Fukushima round up

9 September 2015


Workers at the Fukushima Daiichi NPP have removed half of the ceiling panels covering a damaged unit 1 reactor building, which was damaged by a hydrogen explosion during the 2011 meltdown. Tokyo Electric Power Company (Tepco) is now removing the cover, installed to prevent the spread of radioactive material, in order to clean up debris inside the facility as part of the decommissioning process. Two of the six ceiling panels comprising the cover were removed between late July and early August, after which work stopped to monitor radiation levels. When no abnormalities were found, workers removed a 3rd using a remote-controlled crane. Tepco plans to finish dismantling the cover by late 2016.

Also in early September, Tepco restarted the cooling system at the unit 3 used fuel pool, after a temporary suspension due to a leak of oil from heavy machinery. The pool, located on the top floor of the reactor building, stores 566 used fuel assemblies. The leak occurred during work to remove debris from the pool.

A protective cover will be built over unit 3 as soon as all the rubble has been removed from the reactor building. Fabrication of the cover began in November 2013 at the Onahama works in Iwaki city. It is made in sections to shorten the assembly time at the site, minimising workers' radiation exposure. A separate structure will facilitate removal by crane of used fuel from the pool. On 2 August, Tepco announced that it had removed the damaged fuel handling machine from the pool clearing the way for the remaining rubble and used fuel in to be removed. The fuel will be packaged for transport to the site's communal fuel storage pool, after being inspected and flushed clean of dust and debris.

Meanwhile, engineers have tested a new robot designed to remove radioactive substances from high places in the reactor buildings. Tepco plans to use the device in October to speed up decontamination work at unit 3. The robot, made by Toshiba, emits high-pressure jets of frozen carbon dioxide grains onto walls and scrapes off the coating along with radioactive substances. It can reach up to 8 metres. Another robot developed by Toshiba and the International Research Institute for Nuclear Decommissioning will investigate conditions inside the unit 2 primary containment vessel and facilitate debris removal. A telescope equipped with a dosimeter will be used to investigate whether its path is clear.

Meanwhile, attempts are continuing to control the flow of groundwater into the reactor buildings and port area of the plant. Tepco said umping up of groundwater by 20 wells in the subdrain system began on 5 September.

The subdrain system comprises 41 wells installed near the reactor and turbine buildings with the aim of significantly reducing the flow of water into the buildings. Groundwater flowing into the port area is held back by the coastal impermeable wall and pumped up by another group of wells, the groundwater drain system.

The collected groundwater is temporarily stored to check its quality and then discharged into the port area, following treatment. Tepco said it expects the water pumped up by the subdrain and groundwater drain to be slightly more contaminated than water from the existing groundwater bypass (which intercepts water on the land side of the reactor buildings). However the water will be treated to meet more stringent quality than for the groundwater bypass and will also be monitored more frequently to verify its quality for discharge. Tepco required the approval of prefectural and national fishermen's associations for use of the system.

Tepco estimates the subdrain will reduce the flow of groundwater into the buildings to 150 cubic metres a day from 300 cubic metres at present. In the longer term, the pumping systems and seaside wall are expected to be joined with the land side frozen soil wall, currently under construction which should further reduce the intrusion of groundwater, although the project has faced technical difficulties.

The Fukushima site has experienced regular leakages of contaminated water from drainage ditches and holding tanks. Reports at the end of August said contaminated rainwater had flowed from a drainage ditch by the reactor buildings into the sea on five occasions in the previous four months. The ditch was intended to channel clean rainwater from the plant grounds into the ocean, but in February highly contaminated rainwater from the top of the unit 2 reactor building had flowed into the ditch and then into the ocean. Tepco installed a dam in the ditch, as well as eight pumps to move water to channel.

The pumps were started in April but within days suffered a power loss allowing contaminated water to reach the sea again. Similar problems occurred in July. Concentrations of radioactive caesium and other radioactive materials in the rainwater ranged from 20 to 670 times the safety level set for the subdrain. The Fukushima Prefectural Government on 27August issued a new request to Tepco to introduce leak prevention measures. And Tepco began work to build a new ditch.

Also at the end of August, Japan's Environment Ministry reported abnormalities in fir trees near the Fukushima Daiichi site. The ministry has been monitoring about 80 species of wild animals and trees near the plant since 2011.The National Institute of Radiological Sciences analysed fir trees in areas where radiation levels are relatively high and published the results which showed a significantly increased number of morphological defects, including deletions of leader shoots of the main branch axis. The study shows that 98% of fir trees within 3.5km of the plant have defects. The radiation dose in the area is about 34 microSieverts per hour. Some 44% of fir trees have defects within an 8.5km zone with 20 microSieverts, and 27% in a 15km zone with 7 microSieverts.

The Ministry says no abnormality has so far been confirmed in other animals and trees.



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