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.

Ono described the data up for review as “massive” and said the utility plans to start the review from the beginning of the nuclear crisis in March 2011 up to October last year, when it started preparing manuals on proper measurement procedure.

Among the data that need to be examined are the readings for around 300 tons of water that inexplicably vanished from a storage tank in August last year. Tepco had detected 80 million becquerels per liter of beta radiation from the leak, part of which is believed to have ended up in the Pacific.

Tepco blamed the measuring errors on what it calls the “counting miss” phenomenon, which occurs in sensors when radioactivity in a sample is too high. In such cases, the proper procedure is to dilute the water sample so the sensors can correctly detect the radioactivity.

If the “counting miss” is not taken into account, the readings will be too low, Ono said.

Tepco meanwhile believes that its data on seawater and other less contaminated liquids is probably sound.

The 16-meter observation well is located between reactors 1 and 2, about 25 meters from the Pacific. It is also about 6 meters from an underground channel from which highly radioactive water was found to be seeping into the sea shortly after the nuclear crisis.