Nuclear

The Truth about Radon

Matt Herod February 22, 2013

There are few things on Earth that evoke more fear than radioactivity. Most people’s response to radioactivity is one of immediate fear and confusion and I can’t say I blame them. There is something very frightening about a substance that shoots off invisible rays that can kill you if you’re exposed to enough of them. However, most people really don’t need to worry about being exposed to large amounts of radiation in their lifetimes. That said we are all exposed to radiation all the time. There is no escaping radioactive materials. They are in the food we eat, the water we drink and the air we breathe.

This constant exposure to radiation is called background radiation and there is simply no way to avoid it. The largest portion of background radiation comes from the radioactive gas, radon, which composes around 50-60% of our background radiation exposure. Radon is one of the most innocuous, yet ubiquitous radionuclides around and it is crucial that everyone be conscious that we all inhale some radon and what this means for our health.

The chemical symbol for radon (Source)

As with most things radioactive the first questions asked are: what is it and where did it come from? How does it behave in the environment and what does this mean for me and others (aka: is it dangerous)?

What is it and where does it come from?

Radon, chemical symbol Rn, is an odourless, colourless radioactive gas. In order to understand where radon comes from we have to discuss the radioactive decay of uranium, which is its source. Uranium is a radioactive metal that occurs naturally in almost all places on Earth. The major isotope of uranium is 238U. 238U comprises 99% of all uranium isotopes and has a half life of 4.47 billion years. One of the daughter products produced by the decay of uranium is radon.

Figure 1 shows the decay chain of Uranium-238, the most common form of uranium. Here α is Alpha radiation and β is Beta radiation

The decay chain of Uranium-238 (Source)

As the diagram above shows, the decay of 238U occurs over many, many years and there are several steps to be taken before radon is produced. There are two modes of decay shown in the diagram above: alpha and beta. Alpha decay is the radioactive decay process that occurs when the nucleus of a radioactive element ejects two neutrons and two protons (a helium nucleus). Alpha particles have a low penetration ability, but can be very energetic. This means that they cannot travel very far or through objects, but they have a high enough energy to be dangerous to living things when they are emitted very near them. Ingesting or inhaling alpha emitters is dangerous to people but, merely being in their presence is not as dangerous since alpha particles cannot penetrate skin. The other type of radiation emitted during the decay of uranium is beta. Beta decay involves the emission of an electron from around the atom and it is more penetrating than alpha radiation. Gamma radiation is also emitted at several steps in the uranium decay process.

As these alpha particles and electrons are emitted, the decay progresses and eventually radon is produced. The radioactivity that comes from radon when it decays into Polonium-218 constitutes an alpha particle that has an energy of 5.59 million electron volts, which is quite high.

How does radon behave?

The meaning of the question how does radon behave encompasses several other questions such as: how does it travel and how does it interact in the environment and where in the environment is it produced?

The noble gases. Notice radon is the only one sitting, because he is the heaviest. Haha, a little chemistry humour there. (Source)

Radon is a noble gas. This means it is very un-reactive in the environment and does not interact readily with other compounds or elements around it. This also means that in the environment radon travels all on its own and does not attach itself to other elements as a way to get around. This independent behaviour does not hinder the ability of radon to transfer from air to water and back again, in fact, radon transfers very readily between the two. As a gas radon is present in the atmosphere, in the gas trapped between soil grains, it can even be found dissolved in groundwater or surface water.

