Hafnium178m2 isn't a natural isomer
You cannot find Hafnium-178, isomer m2, in nature. You need to "charge" it so to speak using cyclotrons, and you always need to put a little bit more energy into charging it than what you get from discharging it through induced gamma radiation. It's basically a nuclear battery, with the advantage of being capable of storing 5 orders of magnitude more energy than any electrochemical battery ever will.
Thorium, on the other hand, is about 3 orders of magnitude denser in energy than Hf-178m2. Converting thorium-232, the only natural isotope and which is not fissile, into uranium-233, which is fissile, requires one somewhat energetic neutron. A second neutron can fission the U-233, which produces 2.5 neutrons on average per fission plus much, much more energy than what was required to set all this up. Thorium is basically like Hf-178m2 if that was naturally occurring and if it required neutrons instead of photons to induce energy release.
