On September 30, 1999, three workers at the JCO plant in Tōkai, Ibaraki Prefecture, were preparing a small batch of nuclear fuel. They had dissolved uranium in stainless steel buckets and tipped the solution into a precipitation tank, with no proper control of the amount. It became Japan’s first criticality accident. Two of the three workers died.
The approved procedure dissolved the uranium in a dissolution tank. Three years earlier, the company had changed its work procedure without permission from the regulators, so that buckets could be used instead. According to a report from Kyoto University, the plant’s own manual allowed the buckets.