Summary
I took a 480 MW reactor blueprint, and asked a calculator what it would take to make in an hour. (I don't provide reactor blueprints. Any reactor with 48 Heat Exchangers and 96 Steam Turbines will be fine.) All smelting and intermediary components are included! It does everything in 1 hour, without having to leech off or compete with other builds.
It's difficult to need nuclear built faster than this. That's the secret of this build: slow and steady, and you will have nuclear built on time. Trying to make it faster just makes a mess of things.
Setup
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Give it priority access to ores at the start of a bus. The iron needs 0.62 of a yellow belt. The others need < 0.5 and can be half-belts.
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It can make centrifuges (20/hour) instead of reactor parts, using a small hack. So you build it early, before reactors are needed, and make centrifuges first.
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If there's any chance of other builds / players leeching the Roboports and Substations, change them to non-networked chests.
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If you want it to buffer more nuclear parts, change the -4 in the constant combinator to a -8, etc.
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If making 2 copies, it's recommended to join the Light Oil networks. They connect easily with an underground pipe, if the builds are offset by 1 tile. This avoids concerns that uneven consumption starves one build of the chance to make fuel for its smelters.
Warning
Plastic must flow, or there won't be fuel for the smelters. If asked to provide Steam Exchanger but not Nuclear Reactor, the build will break. (Don't let a different build "help" provide Nuclear Reactors.)
It's a rush build, so I don't care about weird ways of breaking it. The easiest proper solution is to get coal onto the fuel belts, when Solid Fuel is short. The layout would need to make room for this, and it's already stupidly long.
Documentation
Tested to work in 1 hour, without needing inserter stack bonus. The first iteration needs +15 minutes to fuel the furnaces, if they aren't manually primed.
Balanced production. Waits for the first 1/4th of all parts to be finished before moving on to the 2nd 1/4th.
A wired inserter makes iron/copper smelters get fuel before steel smelters. This is needed when bootstrapping, so the red circuits start up first, consume plastic, and keep oil flowing.
The (light oil > 1000) value on the pump came from experimentation. If it's too high, the build wastes time buffering light oil and starts up slowly. If it's too low, it won't bootstrap correctly.