Nuclear

Tracking the Fallout and Fate of Fukushima Iodine-129 in Rain and Groundwater

Matt Herod December 21, 2015

A recently published paper (by myself and colleagues from uOttawa and Environment Canada) investigates the environmental fate of the long lived radioisotope of iodine, 129I, which was released by the Fukushima-Daichii Nuclear Accident (FDNA). Within 6 days of the FDNA 129I concentrations in Vancouver precipitation increased 5-15 times above pre-Fukushima concentrations and then rapidly returned to background. The concentrations of 129I reached were never remotely close to being dangerous, however they were sufficient to distinguish the impact of the FDNA on the region.

Subsequent sampling of groundwater revealed slight increases in 129I concentration that were coincident with the expected recharge times. This suggests that a small fraction of the FDNA-derived 129I may have been transported into local groundwater after infiltrating through soils.

Sample map showing location of all three precipitation sampling locations as well as the location of both wells used for groundwater sampling. The surface expression of the Abbotsford-Sumas Aquifer is shaded.

What is iodine-129 and where does it come from?

Iodine-129 is the longest lived isotope of iodine with a half-life of 15.7 million years. It is radioactive and occurs everywhere throughout the environment. It is produced in three ways. The first two are natural and the third is by the nuclear industry.

The natural production of 129I occurs in the atmosphere and in soil/rocks. The atmospheric production happens when a cosmic ray proton hits a xenon-129 nucleus and removes a neutron, replacing it and creating an iodine-129 nucleus. The production in soil and rocks happens when a uranium-238 nucleus spontaneously fissions and one of the halves it releases has a mass of 129 ala, iodine-129.

The anthropogenic production occurs because when uranium fissions in a nuclear reactor sometimes one of the parts is 129I. This anthropogenic production is by far the largest source in the environment as substantial amounts have been released by nuclear fuel reprocessing. This 129I that has been released can trace a host of environmental processes and inform us about what happens to 129I or the much more dangerous, 131I. The current levels of 129I are much too low to pose a health threat to humans or the environment, but do allow 129I to be used as an environmental tracer.

129I from Fukushima is present in Vancouver, B.C. rain

The purpose of this research was to discover the fate of 129I in the released by Fukushima, which although a small amount, was isolated in time and space. We measured the 129I deposition in rain and its subsequent movement though soils and see if it reached groundwater. The results tell us about the impact of Fukushima, how 129I moves, where it is attenuated, and how quickly contaminants in this aquifer move from the ground surface to the water table. This knowledge can then be applied to understand 129I behavior in other settings such as nuclear waste repositories and watersheds or it can be used to learn about the behavior of other types of contaminant in this aquifer and how vulnerable it is to contamination.

The results in rain show an increase in 129I concentrations of up to 220 million atoms/L*. This increase was seen ~6-10 days after the emission from Fukushima began and are 5-15 times higher than rain samples collected before Fukushima. Following this increase 129I concentrations returned to background with a few weeks. This agrees with other studies monitoring the fallout of Fukushima derived radioisotopes [Wetherbee et al., 2012]. Furthermore, atmospheric back trajectory modelling shows trajectories for air parcels arriving in Vancouver from over the Pacific ocean and Japan.

Variation in the concentration of 129I and the 129I/127I ratio in precipitation from Vancouver, Saturna Island and NADP site WA19 over time. The time range that each NADP sample integrates is displayed using horizontal error bars. 1σ error is contained within data points if not visible. The dashed vertical line shows the date of the FDNA relative to samples.

We also calculated the mass flux of 129I from Fukushima. That is the actual quantity of 129I that was deposited on the region in grams, or in this case in atoms/m2. This was calculated by simply multiplying the concentration of 129I in rain by the amount of rain that fell. We found that only about 15% of the annual 129I deposition in the Vancouver region could be directly linked to Fukushima affected rain events. The total mass deposited by Fukushima was ~0.0000000000002 (2 x 10^-13) grams. This is a negligibly small quantity with respect to radioactive risk.

Despite the fact that the deposition of 129I from Fukushima was infinitesimally small it was still measurable. Therefore, the question became where did it go and can we learn about local groundwater resources using 129I as a tracer?

