North Korea currently has only one publicly known uranium mine—the Pyongsăn uranium mining and milling complex—that serves as a first step in the country’s pathway towards nuclear weapons.

Using a combination of multispectral imagery sourced from the European Space Agency’s Copernicus Sentinel-2 satellite and a review of geological analyses dating back to 1955, a new study from Stanford’s Center for International Security and Cooperation (CISAC) in Jane’s Intelligence Review by geological sciences postdoctoral fellow Sulgiye Park (PhD ’17) and CISAC honors student Federico Derby (BS ’19) looks for evidence of uranium mining in North Korea, going beyond what is currently available in open sources in order to estimate the uranium resources and their locations in North Korea.

The peer-reviewed CISAC study has identified around 18 additional sites in North Korea where the hyperspectral signatures and geological profile combine to suggest the possibility of uranium mining. Nevertheless, CISAC and Jane’s stress that the presence of these ‘hotspots’ does not imply the presence of an active uranium mine or related facility, but rather a site that warrants further analysis.

In this Q&A with Katy Gabel Chui, researchers Sulgiye Park and Federico Derby discuss their work on the project:

How did you land on this project? What made you think to look for more mining sites?

Sulgiye Park (SP) and Federico Derby (FD): Very little is known about the front-end of North Korea’s nuclear fuel cycle, particularly when it comes to the mining and milling processes of uranium production pathway. To date, assessments of this portion of North Korea’s nuclear fuel cycle have been mostly conducted through traditional (electro-optical) satellite imagery observations---the type of imagery that you can access through Google Earth, for instance.

We wanted to get a more complete grasp of North Korea's uranium mining and processing capacity by conducting a multi-disciplinary approach that combines both the visible signatures from multi-spectral satellite imagery and a geological dataset that contains information such as mineralogy and geochemistry. The two individual methods come together at the end to provide information that encapsulates the potential regions likely to host uranium deposits and mines.

What is multispectral imaging? How would it ordinarily be used, and how did you use it for this project?

SP and FD: Traditional electro-optical satellite imagery exploits only three portions of the electromagnetic spectrum; namely, the blue, green and red bands. In general, when using the term “multispectral” within the satellite imagery community, we are usually referring to a satellite system that covers a few to tens of different bands in the electromagnetic spectrum.

Multispectral imagery is used in a wide variety of industries, to measure things like water turbidity, crop healthiness, vegetation quality, etc. For this project, we focused on using spectral fingerprints. Basically, every object – whether it be a mineral, a living thing, water, etc. – has a(n in theory unique) spectral fingerprint. Spectral fingerprints are measured as the intensity of the object’s reflectance of light at a specific wavelength. Varying across wavelengths – hence the importance of having a multispectral system that can give you access to different ranges of the electromagnetic spectrum – you ultimately get a spectral curve that is unique to the item you are studying.

The spectral fingerprints you collect on a specific image can be compared to previously collected fingerprints stored in what is usually termed a spectral library, for classification purposes. Basically, if my spectral curve of a given pixel (or set of pixels) looks super similar to that of gold (for which I obtained a reference spectral curve from a spectral library), then it is probably gold. Obviously, this matching is performed in a more rigorous manner, but you get the idea of how the process works.

In this project, we used the Pyongsan uranium mine in North Korea (arguably the only well-identified uranium mine in the country) as my reference spectral curve. Essentially, using various imaging techniques, we traversed North Korea looking for pixels whose spectral curves are similar to that of the Pyongsan uranium mine. Those are the ‘hotspots’ we identified.

What most surprised you in both your work and your findings?

SP and FD: The fascinating match between the 'hotspots' identified through satellite imagery analysis and the geologic information available in maps and reports. The majority of the 'hotspots' appeared adjacent to the limestone formation from the Ordovician period (circa 445-485 Ma) that are in contact with a specific sedimentary rocks of upper Proterozoic group. Part of the geologic characteristics of the 'hotspots' regions were similar to what had been observed in the Pyongsan (the most well-known) uranium mine of North Korea.

What was most surprising in the work itself? What was difficult in doing the work?

SP and FD: It was surprising to see how much we still don't know about North Korea despite the amount of effort that had been invested. There is no consensus reached regarding the location and the total number of uranium mines in North Korea.

One of the bigger difficulties we had was finding credible geological data and information.

