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Comment by timerol | Hacker News Reader

These are pretty big SMRs - 300 MWe, compared to 77MWe from NuScale. Definitely an iteration of reduced size from their 1 GWe model, as opposed to a "as small as reasonable" design.

It's cool that they call out 15 MWe / min "load following" capability. Ramping up and down in response to renewables will be an important function of any nuclear reactor installed today.

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> Ramping up and down in response to renewables will be an important function of any nuclear reactor installed today.

Realistically, it's not going to happen. PWRs are inherently stable, that's one of their big selling points. If anything goes wrong and the reactor becomes too hot, the water expands, moderates less the neutrons and they don't slow down enough to trigger fission events, so the rate of fission decreases. If for some reason the reactor stops producing enough heat, the water cools down and more fission starts happening. And there are other negative feedback processes in place too, for example Doppler broadening [1].

This is great for safety, but not so great for load following. If you want to load follow, it's probably going to be many times cheaper to just invest in a bunch of batteries.

[1] https://en.wikipedia.org/wiki/Doppler_broadening#Application...

That depends on what they mean by load following. While you can load-follow by changing reactor power (like the French do extensively). Westinghouse has long been promoting thermal storage based load following in their other reactor designs [1]. Where instead of perturbing the reactor's power, you divert the thermal output to a molten salt thermal battery when you want to decrease power suddenly, and use the battery to pre-heat feedwater when you want to increase power suddenly. For their LFR design they are claiming they should be able to load follow within 65-125% of nominal full power (ramping at 10%/minute). As long as the load-following averages out to 100% power over a long/short enough time period the reactor never has to change power level.

The only really needed to do this at any thermal plant is to over size the steam turbines, install some piping, and build an insulated salt tank.

Of course Westinghouse hasn't built any plants with that feature since it doesn't make economic sense without variable energy pricing.

[1] "Status Report – Westinghouse Lead Fast Reactor," (Westinghouse Electric Company LLC, United States of America), https://aris.iaea.org/PDF/W-LFR_2020.pdf

Light water reactors (like the AP300) don't get hot enough for molten salt storage. That requires one of the high temperature Gen IV reactors.

That just means for LWR applications you need to switch the storage media. Any thermal plant could implement heat storage if they identify a medium with a large latent heat of fusion that is around where you want to preheat your working fluid to.

The salt used for energy storage is normally a eutectic mixture of ~60% sodium nitrate and ~40% potassium nitrate (not NaCl) and has a melting temperature of ~260 °C. The secondary on a PWR has a maximum temperature of 275 °C. So within the liquid range of typical thermal storage salts, though I suspect finer tuning of the salt composition would be used to reduce the melting temperature to closer to the feed water temperature of ~220 °C.

Heat storage is technically possible at any temperature, but it's much more cost effective at 600°C than 300°C.

And if you're going with batteries, you might as well charge them with a source that has low levelized cost. That's not nuclear.

> 15 MWe / min "load following" capability

I know nothing of nuclear. What sets this rate? Is it some system level thermal gradient limitation? Or maybe the complexity of the safety around the movement?

Mostly it is the half lives of the reaction products, sometimes the half lives of the decay products or intermediate products. Ramping up and down was historically somewhat difficult, famously one of the contributing factors to the Chernobyl accident was the reactor having been turned down to a very low level on the previous day. You have to be careful not to accidentally poison your reactor during the day when the solar farms are chugging out megawatts only to not be able to ramp up in the evening when the sun sets.

Part of it is also that whole system have large amount of "inertia". We are talking about megawatts of power. First in heated water and then passed through a massive turbine. This whole process carries for a bit. And you want it to run the grid and not the other way around.

> And you want it to run the grid and not the other way around.

As simple as it is, this really blew my mind. I never considered that all the plants need to work together, as a system, each with "inertia", but all without ringing/oscillations. Thanks, I'll need to dig into this deeper!

I can only assume that having grid batteries can really clean things up.

The turbines have some perks in this. Batteries can supply power, but they are not as good in smoothing things out than actual large physical masses in thermal power plants and hydro plants.

But it is kinda amazing to know that all of these generators run essentially in sync. If one slows down all of them slow done.

The so far most successful SMR, BWRX-300, is also that size, and also a scaled down version of a long lineage of reactors. They are shooting for the same price target as well. So they are just going where the market is going.

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