Ukraine is marking 40 years since the explosion at the Chornobyl Nuclear Power Plant, the world’s worst civilian nuclear disaster, with survivors of the cleanup operation returning to the site amid renewed debate over its human and environmental toll. At 1:23am on April 26 (22:23 GMT, April 25), 1986, a botched safety test triggered a catastrophic blast in reactor four at the Chornobyl plant in northern Ukraine, then part of the Soviet Union. The explosion tore through the building and sent an enormous plume of radioactive smoke into the atmosphere. Nuclear fuel burned for more than 10 days as helicopters dumped thousands of tonnes of sand, clay and lead in a desperate bid to smother the fire. The International Atomic Energy Agency (IAEA) later blamed “severe deficiencies in the design of the reactor and the shutdown system” as well as violations of operating procedures. Radiation heavily contaminated large areas of Ukraine, Belarus and Russia, before spreading across Europe.…