The purpose of this post is to answer the question posed in the title by summarizing a recently published peer reviewed study in the journal Nuclear Engineering and Design. The diary is part of an ongoing effort to communicate results from scientific studies aimed at understanding the impact of the Fukushima Dai-ichi meltdowns on the environment. The paper by Jäckel compares measured and predicted activities of reactor products 134-Cesium (134Cs half life ~2 years) and 137-Cs (137Cs half life ~30 years) in the reactor cores and spent fuel to measurements in the spent fuel pools (SFPs) of Units 1, 2, 3 and 4 at the site to determine how much spent fuel radiocesium was released after the accident. The comparison indicates that only very minor damage to the spent fuel occurred during the accident and subsequent clearing work such that at most about 1% of the Cs inventory from a single bundle (in Unit 2 SFP) was released to the cooling water. The short answer to the question is that not very much of the spent fuel was released at all and the bulk of releases originated from the reactor fuel in Units 1, 2 and 3 at the time of the accident. This is consistent with the results of measurements made of Fukushima derived radionuclides in air, soil and water worldwide since March 2011. Continue reading Status of the Spent Fuel At Fukushima Dai-ichi: How much was released to the environment?
This post reports on a recently published peer reviewed study by Steinhauser and colleagues in the journal Science of the Total Environment (behind pay wall) comparing the Chernobyl and Fukushima nuclear accidents. The post is part of an ongoing effort to communicate the results of scientific studies into the impact of the Fukushima disaster on the environment. A majority of the radioactivity released from both Chernobyl and Fukushima can be attributed to volatile radionuclides (noble gases, iodine, cesium, tellurium). In contrast, the amounts of more refractory elements (including actinides like plutonium), released by Chernobyl was ~four orders of magnitude (10,000 fold) higher than releases from Fukushima. The most cited source term for Chernobyl is 5300 PBq (excluding noble gases) while a review of published studies of Fukushima carried out by the authors above allow an estimate for the total atmospheric source term of 520 (a range of 340–800) PBq. Monitoring of air, soil and water for radionuclides after the respective accidents indicate that the environmental impact of Chernobyl is likely to be much greater than the Fukushima accident. The post is relatively information dense as I have provided data tables for those who are interested in the estimates and the peer-reviewed studies from which they come. Apologies up front to those who find such information tedious. Continue reading Comparing the Environmental Impacts of the Chernobyl and Fukushima Disasters
The purpose of this post is to address an error in a recently published review of current release estimates from the Fukushima Dai-ichi nuclear power plant disaster that began in March 2011. The post is part of an ongoing effort to communicate results of scientific studies aimed at understanding the impact of Fukushima on the health of the North Pacific and residents of the west coast of North America. In a recent review paper published in Progress in Nuclear Energy by Koo and colleagues this July, compiled estimates of atmospheric and ocean releases from Fukushima were presented. Due to an error in interpretation they suggest that direct ocean releases were a factor of 4 greater than atmospheric releases of radiologically significant isotopes like 131-I (~8 day half life) and 137-Cs (~30 year half life). This error inflates release estimates and has been reported on to suggest Fukushima releases exceed Chernobyl’s. Accurate estimates of releases from Fukushima suggest that they are about an order of magnitude less than those from Chernobyl in 1986.
The study of Koo and others (link to a ResearchGate upload) estimated atmospheric releases of 131-I, 137-Cs and the noble gas 133-Xe (half life ~ 5 days) from the Fukushima Dai-ichii nuclear power plant. Their estimates compared with previously published estimates are reported in the following table (Table 2 from paper):
Similar to previous work, for example, they estimate the atmospheric release of 137-Cs from the plant to be 10-50 PBq or somewhere between 3 and 17 kg of the isotope. Given the core inventories of reactors 1-3 this release represents about 4% of the inventory at the time of the meltdowns in March 2011.
The authors make a significant error when they begin their estimate of direct releases from Fukushima to the ocean when they state the following in section 2.2.2. Release from the primary system into the sea:
It is reported that, of the total radioactivity released from the units 1–3 into the environment, more than 80% of it flowed into the sea (Hoeve and Jacobson, 2012 and Christoudias and Lelieveld, 2013), implying that 4 times more radioactivity was released to the sea than to the atmosphere.
Bolds are mine. In stating that 80% of the total radioisotope releases flowed into the sea they fundamentally misinterpret the studies they cite. What the study of Christoudias and Lelieveld (2013), and other studies not referenced here in the diary, actually show and establish is that (quoting from the Christoudias and Lelieveld work):
We calculated that about 80% of the radioactivity from Fukushima which was released to the atmosphere deposited into the Pacific Ocean.