What is the one thing you think someone should take away from your study?

SP and FD: That there are still many unknowns. While our study identified multiple regions with spectral signatures similar to the uranium tailing piles at Pyongsan, verification of uranium presence is still needed.

What are you working on next?

SP: I am still working on using a geologic approach to glean information on the uranium mines of North Korea. The further evaluation aims to identify a qualitative upper limit of uranium ore grade (quality) and quantity pertaining to all the suspected uranium mines in North Korea.

FD: I co-founded a startup focused on developing deep learning models for credit risk analytics (in Latin America). However, I will still keep in touch with my CISAC peers!

This piece originally appeared at Safecast.

Image

Image above: Secondary electron images from Utsunomiya et al. 2019, of CsMPs discovered in atmospheric particles trapped on a Tokyo air filter from March 15, 2011, with major constituent elements displayed. Via Safecast

An interesting paper  was recently published by a team headed by Dr. Satoshi Utsunomiya of Kyushu University on the subject of Fukushima-derived cesium-enriched microparticles (CsMPs). As many readers will know, several researchers have located and analyzed these microparticles, in which the cesium is often bonded within glass-like silicates and therefore generally significantly less soluble than other Cs chemical species in water, though technically not actually “insoluble.” After an accident like Fukushima, it is much more common to find cesium in water-soluble compounds like cesium hydroxide (CsOH), and predictions about how quickly the cesium will be dispersed through the environment, in soil, in watersheds, taken up by plants and animals, etc, are based primarily on this assumption. The discovery of sparingly-soluble Fukushima-derived cesium microparticles, first documented by Adachi et al in 2013, and since then confirmed by many others, has raised a number of questions. How abundant are they? Does their presence increase health risk to humans? How much do they reveal about the process of the accident itself? From the standpoint of researchers the microparticles are very intriguing.

Utsunomiya et al.’s paper is titled “Caesium fallout in Tokyo on 15th March, 2011 is dominated by highly radioactive, caesium-rich microparticles,” and as noted in a recent Scientific American article, it was originally accepted for publication in 2017 by Scientific Reports journal. Weeks before publication, however, Tokyo Metropolitan Industrial Technology Research Institute (TIRI), operated by the Tokyo Metropolitan Government, raised objections with Scientific Reports. However no questions about the quality of the science or the validity of the paper’s findings appear to have been brought forward. This in itself was highly irregular. Two years elapsed without resolution, and in March of this year Scientific Reports took the highly unusual step of withdrawing its offer to publish the paper, despite the lack of confirmed evidence that would warrant it. Utsunomiya and several co-authors decided that the best course of action was to place the study in the public domain by publishing it via arXiv, a highly respected pre-print website. The paper is now open and free to download. 

This study makes a valuable contribution to the body of scientific literature regarding the consequences of the Fukushima disaster in general and CsMPs in particular. I think it was a mistake for Scientific Reports not to publish it two years ago, especially considering the rapid pace of research into these particles and the tremendous interest in them. To summarize the findings briefly, the researchers analyzed air filter samples from March 15, 2011, in Setagaya, Tokyo, when the radioactive plume from Fukushima caused a noticeable peak in airborne radioactivity in the city. The researchers used radiographic imaging (placing the filters on a photographic plate) to identify any highly radioactive spots. Using these images as a guide they were able to isolate seven CsMPs, which they subjected to atomic-scale analysis using high-resolution electron microscopy (HRTEM) to identify their nano-scale structure and chemical composition. Based on these detailed measurements and quantitative analysis, the researchers concluded that 80-89% of the total cesium fallout in Tokyo that day was in the form of highly radioactive microparticles. The second half of the paper is devoted to estimates of how long such particles might be retained in the human lungs if inhaled, based on previous studies that reported the effects of inhalation of non-radioactive atmospheric particles, and some possible physical consequences. The paper is valuable for the quantitative analysis of the Tokyo particles alone, since it is one of few studies that deal with the issue for Tokyo specifically. Research into possible health consequences of the particles, meanwhile, has gained momentum while the paper remained unpublished, using approaches such as stochastic biokinetics, and DNA damage studies.  In a recent paper, Utsunomiya and colleagues produced estimates of the rate of dissolution of the particles inside the human lung, in pure water, and in seawater. A working group at the Japan Health Physics Society has also devoted attention to the issue, noting the need for further study of the risk from intake of these particles, particularly to the lung.  Likewise, others have been studying the particles to learn about the accident progression and possible consequences for decommissioning.

