That's not really cold fusion... cold fusion is cold compared to the SUN, where it happens. Normal nuclear reactions are fision, not fusion, which needs higher temperatures, but is cleaner and easier to stop than traditional nuclear energy. That said, it's Gensokyo we're talking about... Physics do not apply here
The foothold enabling that next step will be a nuclear fusion reactor that uses the cauldrons of Hell.However, the fusion reactor plans have hit an impasse recently, and the need has arisen to probe into the possibility of a new energy.And that energy is cold fusion!Cold fusion is literally nuclear fusion at room temperature.I forgot to tell you—I'll treat you to a money offering if you make the stuff.It should be possible if you borrow the power of Kanayamabiko-no-mikoto. I'm counting on you. "Prince-Deity-of-Mines", the god of metalsI'd like you to create the palladium alloy necessary for cold fusion.However, as things stand, Gensokyo can't proceed past that next step and develop beyond the outside world.