Radioactive cesium levels surged 13 times in a day in the artificial bay at the crippled Fukushima No. 1 nuclear plant, Tokyo Electric Power Co. said Thursday.
Seawater sampled near a water intake for reactor 2 Wednesday contained 1,200 becquerels of radioactive cesium per liter, up from 90 becquerels the previous day, Tepco said.
The sampling point is inside a silt fence set up to limit the movement of water in the little harbor.
At an embankment around the water intake, Tepco is working to solidify the soil with chemicals to block contaminated groundwater from flowing into the bay.
Cesium levels rose as soil tainted with radioactive substances dropped into the bay during the work, Tepco said.
At a site outside the silt fence, cesium levels more than doubled to 227 becquerels Wednesday from 106 Tuesday.