Environmental Pathway Modeling Part 2

I've touched upon the transfer of radionuclides through air, ground, and water, and in the last part we discussed how organizations like the EPA use pathway models to factor in all three to estimate damage from a contamination (accidental or otherwise), and help direct their most effective course of action in containing, controlling, and removing the contamination. Reading through some of my classmates' blogs brought up an aspect I had not considered, however: the spread of nuclear contamination through organic matter, especially food. 

Radioactive atoms can stay around for a long time. A really long time. We're talking "the Earth has only existed (4.543 billion years) for as long as one half-life of uranium-238 (4.468 billion years)" level of long time. Now, uranium-238 is mostly harmless, as that long half-life makes it essentially stable and not enough to harm humans except in rather large quantities. But the half-lives of all radioactive isotopes make a pretty decent spectrum--ranging from infinitesimally small to unfathomably large--and many fall in the range of months to a few years. This time frame is what we would be most worried about when it comes to food: active enough to give off dangerous levels of radiation, but with enough longevity to stick around and be transferred through food.

When the radiation from a contamination (spill in transport, facility accident, dirty bomb, etc.) gets into the soil and groundwater, it can be absorbed by plants along with their usual nutrients from the soil and water. [1] Of particular note are tritium (hydrogen-3), carbon-13 and -14, technetium-99, sulfur-35, and iodine-129 and -131, among a few lesser others, as all of them are easily absorbed into organic material and by the biological processes of plants and animals. 

The organic material that absorbs these isotopes can have a much higher quantity of radioactive material than the water or soil that supplies it to them, due to the "Bioaccumulation  Factor"[2]. These plants could be in a farm meant for human consumption, or even in a food not typically associated with humans (such as grass) and passed up the food chain into an animal that we will eat (such as a cow), These radionuclides aren't filtered out of the body very quickly or efficiently, especially if it's an isotope of an atom normally expected to be in the body in at least a small quantity (such as carbon, sulfur, and iodine). When these isotopes then decay, the radiation they release can be absorbed by some of the most sensitive parts of the body, like the lungs, brain, or intestines. Parts that normally expect to be protected from radiation exposure by the epidermis. 

Modeling the spread of radioisotopes through the food chain proves to be even more difficult than through the geography of the environment, and the exposure caused can be an even larger danger to humans. As such it is of equal if not greater importance to the EPA for research and development of proper predictions and responses. 

[1]http://www.who.int/foodsafety/fs_management/radionuclides_and_food_300311.pdf

[2]http://www.nap.edu/read/5803/chapter/6#74

The Nuclear Role in the Power Industry

Fair warning, this post is going to be mostly anecdotal.

I just got back from spring break, and during my time of vacation I saw a few old friends and met a lot of new people. Inevitably, I ended up talking about nuclear power and engineering as "what do you do for a living" came up in conversation. Most of the people I spoke with were well educated, but one of many common points of conversation struck me: no one really knew where nuclear power fell in their power grid.

As a general rule, a nuclear reactor doesn't like to change the power level its operating at. It's not like the engine in your car, that you can change dynamically with the press or release of the gas pedal; yes the reactor can be adjusted across a range of outputs, but it's a relatively slow process. Fortunately, there's a place for that kind of power source in the power grid: covering the "base load".

In a given region, the local utility provider(s) will spend a lot of time determining what the minimum draw on the grid will be. For example, in a Florida summer you expect the average household to be running its AC unit, and you expect that unit (which, as household appliances go, is pretty inefficient and a rather large draw of electricity) to draw a certain amount of relatively-constant power. The combination of all of these AC units create the "base load", a minimum amount of power required to keep the grid operational without brown outs or worse. This can be provided quite effectively by a nuclear power plant, which doesn't want to change its power level much, and certainly doesn't want to shut down as may be required in an oil or natural gas power plant (which can change their power levels dynamically, and are used to cover the rises and falls of power during the day, from people's lights, televisions, etc.).

Unfortunately, the characteristics that make a nuclear plant good for the base load are almost all applicable to coal power plants as well, making the two direct competitors in the power market. Both have their advantages and disadvantages, and I hope to do a more factual and detailed comparison between the two later this week.

So if you live on a grid with nuclear a nuclear reactor, chances are you have nuclear power to thank for your AC or electric heater, and hopefully it's making it cheaper as well.