129I variation in groundwater may be due to Fukushima

The results in groundwater show very small 129I concentration increases. Two different wells were sampled. The first had a recharge time, which is the time it takes for water to move from the water table to the well screen, where it is sampled, of 0.9 years and the second had a recharge time of 1.2 years [Wassenaar et al., 2006]. The exact time it takes for water and dissolved contaminants to travel through the unsaturated zone was unknown. However, the sediments of this aquifer are very coarse and are known for their ability to rapidly transport contaminants, such as nitrate [Chesnaux and Allen, 2007]. Therefore, if we were going to see 129I from Fukushima this was an ideal location.

The increases in groundwater 129I concentrations were seen in two different wells (ABB03 and PB20) located close to one another. The two wells also had slightly different recharge times. The first was 0.9 years and the second was 1.2 years. The 129I anomaly in the first well occurred at 0.9 years and in the second well at 1.2 years. These 129I anomalies, which occurred exactly when the recharge age predicted they would, suggests that some of the 129I deposited by Fukushima was reaching the wells and causing these increases.

Temporal variation in the concentration of 129I in groundwater in ABB03 and PB20. The solid vertical line shows the date of the Fukushima accident and the dashed horizontal line shows the median of each dataset respectively. The 3H/3He ages from [Wassenaar et al., 2006] of groundwater in each well and their uncertainty is pictured as the solid arrow which is aligned with the 129I anomaly possibly caused by the FDNA. The dashed arrow covers a 40 day (0.11 year) time span and represents a possible vadose zone transport time.

In order to verify if it was possible for 129I to travel from the ground surface to the water table in the time required to produce the variations observed we modelled its transport time and attenuation through the unsaturated zone.

The time it took for 129I to reach the water table in the model was then added to the previously dated recharge time to get an estimate for how long it might take 129I from Fukushima to reach the wells we sampled. The results show that it is indeed possible for 129I deposited in rain to infiltrate through the unsaturated zone and reach the wells in time for us to detect it. However, this rapid transport assumes that certain flow paths exist to rapidly conduct 129I due to the heterogeneous lithology of the unsaturated zone. There is evidence of such flow paths [McArthur et al., 2010].

To summarize,

Within a week of the FDNPP accident elevated 129I concentrations were observed in precipitation. This agrees very well with work on other radionuclides in air filters and rain.

129I concentrations in rain returned to background within a few weeks. However, discrete pulses of elevated 129I occurred for another several months.

Elevated 129I concentrations were measured in two wells and corresponded with the expected recharge times indicating that 129I from Fukushima can be traced into groundwater.

Vadose zone modeling has shown that 129I can be rapidly transported to the water table and reach the well screen in accordance with groundwater ages.

We propose 129I transport is enhanced by preferential dispersion of 129I that exists due to the heterogeneous nature of the vadose zone.

This results in variability in groundwater 129I concentrations that preserve the variability in the input of 129I via washout with some dampening of the signal due to attenuation and dilution.

Conceptual model showing the possible transport pathways of Fukushima derived 129I which was deposited via precipitation. A fraction of this 129I was rapidly transported through a heterogeneous vadose zone via preferential flowpaths to groundwater where minor 129I variation was detected. The remainder was retarded or attenuated in the vadose zone during transport.

Thanks for reading, if you have any questions or concerns please leave a comment or send me an email to discuss further!

*Note: 100 million atoms/L of 129I is equivalent to an activity of 0.00000014 (1.4 x 10^-7) Bq/L. These quantities are extremely low level and only the most sensitive analytical methods in the world can detect them. This amount of radioactivity is several orders of magnitude lower than the natural background radiation produced by naturally occurring radionuclides in soil and the atmosphere. For more on naturally occurring radioactivity see here. Even a clean rainfall has about 1 Bq/L of tritium (radioactive hydrogen), which remains from atmospheric weapons testing in the 1960’s

Access the full paper here: http://onlinelibrary.wiley.com/doi/10.1002/2015WR017325/abstract

References

Chesnaux, R., and D. M. Allen (2007), Simulating Nitrate Leaching Profiles in a Highly Permeable Vadose Zone, Environ. Model. Assess., 13(4), 527–539, doi:10.1007/s10666-007-9116-4.

