Tokyo Electric Power Co. has detected the highest radiation levels found so far near tanks holding contaminated water used to cool reactors at its wrecked Fukushima No. 1 nuclear plant.

Readings of 2,200 millisieverts (2.2 sieverts) per hour were found Tuesday in an area where levels of 1,800 millisieverts had been detected Saturday, Tepco spokeswoman Mayumi Yoshida said Wednesday. The increase could be due to a slight difference in where the measurements were taken, she said.

“It fluctuates when you move a little bit,” so the higher reading doesn’t necessarily indicate rising radioactivity levels, Yoshida said.

Tuesday’s hourly reading is equivalent to the amount of radiation 44 plant workers may be exposed to in a full year under government guidelines.

A report on the weekend said exposure to 1.8 sieverts could prove fatal in around four hours.

The Tuesday reading was at one of four radiation hot spots near storage tanks reported by Tepco last weekend, one of which led crews to a leaking pipe that was fixed Sunday. Investigators will now deploy more advanced sensors to locate the source of Tuesday’s reading, Yoshida said.

Tepco reported a 300-ton leak last month from a tank used to store radioactive water in what the Nuclear Regulation Authority designated the most severe level since the March 2011 earthquake and tsunami sparked a triple meltdown at the plant.

The utility has subsequently boosted the number of tank-inspection patrols from twice to four times a day and increased its inspection staff to 60 members from 10, it said.