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The Nuclear Furnace (核融合炉 kakuyuugouro) is located at the deepest layer of the Underworld, just below the Remains of Blazing Hell. It produces the heat and power for the Former Hell and that it's connected to the Underground Geyser Center via a very long passageway.
The Nuclear Furnace is the final destination in both Subterranean Animism and Sanae's scenario in Touhou Hisoutensoku, where Utsuho Reiuji exists to power it. It used to lie dormant, but because of the plans of Kanako Yasaka and Suwako Moriya, it is now fully active.
In Wild and Horned Hermit, Kanako and Suwako attempt to develop a better version of nuclear fusion, probably as a way to convert the current Nuclear Furnace into something more efficient.