First I would like to express my gratitude for this interesting mod.
I just reached the point on which I built my first nuclear power station. I chose steam turbine and heat exchanger over Stirling engines because I want to recycle the wastewater from boiling seawater. The wastewater then enter heavy water production factory for more nuclear fuel, though the power station only produce a fraction of the wastewater compared to the amount that is used to produce its fuel (I don't have the breeder cells yet). However I found that compared to vanilla in which a pipe can supply around 10 heat exchanger, the pipe in this mod can only supply six heat exchanger with seawater and only two for steam output, since the steam produced by two heat exchangers is about 2400 units per second, which connects to only two priority turbine. This resulted in a wide and short design, and relatively small amount of cores (I have 4, and I gave up the planned 6 core design because the layout is too wide).
And to solve this it would be nice if you could add a recipe for more energy dense form of steam, namely highly pressurized steam, to be produced in heat exchanger 2. Hopefully this will solve the pipe maximum flux problem, since steam is the bottleneck.

Heat exchangers and steam aren't really tuned for nuclear. Stirling engines work well, especially at tier 3. Heat exchangers were envisioned as a cheaper, lower tech alternative, and intentionally didn't scale beyond mid game, since they don't seem competitive with stirling engine 3's 100% efficiency. In general, I don't want steam to be more energy dense than it already is, since that affects the balance of different energy storage methods.

You don't really need to run steam through pipes, since you can hook up turbines directly to the heat exchanger. For storage, there are thermal tanks, which make more sense than storing steam. Nuclear's 1500 max temperature gives you a decent amount of leeway to use heat pipes before the distance becomes a bottleneck, so you can space out heat exchanger arrays that each directly connect to turbines.

I may consider a higher tier of heat exchanger designed with nuclear in mind instead of geothermal/solar thermal. When heat exchanger was first added, there were so many different boiling recipes, so that makes them a little more interesting. A new heat exchanger 3 could have a higher baseline operating temperature. Then it might be OK to have a higher energy version of steam that you can only get with hotter heat sources. If it's only produced by tier 3 heat sources, not combustion or early game thermal, then it shouldn't interfere as badly with the energy storage balance. At that point you have a lot more consistent energy producers and strong energy storage options.

Great. I really like the idea of heat exchanger 3.

Recently I've built five copies of a powerplant module to use only ground water(basically nothing but electricity) and empty barrels(which is recycled when emptied) as input and output only heavy water barrels. It seems to produce its own fuel from recycling the wastewater it generates while providing about 300MW of power each plant. It is not as efficient as the Stirling engine version because it diverts about a fourth of its power to produce waste water which is later turned into heavy water(the recycle chain is fresh water -> waste water -> heavy water, sludge and saline water, and sludge + fresh water -> more waste water, and saline water -> brine -> salt, salt + fresh water -> sea water, sea water -> more waste water, finally all the fresh water is turned into steam and heavy water). And the signal from steam tanks makes it easy to control the inserters taking out the spent fuel, which sends the signal to the inserter inserting the fuel, making it takes fuel only when there is space for steam. There is no temperature signal from thermal tanks, which means if you want to control the fuel intake lots of accumulator must be used, just to provide signal to the inserters, and at the same time supporting the factory with electricity when the reactors are not fueled. This is basically the reason I don't like to use Stirling engine 3. Adding a signal output on the thermal tanks could help me to control the fuel consumption.

It is a pity I don't have access to discord due to the tight restriction in my country, otherwise I would really like to post a picture of the power plant module.

I think the Inventory Sensor mod, which is compatible with Nullius, has a way to connect thermal tanks or nuclear reactors to the circuit network to measure their temperature. It's not something supported by vanilla, so it's not easy to just turn on that behavior, so a third party mod is a more appropriate way to add the feature than to build it into Nullius.

Oh thanks, I didn't think of third party software. I will try that.

Another way to do it without any extra mods is to have 1 heat exchanger, 1 steam tank, and 1 turbine, near your reactor core, and use that tank as a temperature sensor. The the rest of your generation can be with stirling engines. Stirling engine 3 has a lower minimum operating temperature than heat exchanger, so when your heat exchanger stops working you know it's time to add fuel.

Oh I see that also works, Stirling engine would be still working while heat exchanger stops. Thanks.

If you add a heat exchanger 3, hope it still produce waste water so that I can collect it to produce heavy water. Then the new plant would have higher capacity while still generating some heavy water to be produced into fuel.

Probably a high temperature turbine and a low temperature turbine could be used, the latter feeding on the exhaust of the former? I borrowed the idea from Captain of Industry, so it might cause some trouble, but only if you don't mind.

It would still have the same boiling options and byproducts.

The problem with one turbine feeding off another is that the way turbines work in the Factorio engine it's hard to make them have an output. Space Exploration has a condensing turbine that turns steam back to water instead of making it vanish. But that's a compound entity with many different parts, which is hard to implement. At some point I would like to re-implement how turbines work in Nullius, which would solve several problems. But it's a big project and not an immediate priority.

Wait a second, I discovered a flaw in the second approach.

I checked the description of Stirling engine 3. It has an optimal working temperature range, and unfortunately the lowest working temperature of heat exchangers falls below that. Does that mean when the heat exchanger stops working the Stirling engines would also be working below maximum capacity? I haven't tried that yet, but that would be a serious problem since this makes the core temperature never reaching the optimal range of Stirling engine 3, which makes the whole power plant never reaching its designed output. Is that true or actually there is no problem?

Yeah, that may be a problem. Though it's complicated since it's normally hotter closer to the reactor, and the heat exchanger could be closer to the reactor than the stirling engines.

It's another thing that heat exchanger 3 would be useful for, since it would have a minimum temperature safely above the optimal range of stirling engines.

Heat exchanger 3 is now available in the latest update.

Thank you. I will make good use of it.