In the latest setback for the cleanup at the site of the Fukushima nuclear reactors, a storage tank meant to hold contaminated water from other areas of the site has sprung a leak, dumping approximately 300 tons of radioactive water into the ground nearby. Workers are attempting to contain the leak and remove the remaining water from the tank, but they are limited by the high radiation present at the site.

The problems ultimately stem from the fact that the reactors' containment vessels were damaged during the initial accidents and no longer successfully contain the water used to keep the nuclear fuel cool. As a result, some of the water used for cooling, which ends up heavily contaminated with radioactive isotopes, is leaking into the site. A portion of that is ending up in the local groundwater, where it contaminates the soil and spreads into the nearby ocean. Another portion is being captured by TEPCO (the utility that ran the plant) and pumped into large tanks that have been built on the site.

The tanks are surrounded by dikes to contain the water in the case of failures. But yesterday, "a TEPCO employee on patrol found water leaking from a drain valve of a tank dike." Although TEPCO helpfully notes that "[l]ater, the drain valve was closed," a check of the tank itself showed that approximately 300 tons of water had leaked by that point—or, as TEPCO put it, "It cannot be denied that water stored in a tank has leaked from the tank."

Workers are now attempting to contain the spread of radioactive water from the site of the leak, but they are being hampered by the high levels of radioactivity at the site itself. The BBC quotes a TEPCO manager as saying, "One hundred millisieverts per hour is equivalent to the limit for accumulated exposure over five years for nuclear workers; so it can be said that we found a radiation level strong enough to give someone a five-year dose of radiation within one hour."

Ultimately, the plan is to build a plant on site that can remove most of the radioactive isotopes from the contaminated soil; the remaining water, while still contaminated, will be deemed safe enough to be released into the ocean. But technical difficulties and local opposition have stalled that effort, and the Chicago Tribune notes that the utility is collecting 400 tons of water a day from the site.