It is not hard sci fi, but I need something plausible.
A spaceship falls on a planet and destroys itself. The area surrounding the wreck becomes highly radioactive. It can be because of the leak of radioactive fuel used to propel the ship; or because of the radioactive cargo the ship was carrying; or because of the reaction triggered by the impact with the ground.
This area is now deadly for humans, like going near Chernobyl reactor was, right after the incident.
I need something that could make possible again to go through the zone. An event - something near a hundred years after the incident - that abruptly changes the radiation levels, allowing humans to pass without deadly consequences. Since highly radioactive isotopes usually decay after a longer period of time, and not so suddenly, I thought about a climatic event that could change the features of the area.
A volcanic eruption that covers the radioactive waste, a big flood that carries it away, an earthquake that makes the ground swallow the waste, a rainfall of some sort that lowers, even temporarily, the radiations. It's not the Earth, so it is plausible that a methane or a liquid lead rainfall would reduce the threat, don't hesitate to take it into account.