The Nuclear and Industrial Safety Agency on Wednesday issued an improvement order to Tokyo Electric Power Co. over its failure to manage radiation levels for workers at the Fukushima No. 1 nuclear plant, citing the exposure of six workers at the site to levels in excess of the 250 millisieverts per year emergency limit.

NISA's specific orders include boosting manpower to better manage the radiation discharges, creating a system that enables workers to quickly check their total external and internal exposure, and better educating workers on protective gear.

On June 10, Tepco said two workers were exposed to 678 and 643 millisieverts, respectively. In late June and earlier this month, the utility announced that another worker was exposed to 352 millisieverts, while three more employees were exposed to dosages of 308, 475 and 359 millisieverts, respectively.

The legal annual limit for a nuclear plant worker is 50 millisieverts. However, the emergency limit, which is normally set at 100 milisieverts, was raised to 250 millisieverts, given the gravity of the Fukushima crisis. If someone absorbs 500 millisieverts in one go, experts say the impact causes a reduction in the lymphocyte count.

According to the report Tepco submitted to NISA on June 17, the first two employees were working in the central operating rooms of reactors 3 and 4 during the early stages of the accident. One worker, whose internal exposure is 590 millisieverts, was wearing glasses that likely caused his mask to bulge and allowed in radioactive materials.

The other worker, whose internal exposure was 540 millisieverts, was working near an outside door that could not be shut completely when the reactor 1 hydrogen explosion occurred, spewing radioactive materials.