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 death. 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 produce 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 to 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, their are still alive in some kind of term, so radiation may damage their DNA and tissues.
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.