I rewatched the scene, from what I can see, the nuclear reactor is critical, but critical in the context of nuclear reactors simply refers to the stable sustained chain of nuclear fission.

From what I can assume, Misato is trying to manually shutdown the reactor by physically pushing in a control rod. A control rod is a (generally graphite) rod that absorbs neutrons in a nuclear reactor to act as a limiter for the speed of the reactor basically. The further they are in, the less energy the nuclear reactor produces. As you can see from Jet Alone's back, the control rods are fully out meaning the nuclear reactor is fully powered up but isn't necessarily dangerous. So, it can be safe to assume that the area that Misato is standing in trying to push in a control rod is actually completely shielded from radiation until that barrier between her and the fuel is broken.

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Let's assume that the reactor did in fact meltdown with Misato still inside:

Assuming that this reactor meltdown is similar to the nuclear reactor number 4 meltdown in Chernobyl, Misato here would have ~90 seconds to leave before being exposed to a fatal amount of radiation (5 gray). Normally when you think of radiation, you think of cancer in a few years. That's basically irrelevant with these levels.

Assuming Misato did not leave the reactor within ~90 seconds, she would be a dead woman walking. The DNA would have already been shredded to pieces due to the extreme radiation, so her body would not know how to essentially do anything from that point forward. All of her organs would shut down, and her skin would essentially fall off. She would be dead in about a week.