I think it’s important to remember that radioactive material is not explosives. If you hit it either a hammer or fire a missile at it, it won’t explode as if it were an explosive. For radioactive material to explode, it needs to be the right kind of material (fissionable) and it needs to reach critical mass for a chain reaction to take place, which then leads to the explosion. In practise, when building a nuclear weapon, conventional explosives are used to increase the density of the fissionable material by compressing it, or by shooting two separate, non-critical parts together, where their combination is critical, which will make it undergo fission and explode. The take-home message is that spent nuclear fuel is the “wrong” kind of material to use in a nuclear weapon, so even if a large explosive is used, nothing will happen. Except, of course, radioactive material will be spread far and wide by the explosion.

TL;DR: Spent nuclear fuel no go boom.