A weekend storm caused accumulation of rainwater inside tank barriers that are meant to contain radiation leaks.
The tainted water overflowed 11 barriers on Sunday.At 6 of them, the spilled rainwater contained radioactive strontium above the government-approved release limit of 10 becquerels per liter.
Levels at the most contaminated site were at 71 times that limit.
Tokyo Electric Power Company said that pumps installed to drain rainwater from the barriers didn't have enough capacity to keep up with the rising water levels.
TEPCO says it will add 19 more pumps that can drain up 60 tons of water per hour.
This will boost the system's pumping capacity to about 4 times the current level.
The utility also plans to use larger draining hoses to speed up the transfer of water from the barriers.
The operator says it will measure radiation levels of water in the barriers before removing it to containers. This is to ascertain its toxicity before it is diluted by added rainwater.
A TEPCO official admitted that the company underestimated the amount of possible rainfall. The official said TEPCO will bring in more workers if necessary.
Oct. 22, 2013 - Updated 03:00