We understand improved transmission far better than experimental nuclear reactors. All the types of transmission shown in the graphs above have been built and operated at a commercial scale for decades. Sure, careful planning for transmission is necessary, including sophisticated models, but I find it surprising that a nuclear proponent uses the need for sophisticated modeling as an argument against an investment. Perhaps you think "Generation IV nuclear reactors" should be built using only unsophisticated models? How comforting.
Transmission makes up only 7% of the national transmission bill. According to the REDI report linked to at the start of the article, no currently envisioned transmission expansion scenario would bring that to more than 10%.
Experimental nuclear will only be "low cost post carbon generation" when it has been proven at a commercial scale. Until then, it's just low-cost hope.
For now, the only "low cost post carbon generation" sources we have are wind, geothermal, biomass, and waste-to energy. The last three are limited in supply relative to demand, while even distributed wind requires transmission investments. Solar could easily become low cost before experimental nuclear does, and it too requires long distance transmission even when placed near demand. Again, see:
While the size of the needed transmission investment is huge, is also tiny in comparison to our electric bills as a whole, and any investment in generation, be it nuclear, wind, solar, or even coal.
Regarding ERCOT, one grid expansion that would benefit not only ERCOT but the other