The Nuclear Furnace (核融合炉 Kakuyuugouro) is a super-high furnace that's used for high-temperature heating, in an attempt to tame a power that the Outside World apparently won't master for hundreds of years: nuclear fusion energy. It is located at one of the deepest layers of the Underworld, just below the Remains of Blazing Hell, but one layer above the Hell of Blood Pools.
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. Utsuho Reiuji can be found here to power it and Rin Kaenbyou carts corpses into the Furnace to keep it running. Kanako Yasaka and Suwako Moriya were looking for a high temperature, high pressure furnace to revolutionize the kappa's energy production and thus made use of the Remains of Blazing Hell due to not being used and Utsuho helped them to create the Furnace. It's now being utilized by the kappa of the Youkai Mountain as part of their Mountain Industrial Revolution project. In Wild and Horned Hermit, Kanako and Suwako attempt to develop a better version of the nuclear fusion, probably as a way to convert the current Nuclear Furnace into something more efficient. It used to lie dormant, but because of the plans of Kanako Yasaka and Suwako Moriya, it is now fully active.
- Subterranean Animism
The Nuclear Furnace is the setting of the Final Stage in Subterranean Animism. It is the place that caused the events of the game by creating geysers, but also has an indirect impact on the events of the later games, namely Undefined Fantastic Object and Ten Desires, because it cause the Palanquin Ship to escape from the Underworld. Utsuho misunderstood what Kanako Yasaka had told her, and decided that, using these powers, she would take over Gensokyo. Rin Kaenbyou then drove vengeful spirits above ground to attract strong youkai to deal with her, but a human came instead. After the heroine fled through the Remains of Blazing Hell, fighting vengeful spirits and ravens, Rin will battle once more. After she was defeated, they'll do a little bit more roaming, and finally move upwards into the furnace, where they meet Utsuho, the Final Boss. After discovering that she wanted to take over Gensokyo, they go into a duel, and she was finally defeated. After this event and finding out who gave Utsuho her power, the heroine will go to the Moriya Shrine on the Extra stage as to why this furnace is here, only to find that that the entire event was a misunderstanding.
- Touhou Hisoutensoku
The Furnace appeared as a stage location in Touhou Hisoutensoku. In Sanae Kochiya's scenario, she'll come here after following Hisoutensoku, but instead fights Utsuho Reiuji.
- Touhou Gouyoku Ibun