McArthur, S. A. Q., D. M. Allen, and R. D. Luzitano (2010), Resolving scales of aquifer heterogeneity using ground penetrating radar and borehole geophysical logging, Environ. Earth Sci., 63(3), 581–593, doi:10.1007/s12665-010-0726-9.

Wassenaar, L. I., M. J. Hendry, and N. Harrington (2006), Decadal geochemical and isotopic trends for nitrate in a transboundary aquifer and implications for agricultural beneficial management practices., Environ. Sci. Technol., 40(15), 4626–32.

Wetherbee, G. A., D. A. Gay, T. M. Debey, C. M. B. Lehmann, and M. A. Nilles (2012), Wet Deposition of Fission-Product Isotopes to North America from the Fukushima Dai-ichi Incident, March 2011, Environ. Sci. Technol., 46(5), 2574–2582.

Photo of the Week #49

Matt Herod October 19, 2015

This week’s photo is from my personal research and shows a precipitate that I generated in the lab one day of AgI (silver iodide) for analysis of 129I by accelerator mass spectrometry.

I felt though that I wanted to verify the purity of the AgI so I quickly threw in on our scanning electron microscope to a) check the chemistry and b) take a picture. The image below shows an amalgam of AgI crystals with a scale of 20 microns at 600 times magnification.

By the way, to any fellow Canadians reading this post. Please go vote today and be a part of our democracy!!

ATTA and the Curious Case of Krypton-81

Matt Herod September 14, 2015

Ok, so I took some license with the title. This isn’t really a curious case and neither Krypton-81 nor ATTA are actually people. In fact, Krypton-81 (81Kr) is a radioisotope of the noble gas krypton and ATTA, which stands for atom trap trace analysis, is the revolutionary technique that has made its analysis possible. I recently heard about developments with ATTA at the IAEA Isotope Hydrology Symposium and have been doing some reading about the method and its revolutionary application to the dating of both young and ancient groundwater.

Figure 1. Dr. Z-T Lu working on the ATTA system at Argonne National Labs. Used with permission.

81Kr has long been a bit of a dangling carrot for groundwater dating people like myself. 81Kr is a long lived radioisotope of Kr (half-life: 229,000 years) that is produced by cosmic ray interaction in the atmosphere with other krypton isotopes. This production results in about 5 atoms of 81Kr for every 10^13 atoms of the other Kr isotopes. This 81Kr then settles to the earth surface and is incorporated into groundwater recharge and can then used to date groundwater from 150 thousand to 1.5 million years old. The way this works is that once water reaches the water table no new krypton is added and the clock starts ticking as the 81Kr decays away. In order to use this method we assume that the initial concentration in the recharge is in equilibrium with the concentration of 81Kr in the atmosphere, which is well mixed. ATTA then measures the amount of 81Kr that is left in the water sample compared to the other Kr isotopes and an age can be calculated from the difference between this ratio and the intial ratio.

ATTA can also be used for the short-lived isotope krypton-85 (half-life: 10.8 years). 85Kr is produced by fission in nuclear reactors and is released during nuclear fuel reprocessing. The short half life of 85Kr makes it useful for dating recently recharged groundwater from 1 to 40 years old.

Figure 2. Dating ranges of 85Kr, 39Ar, 81Kr and other established radioisotope tracers. (Source). Used with permission.

The reason krypton is such a useful tracer for groundwater dating is that as a noble gas the interaction of Kr with soils, rocks and the biosphere is minimal whereas other tracers such as 36Cl, 14C or 3H are often subject to retardation during transport or inputs from multiple sources which makes extensive corrections necessary or renders them completely unusable for dating. Furthermore, very few reliable tracers exist in the range that Kr isotopes cover making them extremely useful. One isotope that I haven’t mentioned as much is argon-39, which can be used to date water from 50-1000 years old, is also a noble gas, and can also be measured with ATTA.

Measurements of krypton can also be used for dating of ancient ice cores as well. Atmospheric gases including Kr are trapped in air bubbles in the ice and therefore, using the same method as groundwater dating, an absolute age for an ice core can be obtained. There are several other applications for Kr dating as well such as dating of deep crustal fluids and brines.

Figure 3. Sampling ice cores for Kr analysis by ATTA. Photo: V. Petrenko. Used with permission.

