3, two possible phases of spent fuel elements, in hydrothermal fluids was investigated. β-Cs2U2O7 was unstable under hydrothermal conditions of 100, 200 and 300°C/300 bars releasing substantial amounts of its Cs in deionized water and all of its Cs in a bittern (high-Mg and-Ca) brine. SrZrO3 was found to be quite stable in deionized water but not in a bittern brine. For example, Sr released into solution decreased from 3.9 to 2.8% with an increase in temperature from 100 to 300°C probably because of better crystallization of SrZrO3 at higher temperatures. In bittern brine, 23.3, 94.9 and 100% of Sr was released into solution at 100, 200 and 300°C respectively as a result of acidic conditions generated by the hydrolysis of MgCl2 and formation of brucite. These results suggest that bittern brine which may be encountered in a salt repository is highly corrosive and may release all the Cs and Sr into solution under hydrothermal conditions if the containment were breached. The use of tailor-made overpacks or backfill barriers of highly stable and sorptive materials is essential especially in a salt repository in order to minimize the threat of highly hazardous Cs and Sr radionuclides finding their way into ground waters.