Environmental Pathway Modeling

In the last series of posts, we touched on the concepts behind radionuclide dispersion, through the air, water, and ground, and some of the safety measures in place to make sure that none of it comes from a nuclear power plant or otherwise endangers civilians. But what if something bad does happen? If there's a spill or breach during transport, some form of catastrophic event at a reactor, or even an attack with a dirty bomb? What dictates proper response, and how do you estimate the fallout?

That's where environmental pathway modeling (EPM) comes in. EPM, in simple terms, is the combination of the considerations for radionuclide spread in the air, the ground, and the water, taken into account at the same time. Factors like type of contamination source, whether it's in a container, the local geography, environment, sources of water above and below ground, and many others are considered. The figure below shows a highly simplified diagram for contamination considerations. [1]

The Environmental Protection Agency (EPA) spends a lot of time and money on EPM. They have researched and developed several models for different radiation hazards in different environments. [2] So in the event of a contamination event, what do they do?
First, they make many simplifying assumptions to get a general idea of what kind of hazard they're dealing with. Assumptions such as initially ignoring any shielding that may be around the exposure point and treating the exposed area as homogeneous. This allows simpler, easier models to predict immediate courses of action while specific data relevant to the site is gathered. Once they know more, they can slowly remove assumptions until a model can be developed for the exact problem they face, and the most educated choices can be made in how to contain and control the area, and protect downgrade areas from contamination.
As we discussed before, there is very little (approaching zero) chance of a nuclear power plant endangering you. But in the unlikely event something does go terribly wrong, those in charge won't be leading blind; research is constantly ongoing to be better prepared to predict and react to a contamination.

[1] http://www.atsdr.cdc.gov/hac/phamanual/ch6.html
[2] http://www.epa.gov/sites/production/files/2015-05/documents/540-f-94-024.pdf

Nuclear Power Plants Contaminating Our Rivers?

Spoiler alert: No! They Aren't!

You've been living in your little town for over a decade, and your favorite part about it is the crystal clean, spring-fed river that runs by just out of town. It's great for fishing, canoeing, swimming, or any combination of the three. You read the local paper one day to find out the clearing a few miles up river has been bought by a power company, and they're going to build a nuclear power plant there. You know enough about the power industry to look forward to your power bill going down, but the next time you talk to your neighbor he mentions he's worried, because the power plant plans to use the river for its runoff! Is your clean river in danger of being polluted by nuclear waste?

It really isn't. Unlike farms, chemical processing plants, metal foundries, and other man-made things that pollute local surface and groundwater [1], a nuclear power plant puts nothing back into its water source other than regular, clean water--albeit a little bit warmer.

Yes, a nuclear power plant uses water as its primary coolant, and that water is exposed to and activated by the radiation in the nuclear core. But even in a boiling water reactor (BWR) where the water across the core is turned directly into steam for turning the turbines, that water is not put into the environment. It's cooled off, chemically treated, and placed back into the core. The "cooled-off" part is accomplished through a closed secondary cycle, where "closed" means that the two never come into direct contact with each other; the water on the cold side is never exposed to the radiation from the core or held within the primary water coolant. [2] It's this secondary-cycle water that is drawn from and returned to the local river, lake, ground reservoir, or other source of water in order to maintain the temperatures of the reactor. As mentioned, due to the basic principles of thermodynamics this water is returned to its source a little warmer than it was found, but new reactors always have extensive environmental studies performed to ensure that the net temperature change in the water source will have negligible effects on the surrounding ecosystem.

If the reactor is a pressurized water reactor (PWR), the concern is even less. The primary-cycle water is never brought to a boil, the closed secondary-cycle performing steam generation and running turbines. This puts the cold cycle in a tertiary position, even more removed from the radiation of the core. Your hypothetical self can rest easy knowing that the plant will cause no effect to the river.

[1]http://floridaswater.com/waterbodies/pollutionsources.html
[2]http://world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/nuclear-power-reactors.aspx

Man-Made Radiation in the Air: Emergency Reactor Venting

I'm pleased, because I finally get to stop talking about nuclear-related points or events that are an issue and people may or may not know that they are, to a subject that many people think is an issue but most assuredly isn't.

There have been three nuclear power accidents major enough that the average person on the street might be able to name them: most recently is Fukushima, and turning back time a little farther gets you to Chernobyl and Three Mile Island (TMI). All three accidents were the result of failures in safety systems previously thought impossible, compounded by some pretty major human error. Two of them, Fukushima and Chernobyl, deserve the titles of major accidents and both vie for a top position in "world's most expensive accidents". Chernobyl left a fifth of a country irradiated to an largely uninhabitable degree, and the full effects of Fukushima have yet to be determined, even years later.

