In the USA in 1990 it was also reported that as many as 42 of the 177 underground tanks that are used to store waste from nuclear bomb production are in danger of exploding. Such an explosion could mean the spread of toxic chemicals and radioactive materials over large areas. The risk is due to unforeseen reactions between the chemicals stored and those introduced in an endeavour to consolidate the waste. The ferrocyanide percolating in the tanks is sufficient to cause an explosion equivalent to 36 tons of TNT. In 1957 in the USSR, the explosion of such a nuclear waste storage tank spread radiation over a large area and forced the evacuation of 10,000 people, with some reports that hundreds of people later died. It was also reported that 28 kg of plutonium (equivalent to 7 nuclear bombs) had escaped into the air ducts at the Rocky Flats weapons plant during its 30 years of operation, although it is so toxic that it is usually accounted for in gram quantities.