On Saturday, workers at the Fukushima Daiichi nuclear power plant in Japan were operating a remote-controlled robot to remove debris on the first floor of the Unit 3 reactor building when they discovered a stream of water nearly a foot wide flowing through the first floor the reactor building before escaping into a drain on the floor.
After an investigation found that the water contained levels of radioactive materials equal to highly contaminated water which is accumulating in the basement of the reactor building, Tokyo Electric announced that water leaking inside of the Unit 3 reactor building is likely coming from the containment vessel where it was used to cool the melted nuclear fuel, rather than rain water.
The operators used the robot to sample water flowing into the basement of the reactor building. The investigation revealed some 24 million becquerels per liter of beta ray-emitting radioactive materials, which includes strontium, 700,000 becquerels per liter of Cesium 134, and 1.7 million becquerels per liter of Cesium 137.
Tokyo Electric also found that the temperature of the water, 20 degrees Celsius, is consistent with the same as water found at the bottom of the reactor.
The water is coming from a room which houses a main steam isolation valve, which is causing concern among experts around the world. The latest details have raised questions as to whether the main steam isolation valve or any of its ancillary systems may have failed during the disaster.
There are two main steam isolation valves in each of the four pipes which carry steam from the reactor vessel to the turbine. In case of accident or power loss, the MSIV’s are supposed to fail in a closed configuration.
After the reactor shut down on March 11th, 2011, the main steam isolation valves should have closed with the turbine tripped. Even if workers had manually re-opened the MSIV’s when emergency diesel generators restored power to plant equipment before the tsunami hit the plant, the pressure inside of the condenser should have automatically re-closed the MSIVs shortly thereafter.
The fact that water is leaking from a room which houses the main steam isolation valve may indicate that the valve did not close, or was damaged during the course of the disaster.
In 2011, Dave Lochbaum of the Union of Concerned Scientists drew attention to the fact that water levels in the Unit 3 reactor dropped below zero by 16:00, without any information available that would explain why.
If the MSIV did not close or broke, operators could have been facing a Loss of Coolant Accident without even knowing how the coolant water in the reactor could have leaked out.
If the MSIV was damaged or failed open, this would represent an unanalyzed condition which could affect every other operating Boiling Water Reactor in operation currently, who count on the MSIV’s to work as planned.
Source: Union of Concerned Scientists
Source: Asahi Shimbun