There are two types of nuclear power: fission and fusion. Both types of power rely on the mass-energy and binding energy of elements: iron and nickel have the lowest mass per nucleon and the highest binding energy of any element, making them the most stable (binding energy measures the amount of energy needed to break bonds, meaning that a high binding energy suggests a more stable system that is harder to break). All other elements, under duress, will strive for this low energy state. This is most striking for particularly heavy or particularly light elements, as they'll release the most energy when they are transformed.
We'll talk about fission first, because it's necessary to differentiate the two.
Nuclear fission provides 13.5% of the world's electricity. (If you don't like my description, The Simpsons provides a fairly succinct summary.) The principles behind nuclear fission are the same as those behind an atomic bomb: certain heavy elemental isotopes (such as uranium-235 or plutonium-239) are inherently unstable, throwing off energy and neutrons. If you bring them in close contact, those released neutrons collide with other atoms of (for example) uranium-235, causing them to become more unstable and release more neutrons and more energy. This nuclear chain reaction is a positive feedback loop. If one performs it too quickly, it forms a nuclear weapon; nuclear power requires a more controlled reaction.
This energy is harnessed in a fairly simple way, recognizable to Vitruvius, Hero of Alexandria, Thomas Newcomen, and James Watt. Since these reactions throw off a lot of energy as heat, the fissionable materials are usually shaped into rods and placed in a vessel to transfer the heat to water or liquid sodium. This heat energy is then transferred to water, which turns to steam, which turns turbines, making the whole apparatus a steam engine.
But what about fusion power?
If fission power breaks down atoms into their component pieces, fusion power builds them up. Taking light elements such as hydrogen (typically as heavy deuterium or tritium) or helium (typically as helium-3), one can build these elements to helium-4. This operates on the exact same principle as fission — helium-4 has less mass-energy than heavy hydrogen, and so is more stable. Fusion reactions are extremely energetic, releasing lots more energy than fission reactions (it's why the hydrogen bomb is much more powerful than uranium bombs). From there, modern fusion reactors look a lot like fission reactors — keep it controlled so that it doesn't explode, and the heat energy is diverted to a steam engine.
Of course, fusion reactors have a lot of problems. There's a very successful one that produces most of the energy on Earth, but it's the near black-body called the Sun. The Sun is so successful because it's so massive; a lot of energy needs to go into a fusion reaction, and so far, scientists haven't made it efficient enough to be viable. That's why cold fusion is such a big deal — if scientists could figure out a way to produce fusion reactions under low-energy conditions, fusion would be much easier.
It seems unlikely, though. For the moment.
Fusion reactions require a lot of energy, produce a lot of heat, and require a lot of space. One approach is magnetic confinement, best exemplified by the tokamak reactor. Magnetic confinement uses magnetic fields to contain the superheated plasma, and typically takes place in a torus-shaped reactor. (Compare with inertial confinement, which uses lasers to confine the pellet; inertial confinement can withstand higher temperatures, but magnetic confinement lasts longer.)
Which brings us to the actual point of this article: where can one find a big, indestructible torus among the Planes, complete with its own injection system?
Sigil contains portals to everywhere and everywhen, and some of those portals are either permanently open, or can be opened at will with the proper portal key. Some enterprising scientist with enough manpower could divert power from the portals to the Inner Planes in the Lower Ward to act as the injection and containment center, and then open another portal to allow heat transfer to a site in another Plane, likely building a steam engine on the other end. Containment would likely be a non-issue; it's called the Cage for a reason.
Sure, it would kill everyone and destroy all structures in Sigil, while also venting hot plasma out of every open portal to Sigil, but that's a small price to pay for nearly limitless energy.
Of course, the dabus and the Lady of Pain would likely have something to say about it. As might any enterprising adventurers and Faction-types who catch wind of the plot.
It's also notable that Sigil contains roughly 50,000 residents, while hosting as many as 200,000 transients at any given time. Certainly no entity would consider that a sacrifice of any kind, would they?
Also, didn't Vecna wrest apotheosis and change 2e to 3e by causing mayhem in Sigil? What would happen to the scientist who finally does what Ravel Puzzlewell and Vecna could not?
What happens to the person who breaks the Cage?