TOKYO - Japan's nuclear safety officials reprimanded the operator of Japan's tsunami-damaged power plant Friday and demanded an investigation of how two workers were exposed to radiation more than twice the government-set limit.

The government also ordered the utility to reduce workers' risks of heat-related illnesses as concerns grow about the health risks faced by the people toiling to get the Fukushima Dai-ichi plant under control.

The two men with high radiation exposure worked at a central control room for two reactors when the tsunami struck March 11 and the days that followed. They are not showing immediate health problems but will need long-term monitoring for an increased risk of cancer, said the National Institute of Radiological Sciences, which examined the men.

Soon after the tsunami knocked out the plant's power and cooling systems, the government raised the radiation limit for men to 250 millisieverts from the standard 100 millisieverts so workers could tackle the emergency.

The two men, one in his 30s and the other in his 40s, were confirmed last week as having exceeded that higher limit and were removed from working at the plant. A third man - a senior control-room operator in his 50s - is being tested further after early findings showed high radiation exposure as well, Tokyo Electric Power Co. spokesman Junichi Matsumoto said.

The Nuclear and Industrial Safety Agency reprimanded TEPCO for violating the limit and ordered the company to submit the cause and prevention measures in a report within a week.

The younger man was exposed to 678 millisieverts and the other man to 643 millisieverts - equivalent to about 100 CT scans - mostly by inhaling radioactive particles, NISA spokesman Hidehiko Nishiyama said.

TEPCO already has acknowledged that workers in the earliest, most chaotic, and most dangerous moments of the crisis did not wear masks, possibly other protective gear as well, and lacked dosimeters to monitor their radiation exposures in real time.