Up until now I have been taking it for granted that radon is in the environment, but I have not explained how it gets there in the first place. In order to do this we have to look for the uranium. Uranium is present ubiquitously throughout the environment. It is in soil, it is in rocks, it is in the oceans, it is even in lakes, rivers and groundwater. Of course, as we know, the decay of uranium produces radon and since there is uranium in pretty much everything radon can be produced from all of these places. The amount of radon produced is proportional to the amount of uranium present. And what controls the concentration of uranium in soil and rocks? The geology. I have included a table below showing average concentrations of uranium that one might expect to find in different bedrock types. As you can see they vary wildly from massive concentrations in an ore body, where the radon is practically pumped out of the rock to groundwater, which in carbonate sedimentary regions has almost no radon. Obviously there are lots of exceptions to these situations, but they do provide a general guideline from which to base predictions about the amount of radon that we might find in a house built on any of these rock types. Indeed, it is possible to predict, based on the bedrock alone, whether or not a house is likely to have elevated radon levels in the soil or groundwater. These are not hard and fast rules since so many other factors can affect the radon concentration in a house at any given time such as humidity, air pressure, temperature, etc, but they are at least a starting point.

Material	Concentration (ppm U)
High-grade orebody (>2% U)	>20,000
Low-grade orebody (0.1% U)	1, 000
Average granite	4
Average volcanic rock	20 – 200
Average sedimentary rock	2
Average black shale	50 – 250
Average earth’s crust	2.8
Seawater	0.003
Groundwater	>0.001 – 8

(Source)

Is radon dangerous?

The short answer to this question is: yes, radon is indeed dangerous. The why, how are we exposed and what can we do about it are a bit longer. As I mentioned above radon is an alpha emitter, meaning that it is only dangerous when we are in very close proximity to it or it has been ingested or inhaled. Furthermore, once in our body the radon will continue to decay and produce other daughter products such as 214Pb and 214Bi, which are also highly radioactive and very dangerous. The unfortunate thing is that it is very easy for us to inhale radon making it extremely dangerous. In fact, it is estimated that ~10% or more of lung cancers are caused by radon inhalation, making it an extremely serious threat to human health.

Radon accumulates in confined spaces such as in our houses or other buildings, particularly in basements as radon is heavier than air. In the open air there is no threat from radon, however, Canadians and many other northern cultures spend a great deal of their time inside, especially during winter (it is -20 with wind chill as I write this). This is a major concern as all of this time spent indoors can greatly increase radon exposure.

So how does radon get indoors and why does it accumulate there? Firstly, radon can enter our homes through two main pathways. It can come in as a gas through holes in our basements, sump pumps, windows… essentially any place where our homes are connected to soil or rock, it will even diffuse through walls with ease. It can also enter in our water, especially if we use groundwater. Once radon is dissolved in water it needs to interact with air in order to leave the water so a perfect place is our taps, and showers which cause air-water interaction and force any radon dissolved in the water to de-gas. The source of radon for our homes has to do with the type of soil and bedrock where we live. If there is lots of uranium in the soil or bedrock our homes are built on then there will be lots of radon produced as well. Radon also tends to accumulate in basements by dint of its large mass. It is by far one of the heaviest components of air and therefore tends to sink.

Pathways that radon can take to enter a house (Source)

In Canada the Health Canada limit for radon in air is 200Bq/m^3. Here is a map showing where radon exceeds this level in Canada.

Radon map of Canada showing the percentage of dwellings above 200Bq/m^3 (Source)

The figure shows a mean 222Rn emissions map from soil for 2006. Since I now blog for the EGU I thought I should include a Europe version as well. (Source)

Happily, there are lots of simple remedial actions that you can take to get rid of radon. These generally constitute plugging access point and installing ventilation from the basement so that the air pressure in the basement is greater than outside and the radon will not migrate into the basement. Radon test kits are also readily available in most hardware stores so that you can test the radon levels in your home yourselves.

Case Study: My undergrad thesis….

Radon is not all bad though. There are uses for it too…there is always something. One of my first summer jobs in the geology industry was as a hydrogeology field assistant for a professor of Civil Engineering who specialized in the ridiculously complicated field of fractured bedrock hydrogeology. I worked for several of his grad students during the summer and one, who is now a professor at McGill University, was looking at radon concentrations in the groundwater and surface water of a local watershed. He was looking to see if it was possible to trace and quantify groundwater discharge into surface water using radon.

