The contamination poses no immediate health risk since no seafood from Tokyo Bay has seen contamination levels exceed the government-set threshold. But close, long-term monitoring of the seabed mud is needed, said Hideo Yamazaki, professor at Kinki University's Research Institute for Science and Technology.

"Contamination is flowing into the bay from rivers, including the Edogawa River, where cities with high radiation levels like Kashiwa (in Chiba Prefecture) are located upstream," Yamazaki told The Japan Times.

"Contaminated sludge appears to be . . . accumulating on the bottom at the mouth of the rivers," he added.

Yamazaki, an expert on how radiation and chemical substances impact the environment, and his team took the samples at three locations at the mouths of the Arakawa and Edogawa rivers on April 2 following studies carried out in August.

Samples of mud pulled from 1 meter below the seabed at the sites turned up cesium contamination ranging from 7,305 to 27,213 becquerels per square meter. The August readings were between 578 and 18,242 becquerels per square meter.

Yamazaki noted a thirteenfold rise was detected in a spot where the August readings were relatively low. He said, however, the contamination does not pose a health threat, even if a child were to play in the water.

Although radioactive mud will continue to flow into the bay, the peak contamination concentrations should be within the next couple of years, considering that the half-life of cesium-134 is about two years, Yamazaki said.

"If the contamination were to spread to fish, it is possible that radioactive isotopes could accumulate when bigger fish feed on smaller ones," he said. "We're scheduled to continue our monitoring in the following years" to study such cases.

ref: http://www.japantimes.co.jp/text/nn20120515a4.html

It said readings were detected as high as 72.9 sieverts per hour, which would be fatal to humans in the event of a leak.

That is also enough to affect electronics in robots or other remote-controlled equipment used to probe the reactor ahead of final decommissioning.

Junichi Matsumoto, acting general director of TEPCO's Nuclear Power and Plant Siting Division, said, "We believe it is the effect from the fuel (that melted) and leaked into the containment vessel. For the decommissioning operation, we will need to develop equipment that can withstand high radiation levels."

The March 27 announcement marked the first time TEPCO directly measured radiation levels within the containment vessel of the No. 1 to No. 3 reactors where reactor meltdowns occurred a year ago.

TEPCO workers inserted an industrial endoscope into the containment vessel of the No. 2 reactor and measured radiation levels at eight locations. The highest reading of 72.9 sieverts was recorded about 4.2 meters from the bottom of the vessel.

After the Fukushima nuclear disaster flared, the highest radiation level recorded was about 10 sieverts per hour last August around piping near the main exhaust duct of the No. 1 and No. 2 reactors.

At that time, the high radiation was considered the result of fuel leaking during the venting process. The latest reading is not only much higher but reinforces the possibility that the decommissioning process will be prolonged.

High radiation levels can cause electronic parts to malfunction. The latest measurement throws another spanner in the works: Much more work will need to be done to decontaminate the area to reduce radiation levels.

Decommissioning of the No. 1 to No. 3 reactors will involve the unprecedented process of collecting nuclear fuel that has melted into the containment vessel.

Decommissioning was initially expected to take around 30 years, but that estimate will likely now have to be revised.

ref: http://ajw.asahi.com/article/0311disaster/fukushima/AJ201203280034

Note: By comparison, the mentioned 19.7 microsieverts per hour "reduced reading from snow" is 58 times the background radiation of Boston. After the snow melts they expect it to go back to 30 microsieverts per hour, which is 88 times the background radiation of Boston. Boston's normal background radiation level is 0.34 microsieverts per hour.

The Yomiuri Shimbun

The Fukushima prefectural government has received many inquiries because air radiation levels across the prefecture following the crisis at the Fukushima No. 1 nuclear power plant declined considerably late January and have since remained constant, perhaps due to fallen snow blocking radiation above the ground.

According to monitoring by the Education, Culture, Sports, Science and Technology Ministry and others, the rate of decline was particularly large in the Akougi district in Namie and the Nagadoro district in Iitate, located in the expanded evacuation zone around the nuclear plant.

Radiation measuring found that the Akougi district had a reading of 19.7 microsieverts per hour in the morning of Jan. 25, down from 30 microsieverts per hour recorded in the morning of Jan. 18.

Air radiation levels also decreased to 5.9 microsieverts per hour from 10 microsieverts per hour over the same period in the Nagadoro district.

It is believed there were no major changes in air radiation levels before Jan. 18 and after Jan. 25.

According to the ministry's Nuclear Emergency Response Headquarters, the decline can be attributed only to snowfall since decontamination operations were not conducted in the areas at the time.

The Fukushima Meteorological Observatory said snow accumulation is not monitored in Namie and Iitate, but temperatures and other factors suggest the town and the village had snow from Jan. 20 to 22.

The prefectural emergency response headquarters said radiation levels also declined in the city of Fukushima. While such levels measured 0.84 microsievert per hour at 6 p.m. on Jan. 21 when snow began to fall, at 9 p.m. on Jan. 22, after snowfall, radiation levels in the air measured 0.62 microsievert per hour.

Farmer Masuo Kaneko, 63, who evacuated to the city from Nagadoro district, said after reading the newspapers he thought the radiation levels were dropping rapidly. But he was disappointed to hear the decline was due to snowfall.

"I expected radiation levels to halve in about two years time," he said.

Tokyo Institute of Technology Associate Professor Keiji Saneyoshi said air radiation levels may halve if about 20 centimeters of snow falls in certain areas. "Yet decontamination work needs to continue since the levels will rise again once the snow melts," Saneyoshi said.

Pipe leaks water from reactor 4 fuel pool

Tepco discovers that 8.5 tons of radioactive coolant water leaked in reactor 4 at the Fukushima No. 1 nuclear plant, somewhat higher than its initial estimate of 6 liters, but says none flowed outside the building.