The bulk of the radiation measurements taken at the crippled Fukushima No. 1 power plant since March 2011 will be reviewed because they were taken improperly and are probably too low, Tokyo Electric Power Co. revealed.

"We are very sorry, but we found cases in which beta radiation readings turned out to be wrong when the radioactivity concentration of a sample was high," Tepco spokesman Masayuki Ono told a news conference Friday. Materials known to emit beta rays include strontium-90, which causes bone cancer.

The announcement follows Tepco's finding Thursday that a groundwater sample it had taken from a well at the No. 1 plant last July contained a record-high 5 million becquerels of strontium-90 per liter instead of 900,000 becquerels.