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The radiological effects of the damage to the Kashiwazaki-Kariwa nuclear station.

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It seems that we’re still hearing about Kashiwazaki-Kariwa. In particular, certain anti-nuclear insects are absolutely having a field day with it.

We certainly need to consider the quantitative aspects of the radioactivity releases into the environment, and we need to have the quantitative information investigated and released by the company. These things can’t be measured and calculated straight away of course, which is why there has to be a delay.

OK. Let’s outline everything verifiable that we know about radiological releases at Kashiwazaki-Kariwa.

Of the 67 abnormal conditions and events detected at the nuclear station in the aftermath of the Earthquake, only 15 of them involve radioactive materials at all. For example, a fire broke out at an electrical transformer at the site, but this has got absolutely nothing to do with radioactivity or nuclear safety.

Initially, it was thought that around 1.5L of water was spilled from a spent fuel pool. Later, more detailed reports confirmed a number of different releases of radioactivity.

600 ml of slightly radioactive water leaked onto the third floor of the Unit 6 reactor building. This contained 280 Bq of activity.

From the above link:

“As the results of the Earthquake occurred on July 16, 2007, leakage of water was identified in the non-controlled areas of third floor and medium third floor in the reactor building of Unit 6, Kashiwazaki-Kariwa Nuclear Power Station, which has then been under the periodic inspection outage. The investigation of the leaked water revealed that leaked water at the third floor was about 0.6 liter and its activity was about 280 Bq, while the leaked water at the medium third floor was about 0.9 liter and its activity about 16 thousand Bq.

It was also confirmed that the leaked water from the concerned non-controlled areas was discharged to the sea through drainage path in the station. Its amount and activity were estimated to be about 1.2m3 and about 6×10^4 Bq, respectively. Later on July 18, 2007, during the revision of the calculated results, it was revealed that the value of the activity concentration used for the calculation of the activity had been erroneous. Today, the activity was re-estimated to be about 9 x10^4 Bq.

However, Tokyo Electric Power Company maintains the previous estimation that the contribution of the concerned radioactive materials to the 3 month averaged concentration would be less than 2 x10^-10 Bq/cm3 outside the environment surveillance area due to dilution. This value is evaluated to be largely below 0.2 Bq/cm3 (one billionth) which is specified by the ordinance as to the 3 month averaged concentration limit for the radioactive release to the outside of the environment surveillance area.”

Thus the 90 kBq in 1200L was the total of all the radiologically contaminated water spills at the plant.

On Wednesday June 18, at Unit 7, radioactive Iodine (We don’t know the isotopic composition) was found to have been released from an exhaust pipe by a government inspector. The release began between Tuesday and Wednesday and was confirmed to have stopped by Thursday night. The maximum amount of radioactivity released into the air was about 400 MBq. It is estimated that this caused an unintentional dose of, on average, 0.2 nSv (nanosieverts) per person.

The limit for dose to the public from the operations of a nuclear plant in Japan in one year is 1.1 mSv. For reference, the Three Mile Island accident caused a average dose of around 0.08 mSv to every individual living within 10 miles of the plant.

Here’s an extract from TEPCO’s report:

“Iodine and particulate radioactive materials (Chromium-51, Cobalt-60) has been detected at Unit 7 of Kashiwazaki-Kariwa Nuclear Power Station during the regular weekly measurement of the main stack on July 17. The estimated total amount of detected iodine activity was about 3 x 10^8 Bq and that of detected particulate radioactive materials was about 2 x 10^6 Bq. (As announced on July 17)

After the above event, the measurement frequency was increased from weekly to daily and it was found that the measurement conducted on July 17 and 18 has detected iodine release with the estimated total amount of about 2 x 10^7 Bq. (As announced on July 19)

Today, it was found that the measurement conducted on July 18 and 19 hasn’t detected any release of iodine and particulate radioactive materials.

This results in the total amount of release since July 9 to be 4 x 10^8 Bq (rounded up from 3.12 x 10^8 Bq) for iodine activity and 2 x 10^6 Bq for particulate radioactive materials.

NISA has assessed the situation and concluded that there is no possibility of leaking from fuel assemblies as significant change of iodine concentration in the reactor coolant hasn’t been observed during plant operation and measurement after the plant shutdown on July 18. The release of iodine is estimated to occur in a way that iodine accumulated in the main condenser was vented via the turbine gland steam ventilator to the plant main stack.

The dose by the activity released is about 2 x 10^-7 mSv and the impact of this dose on individuals is calculated to be very low or 2 x 10^-7 times the maximum dose (1 mSv) of the general public allowed by the Japanese regulation.

No significant reading has been identified at the radiation monitors of the main stack and the monitoring posts. “

About 400 drums containing low-level radioactive waste stored at the plant were knocked over by the aftershocks, 40 losing their lids. The number of spilled drums was previously reported as 100. [Reference here]

“Several hundred drums in the solid waste storage warehouse tipped over and several tens of drums were found with their lids open.

Radioactive material concentration in the air was measured at four locations, no radioactive material was detected.

It was confirmed that water leaks from the fallen drums and the amount of leaked water was 16 litres. No radioactivity was detected.”

So, that’s the story on radioactive releases. The radioactive emissions – all the radioactive emissions – into the environment were about 10.8 millicuries of mainly Iodine isotopes, with a comparatively infinitesmal amount of Co-60 and Cr-51, causing a dose of 0.2 nSv per person, and the far more widely publicised release of 2.43 microcuries of radioactivity, in 1200 L of water, into the sea of Japan.

From here, we find that 1 cup of milk contains 12.6 Bq of activity, or about 50.4 Bq/L.

Therefore, the 90 kBq released into the ocean corresponds to about 1786 L of milk. Or, putting it slightly differently, the water released into the ocean at Kashiwazaki-Kariwa is around 1.5 times as radioactive as milk.

Now, I can’t find any information about what radionuclides are present in that water that was spilled from the spent fuel pool, so for the purposes of demonstrating a general point, i’ll work on the hypothetical assumption that all the radioactivity is in the form of Caesium-137, a quite radioactive, moderately long lived, quite radiotoxic beta- and gamma-emitting fission product which is produced in used fission reactor fuel.

If a person drinks, say, 10 L of the water released at Kashiwazaki-Kariwa, they’ve ingested 750 Bq of Cs-137, which, given a 50-year CEDE of 0.05 rem/uCi for ingested Cs-137, corresponds to a dose of 1.014 millirem, or 10.14 microsieverts.

Given the rapid and vast dilution of this already small amount of radioactivity in the ocean, it is unlikely that any person could possibly be exposed to such a quantity of radioactivity. Keep in mind that you can not, of course, actually drink seawater.

I’d hazard a guess that people around the nuclear plant have been exposed to a maximum possible dose of perhaps 0.5 nSv.


Written by Luke Weston

July 26, 2007 at 12:21 pm

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