Police on Wednesday sent investigation papers on three employees of JCO Co. and the company itself to prosecutors on suspicion that the nuclear-fuel processing firm illegally changed production procedures at its plant in Tokai, Ibaraki Prefecture.
Illegally rewriting the procedure manual led to Japan's worst nuclear accident last year.
Kenzo Koshijima, 54, head of the plant at the time, Hiromasa Kato, the 61-year-old former chief of the production department, and former planning group leader Hiroyuki Ogawa, 43, are suspected of violating the Law on the Regulation of Nuclear Source Material, Nuclear Fuel Material and Reactors.
The three employees of JCO, a subsidiary of Sumitomo Metal Mining Co., are among six company officials arrested last month on suspicion of negligence resulting in the deaths of two plant workers in the accident, which occurred Sept. 30 1999.
The nuclear fission chain reaction was triggered when workers bypassed several required steps in mixing an excessive amount of uranium with nitric acid.
JCO began using illegal methods to process uranium in 1993, police said. Koshijima and others approved the procedures at an in-house safety committee in 1995, leading to the compilation of an unofficial manual in 1996 that recommended the use of buckets to make the solution.
Operators of nuclear facilities are required by law to obtain approval before changing production methods, but JCO did not seek or obtain any such permission, investigators said.
The Mito District Public Prosecutors Office is expected to indict the three on charges of negligence resulting in the two deaths and violating the law regulating nuclear reactors.
JCO worker Hisashi Ouchi died last December, while Masato Shinohara died in April, both of radiation sickness.
At least 438 people were exposed to higher-than-normal levels of radiation due to the accident.