I will likely go into detail on at least one of these in a later post, but for today we're talking about TMI.

The Accident

I'll keep this brief, as accident analysis isn't the point of the post. In 1979 A stuck valve in the non-nuclear secondary system (the part that actually makes steam and turn turbines) lead to the inability to remove heat from the reactor, even after a full shutdown via SCRAM. Secondary pumps were unable to be started due to human operator confusion over valve status and improper maintenance performed the night before. Eventually heat built up enough to cause a partial core meltdown. In order to prevent further issues the operators were forced to vent nuclear products into the air, exposing the public living near the reactor to the material.

The Aftermath

A voluntary evacuation was suggested to pregnant women in the area. Within hours of the accident, several organizations, ranging from the EPA, the national lab, college research groups, and privately funded research, began a series of extensive tests to determine the full effects of the radioactive release.

Now, if you ask your parents, or maybe your grandparents, you might here some ridiculous stories about babies with multiple extra limbs, or fish in the local river with three eyes, or the more believable anecdote of increased miscarriage or other pre-natal issues in the area. However, the honest truth is that there was no determinable effect. Based on soil and air samples it is estimated that the two million people living in the vicinity of the plant received an average dose above background of 1.4 millirem. By comparison, a chest x-ray is 3.2 millirem, and people receive those all the time without issue. Now, there is a big difference between the voluntary 3.2 mrem from the x-ray, and the involuntary exposure to the 1.4 mrem from the power plant. But after long, extensive study by the EPA and the other groups, it was determined that not even one additional cancer death occurred as a result of the venting at TMI. [1]

What's more, we learned a lot of important lessons from TMI. Lessons in redundancy, and how to further remove the potential for human error in an accident scenario. Thanks to those lessons, America has never had another significant reactor event, and certainly no major exposure to the public. Unfortunately, we determined all of this far too late to prevent a panic in the public and in the government, which led to a moratorium on new reactors in America that lasted until just a couple years ago. It, along with Chernobyl, instilled a fear of nuclear in the general public that had just started to die off when Fukushima occurred.

The final takeaway is this: with only a handful of exceptions, the nuclear power plant down the road from you, or in the next town over, or wherever, is not exposing you to atmospheric radiation. Even in America's most significant reactor accident, no adverse health affects occurred.

[1] http://aje.oxfordjournals.org/content/132/3/397.full.pdf+html

Man-Made Radiation in the Air: Weapon Fallout

As a soon-to-be nuclear engineer, I feel the need to defend my field as much as possible from undue criticism about the danger of nuclear power and the aspects surrounding it. I feel like a broken record stating that this defense is the original idea of this blog, but I have to seeing how the last couple posts have been about legitimate nuclear dangers, and this post is no different. Today we're talking about nuclear weapons.

Now, I'm not an idiot, and I don't assume anyone reading this blog is an idiot. Obviously, nuclear weapons are bad. I'm not going to even attempt to defend their existence (outside of their creation leading to the first development of nuclear power). I also don't think I need to explain how dangerous to human life they are; I feel that most people have at least some idea of their local destructive power as well as their lasting radiation. Most importantly, this series of posts is supposed to be on atmospheric dispersion of radionuclides in the air, and so that's what we'll discuss: the morbidly fascinating reach of a nuclear weapon.

Basics of Fallout

A nuclear weapon unleashes a large amount of energy when it detonates. This energy vaporizes most things around it, organic or not, within instants of the detonation. A large amount of this energy also throws nuclear material far and wide, whether it's produced directly by the fission reaction in the bomb, or indirectly by making the dirt and ash thrown into the air radioactive or at least ionized by the energy and radioactive processes of the detonation. This dirt and ash are pushed and pulled by the charged air currents, resulting in the iconic "mushroom cloud" of a nuclear weapon. This cloud of radioactive ash is thrown into the atmosphere where it is free to spread far and wide, and settle wherever the winds and rains take it. 

Nuclear Weapons Testing

Fortunately, only two nuclear weapons have ever been used in anger: the bombings of Hiroshima and Nagasaki that ended WWII. Unfortunately, that was followed by the Cold War, and from the period of 1945-1980 at least 500 nuclear weapons tests were performed by various world powers underground, underwater, and--most importantly--in the air. [1]

Any particle in our atmosphere has the ability to travel anywhere in the world. So, given enough sources, the nuclear material from a nuclear weapon can appear anywhere. If you analyzed a cup of soil anywhere in the world, you would almost certainly find cesium-137 in small quantities. But if you somehow had a soil sample preserved from before 1945, you would not find this isotope of cesium in it, as it is a non-naturally occurring isotope; it's created by nuclear processes, either in a reactor or in a bomb.

