Comment by credit_guy | Hacker News Reader

Comment by credit_guy | Hacker News Reader

Parent Full thread

This reactor indeed has lower power density than a PWR, but not by a factor of 12. I compared it with NuScale's reactor, which is a PWR SMR. Details about both can be found in [1]. The HTR-PM reactor pressure vessel has a volume of about 640 m3, and yields 105 MWe, while NuScale has a volume of 101 m3 and yields 77 MWe. The power densities come to be 6.1 m3/MWe vs 1.3 m3/Mwe, and the ratio is 4.7x.

Still, this is a good price to pay for getting a meltdown-proof reactor.

> Pebbles themselves are also problematic, they tend to swell, crack, and they can't be reprocessed using the current technologies.

It is simply not true that pebbles tend to swell and crack. Quite the opposite happens: fuel elements in the current generation PWRs tend to swell, crack and burst. This happens because some fission products and decay products are gasses, such as xenon, kripton, radon. They build up in time and create internal pressure. The same happens inside the fuel kernels in the pebbles used in this reactor, but those kernels are specially built to withstand much higher internal pressures.

Here's a relevant quote from [2]:

  > As the pressure vessel size is reduced, the more efficient spherical geometry can be adopted, and the required wall thickness drops dramatically to the point that a 35 μm SiC layer can indefinitely contain gas pressures in excess of 100 MPa. This compares to the main reactor steel pressure vessel which may go as high as 20MPa or the Zircalloy cladding which can have pressures up to 10s of MPa in limited conditions – temperatures far below what the ceramic pressure vessel tolerate

As for the reprocessing part, I think you are jumping the gun. There is no reprocessing done in the US, at all, for any type of fuel. Even where reprocessing happens, as in France, the benefit is quite reduced. One can extract some plutonium and unburned uranium, but in the end that will allow you to extract maybe 10-20% more energy from the original amount of natural uranium. It will not make you extract one hundred, or 10 times, or even just twice as much energy. Reprocessing is simply not a game changer. It is not clear at all if it makes economic sense to build the highly complex facilities that do reprocessing, for the limited benefit.

[1] https://aris.iaea.org/publications/SMR_catalogue_2024.pdf

[2] https://www.usnc.com/triso/

> As for the reprocessing part, I think you are jumping the gun.

The problem is that if the pebbles aren't reprocessed, you now have to store their very large volume. The moderator, graphite, is integrated into them. This is unlike a LWR, where the moderator is water that the spent fuel can be simply lifted out of (after cooling). LWR fuel can be stored after a few years into dry casks; the equivalent for pebbles would be vastly larger and more expensive, and involves storing the spent fuel with its moderator, increasing concerns of criticality (although I imagine they'd be doused with borate or something to prevent that).

PWR pressure is on order 2-3x higher than an HTGR, so that’s a 2-3x thicker vessel. So good savings there

Cost of a pressure vessel is roughly proportional to pressure x volume. HTGR doesn't come out on top by that metric.

Thickness of a pressure vessel wall is proportional to pressure x linear dimensions of the pressure vessel, increasing at a given pressure as the vessel is scaled up.

Is NuScale’s design a good one to compare against? Isn’t it a theoretical design only?

I held their stock for a while until I realized they don’t exist to make a reactor. They exist to get funding.

Sounds crazy, but look through their actions. All press releases are just talk about what they will research and with whom they talked or made a “memorandum of understanding”. The CEO CV is also interesting since it lists a whole lot of board positions and titles but it’s not clear what he has actually done.

This is a very cynical take.

I picked NuScale because it's a PWR SMR, and is the only SMR design that was approved by the Nuclear Regulatory Commission. HTR-PM is an SMR, and comparing it with a full scale PWR reactor is not entirely fair, because reactors benefit from the square-cube law: the larger a reactor is the more efficient its neutron economy is, so you can extract more power per unit of volume.

As for NuScale being a scam, it would be probably the most elaborate scam in the history of financial scams. They were founded in 2007 and went public in 2022. People don't spend 15 years to run a scam, especially if this involves not one by two government agencies (the NRC and the SEC). There's a huge probability that such a scam would not work in the end, with an additional likelihood that you get relocated to a correctional facility (see Elizabeth Holmes).

The power density of the reactor vessel of a conventional large PWR is close to 20 MW/m^3 (inverse of 0.05 m^3/MW); a BWR is around half that. The peak power density in a PWR's core is around 100 MW/m^3.

NuScale integrates the steam generator in the "reactor" so the volume is larger, but this means using it is comparing apples and oranges. NuScale's design is also intended to use natural convection instead of forced circulation in accident conditions. This further reduces the power density allowed.

NuScale isn't a scam, but it appears to be founded on faulty principles (that the thing holding back nuclear power was safety concerns, rather than cost) so its business case doesn't appear to be working.

> As for NuScale being a scam, it would be probably the most elaborate scam in the history of financial scams. They were founded in 2007 and went public in 2022. People don't spend 15 years to run a scam, especially if this involves not one by two government agencies (the NRC and the SEC).

I wouldn’t call it a scam, but something that runs for 15 years with the involvement of 2 government agencies (read: large bureaucracies) is not unlikely to produce very little.

For example, see NASA’s space program, the European space program, and Boeing.

interesting comment - made me look up the stock and it just jumped. Propelled by this news perhaps?

price != value

Otherwise Buffett wouldn't be so rich.

> This reactor indeed has lower power density than a PWR, but not by a factor of 12.

It's more.

> while NuScale has a volume of 101 m3 and yields 77 MWe

VVER1200 has the inner vessel _diameter_ of 4.2m, height of 11m for the internal volume of 153m^3, and 1200MWe capacity (so around 3GWt).

_THIS_ is what you're comparing it with.

> Quite the opposite happens: fuel elements in the current generation PWRs tend to swell, crack and burst

Nope. A swollen or a ruptured fuel rod in a regular reactor is a reason for SCRAM. The water inside the reactor vessel is constantly monitored for fission products. The individual fuel tablets swell, but they are contained inside zirconium rods.

Reply on news.ycombinator.com