Published on Jan.21 ,2013 : The Denki shinbun(The Electric Daily News)
Dr. Michio Ishikawa
Chief Adviser(Former President & CEO of JANTI

Another problem with emergency response following the previous issue (article of November 19, 2012) is the failure of RCV venting at Unit 2. But for this failure, the radiation level at the site premises would have been small, and, although it depends on the decision of the government, a situation that resulted in large scale evacuation of residents could have been avoided.

The measurement results of the radiation level near the main gate of the power station during the accident show that the local radiation level increased twice. The first was on the 12th, which increased from the normal value of approximately 0.1μSv/h to approximately 3μSv/h. The second was on the 15th, which increased by almost 100 times to approximately 300μSv/h.

The reason for the different amount of increase is according to whether radioactivity was released from the vent system or from the damaged portion of the RCV. It has already been mentioned (in the article of October 2, 2012) that the vent system is a system to release gases from the RCV. When core cooling fails, RCV pressure rises due to the high pressure steam and radioactive gas from the reactor, eventually leading to damage. The vent system releases such gases before such damage occurs.

At Unit 1, the vent system opened before the explosion. The same went for Unit 3, where gas was successfully released and the RCV was not damaged. However, the vent system at Unit 2 did not successfully open, resulting in overpressure of the RCV that led to partial damage. It is said that this is because the rupture disk separating the vent pipes did not break or because the isolation valve did not stay open.

The first increase of the local radiation level occurred around 5:00am on the 12th when Unit 1 was being suspected of melting. The radiation level did not change even after the melting and explosion at Unit 3, and stayed around 3μSv until late into the night of the 14th.

The radiation level during the accident jumped to tens of thousands μSv/h due to the impact of the explosion and venting, but this was only for a short period of time and the radiation level returned to the original state shortly after.

The situation changed in the hours between the night of the 14th and the morning of the 15th, when the Unit 2 reactor melted and the RCV was damaged. After fluctuating for some time, the local radiation level settled at around 300μSv/h in the evening of the 15th.

It is obvious that the sudden rise of the radiation level from 3μSv/h to 300μSv/h was due to different release routes.

Just for reference, it was only last spring that the 300μSv/h local radiation level decreased to around 3μSv/h, a year after the accident.

Radioactivity released from the vent pipes travels through the puddle of water at the bottom of the RCV before being discharged from the stack. Therefore, the released radioactivity is low in its level since it is discharged as if being rinsed.

This rinsing effect is the reason for the local radiation level staying at approximately 3μSv/h even after the melting and explosion at Unit 3. In contrast, the radioactivity of the Unit 2 reactor is high as it was released directly from the crack of the RCV. This difference resulted in the 100 times difference of the radiation level near the main gate.

Moreover, 3μSv/h is equivalent to 26mSv/year. This is in the lower range of the reference levels for emergency exposure situations (20-100mSv/year), recommended by the International Commission on Radiological Protection (ICRP) to the Government of Japan.

If the venting at Unit 2 had been successful, the rinsing effect would not have caused much change in the local radiation level and would have remained at around 3μSv/h.

The radiation level in many of the residential areas located farther away from the power station than the main gate is obviously lower than that at the main gate, implying that it did not exceed 20mSv/ year, the level recommended to start evacuation. This means that it did not call for evacuation. The same can be said for the distant Iitate Village.

Why did venting fail? Considering the prevention of radioactivity release, the rupture disk and isolation valve were designed with the risk of failing to conceal the RCV due to the accidentally opened vent system in mind. However, the role of the vent system is to act as a safety device by preventing such concealment. It should have been designed so that it forcibly opened in case of an emergency. It was a major mistake of the plant designer and utility to have a design that focused less on the main objective and more on a petty matter.

The failure to activate the isolation condenser was due to the fear of a minor radiation leak. It all comes down to the same thing. The awareness on radioactivity must be reformed for the nuclear industry as well.

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