The development of atom trap trace analysis was first reported in Science in 1999 and since then has undergone several substantial improvements primarily aimed at reducing the required sample size required for an analysis of Kr. ATTA (Figure 4) works by trapping Kr atoms with a laser which causes a slight and temporary change in their atomic structure which lasts for about 40 seconds. During this period the Kr atoms in the laser beam are focussed and slowed and then trapped in an MOT (magneto optical trap) where they are held in place for an average of 1.8 seconds. Once the Kr atom is in the MOT it fluoresces as it returns to its original state. This fluorescence is detected by a camera which is sensitive enough to detect the emission from a single atom (Figure 5)!

Figure 4. Schematic layout of the ATTA-3 apparatus. (Source). Used with permission.

One of the key features of ATTA is that this laser induced fluorescence within the MOT occurs uniquely for every isotope as the laser frequency is tuned specifically! This means that only atoms of of the desired Kr isotope are trapped. Furthermore, this means that ATTA is completely immune to interference from other elements, isotopes, isobars, or molecules. In essence nothing can confuse the detection of the 81Kr atom once it fluoresces and therefore there is no background of spurious detections that need to be corrected for. Among low-level analytical techniques this is unique and a really big deal! As a user of AMS, which is another low level analytical method that does suffer from these issues, this is statement is an eye-catcher.

Figure 5. A CCD image showing the response of an atom in the MOT. Used with permission.

Since its invention ATTA has evolved considerably. We are now on the 3rd iteration of ATTA and significant improvements have been made that make ATTA much more practical for routine use. Specifically, the amount of sample required for an analysis has been reduced drastically. The first version of ATTA could only be used for atmospheric measurements as the quantity of Kr needed was too large to be extracted from water. ATTA-2 required ~1000 kg of water to extract 50uL of Kr gas. Now, ATTA-3 only requires 5-10uL of Kr which can be obtained from only 100-200 kg of water or 40-80 kg of ice. This advancement means that ATTA is now usable for groundwater dating applications never before possible. This has been demonstrated by the use of ATTA to date groundwater in Egypt to around 500,000 years old and even older water in Brazil up to 800,000 years. Other dating methods have confirmed that ATTA measurements are accurate.

Now that the sample sizes required for an 81Kr or 85Kr analysis have been lowered so dramatically the method is even more useful to the geoscience community. One of the messages from Dr. Lu’s talk at the IAEA meeting was that this technique is open for business and the geoscience community is strongly encouraged to reach out for collaboration and discussion. Furthermore, it may also be possible to use ATTA to measure argon-39, calcium-41 and potentially lead-205, strontium-90 and cesium-137,135 at extremely low levels.

Note: During the writing of this blog I corresponded with Dr. Z-T Lu, one of the creators of ATTA. I would like to thank him for allowing me to use his personal photos in this post. Dr. Lu is now establishing a radiokrypton dating centre at the University of Science and Technology of China.

References

Lu Z-T, Schlosser P, Smethie WM, Sturchio NC, Fischer TP, Kennedy BM, et al. Tracer applications of noble gas radionuclides in the geosciences. Earth-Science Rev. 2014;138:196–214.

Chen CY. Ultrasensitive Isotope Trace Analyses with a Magneto-Optical Trap. Science (80-). 1999;286(5442):1139–41.

Du X, Purtschert R, Bailey K, Lehmann BE, Lorenzo R, Lu Z-T, et al. A New Method of Measuring 81Kr and 85Kr Abundances in Environmental Samples. Geophys Res Lett. 2003;30(20):2068. Available from: http://arxiv.org/abs/physics/0311118

Aggarwal PK, Matsumoto T, Sturchio NC, Chang HK, Gastmans D, Araguas-Araguas LJ, et al. Continental degassing of 4He by surficial discharge of deep groundwater. Nat Geosci. 2014;8.

Lu Z-T. Atom Trap, Krypton-81, and Saharan Water. Nucl Phys News. 2008;18(2):24–7.

GeoPoll: What should we do with radioactive waste?