This project then led me to propose a radon in water investigation for my honours project, although the aim was a bit different. I was looking to see if radon could be used as a tracer of radioactive waste in groundwater and surface water. I was working in Port Hope, Ontario at a low-level radioactive waste site and sampling the adjacent creek and installing some mini-piezometers. I have added the abstract below so you can see what I found. Long story short though, radon was elevated in both surface and groundwater although not where I expected it to be.

Port Hope, Ontario is home to 1.5 million cubic metres of low-level radioactive waste. This waste decays to produce the noble gas radon. Radon can be used as a tracer of waste migration in groundwater and surface water. In this experiment radon was sampled in a creek adjacent to a waste site in order to determine if elevated radon was produced by low level radioactive waste and if it could be found in water. Previously measured dissolved uranium concentrations were also used to determine if uranium and radon were linked in surface water. Background levels of radon in an uncontaminated local river was 18.1 +/- 6.28 pCi/L. Radon in water was detected in the field using a radon-in-air analyzer with an alpha spectrometer and outfitted to analyze water samples. The water samples were placed in a sealed chamber and forced to degas. This gas then entered the radometer and was analyzed. Elevated radon levels were detected along the length of the creek with the highest readings being 115 pCi/L upstream of the waste site and progressively dropping along the reach to a low of 45 pCi/L due to degassing. The trend in the radon was opposite to the trend in the uranium data with a high of 0.032mg/L adjacent to the waste site and a low of 0.017mg/L upstream at the same point of highest radon. The source of the radon is therefore hypothesized to be previously contaminated groundwater entering the creek at a point upstream of the waste site. The source of the uranium is solid waste that is present in the creek as well as aqueous complexes that have undergone redox transformations in organic mud present at the waste site. The flow of the creek is too fast for radon and uranium to achieve equilibrium. Therefore, these two contaminants are not linked in this system. This has implications for understanding the movement of uranium and radon in natural systems and how they may be related in nature from a hydrogeological and geochemical perspective.

By the way, in case you were wondering 1pCi/L is equivalent to 37Bq/m^3. Therefore a measurement of 115 pCi/L is 4,225 Bq/m^3. Remember, this is in water though so the transfer to air changes things a lot.

Links:

Health Canada:

http://www.hc-sc.gc.ca/ewh-semt/radiation/radon/index-eng.php

Canadian Nuclear Safety Commission:

http://www.nuclearsafety.gc.ca/pubs_catalogue/uploads/February-2011-Radon-and-Health-INFO-0813_e.pdf

Matt

Note: This post was previously published at my pre-EGU blog Geosphere on March 4, 2011. Although several changes and new sections were added to this post.

Research Highlight – Variations of 129I in the atmospheric fallout of Tokyo, Japan: 1963-2003

Matt Herod December 13, 2012

I occasionally like to focus in on what I view as a key paper either in my particular field of iodine geochemistry or in the geochemical world at large. In this instance I have decided to highlight a paper in my field that releases a fantastic wealth of data that is matched nowhere else in the literature I have seen. This paper also reflects incredible dedication and planning to answer a scientific question that far exceeds that of most papers in terms of patience and structure. The reason I say this is because most datasets in papers today may reflect a few years of sampling. In my opinion the average is around 2-5 years of data for papers that concern modern processes. This is great, however, datasets such as this only represent a small snapshot of these processes and do not allow one to observe long term trends with much confidence. Indeed, most of these papers finish by saying more data is needed to reach conclusions about long term changes. The reason for this is usually money, grad students wanting to finish, etc. These are good reasons, particularly the latter, which I can really empathize with, however, the problem is that such short time periods are not really long enough to make robust conclusions about changes in the environment or that factors that are causing such changes. Most researchers overcome their piddly datasets by adding in the work of others to broaden their time periods. Obviously, there are problems with doing this such as differences in sampling location, sampling techniques, lab practices…the list goes on and on with reasons why two datasets from different researchers may not be exactly comparable. This practice is very common nonetheless. However, rarely papers come out that present many years worth of data all analysed by the same person, in the same place, and collected the same way. Such datasets are invaluable in the geochemistry world! The paper I review here is such a paper. In fact, it presents data for a 40 year period!!

