Issues with using salt domes to store nuclear waste:
Salt is generally regarded as being self healing. Any small cracks that may develop within the salt deposit will close. This applies to small cracks, but may not apply to very large cracks.
Salt domes are generally considered to inhibit the flow of fluids through them. However, under conditions of high temperature and pressure, as exists deep underground, salt can become more porous allowing more fluids to pass through it. This can be problematic if in flowing water was to pick up radioactive material and start spreading it around the region of the salt dome.
The nature of the nuclear waste to be stored needs to be considered. Salt domes would be better at storing solid waste. Some waste is liquid which can more easily contaminate storage area via leaks and spills. Such waste would need to be thickened so it contained 35 percent solids.
If high level nuclear waste is to be stored, the amount of heat generated would be 1 - 3 Btu/h per US gallon (278.7 - 836.1 J/L). An acre-foot (1233.48 m3) of such waste would produce 1 MBtu per hour (1055 MJ per hour). The equivalent of burning 700 lb (317.5 kg) of coal.