TEPCO has announced new work at Fukushima Daiichi related to the unit 1-2 vent tower. This location has the highest radiation levels outside of the reactor buildings. The effort is the newest chapter in the problematic history of this tower.
In 2011 a worker without any body shielding, using a roughly 10 foot probe stood near the tower and took readings of the SGTS pipe as it enters the vent tower. The official radiation reading given by TEPCO was 10 Sieverts/hr. It may have been higher, workers from the plant speculated that 10 Sieverts was the upper limit of the meter used. There were concerns this worker is the same worker who died of acute leukemia just over a week after this work took place.
In 2013 a long boom was used to take radiation readings in the same location that found 25 sieverts/hr in the SGTS connecting pipes.
In 2015 a worker in a lead suit approached this same pipe and found 2 Sieverts/hr
In 2016 highly radioactive water was found in the drain sump for the vent tower. Thick concrete shielding was placed and a boom truck used to sample the water as the ambient radiation levels at the base of the tower were so unsafe.
Now TEPCO plans to remove the entire run of SGTS piping between the tower and the two adjacent reactor buildings. The SGTS (standby gas treatment system) pipe is the downstream piping for a system used during normal operations. Gasses can be vented through this system by first flowing through a pair of industrial HEPA filters. This system is not used as part of the emergency venting system, identified by the large pipe that connects to the vent tower. It is not clear why the SGTS pipes are highly contaminated, both systems have closed valves per TEPCO’s report. The emergency vent line connects to the stack high above the SGTS piping connection.
TEPCO vaguely cites the reason for the SGTS pipe removal plan is that the drainage sump water from the vent tower has been consistently radioactive. They expected the radiation levels to go down over time. By some mechanism the SGTS pipe is assumed to be the cause of this. TEPCO cites concerns that damage to the reactor waste building roofs could be contributing to this but they don’t specify how that would happen.
The first step of this plan is to insert a camera and take swipe samples inside the base of the vent tower. To accomplish this TEPCO has devised what looks like a lead shielded phone booth on wheels.
The nuclear phone booth would be wheeled close to the vent tower base. A hole would be cut in the offgas pipe that protrudes from the ground before reaching the vent tower. Through this hole they plan to place the camera and take radiation swipes. TEPCO estimates the total exposure to be 83 mSv per person. The maximum worker radiation exposure at Fukushima Daiichi is 100 mSv before a worker has to retire from work at the disaster site. This work is planned for the month of May.
To date a radiation meter has been inserted into this area of the vent tower and a partial set of readings were taken. These found lower levels of radiation in the off gas pipe compared to the reading taken outside of the SGTS pipe in early inspection work.
Work to begin removing the SGTS pipe is currently scheduled to start in December 2020. This could change based on research and mock up testing.
A machine translated version of the TEPCO report can be found here
The original Japanese version can be found in this larger report
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