[hjyeziee] Energy cost effectiveness of thermonuclear bombs

Suppose we wish to extract useful energy, probably generate electricity, from thermonuclear explosions by blowing up H-bombs.

The motivation is, both magnetic and inertial confinement fusion might be too difficult to ever be commercially successful.  However, if we want fusion, we do have a proven way of doing it now.

How cost efficient is it, counting only the cost of the consumables?  Unfortunately the things that go into an H-bomb, and their costs, are closely guarded military secrets.  Weapons-grade uranium or plutonium for the primary stage.

Bigger bombs are probably more efficient: the fuel of the secondary stage (and later fusion stages) are likely much cheaper than the primary.  Tsar Bomba proved large bombs do work, though we might want to go even larger for cost efficiency.  How large of a nuclear bomb can be built before hitting difficult engineering or science problems?

Of course, harnessing the huge amount of energy released by the bomb in the very short amount of time is a mechanical engineering challenge.  But internal combustion engines harness explosions all the time.  Bullets and Project Orion are other examples of the energy of explosions harnessed to do useful work.  We imagine an extremely large blast chamber, large enough so that the explosion doesn't destroy it, then heat is collected on the outer surface to generate electricity.  How large does the blast chamber need to be?  Because of the inverse-square rule, there's definitely a size that's large enough. I'm guessing it might be comparable in size to the planet.  What material should the blast chamber be filled with?

If the nuclear explosion could be made anisotropic, a shaped charge, then this could make the blast chamber easier to build, for example, building a hemisphere instead of a giant sphere.

Is the largest hurdle to doing this the mechanical engineering difficulty of containing the blast and extracting energy, or the political / national security traditions of keeping nuclear weapons and information about them off limits to everyone?  What's stopping us from having unlimited energy?