TEPCO robots find leaks in unit 1 and unit 3

5 June 2014

Two separate inspections have finally located leaks in the Fukushima Daiichi unit 1 and unit 3 primary containment vessel.

It has been known for three years that cooling water pumped into the reactors was escaping into the basements of the reactor and turbine buildings, but despite extensive investigations the principal leaks have not been found.

Finding, and fixing, the leaks is an essential step in the long-term plan to remove the fuel and decontaminate the damaged reactors. Once the leaks are plugged, the reactor vessels and primary containment vessels will be able to be stably cooled by flooding the primary containment vessels. At the same time, the reactor buildings can be dried out, and the huge array of storage tanks for contaminated water will no longer be necessary.

Discovering the source of the leaks

In unit 1, a leak of 0.75-1.5 tons of water per hour has been photographed by a remote camera in the upper part of the torus suppression chamber. The leak is coming from a 800mm-diameter flexible joint in a vacuum breaking line that comes from containment. The joint is thought to be corroded.

The leak was detected on 27 May by a Hitachi-GE robot camera (Hitachi-GE tele-runner) that was lowered 2.7m into the suppression chamber through a hole drilled in the floor above and controlled remotely, TEPCO said.

The general location of the leakage was first identified in November 2013 by a remote-controlled boat that took photographs while floating on the accumulated water in the basement, outside the containment vessel. At that time, it became clear that it would be necessary to examine more deeply around the suppression chamber, which is the outer part of the containment vessel, to identify the source of the leaks with more precision. Because radiation is high around the vessel, it could only be safely examined by remote control.

The camera drove along the outer catwalk above the circular torus to the leak. A second survey on 30 May travelled around the other side of the torus, where it found dirtier pipes and higher radiation levels, but no more leaks. Maximum radiation was approximately 2400 mSv/hr some distance away from the leak, where the pipes were seen to be dirty. Near the leak location radiation was 500 mSv/hr, compared with 200-400 mSv/hr elsewhere in the suppression chamber. A piece of broken metal insulation blocked the catwalk and prevented the robot from completing an entire circuit.

Another leak was found, in a pipe coming out of the unit 3 primary containment vessel, on 15 May. The spot was approximately 4m away from the surface of the PCV, and the scale of the leakage was about several centimeters in diameter. TEPCO said it was the first time it had pinpoined a leak at unit 3, and said that after it assessed the amount of water leaking would come up with a plan to plug it.

Ice wall construction begins

In other Fukushima news, construction of an underground ice wall around the Fukushima Daiichi reactors has begun. The principal benefit of the frozen barrier compared with a physical barrier is that it avoids the challenges of building a wall around such underground obstacles as pipes, and creates a seamless barrier, TEPCO said.

On 2 June, workers began installing equipment to freeze the soil around the four reactor units. Pipes for freezing the soil are being implanted, starting at the site's northwest corner. The start of construction followed approval from the Nuclear Regulatory Authority on May 30. Construction is expected to be completed in early 2015, TEPCO said.

The barrier is being funded by the government, with construction being undertaken by Kajima Corporation. Chilled brine will flow through the pipes to a depth of 30 meters, freezing the surrounding soil and forming a barrier around the reactor facilities. The circumference of the wall is approximately 1500 meters, TEPCO said.

Water treatment update

Finally, one of three trains of the ALPS system that removes radionuclides from contaminated water has been restarted. Since its installation in October 2012, it has been under test operation, processing total of approximately 85,000 tons of water, TEPCO said. The facility is designed as a "secondary" system, to remove most remaining radioactivity after the primary treatment system removes caesium.

Unit B had been shut down on March 18 after monitoring detected radiation in output water that exceeded limits. The water was safely stored on-site and did not flow to the sea. Inspection revealed that a part of the gasket inside the filter was deteriorating due to exposure to radiation. The original Teflon gasket has been replaced with one made from a synthetic rubber that is expected to withstand radiation more effectively. Unit B restarted operation on May 23 and so far no problem has occurred. The equivalent filters have also been replaced in the other two units, A and C, which should restart in June, TEPCO said.

Photo: View of vacuum breaker pipe through handrail

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