This is a fundamentally different than the interpretation Koo and colleagues use in their study. By wrongly interpreting that atmospheric releases represent 20% of the total release they assume that direct ocean releases are 4 fold greater than the 4% of core inventories (10-50 PBq) or 16% of core inventories of 137-Cs in March 2011. This error greatly increases the estimated total releases from the plant (atmosphere + direct ocean).
It is very likely that this incorrect approach will lead others to conclude that total releases from Fukushima are greater than those from Chernobyl. For example a back of the envelope calculation assuming the 4% of the total core inventory of 137-Cs (760-820 PBq according to the table above) was released to the atmosphere and 16% to the ocean would lead to a total release of ~152-164 PBq. Such a calculation was done by a popular news aggregator and editorial site that has a history of misinterpreting and misinformation the public about Fukushima. This estimate, not surprisingly, is at great odds with existing estimates based on measurements and modeling.
Best estimates to date suggest that:
1. atmospheric releases of 137-Cs were 19.4 +- 3.0 PBq through the end of March 2011
2. direct ocean discharge of 137-Cs to the Pacific in addition to atmospheric deposition are in the range 2.3 to 26.9 PBq
3. About 19.5 +- 5% of releases were deposited to land while about 80% ended up in the Pacific Ocean
A report reviewing the most recent peer reviewed studies which reaches these conclusions was summarized in a post here.
Releases of isotopes that represent potential radiological health threats given their respective total activities and/or their significant half lives (e.g. 131-I and 137-Cs) were about an order of magnitude (factor of 10 times) lower than the releases from the Chernobyl disaster in 1986 (see reports here and here for example). More and more observations are being made globally by the international scientific community which will help to improve source term and release estimates. I will report on these studies as the data becomes available.
I have contacted the authors to bring their attention to this problem with their study.
The purpose of this post is to summarize results from various studies that monitored the timing of arrival and activity of radioactive iodine falling from the atmosphere in western North America following the Fukushima disaster in 2011. Determining the activity of 131-I (half life ~8 day) in rain and seaweed, which serves as a biological monitor, is important because of the isotopes short half life and its propensity to concentrate in the human body, specifically the thyroid gland. This combination of rapid energy release and biological tissue targeting can represent a potential radiological health risk. Measurements of 131-I in rain collected in the San Francisco Bay area and southern British Columbia, Canada indicate that the atmospheric transport brought contaminated air from Fukushima to North America by March 18 roughly 1 week after the earthquake and tsunami. Depending on location, activities of 131-I in rain peaked between March 20-24 and were observed to decrease to background levels in the first week of April. Peak activities in seaweed occurred later on March 28 and were observed to return to background levels in mid-May. Maximum 131-I activities in rain resulting from Fukushima were a factor of 10 lower for rainwater and a factor of 40-80 lower for seaweed compared to similar measurements made following the Chernobyl disaster in 1986. Observed 131-I activities suggest that the upper limit of radiation dose to the public resulting from Fukushima was similarly an order of magnitude lower than that from Chernobyl suggesting that the short and long-term impact on human health in western North America is expected to be minor.
How Scientists Talk About Radioactivity
Scientists use a variety of units to measure radioactivity. A commonly used unit is the Becquerel (Bq for short) which represents an amount of radioactive material where one atom decays per second and has units of inverse time (per second). Another unit commonly used is disintegrations per minute (dpm) where the number of atoms undergoing radioactive decay in one minute are counted (so 1 Bq = 60 dpm).
131-Iodine Releases From Fukushima
As a result of the great eastern Japan earthquake and tsunami on March 11, 2011 three of six reactors melted down resulting in releases of radionuclides from the Fukushima Dai-ichi nuclear power plant to the environment. In terms of absolute activity released and potential for causing harm to organisms, 131-I (half life ~8 day) was one of the most significant. Given its volatility and the damage to reactor fuel rods large releases of ~2000 PBq (petaBequerel = 10^15 Bq) to the atmosphere and ocean occurred in the weeks following the disaster. Prevailing atmospheric circulation brought this plume of contaminated air to North America within one half life of 131-I where rain and fallout of aerosol particles delivered 131-I and other isotopes to land and coastal waters.
