This is indeed the elephant in the room. When I was reporting on NPPs in Japan, we took a tour inside the Hamaoka plant near Tokyo. As we entered the restricted area, we noticed a large concrete bunker being constructed. We asked what that was, and our guide said that he actually didn't know. It was being constructed by the defense agency and the power plant operator was not being informed what it was for or how many people would man it. It is quite clear that an attack on that plant (which has five reactors) could effectively end Japan, as 80% of the total Japanese population lives within 200 miles of it (Tokyo, Yokohama, Shizuoka, Nagoya, Osaka, Kyoto are all in that radius).

Even if the reactor buildings themselves are heavily reinforced, the cooling systems and associated piping and generators are not. All one would need to do is to knock out power to the plant in fact: a full station blackout could cause meltdowns, even with the passive core cooling systems in place (as happened at three reactors in Fukushima). Even more scary, if a spent fuel pool with hot rods were to be cracked and emptied, it would create an uncontained nuclear fire that could not be extinguished. It would emit about 10x more radiation than Chernobyl. The area would remain unapproachable for at least 300 years, and even after that would remain very hot, contaminated by radionuclides with half lives into the millions of years (U-235 has a half life of 700,000,000, and U-238 has a half life of 4.5B years).

Originally designers decided to create complexes of reactors, in order to share resources such as SFPs at a single site. What they realized with Fukushima was that if there is a critical problem at one reactor that caused the abandonment of the site, it could lead to a chain failure that would be beyond catastrophic. There are six reactors at Daiichi. Three suffered meltdowns and explosions, with a fourth not in operation also exploding. At one point the plant operator asked for permission to abandon the plant, but this was refused by the Prime Minister. As it was, only a relatively small amount of the potential radiation was released into the environment, about 5.5x10^18 becquerels (a becquerel is one radioactive decay per second). That is about half of what was supposedly released in Chernobyl (estimates vary). And that is about 1/5 of the total radiation in a single reactor core.

If Fukushima had been abandoned, all six reactors would have eventually melted down, releasing something like 15 times what was released in Chernobyl. Further, there would have been large airborne releases, much larger than Chernobyl, which would have gone wherever the wind took them. It would have been huge amounts of radiation spreading far and wide, impossible to clean up. Even in Fukushima, in which 99% of the airborne releases were blown out over the Pacific, the remaining radiation that settled in the 20 km zone around Fukushima required decontamination. Just decontaminating all the areas within six meters of human activity (ignoring the forests and fields, which make up most of the area, which is very rural), required scraping and replacing the first few inches of topsoil. That ended up being 92,000,000 bags of soil, each half the size of a dumpster. Those have to be stored somewhere indefinitely. They are still all in temporary storage, rotting in large fields across the area. Decontamination of hotspots had to be done as far as 200 miles away. Now imagine trying to decontaminate NYC.

And if Fukushima #1 had melted down, it would have meant the abandonment of Fukushima #2, only a few miles away. That has another four reactors (which also almost melted down, but that is another story that has been suppressed). A meltdown there would most probably have forced the abandonment of the northern part of Japan, which would have forced abandonment of the Tokai plant, which would also have melted down. Tokai is right next to Tokyo. This would have spelled the end of Japan, or at least the part on the lee side of the Japan Alps. There are 54 reactors in Japan. What would happen to the rest?

The potential for disruption is clearly not lost on terrorists and enemy states. The official position is that we'd better not speak of this, since it takes 40 years to decommission a nuclear reactor. One well-placed missile strike, or an EMP weapon that disrupted the grid would be an unimaginable catastrophe for a country. And the area rendered uninhabitable would remain so for hundreds, if not thousands, of years.

Toby, I will review this and pass this along for comments.

One thing I would mention that is not commonly understood and often mentioned to overstate the danger of a radioactive release is that isotopes with the longest half-life are the ones with the lowest radiation - isotopes with the shortest half lives are the most dangerous. Think of it as there is x amount of radiation which can be released at a very high rate very quickly or at a very low rate for a very long time. I have held a block of Uranium in my hands several times. The billion years sounds super scary, but in fact, it’s the long half life elements that are the least dangerous. Elements that are released in an accident such as Chernobyl and are measured and reported are Iodine 131 (half life 8 days), Strontium 90 (28.8 years), Cesium 137 (30.7 years) and Plutonium 241 (14.1 years). Here is the data from the Chernobyl accident. As you can see, the largest release in terms of Bq is Iodine 131 with a half life of 8 days. That puts things in a very different perspective :

Major radioactive substances released by the Chernobyl accident

Radioactive substance half-life EBq
iodine-131 8.04 days. 1.760
caesium-137 30 years 0.085
strontium-90 29.12 years 0.010
plutonium-241 14.4 years 0.003

Btw, on a personal level, I am not naive to the risks. I live within 20 miles of the Sharon Harris Nuclear plant, and we have one of the largest above ground repositories of spent fuel (because we cannot get past the politics of opening the Yucca Mountain Repository), and I have a package of Iodine pills in my emergency disaster kit. But, in a professional capacity, I have visited several Nuclear plants and been inside the containment of one, and I can tell you that the security and safety precautions are world-class, right up there with the Polaris Missile Facility in Goose Creek, SC, where nuclear warheads are loaded onto Polaris and Poseidon submarine launched missiles (deadly force will be used against all intruders).