The paper is by the AMS research group in the department of chemistry at Gakushin University in Tokyo, Japan. The authors are: Chiaki Toyama, Yasuyuki Muramatsu, Yuka Uchida, Yasuhito Igarashi, Michio Aoyama, and Hiroyuki Matsuzaki.

Why 129I

I thought this might be a good introductory section to add in order to bring everyone up to speed on what the value of researching 129I is. The main reason is that 129I levels are continually increasing globally and it is an emerging contaminant which we don’t want our environment, particularly 129I, which we don’t understand very well. The next reason is that we are still learning how iodine and 129I behave in the environment. They are tricky little elements that just won’t stay put no matter what the chemical conditions. Therefore, we need to better understand how they move and over what time scales. Lastly, 129I is produced by nuclear reactors and is present in large amounts in radioactive waste. This means that when we dispose of it and store it there is a very mobile isotope in there with a 16 million year half life. It is essential that we understand how it behaves and where it comes from.

Purpose

The purpose of this paper, that is to say the research questions that they authors are attempting to answer is:

1. What variations exist in the fallout rate of iodine-129 over time in Tokyo and what factors influence this fallout?

2. What are the sources of 129I in Tokyo?

3. How have nuclear accidents, such as Chernobyl, affected 129I concentrations in Tokyo?

4. What is the background concentration of 129I in Tokyo?

Sampling Methods

The samples were collected by the Japanese Meteorological Research Institute. Precipitation and particulate fallout samples were collected on a monthly basis from 1963 to 2003 and iodine was extracted as silver iodide and analyzed using accelerator mass spectrometry. Total iodine was analyzed with ICP-MS.

Results and Discussion

Unfortunately I cannot reproduce any figures from the paper…even though it has a graphical abstract…since Elsevier wants to charge me $20/figure to use them.

Anyway, I’ll make do without and try and describe the results without being too long winded.

– The 129I/127I ratio increased drastically during the 70’s and 80’s. It peaked at 4 orders of magnitude, or 10,000 times higher than the pre-anthropogenic ratio of 1.5 x 10^12. This increase is likely initially due to bomb testing, but the steady increase through the 80’s, which post dates most bomb testing means there is another cause.

– The deposition of 129I initially mimics the deposition of strontium-90 and cesium-137, both produced exclusively by bomb testing and nuclear reactor accidents. However, over time the 90Sr and 137Cs inputs decrease while the 129I remains steady. Also, there is a visible Chernobyl pulse for 90Sr and 137Cs, but not for 129I, although this is mainly since very little 129I was released from Chernobyl.

– The authors then compare the 129I deposition to the releases of 129I from the nearby Tokai Reprocessing Plant. The correlation is excellent. In order to guage the influence of European reprocessing plants the authors compared their results to another study looking at 129I in ice cores from the Alps, which found that there was a strong relationship to reprocessing releases. The Japanese deposition fluxes did not match the European very well. The suggests that 129I deposition in Tokyo is mainly from the Tokai plant.

– There was a consistent seasonal variation in 129I deposition in Tokyo. Indeed, the deposition of 129I was about 2 times higher in the spring than in the fall. This is consistent with other studies that have noticed a “spring peak” in radioactive fallout in mid-latitude regions. This is explained by a change in stratosphere-troposphere mixing in the spring (I have the reference if you want it). However, the authors also note that the air mass above Tokyo in the spring comes from Europe the higher levels could represent an influence of the European or Russian fuel reprocessing plants.

Conclusions

The principle conclusions of this article are:

1. The current ratio of 129I/I in atmospheric fallout is 10,000 times higher than the pre-anthropogenic, natural 129I/I ratio. This is primarily due to atmospheric weapons testing in the 1950’s and 60’s.