Now, this isn't always a bad thing. Nuclear materials like cesium are used for some cool applications, like atomic clocks. And cesium has even been used to detect wine fraud [2]. Because if it's in the soil, that means it can make it into our crops, like grapes. But if it's in our crops, that means it's in us! Similar to cesium-137, iodine-131 is a non-natural isotope that exists from the fallout of nuclear weapon tests. The CDC estimates that due to those 500+ tests mentioned earlier, at least 11,000 excess deaths have occurred due to thyroid cancer caused by exposure to ingested iodine-131. [1]

Thankfully, nuclear testing has been banned through the Comprehensive Nuclear-Test-Ban Treaty, which has been signed by all nuclear powers except the craziest (looking at you Pakistan and India) [3]. Still though, the world has been changed, potentially permanently, through exposure to non-natural isotopes caused by nuclear weapons, and spread through natural atmospheric dispersion. Thankfully, the average increase in background exposure is tiny, and 11,000 excess deaths in 35 years is not that bad from a cold, utilitarian standpoint. But the global effect of nuclear weapons and their release of nuclear material to the atmosphere, even when not used in anger, is intimidating. Hopefully, in time, humanity can finally phase them out of existence.


[1]National Research Council. Exposure of the American Population to Radioactive Fallout from Nuclear Weapons Tests: A Review of the CDC-NCI Draft Report on a Feasibility Study of the Health Consequences to the American Population from Nuclear Weapons Tests Conducted by the United States and Other Nations. Washington, DC: The National Academies Press, 2003. doi:10.17226/10621
Available: http://www.nap.edu/catalog/10621/exposure-of-the-american-population-to-radioactive-fallout-from-nuclear-weapons-tests

[2]http://www.npr.org/sections/thesalt/2014/06/03/318241738/how-atomic-particles-became-the-smoking-gun-in-wine-fraud-mystery

[3] https://fas.org/sgp/crs/nuke/RL34394.pdf

Man-Made Radiation in the Air: Primer

This post is going to be the first of a three-part series on "atmospheric dispersion of radionuclides". The intention is to give some basic information on the topic that would otherwise clutter the other two topics, weapon fallout and power plant release.

Why do we care?

Man-made radiation releases to the atmosphere are a top priority for many nuclear regulatory and research organizations around the world. [1] The primary reason being that the air provides both the fastest and widest-reaching medium for radioactive particles and their radiation to spread to and around an area. Though radiation can and will travel through water, soil, and underground, none of it beats the speed or area coverage of radiation in the air. Plus, humans have a tendency to breathe the atmosphere; as we discussed with radon, radioactive material in your lungs is something you should avoid whenever possible.

The ability to avoid it is another reason why it's so important--or more accurately, the lack of an ability to avoid it. Depending on the source event, radiation in the atmosphere can travel very quickly over a very large area. If that area happens to include where you live, unless you evacuated a while ago chances are you're going to be exposed to something.

What's in the air?

That "something" depends on the source event. Many radioactive nuclear byproducts exist naturally in a gaseous state. We've already discussed radon, but alongside it can be krypton, xenon, radioactive isotopes of oxygen or nitrogen, and a few others, none of which are going to be very good for you if you breathe them in. Depending on the energy behind the release (e.g. a bomb versus a reactor venting, which we'll get into in the other parts of this series) there may be heavy elements flying through the atmosphere as well. These elements, which can be decaying with all kinds of nasty radiation from high-energy gammas that can do damage no matter where they hit you, to alpha particles that can really hurt the interior of your lungs when you inhale them. They also tend to be part of long decay chains, which means they and their "daughter" elements can still be highly radioactive even after a large amount of time has passed (we'll discuss this further in the subsequent posts).

To be continued

I know the stated idea of this blog is to try and assuage some fears about nuclear, but it sure sounds like I've done some fear mongering here. I just wanted to build a bit of a teaser for the more in-depth posts to come, on the atmospheric radiation from nuclear weapons and non-weapons. I will say that thankfully, the number of man-made events resulting in the release of radionuclides to the atmosphere are quite few; unfortunately, most of them have been very serious.

Up next will be a discussion of nuclear fallout from weapons, and its lasting effects.

[1] https://rem.jrc.ec.europa.eu/RemWeb/activities/AtmosphericDispersion.aspx