Nuclear fission fuel is inexhaustible (2022)

The trick is that they keep building the same obsolete US-based design instead of re-inventing the entire thing from scratch for each plant.

Imagine how much more accessible computers would be if you could just copy the operating system from one "printed" circuit board to another, instead of hand-wiring all the transistors, then hand coding process scheduling and I/O.

The French did this totally unprecedented novel thing where they manufacture more than one identical part at a time in a line of assembly stations, and the parts of the plants are interchangeable. I doubt such things transfer to other countries or industries though.

Thermal-spectrum (slow neutron) nuclear reactors are much easier to control than fast reactors because they have much slower reaction times. But this isn't particularly relevant because thermal-spectrum supercritical water reactors are possible; supercritical water reactor doesn't implies fast reactor. Let's put fast reactors aside.

In thermal-spectrum reactors, the moderator is not in itself a safety feature. The moderator is necessary to slow the neutrons down into the thermal-spectrum. Thermal-spectrum neutrons are far more likely to be absorbed by the fuel than fast neutrons, so if you lose the moderator the reaction stops (and decay heat continues...) Fast reactors don't use a moderator and don't need one because they use a higher grade of fuel which can sustain a chain reaction with fast neutrons, not needing thermal neutrons.

In some thermal-spectrum reactors (particularly BWRs and PWRs), the moderator plays double duty as a coolant. In these reactors the moderator is a safety feature insofar as it's the coolant, not because it's a moderator. In other kinds of thermal-spectrum reactors, the coolant and moderator may be different; for instance RBMKs use graphite as the moderator and water as the coolant. Modern CANDU reactors use heavy water as a moderator (which is less efficient as a moderator, but captures fewer neutrons and therefore allows for a lower grade of fuel), but this moderator is unpressurized in the calandria and remains cool; the water in the coolant loop is hot, pressurized, and doesn't provide sufficient moderation to keep the reaction going. If you drained the moderator but kept the coolant loop running, the reactor would stop. If you kept the moderator but drained the coolant loop, it would eventually melt down (probably after the water in the calandria boils off). The coolant is what removes heat from the reactor and the moderator is what slows the neutrons so they're more likely to cause fission.

tl;dr: The moderator increases reactivity. Coolant removes heat.

In principle you could make a thermal-spectrum supercritical water reactor using any of these, but the research is aimed towards making a supercritical reactor with the PWR or CANDU designs. The reason supercritical water reactors aren't used yet is because metallurgy isn't up to the task. Supercritical water is already used in traditional power plants, but those don't have to deal with neutron radiation which structurally degrades anything it runs into (some alloys/materials more than others.) Finding alloys which can hold up to both supercritical water and neutron radiation is the major hurdle to clear.

Also, because it's a global problem, the solution has to be something that everyone wants to have, and that everyone wants everyone else to have too, which means we have to care that what powers Iran is green and also Iran will care about not triggering another airstrike from Israel fearing it's a secret atomic bomb project.

PV in particular is also useful for being a continuous roll-out, so even if it worked out at exactly the same cost/joule as nuclear and taking exactly as long to reach the same final total average power output, getting the first joules sooner displaces more of the existing CO2 emissions.

Nuclear is a niche category now. But nuclear technology is a fashionable investment that VC limiteds want in their portfolio. So you get articles like this that position uranium fission energy as "renewable."