Monitoring of 131-I in the environment is important because as an essential nutrient when concentrated in the human body in the thyroid gland the decay of the isotope can cause damage resulting in negative health impacts like cancer. After the triple meltdowns stations in North America began monitoring the activities of released radionuclides in air, rainwater and seaweed to determine the risk to public health.
Rainwater activities of 131-I
Rainwater in the San Francisco Bay area was monitored and the results published in the open-access, peer reviewed journal PLOSOne in 2011 by Norman and co-workers. Measurements were made for the period March 16-26, 2011 on rainwater collected in Oakland, Berkeley hills and Albany, CA. Results of these measurements are summarized in the figure below:
Similar activities and timing of arrival of the atmospheric plume of 131-I were observed in southern British Columbia, Canada. Chester and colleagues published this work in the peer-reviewed Journal of Environmental Radioactivity in 2013. Maximum activity of 131-I of 5.8 Bq/L was detected in the Vancouver, BC area nine days after the Fukushima disaster on March 20, 2011. The activity of 131-I returned to background by early April 2011. Results are summarized in the figure below.
131-I levels in seaweed are known to correlate strongly with levels in rain and seaweeds are useful monitors for human made radionuclides in the environment as the concentrate isotopes from their surroundings and are geographically widespread. Seaweeds were collected along the Canadian west coast by Chester and colleagues and analyzed for 131-I following the Fukushima disaster. Results of these analyses are summarized in the figure below:
Maximum 131-I was detected in BC seaweed on March 22 near Vancouver and on March 28 off the west coast of Vancouver Island some 250 km to the west of the city. Peak activities were 130 and 67 Bq/kg respectively. By mid-May activities had returned to background activities in the seaweed.
Summary: Health Implications and Comparison to Chernobyl
The maximum levels of 131-I in rainwater can be compared to limits allowed in drinking water in both the USA and Canada. Maximum activities in rain were in the range ~6-16 Bq/L. For example the maximum allowable concentration (MAC) or activity allowed in Canadian drinking water is 6 Bq/L. The MAC for 131-I is calculated using a reference dose level of 0.1 mSv (where mSv = 0.001 Sv) for 1 year’s consumption of drinking water, assuming a consumption of 2 L/day at the MAC. This compares to an effective dose received by someone living in Vancouver of about 1.3 mSv. Given the short half-life of 131-I of ~8 days the actual dose attributable to Fukushima fallout in precipitation is likely to be much lower than the 0.1 mSv upper limit on which the drinking water MAC is based.
Indeed, comparing measurements in the studies above to measurements made on the west coast of North America in the aftermath of the Chernobyl disaster in 1986 suggests that doses experienced by the public post Fukushima fallout were an order of magnitude lower. Measurements in the same species of seaweed in 1986 (behind paywall) are compared to the measurements of Chester and others here:
The calculated dose estimates to Canadians following the Chernobyl disaster were on the order of ~1 micoSievert (0.000001 Sv) (What is a Sievert, Sv?) while the peak 131-I activities present in rainwater after Fukushima suggest an upper dose of 0.1 microSv which is an order of magnitude lower dose.
These data have led health professionals in the US and Canada to expect that short-term and longer-term impact of Fukushima on public and environmental health to be very small compared to other radiological impacts from natural and legacy sources of radiation.
A Twitter user interested in the InFORM network asked the following question today:
“…do you have a public list of all products in Fukushima emissions?”
It is important to realize that release estimates are not fully constrained and that estimates improve as more and more measurements are made in the environment. A useful, peer-reviewed, open access study that summarizes release estimates is Povinec et al. (2013) published in the journal Biogeosciences. Table 1 from this study is shown below and can be maximized by clicking the image:
Full references for the table are available in the Povinec paper by following the link given above. Estimates of plutonium released by the Fukushima disaster are provided in this summary with peer-reviewed studies linked to within the document. Measurements of Pu in soil, air and water suggest that 1 to 1,000,000 fold less Pu was released from Fukushima compared to 137-Cs emissions given above.
Below we present near comprehensive inventories of radionuclides in the reactor cores of Units 1,2 and 3 and in the spent fuel pools of Units 1,2,3 and 4 at the time of the accident as reported by:
Nishihara, K., H. Iwamoto, K. Suyama. 2012. Estimation of Fuel Compositions in Fukushima-Daiichi Nuclear Power Plant. Japan Atomic Energy Agency, Tokai. Japan.
The following information has been modified from:
Tables in the pdf file given just below contain information on the radionuclide, grams of radionuclide present, half-life of element, activity in Bq and ratio to 137-Cs in the core or spent fuel respectively.