On 26 April 1986, reactor 4 of the Chernobyl Nuclear Power Plant, located near Pripyat, Ukrainian SSR, Soviet Union (later Ukraine), exploded. With dozens of direct casualties and thousands of health complications stemming from the disaster, it is one of only two nuclear accidents rated at the maximum severity on the International Nuclear Event Scale, the other being the 2011 Fukushima nuclear accident. The response involved more than 500,000 personnel and cost an estimated 18 billion rubles (about US$84.5 billion in 2025). It remains the worst nuclear disaster and the most expensive disaster in history, with an estimated cost of US$700 billion. The disaster occurred during a test to simulate cooling the reactor during an accident in blackout conditions. The operators carried out the test despite an accidental drop in reactor power. A design issue when attempting to shut down the reactor in those conditions resulted...