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Sometimes ships really are unsinkable. You can build a small boat out of materials that are themselves buoyant. If you make a boat of a foam material or some woods, you can submerge that boat at the bottom of a lake, release it, and it will pop right back up to the surface. It's hard to build an ocean liner this way, but there are truly unsinkable boats. There is a difference between safety by backups/safety mechanisms and intrinsic safety. Your car's engine cannot explode in a nuclear fireball. It's resistant to nuclear explosions not because of some elaborate series of safety mechanisms and backups, but simply because it lacks the capability to generate any kind of atomic reaction. Physics, not engineering, provides for the safety of unsinkable boats, your car's lack of nuclear explosiveness, and fusion-fission reactors.

You speculate that the beam may not be shut off fast enough. But there IS no "fast enough" in this context. This is not some system that has the capability of spiraling out of control. Imagine you had a combustion engine that was provided air by a blower motor. The blower motor can supply a certain m^3/min, and this is all the air the engine receives. The motor can only supply the engine so much air; it is fundamentally incapable of spiraling out of control.

There's no way for a fusion-fission reactor to explode in some runaway process. You design the neutron beam so that its absolute maximum power is still well below what would be required to turn the fission reactor into a pile of slag, like orders of magnitude below. You don't put some big honking fusion reactor in this system. You build your fusion portion so that it's only capable of providing enough neutrons for a gentle slow fission burn. There simply will never be enough neutrons in the system for the fission pile to experience runaway fission.