power reactors aren't atomic bombs - Maya is keeping this simple for Misato's and brevity's sake, but it's mostly true. There is a theoretical possibility that, in a core meltdown, gravity could cause the collapsing fuel to assume a shape that accidentally assembles a supercritical mass that will try to go off like a "Little Boy"-style A-bomb, but such a shape would immediately disrupt itself by starting to explode, which would end the chain reaction before the explosion could reach what we might think of as "nuclear bomb" proportions. It would still be a substantial and dangerous explosion, but on the "blow up a building" scale, not the "eradicate a city center" one.

This phenomenon is called a fizzle, and some nuclear scientists believe it may be what the first of the two explosions at Chernobyl was, rather than the more commonly accepted view that it was a steam explosion from the hot fuel falling into the coolant reserve underneath the reactor.

750 megawatts of electricity... 2500 megawatts of heat - Nuclear power reactors make electricity by heating water to make steam that drives turbines, just like coal-fired power plants do. These generating systems rarely have an efficiency greater than about 35 percent, so the reactor has to make far more heat than the final electrical power figure suggests.

As an aside, 2500 megawatts (heat) is a pretty big power plant—gigantic for one used in a mobile application. By comparison, the A4W reactors fitted to Nimitz-class aircraft carriers are rated at 550 megawatts (heat) each. Even in a fixed installation, a 750-MW (electricity) reactor would be considered quite substantial. The RBMK-1000 units at Chernobyl were so called because they were rated to make a gigawatt (i.e., 1000 megawatts) of electricity apiece at full power, and they were considered enormous in their time. (There's an anecdote in Adam Higginbotham's Midnight in Chernobyl about a Soviet Navy veteran, trained in operating shipboard reactors, seeing an RBMK for the first time and demanding of its operators, "How can you possibly control this hulking piece of shit? And what is it doing in civilian use?")

it keeps making heat from fission waste decay - This is how reactors that have been shut down safely, such as the ones at Fukishima Daiichi, can still melt down if their cooling systems fail. Uranium fission produces a range of radionuclides which spontaneously generate heat as they progress down their individual decay chains. This means that the reactor will not just stay hot, but keep generating heat, for some time after shutdown—not as much as they were when fission was underway, but in a big reactor, even 10 percent of active output is a lot of heat to manage.

The only upside is that the initial fission products generally have short half-lives, which makes them very radioactive and kick out a lot of decay heat, but also means they're mostly gone within a short time. This means the core only keeps making significant heat for a few days, but during those days—especially the first few hours after shutdown—it's still fully capable of exceeding the fuel's melting temperature without active cooling.

This stuff, by the way, is the dangerous part of nuclear waste. It gets progressively less dangerous over time, as more and more of it decays into things that are stable and, if not harmless, at least a lot easier to manage (like lead), but some of the decay products fall into an inconvenient middle ground where they're just radioactive enough to be hazardous but not radioactive enough to decay quickly.

and starts making power again, if you're really unlucky - This is a similar phenomenon to the one that can cause a fizzle: the melted fuel happening to fall into a shape (say, by pooling in the hemispherical bottom of a reactor vessel) that can achieve spontaneous criticality without a neutron moderator. It won't explode in this condition, but it will make a ton of fissions, get very hot, and throw out a lot of neutrons and gamma rays.

In the old days, people used to think that if this happened, the spontaneous reactor would burn its way into the ground until it either reached the water table and went up in a giant steam explosion, or, rather fancifully, melted completely through the planet (this is where the phrase "China syndrome" comes from). Evidence from the Chernobyl accident suggests that, in practice, such a mass is too unstable to remain critical once it breaches containment and starts flowing. The basement of the Unit 4 ruins is full of melted reactor guts that did resume criticality for a while, but once it burned through the bottom of the reactor, it was no longer the right shape, stopped fissioning, and has been slowly cooling into weird shapes ever since. It's still so radioactive no one can get near it, but that's all decay radiation, not from fission.

plated with a zirconium alloy - This is a real thing (or rather family of things)! It's called Zircaloy, and was originally developed for the fuel cladding in submarine reactors in the 1950s. It works well in this application because it has a high melting point and a very low neutron capture cross-section (which means it doesn't interfere with the fission process). Nuclear engineers figure this outweighs its bothersome habit of causing nitrogen build-up if a mishap does manage to melt it.

A similar alloy is sometimes used to plate medical implants, since it doesn't cause immune reactions.

a puddle of red-hot crap - The technical name for this substance is, I kid you not, corium. It's like the nuclear equivalent of fordite, only much worse.

"I don't know why I'm even talking to you." - This instantly became one of my all-time favorite Misato lines.

ahh, zum Donnerwetter - Literally, "Ah, to the thunder-weather." Idiomatically similar to "ah, to hell with it."

--G.
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Benjamin D. Hutchins, Co-Founder, Editor-in-Chief, & Forum Mod
Eyrie Productions, Unlimited http://www.eyrie-productions.com/
zgryphon at that email service Google has
Ceterum censeo Carthaginem esse delendam.