The power plant at the centre of the biggest civilian nuclear crisis in Japan's history contained far more spent fuel rods than it was designed to store, while its technicians repeatedly failed to carry out mandatory safety checks, according to documents from the reactor's operator.
The risk that used fuel rods present to efforts to avert disaster at the Fukushima Daiichi plant was underlined on Tuesday when nuclear safety officials said the No 2 reactor's storage pool had heated to around boiling point, raising the risk of a leakage of radioactive steam.
"We cannot leave this alone and we must take care of it as quickly as possible," Hidehiko Nishiyama, of the nuclear and industrial safety agency, said.
According to documents from Tokyo Electric Power (Tepco), the company repeatedly missed safety checks over a 10-year period up to two weeks before the 11 March disaster, and allowed uranium fuel rods to pile up inside the 40-year-old facility.