Iodine-131 concentrations 7.5 million times the legal limit (300,000 Bq/cc) were detected in seawater sampled near the water intake of the crippled Fukushima Daiichi unit 2 on April 2.
Those levels fell to 200,000 Bq/cc on Monday April 4. Monday's sample also contained 1.1 million times the level of cesium 137, according to a report from Japanese newswire NHK, republished by the Japan Atomic Industrial Forum.
It is possible, although not certain, that a leak found in a unit 2 culvert contributed to the high levels of radiation. That leak has now been confirmed to be plugged, after workers injected a hardening agent into drilled holes around the pit. They infused liquid glass into gravel below the pit on Tuesday 4 April, NHK reported. It also reported that a test using a dye suggested that radioactive water is leaking from a cracked pipe and seeping through gravel into the concrete pit. In addition, TEPCO is planning to further isolate the plant from the sea by boarding up sections of an offshore dike that have been breached, and is considering building underwater silt barriers, including a dyke near the unit 2 water intake. The barriers would have fibre curtains extending to the seafloor held in place with weights.
Small fish called sand lances caught off Ibaraki prefecture, south of the Fukushima-Daiichi plant, were found to contain radioactive caesium in quantities greater than the legal limits (526 Bq/kg, compared with a legal limit of 500 Bq/kg), according to NHK. The finding is the first time since the accident that fish have been found with levels higher than the legal maximum. As a result, sand lance fishing has been suspended in the prefecture.
Fishing has not resumed in Fukushima prefecture, according to Japanese government chief cabinet secretary Yukio Edano, according to NHK. He also said that fish caught off the coast of Ibaraki prefecture to the south would be monitored. He also said that steps are in place to strengthen monitoring systems of seawater.
In other news, TEPCO is considering injecting nitrogen gas into unit 1 containment to prevent a hydrogen explosion, according to JAIF.
FilesFukushima-Daiichi parameters 7 April Reactor-by-reactor, system-by-system summary from JAIF on 7 April