Safest or cheapest? And on what time scale? Because the safest long-term means of disposal is to launch it into the sun, but that's prohibitively expensive and

Safest or cheapest? And on what time scale? Because the safest long-term means of disposal is to launch it into the sun, but that's prohibitively expensive and is short-term extremely dangerous (e.g. the rocket exploding or crashing shortly after lift-off).