Cooling fuel pools at Fukushima

13 June 2011

While progress has been made in cooling the used fuel pool at Fukushima Daiichi unit 2, an inspection of the reactor building at unit 4 suggests workers may face problems setting up a similar cooling system there. Meanwhile, tests have confirmed that two workers have received radiation doses of over 650 millisieverts.


Tokyo Electric Power Company (Tepco) continues its efforts to make conditions within the reactor building of unit 2 at the Fukushima Daiichi more bearable for staff. Work has been all but prevented at the unit due to the very high humidity, up to 99.9%, originally thought to be due to the warm cooling pond in the top of the sealed building.



Fukushima Daiichi 2 turbine building (Tepco)
A photo, taken through the camera's protective cover, shows workers in the turbine building of unit 2 (Image: Tepco)



An alternative circulation system for the used fuel pool at unit 2 began operation on 31 May and brought temperatures down to 31°C by 10 June - much closer to normal temperatures of around 25ºC. This had been expected to reduce humidity within the building, but this has not been the case. Instead Tepco now thinks the primary source of humidity is the damaged torus suppression chamber in the basement of the building.


Tackling a separate issue at the unit, Tepco has announced the operation of air filters to ventilate unit 2, while filtering out airborne radioactivity. The filtered air is being fed back into the reactor building. The company said that it will continue this for about three days, when it will again check radiation levels. Should internal levels be found to have dropped significantly, Tepco will open the doors to the building to allow workers to enter.


Workers have now been able to enter the third floor of the reactor building of unit 4 for the first time since the accident happened, reporting a large hole in a wall created by the sequence of several hydrogen explosions after an inflow of the gas on 15 March. In addition, an existing water pipe that is required for the alternative circulation system was found to be badly damaged. The company may have to reconsider its plans for installing a fuel pond cooling system at unit 4 as the amount of damage and rubble surrounding the fuel pool may make repairing the damaged pipe difficult.


The temperature of the water in unit 4's fuel pool is above 80ºC, much higher than at the other units on the site primarily because it contains many more fuel assemblies.


Worker over-exposure


Meanwhile, Tepco has released the results of further examinations of two control room operators at the Fukushima Daiichi plant previously reported to have been exposed to radiation amounts exceeding the safety limit of 250 millisieverts set by the government.


Tests at the National Institute of Radiological Sciences (NIRS) indicate that one of the men, in his 30s, has received a total dose of some 678 millisieverts (an external dose of 88 millisieverts and an internal dose of 590 millisieverts), while the second worker, in his 40s, received a total dose of 643 millisieverts (103 millisieverts externally and 540 millisieverts internally).


Examinations by NIRS also confirmed that the two workers had no health problems as a result of their exposures.


Tepco said that a third worker has been found to have received a radiation dose above the regulatory limit. The company said that it will conducted examinations to determine more accurate levels of the man’s exposure dose.


Researched and written

by World Nuclear News