No, you can't get a "nuclear explosion" (as in a nuclear bomb) out of a reactor. Wrong fuel, wrong geometry, etc. The physics doesn't work.

What you can get are steam explosions caused by "power excursions". This is what happened to Chernobyl and why we have containment structures. The reactor starts producing enormous amounts of heat and in a few seconds flashes all of the cooling water to steam, along with any other volatiles nearby. The result is like a steam boiler explosion, only with potentially a lot more energy and radioactive volatiles mixed in...

Much goes into preventing this sort of thing, and other problems. Terrorist attacks aren't likely to accomplish anything since the first thing the operator does in the control room is hit the SCRAM button to shut things down. A more likely failure is enough people making enough stupid mistakes the designers never thought to explicitly disallow in the design.

The biggest problem is likely to be panic in the public. If the containment structure isn't breached you're more likely to die on the highway in a traffic accident trying to get out of town than from anything from the reactor. Even reports of "radioactive steam" release aren't something to panic over.

Now, if there's a steam explosion that blows the containment structure off and chunks of graphite are found in the lake that is the local water supply, then it's time to panic, just like any other time a city loses its water supply.

KI4U sells Potassium Iodide and radiation meters/detectors of various kinds.

PS. Three Mile Island was a serious accident in that the reactor core/fuel partially melted. But containment worked and nobody was injured or killed.