Published on Nov.12 ,2013 : The Denki shinbun(The Electric Daily News)
Dr. Michio Ishikawa
Chief Adviser(Former President & CEO of JANTI

While looking at a radiation dose rate chart of the Fukushima accident, I suddenly wondered whether filter vents were necessary.

It's too bad that I cannot show you on a diagram, but the hourly dose rate at the main gate of the power station sharply rose to approximately 4µSv at around 4:00 on the 12th, which was the morning after the accident, and after remaining steady for about 3 days, climbed again to approximately 300µSv late at night on the 14th.

The first dose increase is undoubtedly due to radiation from Unit 1, both in terms of the time of explosion at Unit 1 and the vent record.

The second increase is from Unit 2, and can be proved both in terms of the time of the spike and pressure changes of the containment vessel. The dose rate is high because radiation from the molten core directly leaked out of the containment vessel due to a venting failure.

To give a bit of background, the dose rate after the accident repeatedly spiked. These spikes appeared each time there was an explosion or venting, and the dose rate increased up to a maximum of about 1000µSv, but quickly returned to the background dose rate of about 4µSv.

To elaborate, the amount of radiation released during the spikes was limited and did not affect the overall background dose rate. This therefore means that the large quantity of radiation release that affected the background dose rate was the first 4µSv and the second 300µSv.

The amount of the second release is overwhelmingly large compared to the first. It is common sense to attribute this difference to the presence of venting. This is because the radiation that passes through the vents is washed off as it passes through the water at the bottom of the containment vessel, and becomes less concentrated.

Therefore, the effect of removing radioactivity by venting can be seen when the two dose rates are compared. The ratio is 300µSv to 4µSv, so the decontamination effect is 75. This shows that the radioactivity concentration was reduced to 1/75th by passing the gas through the water in the containment vessel.

This is a large decontamination effect. Again, if the Unit 2 vents had been open, the background dose rate would surely have stayed at approximately 4µSv. This dose rate is close to the lower limit of the ICRP evacuation order of 20mSv a year.

I was thinking loosely in this way until I realized an error. The time of the first increase in the radiation dose was 4:00, which was a whole five hours before the time of the Unit 1 venting at 9:00, so this increase was unrelated to the venting. Furthermore, it is strange that while the emissions doubled during the Unit 3 venting, the dose rate of 4µSv did not change. This means that the venting is not the culprit.

After reexamining the matter, I found in the TEPCO report that at the same time, a fire engine was being connected to the pipe that was directly linked to the reactor, but this was suspended due to an increase in the dose rate. Based on the time, this seems to be suspicious.

The joint of the fire engine hose is not gas-tight. This is a bit technical, but hydrogen generation from the molten core due to water injection can also be considered. It is presumed that radiation was leaked directly from the building following water injection by the fire engine. Although the first increase was minute, it was due to a direct leakage from the Unit 1 building.

This means that the decontamination effect of the water in the containment vessel was even greater. The amount of radiation of the venting must be even lower than the amount of direct leakage, and unless it was about 1/10th of the direct leakage, the fact that the dose rate stayed constant cannot be explained. If this applies, the decontamination coefficient will be 750.

If the speculation above is correct, the dose rate during the accident would have remained at about 2mSv per year if only the vents had been open, and evacuation would not have been necessary.

Well, the proof is in the pudding. The BWR group should verify the decontamination effects of the containment vessel through experiments. This is to firmly establish the safe performance of existing equipment. If results are achieved, filter vents will no longer be necessary. Excessive equipment is harmful to safety.

In addition, the emergency procedures should also be revised. If vents are opened at the same time as isolation of the containment vessel, radiation would be washed by water and would not be released even in the event of a core meltdown. The dose rate would also be low. The folly of seeking the government's permission for venting will also be unnecessary.

I strongly request the BWR group to consider this matter.

end