#nuclear reactors

The people most directly effected by nuclear colonialism is of course indigenous peoples around the world. The majority of testing, refining, and disposal is on sacred lands, reservations, and island nations. The cancer, birth defects and disability rates of communities near these sites are extremely high. The US military leaves behind nuclear waste that kills people for generations after.

Top countries by nuclear reactor capacity (in gigawatts electric)

Like, a Los Angeles class fast-attack submarine is 350 ft long. Almost half of that space is taken up by the power plant system. That's 175 ft, or over 50 m. That's roughly equivalent to a 14 story building. Gypsy Danger is 260 ft tall. If 1/3 of its height is the torso, that's ~80 ft. Not enough room for a small nuclear power plant. And that's with what we were capable of building in the early 2010s. That space also doesn't take into account all of the coolant needed, whether it's a water-cooled reactor or a sodium-cooled reactor. And that's not even considering the weight of all of that stuff. Plus the fact that, while a sodium-cooled reactor may be more efficient, it is also highly flammable. There's a reason Jimmy Carter refused to allow sodium-cooled reactors on US nuclear submarines, and he was basically considered the father of the nuclear navy.

On top of all of that, nuclear reactors are fussy. They don't like getting knocked around. There's a lot of delicate instrumentation, and the entire system is running at high temperatures and high pressures. We're talking 250 to 300 °C (482 to 572 °F) and >2000 PSI (136 atm, or 136 times normal outside pressure). You need a very robust system of pipes and pumps to handle that sort of thing, and if any of it breaks, at best you're getting irradiated water and at worst you could be looking at a hydrogen or sodium explosion depending on the type of cooling system.

All of these concerns regarding weight and size are basically why nuclear-powered jet engines never took off (hah). Yes, they were actually being designed, by both Pratt & Whitney and General Electric, the two major jet engine manufacturers in the US. While the GE design was "relatively" small, it was a direct air cycle, which means that the air powering the turbine was heated up directly by the nuclear reactor, meaning that the exhaust from the engines were irradiated. So using it meant spewing radiation behind it.

That's not even taking into account that the first nuclear-powered jaeger was built in ~2014, a time when nuclear power was especially contentious. If it had been pre-Fukushima, when the "nuclear renaissance" was still holding strong, I could believe that they may have tried for a nuclear-powered jaeger. But post-Fukushima? Not so much.

So yes, it's highly unlikely that we would have developed nuclear-powered mecha. What's more likely is that we would have seen huge advances in rechargeable batteries, fuel cells, and electric motors. We are just now seeing in the early 2020s an electric motor that can generate power on the same scale as a combustion jet engine, but the amount of power to run that motor is still beyond the capacity of what we have for battery storage. And the problem we see, of course, with the rechargeable batteries being used in electric cars is that they tend to catch on fire.

All that being said, yes, I recognize that it is a sci-fi movie and the science aspect is more or less just ignored. But this is what happens when you have a bored writer who has also been in the power generation industry for nearly 17 years.