Why did Tokyo Metropolitan Industrial Technology Research Institute object to the paper’s publication? When we first heard that publication of the paper was being held up by Tokyo Metropolitan Government, we thought politically-motivated suppression was a likely explanation. Since then the public has learned that the actual complaint given to Scientific Reports stems from a chain of custody issue of the original air filter samples. We don’t want to speculate further about Tokyo’s motivation, because we have seen no direct evidence yet of political suppression in this case. But based on past occurrences with other government institutions, we would find it plausible. We will let readers know if TIRI responds to our inquiries.

We spoke with Dr. Utsunomiya and co-author Dr. Rodney Ewing recently. I was aware of their co-authorship of several strong papers on CsMPs, including Utsunomiya’s plenary talk at the Goldschmidt Conference in Yokohama in 2016, which I attended. I asked how this new arXiv paper fits in with their other papers, and where they think this research is heading next:

Satoshi Utsunomiya:

Thank you for asking. The Tokyo paper was actually our first paper regarding CsMPs. As I mentioned, the paper was accepted two years ago. There were no previous papers of ours on CsMPs that time. Currently we are working on several topics on CsMPs. I cannot reveal the content yet, as we are thinking about a press release for the next paper. But I think it is important to continue this kind of research, providing some insights for decommissioning at Fukushima Daiichi Nuclear Power Plant.

Azby Brown:

I didn’t realize that this was your first paper on the subject.  How does it relate to the one presented at the Goldschmidt Conference in Yokohama in 2016? “Cesium-Rich Micro-Particles Unveil the Explosive Events in the Fukushima Daiichi Nuclear Power Plant.” Didn’t that paper receive a prize?

SU:

My talk at Goldschmidt briefly covered the story described in the two papers that were accepted for publication at the same time. One was published in Scientific Reports. The other one was not published. There was no prize. It was a plenary talk.

AB:

I see. I recall that it received a lot of attention. Now it makes more sense to me.

Can you tell me a little bit about the specific characteristics and focus of your research, and how it differs from papers like Adachi 2013, Abe 2014, etc? Generally speaking, that is. I’d like to help people understand the different aspects of the field.

SU:

Adachi reported the discovery of CsMPs. Abe demonstrated X-ray absorption analysis on the CsMPs. We focused on the nanotexture inside CsMPs. We are particularly interested in the detailed evidence remaining within the microparticle, which can provide useful information on the development of the chemical reactions during the meltdowns, because it is still difficult to directly analyze the materials inside the reactors. We, for the first time, succeeded in performing isotopic analysis on individual CsMPs. More specifically, the occurrence of uranium can directly tell the story of how the fuel melted. Our research has two directions: one is to understand the environmental impact of CsMPs, and the other is to provide useful information on the debris properties to help decommissioning at FDNPP. We are also interested in the implications for health.

AB:

Can you tell me a little bit about your working relationship? Satoshi went to the US to work in your lab, right Rod? When was that, and what were you working on?

Rod Ewing:

Satoshi and I have known each other since 2000, when he joined my research group as a post-doctoral fellow at the University of Michigan. He was a member of the research group until 2007. We collaborated on a wide range of topics that had to do with radioactive materials, such as the transport of plutonium at the Mayak site in Russia to the identification of uranium phases within C60 cages, so called buckyballs, that were formed and released from coal power plants. Once Satoshi returned to Japan to take his position at Kyushu University, we continued to collaborate, particularly on topics related to Fukushima Daiichi.

AB:

How did you both get interested in CsMPs?

RE:

Once discovered, CsMPs were clearly of high interest. They had not been noted in earlier reactor accidents. Satoshi is a master with the transmission electron microscope – exactly the tool/technique needed to study these particles.

AB:

For people who aren’t familiar with what’s involved in a research experiment like yours, can you describe the overall process? What were the technical challenges?

RE:

I would just emphasize that it is very difficult to find and characterize these particles. Considering the full literature and efforts by others as well as our team – the results are impressive. It is rare to have both the TEM characterization and the isotopic data.

