Thursday, March 22, 2012

Wet Depostion of Fission Products in North America Released by Fukushima

We are all familiar with the Fukushima Dachaii Nuclear Power Plant (FDNPP) disaster that was the result of the earthquake and subsequent tsunami on March 12, 2011. We have read countless reports on the status of the power plant and the spread of radionuclides as a result of the explosions in the reactors. A quick recap is necessary though. On March 11, 2011 a magnitude 9.0 earthquake struck just off the Pacific coast of Japan triggering a massive tsunami which destroyed everything in its path. The tsunami caused massive flooding which killed the backup power at the plant and caused the reactor cooling systems to fail. The failure of the cooling system led to a meltdown and several hydrogen explosions which released radionuclides into the environment. The three nuclides of greatest concern are iodine-131 (half-life: 8 days), cesium-134 (half-life: 2.1 yrs) and cesium-137 (half-life: 30.2 yrs). Other radionuclides, such as iodine-129, were also released, however they have not been the subject of much study...yet.

The big fear in North America was: will these three radionuclides, all of which can be absorbed into our bodies, reach us, and will there be enough present to cause harm? A recent article by a group from the USGS and the University of Illinois tackles this problem and presents one of the first summaries of radiation monitoring data that I have seen to date. The article presents data from several sources but the primary ones are the US National Atmospheric Deposition Program (NADP), which primarily samples rains and chemical particulates in rain. The other main source was the US EPA National Air and Radiation Environmental Laboratory (RadNet) which operates radiation air monitors across the US. Some samples from the Canadian Air and Precipitation Monitoring Network (CAPMoN) were also used. A comparison to the Chernobyl disaster is also included.

131I - Background and Results

Iodine-131 is a radioactive isotope of iodine that has an eight day half life. It is definitely a dangerous isotope to humans and releases a substantial amount of radiation in the form of both beta and gamma rays. The specific activity of 131I is 4.81x10^15 Bq/g. Its risk to human health is further exacerbated by the fact that our bodies love to absorb iodine from the environment around us and that iodine is an extremely mobile element. In fact, iodine can exist as a gas, a solid or dissolved in water and it can transfer between these three states very readily depending on the conditions of the environment around it. Furthermore, iodine is attracted to organic molecules making it even more mobile. It is also en essential nutrient of life. In fact, all salt has iodine added to it so that we do not become iodine deficient. In the case of salt, however, the isotope added is iodine-127 and is not radioactive. If our bodies absorb too much 131I we are being subjected to a high dose of radiation from within us. Iodine concentrates in the thyroid gland so exposure to 131I will greatly increase the risk of cancer particularly thyroid cancer. The maximum dose of 131I that is allowed is 4 Bequerels per litre (Bq/L) for drinking water and 3.7 Bq/L for air according to the US-EPA. According to the Canadian Nuclear Safety Commission (CNSC) the maximum acceptable concentration for drinking water is 6 Bq/L and is 0.2 Bq/m^3 for air.

As a result of the releases from Fukushima both RadNet and the NADP sampling network showed hits of 131I in their precipitation and bulk air samples. The data presented for the detected levels for 131I ranged from 0.1-40 Bq/L for all North American rain samples and from 60-5100 Bq/m^2 for deposition from air. Obviously the highest value for water is ten times the allowable limit of 131I in drinking water. It is important to note that due to the short half life of 131I many of the samples were measured after too much time had passed to detect 131I, causing it to appear less ubiquitous than perhaps it was. Also, the paper points out that concentrations of 131I decreased from west to east due to decay, deposition of the 131I and dilution of the plume over time.  This suggests that geographical distance played an important role, as the sites that were further away from Fukushima had smaller deposition of radioactive material.  The comparison of this data to that from sites around the globe after the Chernobyl disaster shows that levels measured in North America were within the range of data from the Fukushima fallout. However, samples collected from sites closer to the Chernobyl disaster are far higher for 131I in air.

134Cs - Background and Results

Cesium-134 is another isotope that was released from Fukushima. It has a 2.1 year half life and also is quite radioactive and has a specific activity of 4.81x10^13 Bq/g. It emits both beta and gamma radiation. Cesium differs from iodine primarily by its different behaviour in the environment, as it is much less mobile and often sticks to soils. It can still be found dissolved in water and stuck to particles carried in the atmosphere. Cesium can enter our bodies in food, drinking water or through inhalation, giving it the triple threat of exposure pathways. Furthermore, once in our bodies cesium behaves like potassium, which is not surprising given they are both alkali metals. Finally, once in our bodies cesium tends to concentrate in the muscles. The silver lining here however, is that once cesium is in our bodies it does not reside there for long. It has a biological half life of ~110 days meaning half of what is absorbed will be secreted within 110 days. The maximum acceptable concentrations for 134Cs in water are eluding me on both the EPA and CNSC websites, but if anyone does know them or stumbles across them please comment below. I was successful in finding a maximum allowable concentration in air though from the CNSC, which is 0.2 Bq/m^3.

The results from the measurement of 134Cs in water range from 0.01-2.0 Bq/L for both RadNet and NADP samples and for deposition from air the values range between 0.47-180 Bq/m^2.

137Cs - Background and Results

The overall behaviour, risks, and absorption pathways of cesium-137 are the same as 134Cs. The main difference is that 137Cs has a much longer half life of 30 years and emits only beta radiation. It has a specific activity of 3.26x10^12 Bq/g, slightly less than that of 134Cs, which makes sense given the slightly longer half life. However, 137Cs decays into Barium-137 which emits both beta and gamma radiation and has a specific activity of 2.0x10^19, making it extremely dangerous due to its very short half life of 2.6 minutes. The CNSC drinking water limit for cesium-137 is 10 Bq/L and the EPA limit is still impossible to find (this is really annoying!). The CNSC also have a limit for 137Cs in air which is 2.6 Bq/m^3.

The concentrations of 137Cs that were measured by the radiation survey fell between 0.03-1.4 Bq/L for both RadNet and NADP samples in water and 0.78-240 Bq/m^2 for deposition from air. Once again the levels for both cesium isotopes are far less than those recorded after Chernobyl.


Overall the conclusions of the paper were that fairly high levels of radionuclides were transported from Fukushima. However, these levels are far less than those generated in the US during nuclear weapons testing and the radionuclides released from Chernobyl. This paper also demonstrates the sensitivity and effectiveness of radionuclide monitoring in North America and is proof that a rapid response to a nuclear disaster can be executed. I would add that despite some exceedance of maximum allowable standards by the measured values of radionuclides there was no health threat to people in North America from Fukushima. I would really encourage people to comment and share their opinions on this paper and the impact of the Fukushima disaster.


CNSC Standards:


Iodine Fact Sheet:

Cesium Fact Sheet:

Wetherbee, G. A., Gay, D. A., Debey, T. M., Lehmann, C. M. B., & Nilles, M. A. (2012). Wet Depostion of Fission Product Isotopes to North America Released from the Fukushima Dai-ichi Incident, March 2011 Environmental Science and Technology

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