"Static, Not Stable" - What Could Happen in the Second Month of the Fukushima Daiichi Nuclear Accident in Japan?
Posted April 19, 2011
US Nuclear Regulatory Commission Chairman Gregory Jaczko used these words – “static, not stable”– to describe the current situation at Fukushima Daiichi in a Senate hearing this past Tuesday. I was in the hearing room of the Committee on Environment and Public Works to listen to Jaczko’s thoughts on the accident, and to be present when my colleague Tom Cochran testified in a subsequent panel that afternoon on implications of the Japanese accident for US nuclear power. The day before, on April 11th – exactly one month after the earthquake and tsunami – the Japanese government raised the classification of the accident to put it on par with the 1986 Chernobyl accident, and increased the evacuation zone around the damaged reactors, telling residents of five municipalities to leave within a month because of radioactive contamination that is likely to pose a danger to health over the longer term.
According to the Mainichi Daily News, the municipalities covered under the Japanese government’s new "planned evacuation area" are Katsurao, Namie, Iitate, part of Kawamata and part of Minamisoma. These municipalities were chosen because the annual expected radiation dose would be higher than 20 milli-Sieverts (mSV), or six-and-a-half times the average background dose Americans receive from natural sources of radiation over a year. To get a sense of the risk from long-term exposure to 20 mSV, in a group of 100 people exposed to 20 mSV per year over their lifetimes, about 16 would get cancer as a result of this radiation exposure, and 8 would die from their cancer.
I was curious to find where these municipalities were located in Fukushima Prefecture, so I used Japan’s National Atlas to locate them. And to get a sense of the radiation levels in these municipalities, I mapped up the “planned evacuation area” on the US National Nuclear Security Agency’s (NNSA’s) aerial survey of radiation around the damaged reactors. In the map below, you can see that these five municipalities fall along a “plume” of radiation extending in the northwest direction from the Fukushima Daiichi plant.
These NNSA aerial measurements (data from March 30 to April 3) had a maximum value of 0.3 mSV per hour, and for the centerline of a plume, values shown in yellow and orange are between 0.0119 to 0.125 mSV per hour. The outer band of the plume shown in green on the NNSA map had a minimum dose rate of 0.0025 mSV per hour. So therefore the minimum annual expected dose in these areas would be:
NNSA Green dose rate (0.0025 mSV/hour) –> annual expected dose = 21.9 mSV
NNSA Yellow dose rate (0.0119 mSV/hour) –> annual expected dose = 104.3 mSV
NNSA Orange dose rate (0.0217 mSV/hour) -> annual expected dose = 190.2 mSV
So the outer edges of the green areas roughly correspond to the current guidelines for the “planned evacuation area” of 20 mSV annual expected dose. The five municipalities for the “planned evacuation area” are mapped in red on top of the NNSA radiation measurements. Note that the NNSA green areas – and even some yellow and orange areas - fall outside of the new “planned evacuation area,” but from the NNSA data residents there would expect to receive an annual expected dose greater than 20 mSV. And in the orange areas of the plume the mid-range cumulative dose over one month would be 53.6 mSV, so it would seem to me that the residents should leave sooner in those areas than the Japanese government is asking them to.
For Tom Cochran’s testimony before the Senate, he and I looked at the radiation measurements published for prefectures in the region around the Fukushima Daiichi plant. The Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) has posted hourly dose rates by prefecture on its website. In the chart below, I show the hourly dose rates for Ibaraki and Tokyo prefectures for a three-week time interval beginning several days after the earthquake. Ibaraki Prefecture borders Fukuhima Prefecture to the southwest. Tokyo is located about 250 kilometers (155 miles) also southwest of Fukushima Daiichi. The chart shows “spikes” in the radiation readings during the first week of the accident, corresponding to explosion and fire events, smaller spikes in the second week, and a gradually-decreasing hourly dose rate from the third week onwards.
If the situation at the Fukushima Daiichi plant remains “static” through the second month of the accident, it is likely that further evacuation areas will be established by the Japanese government, perhaps displacing about one-quarter million people altogether from their homes in Fukushima Prefecture. And in the aftermath of the accident at Fukushima Daiichi, extensive dose reconstructions will certainly be undertaken that will include more detailed data on radiation levels, weather, other exposure pathways, and populations, as was done for Three-Mile Island and continues for Chernobyl.
What could occur over the next month to change the picture? The threat of severe aftershocks comes to mind. On April 7th a 7.4-magnitude earthquake struck northeastern Japan, causing the loss of external power to Rokkasho nuclear fuel reprocessing plant and the Higashi-dori nuclear power plant, both facilities relying on emergency diesel power to save cooling capability. For the Onagawa reactors, 3 out of 4 external power lines were lost due to the aftershock, but one external power line survived to maintain cooling capability.
This past Sunday, the utility which owns Fukushima Daiichi - TEPCO - gave the first public time-table for ending the nuclear accident: six to nine months. While it is true that there have been signs of emerging recovery in some areas, like restoration of electrical power and repair to instrumentation, the flow of radioactivity to the open sea continues, as workers pump contaminated water out of diesel generating buildings and elsewhere at the plant towards the ultimate goal of restoring normal powered cooling to the reactor cores and spent fuel pools. Only recently have robots entered the dark, intensely-radioactive reactor building of Unit 3. While in the next month a clearer picture should start to emerge of conditions in the reactor cores at Fukushima Daiichi, it took several years to quantify that half of the fuel had melted at the Three-Mile Island plant.
Comments are closed for this post.