SU:

As Rod mentioned, it is difficult to obtain both TEM and isotopic data from a few micron-sized spots. The isolation of CsMPs from soils is a time consuming process. But to date, many scientists have found and isolated CsMPs. The important thing is what information we can obtain from the analysis of CsMPs. We have been taking various approaches to elucidate the properties, environmental impact, and the role in releasing fissile actinides to the environment.    

As described above, many papers examining various aspects of Fukushima-derived cesium microparticles have been published since they were first identified in 2013. Even so, important aspects remain only partially documented and understood to date. Below is a partial list of relevant publications.

Papers mentioned in this article:

Caesium fallout in Tokyo on 15th March, 2011 is dominated by highly radioactive, caesium-rich microparticles

Utsunomiya, et al., 2019

https://arxiv.org/abs/1906.00212

—————————————————————-

Emission of spherical cesium-bearing particles from an early stage of the Fukushima nuclear accident

Adachi et al., 2013

http://www.nature.com/articles/srep02554

—————————————————————-

Detection of Uranium and Chemical State Analysis of Individual Radioactive Microparticles Emitted from the Fukushima Nuclear Accident Using Multiple Synchrotron Radiation X-ray Analyses

Abe et al., 2014

http://pubs.acs.org/doi/abs/10.1021/ac501998d

—————————————————————-

Dissolution of radioactive, cesium-rich microparticles released from the Fukushima Daiichi Nuclear Power Plant in simulated lung fluid, pure-water, and seawater

Suetake et al., 2019

https://doi.org/10.1016/j.chemosphere.2019.05.248

—————————————————————-

Development of a stochastic biokinetic method and its application to internal dose estimation for insoluble cesium-bearing particles

Manabe & Matsumoto, 2019

https://doi.org/10.1080/00223131.2018.1523756

—————————————————————-

DNA damage induction during localized chronic exposure to an insoluble radioactive microparticle

Matsuya et al., 2019

https://doi.org/10.1038/s41598-019-46874-6

—————————————————————-

Provenance of uranium particulate contained within Fukushima Daiichi Nuclear Power Plant Unit 1 ejecta material

Martin et al., 2019

https://www.nature.com/articles/s41467-019-10937-z

—————————————————————-

Internal doses from radionuclides and their health effects following the Fukushima accident

Ishikawa et al., 2018

https://iopscience.iop.org/article/10.1088/1361-6498/aadb4c

Related papers (by year of publication):

Characteristics Of Spherical Cs-Bearing Particles Collected During The Early Stage Of FDNPP Accident

Igarashi et al., 2014

http://www-pub.iaea.org/iaeameetings/cn224p/Session3/Igarashi.pdf

—————————————————————-

Radioactive Cs in the severely contaminated soils near the Fukushima Daiichi nuclear power plant

Kaneko et al., 2015

https://www.frontiersin.org/articles/10.3389/fenrg.2015.00037

—————————————————————-

First successful isolation of radioactive particles from soil near the Fukushima Daiichi Nuclear Power Plant

Satou et al., 2016

http://www.sciencedirect.com/science/article/pii/S2213305416300340

—————————————————————-

Internal structure of cesium-bearing radioactive microparticles released from Fukushima nuclear power plant

Yamaguchi et al., 2016

http://www.nature.com/articles/srep20548

—————————————————————-

Three-Year Retention Of Radioactive Caesium In The Body Of Tepco Workers Involved In The Fukushima Daiichi Nuclear Power Station Accident

Nakano et al., 2016

http://rpd.oxfordjournals.org/content/early/2016/03/14/rpd.ncw036

—————————————————————-

Monte Carlo Evaluation of Internal Dose and Distribution Imaging Due to Insoluble Radioactive Cs-Bearing Particles of Water Deposited Inside Lungs via Pulmonary Inhalation Using PHITS Code Combined with Voxel Phantom Data

Sakama, M. et al., 2016

https://link.springer.com/chapter/10.1007/978-4-431-55848-4_19

—————————————————————-

Radioactively-hot particles detected in dusts and soils from Northern Japan by combination of gamma spectrometry, autoradiography, and SEM/EDS analysis and implications in radiation risk assessment

Kaltofen & Gundersen, 2017

https://www.sciencedirect.com/science/article/pii/S0048969717317953?via%3Dihub

—————————————————————-

Caesium-rich micro-particles: A window into the meltdown events at the Fukushima Daiichi Nuclear Power Plant

