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Comment by panick21_ | Hacker News Reader

While I don't really think PWR in a module is that huge of an improvment. They did help to develop some certification that will help many other SMR companies.

Congrats on getting this certification threw. Its a huge achievment even with a PWR.

Sadly if it wasnt a PWR it would essentially have been impossible in the US.

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I had to google PWR and it stands for pressurized water reactor https://en.wikipedia.org/wiki/Pressurized_water_reactor

What are the alternatives?

For instance my favourite reactor type - lead bismuth cooled fast reactors. They've only been actually used once, on the Soviet "Alfa" class submarines, and have some interesting advantages (lead naturally blocks gamma radiation, in case of a leak temperatures will go down and the coolant will solidify thus preventing radiation leaks, high efficiency due to the high temperatures, etc.), but are pretty expensive and impractical (the coolant solidifies if temperatures get lower than expected, thus you need specialised equipment to keep them hot/operate them 24/7).

There's also thorium, molten salt reactors.

NaK cooled reactors have an advantage of not freezing when you shut them down since NaK freezes at -12C. However Nak explodes when mixed with water...

The Department of Energy is working with a number of startups on various reactors designs (including NuScale). [1] is a very nice summary.

[1] https://www.energy.gov/sites/default/files/2020/05/f74/Advan...

The other common alternative are boiling water reactors:https://en.wikipedia.org/wiki/Boiling_water_reactor

Gas cooled reactors, CANDU reactors, molten salt (fuel and/or cooling), fast (breeder) reactors, molten metal cooling, etc. Probably some others too.

There is a very, very large amount of different reactor types you can build.

The main decision points are first, do you want to have 'slow'/'thermal' or 'fast' neutrons. If you want to have 'slow' neutrons you need some material to slow them down. This both has advantages and disadvantages.

Then you have to decide what the primary material is you want to use to fuel the reactors, here the options are basically U-238 (natural uranium), U-235 or Thorium.

Then you have to decide, if you are going to slow neutrons down, there are a few options on how to do that. light Water, PWRs are an example, Heavy Water, like CANDU reactors. Other common options are graphite (Molten Salt Reactor Experiment). But there is also more exotic stuff like beryllium.

Once you made that choice you need to somehow cool your reactor. For that you have tons of options all with advantage and disadvantages. Water is the common example but water has to be under a huge amount of pressure for this to work.

There are also salt cooled reactor (like fluride salt), or lead cooled, sodium cooled, gas cooled and so on and so on.

Check Figure 10: https://unece.org/sites/default/files/2021-08/Nuclear%20powe...

There are more options of course, the fuel can be liquid or solid. And if its solid it can be shaped in different forms. Nuclear reactors can work so many different ways.

If you want to see some interesting examples of current commercial reactors check out for example:

- Moltex Energy SSR-W

- Terrestrial Energy IMSR

Here are lots of companies currently engaged with the Canadian regulator:

https://nuclearsafety.gc.ca/eng/reactors/power-plants/pre-li...

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