I dont think it is so easy. Subduction zones have what are called accretionary prisms, which are basically sediment that is sloughed off the top of the subducting plate. Thus, if you want the nuclear waste to be subducted, you have to bury it deep enough, though I can't say quite how far - given the scale of the crust, I'd say you're looking at something on the order of miles at least. Which is extra challenging considering subduction zones are already under thousands of feet of ocean.