2. There is no detectable influence of the Chernobyl accident in atmospheric fallout in Tokyo. This is likely because the signal is hidden rather than not present.

3. The high deposition of 129I in the 1980’s is due to the influence of the nearby Tokai Nuclear Fuel Reprocessing Plant. There is no correlation between 129I and 137Cs and 90Sr, which indicates the recent rise in 129I concentrations are not due to bomb testing.

4. There is also a possibility of distant transport of 129I from overseas reprocessing facilities in Europe, Russia and China and the authors intend to assess this by carrying out further sampling in places throughout Japan.

These conclusions are not especially novel. Many other papers have clearly demonstrated the impact of nuclear fuel reprocessing on 129I fallout and the ease with which 129I can be transported atmospherically. Indeed, I have published a paper demonstrating this in the Yukon Territory. However, most papers, mine included, do not present the quantity of data to verify this conclusion that this one does. This makes this paper a fantastic addition to the body of 129I literature as well as provides an excellent starting point for future conclusions about changes in 129I fallout with time and the factors responsible for them. It also provides excellent context for future work. It is difficult to put 129I deposition fluxes in context over time and this data set opens that door.

Some Speculation

So many papers come out every day in the research community. I have often wondered what each one represents financially. I mean, how much does it cost to produce all this research? How much do the analyses cost, the sampling, the preparation, the time, the analytical equipment? None of these components is cheap, except maybe the grad student hours, and therefore, every paper that comes out represents a substantial financial investment on the part of the supervisor and the funding agencies. So I thought it might be an interesting thought experiment to guess what this paper may have cost to produce. I have no actual knowledge regarding what this paper cost to produce, but I can speculate a bit since I am familiar with the cost of AMS analysis and lab work. So here is my very, very rough cost breakdown. It is likely on the low side though since I am not going to factor in human labour hours, the cost of sampling, or the cost of using the lab equipment besides the AMS.

# of Samples: 80

AMS sample prep: ~$75/sample = $6000

Cost of AMS analysis per sample: ~$250/sample = $20,000

Cost of ICP-MS per samples: ~$25 x 80 = $2000

Grand Total: $28,000

I believe that the paper is sadly trapped behind a paywall. Therefore, if you would like to read the paper in full let me know in the comments below and leave me your email so I can pass it on to you.

Thanks for reading! If you have any questions or concerns I’d love to hear from you. Also, if you have an opinion on the paper or dataset I’d love to hear it as well

Citation

Toyama, C., Muramatsu, Y., Uchida, Y., Igarashi, Y., Aoyama, M., & Matsuzaki, H. (2012). Variations of 129I in the atmospheric fallout of Tokyo, Japan: 1963–2003 Journal of Environmental Radioactivity, 113, 116-122 DOI: 10.1016/j.jenvrad.2012.04.014

Matt

claimtoken-50c9c98387ace

Geology Photo of the Week #14 – Dec 2-8

Matt Herod December 5, 2012

This week’s photo, which is posted mid-week instead of at the beginning is one that I only took this Monday. I was away all day at the Royal Military College SLOWPOKE-2 reactor doing some neutron activation of cesium and calcium. We were making minute quantities of Cs-134 and Ca-41 for research purposes on the accelerator mass spectrometer. This photo is one that I was able to take while we were running the reactor. I am planning on doing a post on the SLOWPOKE reactor in the near future…sometime this month, but I thought I’d show this picture as a start.

The photo is of Cherenkov radiation in the cooling water around the reactor. Cherenkov radiation is caused by a charged particle such as an electron or a gamma photon enters the water at a speed greater than the speed of light in water. This results in a the wavelength of the particle lengthening when it enters the water and causes the water molecules to polarize (gain opposite charges) and revert rapidly back to neutral (normal charges). This change in charge in the water molecules releases the blue glow.

Cherenkov radiation at the RMC SLOWPOKE-2 reactor. (Photo: Matt Herod)

Cheers,

Matt