The nuclear fission landscape in the US is actually pretty exciting.

The leader is of course NuScale, which already obtained the NRC approval for their proposed reactor design. Their technology is the classical pressurized water reactor, but in a Small Modular Reactor (SMR) form factor of about 78 MW per module. Their first contract is for a 6-module power plant. The envisioned technological advantage is simply the fact that manufacturing many small reactors on an assembly line is going to be much cheaper than manufacturing large ones on site. They claim a $45-60 / MWh Levelized Cost of Energy, which is really competitive. Even if they are off by a factor of 2, it's still going to be competitive.

Next ones can be found at the link [1] provided by the Department of Energy. Here's my summary:

1. Sodium-cooled fast reactor (by Terrapower). Currently the Russians operate the largest sodium cooled reactors, the BN-600 and BN-800 [2], which, as the name implies provide 600 and 800 MW of electricity for a total of 1.4 GW. The main advantage of sodium-cooled reactors is that their fission reaction uses fast neutrons rather than slow (or thermal) neutrons. With fast neutrons the nuclear waste is reduced by orders of magnitude, while at the same time the fraction of U235 that undergoes fission increases, for a significant increase of overall efficiency. The negative is that sodium may burn, but sodium burning in air is not the violent, compared with sodium burning (or rather exploding) in water.

2. Triso fuel [3]. These are poppy seed-sized particles that have Uranium in the middle surrounded by 3 layers of some sort of ceramic. They can withstand temperatures of about 1800 deg Celsius for hundreds of hours. In other words, you can drop them in lava or in molten iron, and they are fine. Triso fuel is also envisioned to used medium-enriched Uranium (up to 20% U-235, instead of the usual 4.5% used in the current generation of reactors in the US). When Uranium is more highly enriched, the interval between refueling increases, which reduces operating costs. In the extreme, the naval reactors use weapons-grade Uranium (about 95% enriched), so they need to be refueled either zero or one time during their operating life. With 20% enrichment, you can envision intervals of 5 years between refueling (vs the current 18-24 months).

3. Xe-100 pebble-bed gas cool reactor. This is again an SMR with virtually the same capacity as the NuScale one (76 MW). It will use Triso fuel. It uses Helium as a coolant rather than water, as the current reactors in the US do. There are 3 advantages of Helium. Water is a liquid at room temperature, and you prefer the coolant to not undergo phase transition if possible. But you also want the coolant to absorb as much heat from the fission core as possible, and for water this is difficult, because water boils at 100 deg Celsius. To increase this temperature you increase the pressure. A lot. Like, a lot, a lot. 150 times higher than the atmospheric pressure, or 10 times higher than in a pressure cooker. With Helium you don't have that problem, because Helium is a gas to begin with. Also Helium cannot absorb neutrons and so it does not become radioactive when it goes through the core. Finally, the pressurized water in a PWR can only be heated up to about 550 deg Celsius, while the Helium can be heated up to much higher temperatures, resulting in higher efficiency of the thermal part of the power plant.

4. Salt-cooled reactor, by Kairos Power. Instead of using water, one can use a molten salt as a coolant. Regular table salt (NaCl) is a very stable substance, but Fluoride salts are even stabler, and Kairos (and many other startups) use such salts. Such a reactor is completely unrelated to a sodium-cooled reactor, it is much more similar to a regular PWR reactor (it uses slow neutrons). The big advantage is that it does not need the huge pressures of a PWR reactor. Kairos intends to use Triso fuel.

5. Molten chloride fast reactor (by Terrapower). This is the same company as for point 1 above. Terrapower is famous for having Bill Gates as one of its investors. This reactor design is very different from the one already mentioned. The nuclear fuel is dissolved in the coolant. It is replenished on a continuous basis.

There are a few more projects that the DoE mentions, but I think already these are quite exciting enough.

[1] https://www.energy.gov/ne/articles/infographic-advanced-reac...

[2] https://en.wikipedia.org/wiki/BN-800_reactor

[3] https://www.energy.gov/ne/articles/triso-particles-most-robu...