They assume 3% fizzle yield. It wouldn't work anyway (there will be exactly zero yield), the core will remain intact, ready for insertion into another physics package.
I don’t know the details of how durable a uranium warhead core is (and I suspect it’s very highly classified) but they’re pretty precisely machined bits of metal. Even a 3%-yield fizzle should be more than enough to de-rate one. At that point the uranium could be recycled to build a new core, but that’s about it.
I know some details. (It is plutonium, not uranium). Any yield, even fizzle yield, is impossible by simply igniting the explosives around the core. Again, it is not in pre-critical configuration yet at this point. Arming the device, i.e. assembling the initial configuration, is an incredibly complicated and precise process. It is so beautiful from the engineering point of view.
You don't seem to get it. The object of the exercise is not nuclear yield. BTW, you can physically damage fissile material that needs to be in a particular physical configuration with neutrons and/or x-rays. One is not trying to detonate the weapon, but rather render it in a state where it cannot reach the desired yield (preferably none at all, of course).
You are suggesting that just simple combustion of the HE lenses surrounding the core won't damage it at all? Even if the explosive just burns it would melt the physics package, there won't be any reinserting it anywhere. You could recover the fissile material I suppose.