Matt Herod February 2, 2015

Cherrenekoff radiation is a pretty way to demonstrate radiation. (Photo: Matt Herod)

I don’t think it is any secret that the world is facing an imminent energy crisis. We are trying to generate more power than ever before, but at the same time, we now realize we have to do it in a sustainable way that does not harm the environment or exacerbate any existing issues such as climate change. The problem is these two goals are often mutually exclusive. Most of our power generation methods have environmental implications or are simply at the early technology stage and cannot be relied upon the produce the juice that we all need. Thus, we’re stuck between a rock and a hard place when it comes to generating power. Therefore, the question becomes: we need more power, but at what cost? What are we willing to compromise to generate the energy? The answer is generally something in the environment protection aspect. We now accept that in generating the power we need we will have to affect the environment either to obtain the natural resources required or use huge tracts of land. The trick is how do we best manage the risks and impact of power generation?

The Bruce Nuclear Generating Station in Ontario, Canada. The largest nuclear plant in the world. (Wikipedia Commons – User: Cszmurlo)

This post isn’t about energy policy though. Instead, I would like to write about how we manage the risks of nuclear power, specifically the waste? One of the big knocks against nuclear energy is what do we do with the potentially dangerous and long lived waste that it generates? Over the years a huge range of ideas have been proposed, some silly, and some practical. In this post I intend to explore some of these proposals and see which ones make the most sense given that this is a problem the world is facing now and is going to face for the foreseeable future, especially, if nuclear energy generation expands. One important point is that most radioactive waste is classified as low/intermediate level. This is essentially everything that is not heat generating fuel. The high level waste is the left over fuel. For all intents and purposes the disposal options aren’t different it’s just that high level waste seems scarier and has the potential to cause more environmental or human health damage if it escapes. Either way the waste, no matter what variety, has got to stay isolated essentially forever.

What are the key features that every storage solution must have?

1. Must be immobile/isolated from the environment and people for at least 100,000 to 1,000,000 years.

The reason for this requirement is pretty obvious. Nuclear waste is dangerous and therefore, it must be isolated from the environment for long periods of time. The reason it must be completely isolated for so long is because the half lives of the isotopes within the waste vary greatly. Thus even though the most highly radioactive isotopes decay to almost nothing within the first 1,000 years there are others, like iodine-129, which have long half lives and remain radioactive for millions of years.

This shows the relative activity of each major isotope in high level radioactive waste and how it decays overs time. The top red line shows the total radiation over time. This way you can easily see which isotopes are contributing the most to the total radioactivity of the waste over long time scales.

2. Must be potentially recoverable.

The reason that the waste must be potentially recoverable is two-fold. Should there be a problem with the repository it must be possible to remove the waste and either fix the problem or relocate it. The second reason is that as nuclear technologies advance it may become feasible to re-use the waste for energy generation or some other purpose. Therefore, the waste must remain isolated yet accessible during the lifetime of the repository.

3. Must be able to withstand every possible circumstance for incursion.

This one is clearly a key characteristic of a repository. This waste can be used for potentially nefarious reasons and thus it cannot be simple to access. Furthermore, given that the lifetime of the repository is far greater than human civilization has already existed it must also be able to withstand unintentional incursions by future generations. For example, future generations may not use existing languages. Therefore, it must be communicated unmistakably that you should not dig here.

Deep Geologic Repositories (DGR’s)

The most popular of all solutions for long term disposal of radioactive was these day has to be deep geologic repositories. The idea behind this solution is to bury the waste hundreds of metres underground in an engineered space in a geologically impermeable and stable location. Almost every nation around the world with a nuclear program is investigating this means of disposal. This solution, while certainly expensive, meets the above criteria on all counts. The waste will be isolated for geological time scales. The site evaluation will use a wide variety of methods to confirm this. The waste will be recoverable as it is not that deeply buried and is still on land. In fact, some proposals I have seen for these repositories include an underground lab as well so people will actually be working at the repository site. Finally, the hundreds of metres of solid rock and engineered barriers will withstand attempts to access the waste. Most of these site investigations are looking at igneous rock, however, there is a strong case to be made that sedimentary sites may even be better. The current Canadian site under evaluation for low and intermediate level waste is in sedimentary rock and shows excellent potential as an environment that has been isolated for the last 400 million years.