Furuki et al., 2017

https://www.nature.com/articles/srep42731

—————————————————————-

Isotopic signature and nano-texture of cesium-rich micro-particles: Release of uranium and fission products from the Fukushima Daiichi Nuclear Power Plant

Imoto et al., 2017

https://www.nature.com/articles/s41598-017-05910-z

—————————————————————-

Uranium dioxides and debris fragments released to the environment with cesium-rich microparticles from the Fukushima Daiichi Nuclear Power Plant

Ochiai et al., 2018

https://pubs.acs.org/doi/abs/10.1021/acs.est.7b06309

—————————————————————-

Novel method of quantifying radioactive cesium-rich microparticles (CsMPs) in the environment from the Fukushima Daiichi nuclear power plant

Ikehara et al., 2018

https://pubs.acs.org/doi/full/10.1021/acs.est.7b06693

—————————————————————-

Formation of radioactive cesium microparticles originating from the Fukushima Daiichi Nuclear Power Plant accident: characteristics and perspectives

Ohnuki, Satou, and Utsunomiya, 2019

https://www.tandfonline.com/doi/abs/10.1080/00223131.2019.1595767

Q&A with Professor Rodney C. Ewing, Frank Stanton Professor in Nuclear Security and co-director at the Center for International Security and Cooperation (CISAC) in the Freeman Spogli Institute for International Studies (FSI). Interview with Katy Gabel Chui.

Your previous research with this team helped identify the types of radioactive particles that can become airborne and were transported away from Fukushima during the 2011 nuclear disaster.

This most recent paper goes further to show how these Cesium (Cs)-rich silica particles behave in several types of fluids, including simulated human lung fluid, concluding that the particles are fully dissolved in the latter after more than 35 years. What might that mean for human health in the Fukushima area and beyond?

The first breakthrough was the recognition that such particles, a few microns in diameter, existed, a discovery by Japanese scientists at the Meteorological Research Institute, Tsukuba, in 2013. The particles are important because they were dispersed over distances of tens of kilometers and were “carriers” of highly radioactive Cs. Our team’s previous work, led by Professor Satoshi Utsunomiya, mainly focused on the characterization of the particles and their constituents at the atomic-scale and surveyed their distribution in the area away from the Fukushima Daiichi nuclear power plants. Our earliest work from 2016 can be found online. A good summary of the history of the work on these cesium-rich microparticles was recently published in Scientific American.

This latest paper published in Chemosphere is the 6th in a series of papers on the Cs-rich microparticles. We describe the behavior of these particles when exposed to different types of fluids: ultra-pure water, artificial sea water and simulated lung fluid. The microparticles dissolve in all three fluids, reaching a long-term but a continuing, slow rate of release after just three days. Essentially, the calculated release rate of cesium depends on the rate of dissolution of the silica glass matrix and the initial size of the particles. In the simulated lung fluid, the particles are modelled to fully dissolve after more than 35 years.

What is the simulated lung fluid made of, and how does it work in simulation? How do you estimate 35 years?

The constituents of typical lung fluid were simply mixed to simulate its composition based on a recipe reported by previous studies. The lung fluid is different from the other solutions because it contains organic compounds and has a different chemistry, i.e., higher sodium and chlorine content. The estimates of residence time in the body assumes that the particles are inhaled and find their way to the pulmonary system. The calculation of residence time is based on assumptions about the size and composition of the microparticles, and we used the long-term release rate from the experiments. We assumed a spherical shape and a constant decrease in size as the leaching process continued. The rate can vary depending on the actual shape, internal texture, composition (such as occurrence of intrinsic Cs-phase inclusions), and precipitation of secondary phases that may form a “protective” coating on the cesium-rich microparticles (CsMPs). The rate of release was fastest in the simulated lung fluid.

The important result is to realize that the Cs-rich silica particles dissolve slowly in the environment and in the body. Essentially, the release extends for several decades.

How can nuclear energy experts and policy makers use this research to avoid future risk?

Understanding the form and composition of materials that host and disperse radionuclides is the first step in completing a careful dose calculation. Based on these results, the fraction of Cs contained in the silica particles will not be rapidly “flushed” through the environment or the body, but rather will be released over several decades.