I remember reading a short story, possibly online, approximately 8-10 years ago.
The protagonist is sent undercover, possibly by the police or secret service, to investigate a cult living on an island. I think the island might have been off the coast of Scotland, and it might have been mentioned that the climate there wasn't sufficiently warm to support the population.
The island inhabitants live a largely agrarian existence. The protagonist notices that the innermost circle of the cult are keeping secrets. And all the while members of the cult keep getting sick and dying.
While most of the story is written as a "Lovecraftian horror story", and the protagonist assumes that the cult's practices are causing the illness. They eventually discover that the island was the site of a (covered up) crashed nuclear bomber, and the cult members have been using radioactive material from the bomber to generate heat and light, hence the cause of the sickness.