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Cleanup at the Fukushima nuclear power plant is progressing, but will take a lot of time

Cleanup at the Fukushima nuclear power plant is progressing, but will take a lot of time

15 Mar 2017 10:24
Webnews

Six years after the tsunami that caused such destruction, the situation at the Fukushima nuclear power station is still far from being over. Around six thousand people are working on cleanup in the area continuously. The work will take decades yet – there is no precise schedule.

The owner of the Fukushima plant, Tepco, is responsible for clearance of the area and radiation protection. The biggest and hardest problem is the highly radioactive fuel of the damaged reactors, which were damaged in 2011. There is still no precise information available on the state of the fuel in the reactors.

Tepco is currently planning methods for removing the remaining fuel that is leaking from the reactors.  
The company is trying out these methods using with remote-controlled devices in the area beneath the destroyed Reactor 2. Judging from the photos that have been taken there by robot camera, the damage is more extensive than had previously been assumed. Based on the damage to the grating level of the lower part of the reactor, large amounts of melted fuel have poured from the reactor. Previously it had been estimated that the fuel in Reactor 2 had mostly remained within the pressure chamber.

The radiation emitting from the uncovered fuel is intense, and for the situation to be studied new types of remote-controlled, radiation-resistant devices have to be developed. The intensity of the radiation was too much for the robot camera that was used at the start of the year, and it broke down in the middle of the task. The strength of the radiation being emitted from the areas in the lower part of the reactor has been estimated based on the interference to the camera picture. At its highest, the absorbed dose rate is estimated at about 530 sieverts per hour.

Water both solves and creates problems

The damaged reactors and the fuel in the pools of the reactor buildings must be cooled continually. Cooling water is being pumped into the reactor buildings, but the buildings are not watertight; the water flows into the cellar areas of the turbine buildings. The radiation is most intense in the reactor building of Reactor 2, because the water level there is lower than in the other damaged reactor buildings.

The water that has leaked into the cellar is recycled as cooler water that is reusable by the cleaning process. Because the afterheat efficiency of the fuel is decreasing continuously, the amount of water needed for cooling is decreasing also. By this spring, the amount of water that must be pumped into the reactor buildings in all the reactors had dropped from 4.5 to 3 cubic metres an hour. 

To prevent the contaminated water from flowing from the power plant area into the sea, Tepco has built a watertight steel dam along the sea coast. Attempts are being made to decrease the flow of ground water to the plant area, and the flow out of there, by cooling the ground around the power plant.

The contaminated water that has leaked from the reactor buildings has been removed and stored in reservoirs on the plant area. Since the leak that occurred in August 2013, water has been transferred from bolted-together reservoirs into larger welded reservoirs. According to the company, the amount of water storage needed in the power plant area has been brought under control, and purified water has been released back into the sea.

The storage pools in the reactor buildings also contain undamaged nuclear fuel. The 1533 fuel rod clusters in the pools of reactor building of Reactor 4 were removed in December 2014.

Tepco is doing a survey of the upper parts of the other damaged reactor buildings, and is dismantling them and building in their place temporary structures and installing devices for removing the fuel. This year, the company plans to remove the 566 damaged fuel rod clusters from the pools of the reactor building of Reactor 3. Removal of the fuel from the pools of the reactor buildings of Reactor 1 and Reactor 2 have been planned by Tepco for 2020. The company is building structures to cover the reactor buildings and to protect them from the weather. 

Restrictions are being dismantled

The radioactive materials released into the environment from the buildings that were damaged by the tsunami still continue to contaminate the immediate vicinity of the power plants to such a degree that movement in the area is restricted, and no one is allowed to live in the area. The cleanup work has, however, made progress. The Japanese authorities have been able to decrease the size of the areas that are off limits, and will reduce them further this spring.

The contaminated area extends to a few kilometres from the destroyed plant. The longest stretch of contaminated area extends about 30 kilometres from the power plant area in the northwest direction where winds and rainfall carried radioactive matter during the accident.

Tepco and the Japanese authorities are monitoring the radioactivity of the area, both in the ground waters of the disaster area and in the sea near the area. Cesium (Cs-137 and Cs-134) and the radioactive hydrogen isotope tritium (H-3) have been detected in the ground waters of the disaster area. Upper limits for the water concentrations of these radioactive substances have been set, and the concentrations have remained within those limits.

The radioactivity that has entered the Pacific Ocean decreases very effectively. At a distance of about ten kilometres from the accident area out into the sea, and into the open sea at a distance of 200–300 kilometres from the plant, the seawater concentrations of cesium-137 are a few millibecquerels per litre of water. For instance, measurements carried out in the Baltic Sea show a few centibeqcuerels of cesium-137 per litre, mainly resulting from the explosion of the Chernobyl power plant in 1986.

Foodstuffs sold in Japan are safe

Japanese authorities have set a strict limit on the radioactive levels permitted in foodstuffs. It is not permitted to sell food items with a radioactivity level above 100 becquerels per kilo. For comparison, the corresponding limit imposed by the European Union, and complied with by Finland also, is 600 becquerels per kilo.

In the measurements made in August and September 2016 the ocean area in the vicinity of the Fukushima accident area, none of the radioactivity measurements in fish and other marine life exceeded the aforementioned radioactivity limit. Of samples taken from game in the other prefectures, one sample (accounting for 0.08% of the 1281 samples taken) had a radioactivity level that exceeded the limit.

In foodstuffs other than marine life, radioactive concentrations that exceeded the limit were detected in 0.07 percent of the total of 52,375 samples that were taken.

 

More information:

http://www.tepco.co.jp/en/nu/fukushima-np/roadmap/images/d161222_01-e.pdf
http://www.japan-guide.com/list/e1208.html
http://www.jnto.go.jp/eq/eng/04_recovery.htm
http://www-pub.iaea.org/MTCD/Publications/PDF/Pub1710-ReportByTheDG-Web.pdf

Director Tarja K. Ikäheimonen, tel. +358 9 759 88 596
Inspector Jukka Mettälä, tel. +358 9 759 88 729
Communications Expert Risto Isaksson, tel. +358 9 759 88 208

Nuclear safety
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