Tokyo Electric Power Co. said the radioactive contamination in the water that leaked into the ocean from the Fukushima Daiichi nuclear plant was more than 20,000 times the allowable annual limits.

TEPCO issued an estimate of the total outflow of water over the course of several days. The company said the outflow was found on April 2, though it wasn't clear how long it had been leaking the water before then. The leak was stopped on April 6.

According to TEPCO's estimates, the total amount of radioactive iodine that flowed into the ocean was 2,800 terabecquerels, or 5.5 million becquerels per cubic centimeter. Cesium-134 was estimated at 940 terabecquerels, or 1.8 million becquerels per cubic centimeter. Cesium-137 was at the same level. The total was 4,700 terabecquerels dumped into the sea.

Iodine-131 has a relatively short half-life of about eight days, and after 80 days almost all of it has decayed into non-radioactive elements (primarily xenon). Cesium-134 has a half-life of two years, and cesium-137 has a half-life of 30 years.

Not only is the concentration of contaminants far above the limits that would be safe to drink. The airborne radiation levels of the leaking water were found to be 1,000 millisieverts per hour. That means that if a person were standing in that area, 10 hours of exposure would be absolutely fatal. An hour would be enough to cause immediate illness.

TEPCO said it has taken steps to stop any of the highly contaminated water from getting out. The water is leaking from the inlet canal, which ordinarily allows for wastewater release (that water isn't usually radioactive). The company has put a steel plate in front of the inlet canal for unit 2, where the contaminated water was coming from. Sandbags have been put around the breakwater near unit 4, and bags of zeolite, an absorbent, have been placed in the canals for all four damaged reactor units.

Highly radioactive water from the turbine building of reactor unit 2 is being moved to a treatment facility, to stop any more from getting into the ocean or environment.