The Chernobyl disaster occurred during a safety test at the No. 4 reactor of the Chernobyl Nuclear Power Plant in Soviet Ukraine. A sudden power surge led to a reactor vessel rupture and a series of explosions, releasing a massive plume of radioactive isotopes into the atmosphere that spread across the USSR and Europe. The event is considered the worst nuclear power plant accident in history in terms of cost and casualties. The initial cover-up by Soviet authorities and the subsequent forced evacuation of the city of Pripyat highlighted the systemic failures of the Soviet government. Many historians believe the disaster and the financial and political strain it caused were major factors in the eventual collapse of the Soviet Union. It led to a global slowdown in the development of nuclear energy and a drastic overhaul of safety standards.