A conceptual diagram for the proposed DGR in southern Ontario – Source: NWMO (2011), Descriptive Geosphere Site Model

Dilution in the ocean/Sub seafloor burial

This option is likely not going to be the most popular in my poll below, but in order to give equal treatment to all of the proposals I feel that it should be mentioned. The basics behind this idea are best described by the old and oft-repeated quote “dilution is the solution to pollution”. In essence, the idea is simple: drop the waste into the deep ocean where it can slowly disperse, decay and dilute. Obviously for high level waste this is a bad idea and is certainly not popular among the international community. However, while it may seem that this idea is ludicrous there are numerous operating nuclear facilities that currently release radioactive isotopes into the ocean. In fact, the nuclear fuel reprocessing facilities of Sellafield and La Hague are allowed to release a certain amount every year. These radioisotopes are then dispersed in the ocean currents and diluted. They are released at extremely low levels to begin with. However, their presence has afforded oceanographers the opportunity to use these isotopes as tracers of ocean currents.

Burial of the waste beneath the ocean floor has also been proposed as an option. It bears a similarity to deep geologic disposal, however, in a more remote and isolated environment. This slant does have some merit as long as the waste could still be recovered, however, the cost would be extremely high build the repository and recover the waste. Furthermore, this solution still has the problem of potentially releasing radionuclides into the ocean only with the added difficulty that fixing a leak would involve working hundreds of metres underwater. That said a leak of a small amount of radionuclides would be diluted within the ocean and would be unlikely to result in significant contamination unless it was very large.

Deep sea trenches/Subduction zones

This option might seem kind of similar to the sub seafloor burial option since trenches and subduction zones are part of the seafloor. However, the idea with this option is that the waste would eventually get subducted into the mantle and return to whence it came. Basically this idea is kind of like composting the waste, which admittedly seems appealing. However, this idea has all of the drawbacks of sub seafloor disposal and would certainly make the waste unrecoverable…at least once it had started its journey through the crust.

Blast it into space

This solution certainly meets the permanently isolate criteria as well as withstanding accidental incursion. 2 out of 3 ain’t bad, right? However, it does fall down when it comes to waste recovery for obvious reasons. This is a solution, that while appealing on the surface never really gained significant traction. One of the biggest reasons for this is the extremely costly nature of shuttle launches. The other major issue is where do we aim the rocket? Towards another planet? Into the vastness of space? Into the sun? All of these options are fraught with difficulties. Finally, if the launch should fail there is a possibility that the waste could be released into the local environment as well.

Disposal in ice sheets

Disposal of radioactive waste in ice sheets has been considered as well, although never very seriously. Continental ice sheets, such as the Antarctic and Greenland ice sheets, have been stable for many thousands of years. Additionally, as high level wastes are heat generating storage in glaciers would provide in situ cooling. Win-Win right? At least that was the early logic on the proposal when it came out the 70’s. In practice, this is clearly a pretty bad idea, especially since as the climate warms the long term stability of continental ice sheets is not as certain as it was when this idea was proposed. Furthermore, the heat generating capability of high level waste does not go away quickly, as the figure above shows, meaning that the waste could potentially melt its way out of the ice sheet or mix with the meltwater that was produced spreading radioisotopes. Thus, ensuring the waste’s stability for millions of years into the future is uncertain at best with this storage solution.

A composite satellite image of Antarctica. Source: Wikimedia Commons

Long term above ground storage

The final choice is store the waste in an engineered facility on the surface of the Earth. Generally, this disposal option is considered temporary or only for low-level radioactive waste. However, it has been investigated as a long term solution as well. The idea is the the waste is placed together in a constructed facility that has been engineered to isolate it from the environment and withstand any attempts at incursion. At the same time the facility would still be easy to access and make waste recovery very simple.One issue with this option is that since the waste is on the Earth surface it is impossible to truly seal it. Thus, future generations would also have to be responsible for maintaining the facility and ensuring the waste remained isolated.

An example of a long term above ground storage facility and its engineered barriers. This facility is planned for low-level waste in Port Hope, Ontario. Source

Ultimately, the legacy of nuclear power generation has to be dealt with in a safe and responsible manner for thousands to millions of years into the future. As a society we recognize this and the ideas outlined above range from the practical to the fantastic. I would like your opinion this question. Which of these solutions do you prefer? If the answer is none of above please comment below and encourage discussion on this topic!

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