December 23, 2019
Radioactive water in the sump pit at the base of the unit 1-2 vent tower at Fukushima Daiichi escaped in November.
The base of the vent tower is one of the most radioactive locations at the disaster site. Water in this sump is normally pumped out into a truck to transport it for treatment. An automatic sensor system detects when the sump fills to a certain level then transfers water to the truck’s tank. TEPCO discovered that the water level in the sump had gone down but no transfer of water had taken place. The radioactive water is assumed to have left the sump pit. TEPCO did indicate rising radiation levels in two nearby groundwater control sump wells after the water escaped.
Rainwater is considered to be the main route for water to flow into the vent tower sump pit. When  the upper vent tower removal work is completed, a cap will be placed on the vent pipe to prevent rainwater from flowing into the sump pit. There appears to be some amount of water transfer at the ground level between this sump pit and the surrounding soil. TEPCO did not address this further in their most recent report.
TEPCO document