What would happen if a zombie got caught too close to a reactor meltdown, or a vampire decided to intentionally dose themselves with lethal radiation meant to sterilize machine parts? Would enough radiation delivered over whatever interval it needed to be preserve a corpse or the walking dead, or would it just further damage the tissue? I’m trying to think of ways for a smart undead to prevent further tissue rot and I’m wondering if this might be a better avenue than mummification or vacuum preservation.
Nothing "should" happen
By definition undead creatures are dead. Since a long time ago their vital functions has stopped working, so they don't breath, beat, all their organs are shut down and cells aren't working anymore. They just move by some act of magic... or a lethal virus which produces electrical pulses...
If we attach to that definition radiation won't be more harmful to their bodies than a reactor meltdown could damage a lump of wood, a bunch of coal, or a corpse.
But obviously, you must choose your relation between science and magic to define your undead creatures. If there are moving by magic (complete death) it wouldn't do anything radiation. If their cells just don't die even if you try to kill them (like cancer), or some hybrid between science and magic, they are still alive in some kind of term, so radiation may damage their DNA and tissues.
It depends on how they work.
Ionizing radiation damages molecules, especially DNA. This will mess with any orderly biological mechanisms in a living organism, whether those mechanisms are originally derived from the organism's own body or those of an invasive parasite.
"Realistic" zombies will generally be harmed by radiation
The most realistic zombie stories take ideas from the effects of drugs or parasites. Nature is full of all kinds of mechanisms that can alter the mental state of a living organism, and some parasites can flat-out control simpler animals like insects. Speculation on a parasite that is adapted to manipulate humans is a fun route to go on when trying to develop a realistic or at least semi-realistic zombie story.
The problem, of course, is that "realistic" zombies are not really dead. Whether the parasite simply messes with their brain chemistry or outright replaces existing biological mechanisms with new biological mechanisms, you're still dealing with an organic life form that has all the weaknesses of any other biological life form, including radiation.
The only exception is if you're dealing with a parasite that is specifically adapted to resist and eat radiation, which a few fungi have been known to do (although none of these are parasitic). However even in this case the radiation will still harm the biological mechanisms of the host that the parasite is trying to control, which bodes poorly for the zombie as a whole, unless the parasite has a way of protecting its host as well as itself. This will only happen if the parasite is specifically adapted to operate in a radioactive environment.
Supernatural zombies may be preserved
If the animating mechanism is supernatural in nature, the death of the body or breakdown of DNA might not really matter. However, it is possible that supernatural, truly-dead zombies might still be vulnerable to natural decay. If this is the case, killing putrefactive bacteria or fungi through irradiation would slow the decay process, making the zombie last longer.
It is worth mentioning that enough radiation can damage proteins directly, causing a physical breakdown of the body. However, it takes far more energy to do this than to simply sterilize it.
Exposure to Radiation
Nothing will happen. Radiation, in it's simplest definition is high energy photons moving along a particular wavelength. People tend to think of radiation as poison, but it really isn't. The way it damages your body is that high energy photon hits your DNA and knocks a teeny tinny little bit out of it, or it excites water molecules producing some chemistry that effectively does the same thing. If enough cells have their DNA damaged badly enough they cannot reproduce and as they gradually die your body falls apart. (Or, in smaller amounts, you don't get very sick initially but you get an extremely aggressive cancer and die a few years later.) Since zombies or classical vampires are by definition, animated corpses, cellular division is not occurring and thus these creatures are immune to radiation exposure.
Contamination By Radioactive Material
If your creatures were nearby a nuclear power-plant in full meltdown and were contaminated by microscopic or small dust particles of a radioactive material then they would themselves become radioactive. Well, at-least that is, they would continue to move around and do zombie/vampire stuff while the microscopic bits of plutonium and uranium lodged inside their bodies continued to emit radiation. How radioactive really would depend on what kind of meltdown occurred and how severe it was, and what kind of materials were vented by the meltdown that contaminated them as well as how close they were to it when it happened and how long they were within the contaminated area. It could amount to only just barely enough to set off a Geiger counter all the way up to a walking (well... lurching or lurking?) environmental disaster.
Nothing, since ionizing radiation only affects living tissue. It's why you can sterilize food using gamma rays and the food stays viable as food.
￼Food irradiation is the process of exposing food and food packaging to ionizing radiation. Ionizing radiation, such as from gamma rays, x-rays or electron beams, is energy that can be transmitted without direct contact to the source of the energy (radiation) capable of freeing electrons from their atomic bonds (ionization) in the targeted food. The radiation can be emitted by a radioactive substance or generated electrically. This treatment is used to improve food safety by extending product shelf-life (preservation), reducing the risk of foodborne illness, delaying or eliminating sprouting or ripening, by sterilization of foods, and as a means of controlling insects and invasive pests.
Point being that ionizing radiation only affects living tissue, and the undead aren't living.
If Ionizing Radiation where to contaminate an undead vampire, what would happen to the blood that the vampire has previously consumed from their victims? Wouldn't it hurt the vampire because the blood is technically still living after a little bit of time and the vampire is reliant on that to survive. (I can't say 'alive' because well they're undead. Kind of the whole point.)