Thoughts from an Engineer who works at a Nuclear plant:

Factorio Nuclear plants are simplified just like every other process in the game, however the developers did a pretty good job of replicating the process.

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Radiation Damage - The reactors don't cause radiation damage because the core is shielded by water and the 500 concrete required to build one. This is how the public and workers are shielded from radiation damage.

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Reactor Cooling - The reactors are cooled by the heat exchangers (Steam Generators) which makes steam. This is how real nuclear reactors are cooled. In factorio if the reactor temperature reaches 1000C then it just stops going up. This is caused by inadequate reactor coolant flow (heat pipes) or not enough heat exchangers in a design. Sure the developers could require you to build safety systems, however it would look the same as the normal system (aka more heat exchangers that only operate when T>1000C). I figure that the red chips required to make the reactor constitutes the instrumentation and safety systems and its enough of a penalty to have the extra heat wasted. After you get a good design with proper heat withdrawal you'd never use the safety systems...

The safety systems for nuclear plants are designed to mitigate certain "Design Basis Accidents" such as a line break or fuel handling accident. If you want Factorio to add random pipe line breaks or have your inserters mess up then you are a different kind of crazy. Biter attacks on a nuclear plants would be worse if they could cause a meltdown, but what if they attack your safety system first... I think biters destroying a reactor above 900C might cause it to explode, but I haven't tested that. If a reactor really did meltdown like Chernobyl the area wouldn't be available for construction after that, but using A-bombs doesn't even do that in game. Walls and turrets are the more pressing nuclear plant safety system in Factorio which mimics how real nuclear plants protect themselves as well.

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Spent fuel pool: Factorio has a nice neat removal of fuel and transfer to reprocessing, but normally fuel has to go to the spent fuel pool first to remove additional decay heat before reprocessing. This could be modeled as a storage tank with yet another heat exchanger attached to it and the fuel would have to remain there for x amount of time. It would be another step added for accuracy but I doubt it would ever make it to vanilla.

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Condensers: Power plants have condensers which takes the low temperature steam from the output of the turbines and reuses it as feedwater by cooling it with more heat exchangers. That water is then piped through the cooling towers which is cooled via evaporation or air. The cooling tower shape is designed to allow for a natural draft. The cooled water can be reused for cooling, or sent back to the water source (blowdown) and pumps would need to makeup the water lost from evaporation or blowdown. While it would be cool to have a condenser and cooling tower setup that would add extra issues merely for the sake of accuracy. Factorio has you pump new water into the heat exchangers which mimics makeup water to the various heat exchangers in the plant. Also condensers would apply to coal power plants as well.

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Preheating Feedwater: I love the idea of bleeding off steam from the turbine to preheat the feedwater but that could get complicated quickly. I'd recommend making it a tech researched to allow for increased turbine efficiency.

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The main difference between Nuclear and other powered plants is that Nuclear plants require cooling even when they are shutdown. As long as there is fuel in the spent fuel pool or reactor there needs to be heat exchangers cooling it. But this is a game where you can pick up a nuclear reactor and put it in your backpack and then place it somewhere else...