Unlike the whimsical title of this post, radiation gives little visible sign of its presence. If you can see radiation, there are generally three causes:
- It is affecting some other material, causing it to fluoresce
- Small particles are exceeding the local speed of light (slowed due to water refraction) producing a blow glow know as Cerenkov radiation.
- You're viewing a source powerful enough that you'll be dead in a matter of moments.
Nuclear power plants built in the mid twenty-first century are far better designed than the first and second generation plants of the sixties and seventies. Almost all are passively safe - overheating shuts down the reactor rather than causing a meltdown for example. Almost no civilian reactor had releases of radiation due to The Event.
Nuclear powered warships are a somewhat different matter. Most were sunk at sea, and are sufficiently far underwater to be effectively non-hazards. The aliens did hit some in port, and of course, many ports built to handle such vessels had radiological materials on site for refueling or recently removed.
Fallout is an issue in some areas. Most militaries generally tried to either use high-altitude air-bursts or deep subsurface blasts. Although these techniques minimized the amount of material exposed to radiation and thrown into the atmosphere, short of avoiding atomic weapons, it can't be completely prevented. After five years, almost nothing remains in the atmosphere, and exposed materials are inert, but some long lived isotopes remain. These are unlikely to be directly deadly in the short term, but as mentioned before - high radiation can destroy nano-vac, which leaves people far more open to opportunistic infections.
Another source of hazard are the multitudes of no longer accounted for point sources. Gamma irradiation machines from food processing, various calibration devices, or the cores of radio-therapy machines in abandoned hospitals. (See the Goiânia accident for a real life example of what harm those can do.) Unfortunatly, there are some scavengers who don't know the danger and are acquiring hazardous materials unwittingly. Crossbow teams from Las Vegas are often sent to look for not just nuclear weapons, but old sources as well because of this danger.