Southern Urals reported observing "polar-lights" in the sky near the plant, and American aerial spy photos had documented the destruction caused by the disaster by 1960. This nuclear accident, the Soviet Union's worst before the Chernobyl disaster, is categorised as a Level 6 "Serious Accident" on the 0–7 International Nuclear Events Scale. When Zhores Medvedev exposed the disaster in a 1976 article in New Scientist, some exaggerated claims circulated in the absence of any verifiable information from the Soviet Union. People "grew hysterical with fear with the incidence of unknown 'mysterious' diseases breaking out. Victims were seen with skin 'sloughing off' their faces, hands and other exposed parts of their bodies." As Zhores wrote, "Hundreds of square miles were left barren and unusable for decades and maybe centuries. Hundreds of people died, thousands were injured and surrounding areas were evacuated." Professor Leo Tumerman, former head of the Biophysics Laboratory at the Institute of Molecular Biology in Moscow, disclosed what he knew of the accident around the same time. Russian documents gradually declassified from 1989 onward show the true events were less severe than rumoured. According to Gyorgy, who invoked the Freedom of Information Act to open up the relevant Central Intelligence Agency (CIA) files, the CIA knew of the 1957 Mayak accident, but kept it secret to prevent adverse consequences for the fledgling US nuclear industry. "Ralph Nader surmised that the information had not been released because of the reluctance of the CIA to highlight a nuclear accident in the USSR, that could cause concern among people living near nuclear facilities in the USA." Only in 1992, shortly after the fall of the USSR, did the Russians officially acknowledge the accident. 1968 Criticality Incident In December 1968, the facility was experimenting with plutonium purification techniques. Two operators were using an "unfavourable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution." "Unfavourable geometry" means that the vessel was too compact, reducing the amount of plutonium needed to achieve a critical mass to less than the amount present. After most of the solution had been poured out, there was a flash of light and heat. After the complex had been evacuated, the shift supervisor and radiation control supervisor re-entered the building. The shift supervisor then entered the room of the incident, caused another, larger nuclear reaction and irradiated himself with a deadly dose of