UPDATE - This article was updated on 24 February following a revision by Tepco indicating that the water had not yet passed through the desalination plant and was intended to be fed into tanks in a different area on the site.
An investigation is underway into why a storage tank at the Fukushima Daiichi site overflowed, leading to a spill of partially treated radioactive water. Clean-up of this water, which does not appear to have flowed into the sea, is underway.
|Work is underway to clean up the overflowed water (Image: Tepco)
Tokyo Electric Power Company (Tepco) said that, just before midnight on 19 February, workers discovered contaminated water overflowing from one of many storage tanks on the site. The tank holds water that has been used to cool the damaged reactors. This water had already passed through the caesium removal facility, but other beta-emitting nuclides had yet to be removed. The water had not also been through the desalination plant.
The company said that the tank appears to have overflowed due to "the improper flow of water through a pipe from the processing facility to the tank." It originally thought that two of the three valves along the pipe were improperly left open, while the third valve was closed but failed to function. Some of the water overflowed from the tank into the dike surrounding the tank, but the rest flowed along a rainwater drainpipe to the ground beyond this dike. The beta radioactivity of the water in the drainpipe was 230 million Bq/L while that in the dike was found to be 3 million Bq/L.
However, in an update Tepco said that the water was supposed to flow into storage tanks within a different area of the plant site, but the valve in the pipe leading to that area was found to be shut while those leading to the storage tank that overflowed had all been left open. It continues to investigate how these valves had come to be in this state.
It took workers just over six hours to stop the water overflowing from the tank by shutting off the open valves and reducing the level of the water in the tank. By this time, some 100 tonnes of water is estimated to have leaked from the tank, causing a puddle measuring some 30 metres by 3 metres to form adjacent to the tank but outside of the dike.
Tepco stressed that there is no pathway for the water to flow from this area to the sea, which is about 700 metres away. The company immediately began work to remove the spilled water and the soil that it seeped into.
Checks have shown that no other tanks on the site - which together store some 340,000 tonnes of water - have shown signs of overflowing.
Tepco director and executive vice president Zengo Aizawa commented, "We are deeply embarrassed that this sort of unacceptable event would occur after the many steps we have taken to improve the management of stored water." He added, "We will therefore conduct a thorough investigation into what occurred and determine what additional steps must be taken to prevent any similar occurrence in the future, and will further strengthen field management of stored water. It also demonstrates the need for a permanent solution to the contaminated water issue."
Researched and written
by World Nuclear News