Comment by credit_guy
16 days ago
> because the temperature is higher, more expensive materials are needed
Is that a fact or an educated guess? Nuclear reactors do not use ordinary steel, they use nuclear grade steel. I don't know much about steel, but with a bit of googling I found out that the 316L steel (on of the 2 most common nuclear grade steels, the other being 304L) has much higher creep temperature than ordinary steel. This is the steel used in making the pressure vessels of PWRs and BWRs.
The steel used in the HTR-10 (the precursor of the 2 HTR-PM rectors that China recently hooked to the grid) has a composition [1] that seems to me to be almost identical to the 316L [2]. The HTR-10 has an average helium temperature at the outlet of 700°C.
Do you have some more concrete sources of information that would indicate the steel in HTGR is different and significantly more expensive than the steel in PWRs?
[1] https://www-pub.iaea.org/MTCD/publications/PDF/te_1382_web/T...
Well, requiring a material to withstand a higher temperature is an added constraint, so of course it's going to increase cost. Otherwise, the more exotic material would have been used in the lower temperature application too.
But that's all secondary to the observation you ignored, that the apples-to-apples comparison you're trying to make ignores power density, and when that's included HTGRs require much heavier pressure vessels.
> Well, requiring a material to withstand a higher temperature is an added constraint, so of course it's going to increase cost.
This reasoning sounds good in theory, but practice and theory sometimes differ. The steel in reactors is subject to neutron embrittlement. In order to avoid that, you need special steel. Which happens to also withstand much higher temperatures. So no, it's not an additional constraint.
> that the apples-to-apples comparison you're trying to make ignores power density.
The strange thing is that the second part of your argument had nothing to do with TRISO. Not sure why you brought it up. You said that this is a tradeoff of passive safety. If you want passive safety, all things being equal, you need to accept lower power density.
However, all things are not equal. When you use TRISO as a fuel, that can withstand absolutely huge temperatures, in excess of 2000°C. Since radiative cooling is governed by the Sefen-Boltzman equation (i.e. the radiated power is proportional to the 4th power of the temperature), if the pebbles reach high temperatures, they get rid of a lot of heat by radiation, which is then absorbed by the steel pressure vessel and conducted outside.
Additionally, when you use TRISO in the form of pebbles, you recirculate the pebbles. You take them out and let them "rest", and then put them back later, so you limit the decay heat.
Anyway, enough theory, let's look at actual numbers. The American designs Xe-100 and NuScale are perfect for our comparison. They both have a thermal output of 200 MW. NuScale is a PWR and has a lower efficiency, so the electrical output is 60 MW, while the Xe-100 produces 82.5 MWe. NuScale's reactor pressure vessel has a volume of 129 m3, and Xe-100 has 390 m3, so larger by a factor of 3. If we only care about the electricity output, then Xe-100 needs 4.73 m3/MWe, while NuScale 2.15 m3/MWe, the ratio is only 2.15.
However, because the Xe-100 has to withstand much lower pressures, the mass of the pressure vessel is 274 tons, vs 260 tons for NuScale. Per MWe, the Xe-100 vessel is actually 23% lighter.
[1] https://aris.iaea.org/Publications/SMR_Book_2020.pdf
[2] https://aris.iaea.org/PDF/NuScale-NPM200_2020.pdf