Fukushima Daiichi Vent Systems Inspected
TEPCO recently completed a series of inspections of the SGTS (standby gas treatment system) rooms and filter trains for units 1 through 4. These systems include the HVAC and venting system used during normal operations of the reactors and the emergency venting system that was used during the initial meltdowns.
The venting systems are a key piece of understanding the events of the initial disaster. Earlier work has been completed to inspect the vent lines outside of the reactor buildings as they route to the vent towers.
The above diagram shows the vent system in relation to the reactor systems and what the SGTS room located in the accessory building between the reactor building and turbine building looks like.
* the diagram shows the pink line as the “emergency” vent line. The green line has the rupture disc on it, indicating it is the emergency vent line. The error in the diagram may be a translation issue.
Inspections took place for units 1 & 2 in August and September. Units 3 & 4 were conducted in September and October of 2020. The inspections included taking radiation readings, using a y-ray imager and in some cases, opening the filter trains to look inside.
Units 1 & 2 had high radiation levels in the SGTS rooms when they were initially checked in 2011 and 2014 respectively. Units 3 and 4 had lower levels of contamination in these early surveys in 2011.
From these most recent inspections it shows that unit 1 was the highest in contamination. Unit 1’s ventings may have been a significant cause of environmental contamination. Unit 2 appears to have not successfully vented, but moderate levels of contamination did back flow into the SGTS filter trains. Unit 3 vented but only one filter train became notably contaminated. Unit 4’s inspection confirmed earlier findings about unit 3’s vent gasses flowing into unit 4 via the venting systems and HVAC ductwork in the building due to the lack of backflow valves in the shared vent system piping outside of the reactor buildings.
The unit 1 SGTS filter trains are the most contaminated of the four units inspected. This room had been off limits for years due to high radiation levels even at the door to the room.
Readings taken inside unit 1’s SGTS room showed levels over 1 Sievert near the filter trains.
A y-ray image of the vent pipes near the rupture disc was taken at a distance. No radiation readings were collected in this area.
A y-ray image and a radiation reading were obtained for the end of the filter trained. The SGTS pipe entering filter train A was found to have over 1.5 Sieverts of contamination at this pipe.
Radiation readings found near unit 2’s SGTS filter trains were less than half of what was found at unit 1, yet these readings are significantly higher than readings found at units 3 and 4.
The above y-ray image shows a hot spot at the end of one of the filter trains in unit 2. The end of the filter trains are where radiation readings found the highest levels.
A second y-ray image taken from the other side of the same end of the filter trains shows a similar hot spot. The end of both filter trains appear to be a point of higher contamination.
Radiation readings taken around the rupture disc show both sides to be low in contamination. Unit 2’s venting attempts had been characterized by TEPCO as having failed. These readings appear to confirm this. Upstream systems appear to have not allowed vent gasses to reach the area of the rupture disc.
A leak was found coming from the SGTS B filter train. This is near the area of highest contamination in the SGTS room. The leak appears to have been from contaminated water that has since dried up.
The y-ray imager found contamination in an upper pipe between the two filter trains and one in what appears to be an open area or the end of a filter train. Neither location was extremely high in radiation.
The second y-ray image for unit 3 looks at the green emergency vent line and the location of the rupture disc. It shows that the emergency vent line is contaminated including the section past the rupture disc. The radiation readings are fairly low.
The new radiation readings for the area around the rupture disc show higher levels downstream of the rupture disc. The rupture disc appears to have worked based on the radiation readings. Early documentation of the disaster showed at least one successful venting of unit 3 during the meltdowns.
Both the A and B filter trains inside unit 3 were opened and inspected. Each area was sealed off with plastic sheeting and a portable negative air pressure filtration system was used to prevent any contamination releases while this work was conducted.
The A filter train appeared to be mostly clean as if little had flowed through this side. This train had significantly lower contamination within the filter systems. The B filter train showed notable filth and higher radiation levels. These higher levels are still far below those found elsewhere.
Unit 3 A Filter Train
Unit 3 A Filter Train
The unit B images above show that the demister is dark, dirty and warped or dislodged. The prefilter has black stains.
The first HEPA filter downstream of the prefilter shows staining, the last HEPA filter appears clean.
The B filter train had standing water inside it. The water level was slightly over the doors used to access the filters. This water was collected for analysis, no results have been posted to date.
Unit 4 was inspected in this latest round of work due to hydrogen infiltration from unit 3 via the vent system during the meltdowns.
Y-ray imaging in unit 4 found a hot spot downstream of the rupture disc. Vented gasses from unit 3 flowed backward through the outdoor portions of the vent system into unit 4. A concentration of contamination where gasses would have been blocked by the closed rupture disc is not an unusual finding.
This y-ray image looks at the end of the two filter trains in unit 4. Previous inspections found contamination in these filter banks caused by the gasses that flowed into these systems from unit 3.
The unit 4 rupture disc area appears to be contaminated on both sides of the closed rupture disc. The contamination levels are low. The contamination pattern may be due to contamination from unit 3 that flowed through the HVAC ductwork of unit 4.
The A filter train of unit 4 showed some staining in the prefilter but over all contamination levels are low.
The filters that are on the vent stack side showed significantly higher contamination, closer to the levels found inside unit 3’s B filter train.
The B filter train for unit 4 shows relatively low contamination, the condition of the prefilter isn’t available due to the stuck door.
The downstream portion of the B filter train shows the HEPA filter closest to the outside vent system to have higher contamination, both show some light staining. The findings at unit 4 are consistent with the previous assumptions that unit 3’s vent gasses flowed back flowed into unit 4’s venting and HVAC systems.
Machine translated document in English:
Original document in Japanese:
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