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GeoPoll: What should we do with radioactive waste?

GeoPoll: What should we do with radioactive waste?

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.

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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

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.

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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!

[polldaddy poll=”8130488″]

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Matt Herod is a Ph.D Candidate in the Department of Earth Sciences at the University of Ottawa in Ontario, Canada. His research focuses on the geochemistry of iodine and the radioactive isotope iodine-129. His work involves characterizing the cycle and sources of 129I in the Canadian Arctic and applying this to long term radioactive waste disposal and the effect of Fukushima fallout. His project includes field work and lab work at the André E. Lalonde 3MV AMS Laboratory. Matt blogs about any topic in geology that interests him, and attempts to make these topics understandable to everyone. Tweets as @GeoHerod.

4 Comments

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    Other poll answer: pop it back where it came from – Cigar Lake Style

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    There are two problems with this poll.

    The first problem concerns the graph. The unit Bq is not suited for this discussion, as it disregards the effect on the human body. Bq merely tells you how many atoms undergo radioactive decay per second. But when an atom undergoes nuclear decay, it will release different amounts of energy in the shape different kinds of radiation, depending on what kind of atom it is.

    To pick two typical cases from the graph: Pu-239 undergoes alpha decay with an energy of about 5250keV. Alpha particles are about 20 times more harmful than beta or gamma radiation with the same amount of energy. So in terms of harm done to biological tissue, it has the equivalent of about 100,000keV of beta or gamma radiation.

    Zr-93 undergoes beta and gamma decay with 90keV and 30keV of energy respectively. So you would expect 1Bq of Pu-239 to be about 800 times as harmful as 1Bq of Zr-93. The decay energy released from 1Bq of I-129 is similar, with about 150keV per decay. Contrary to the graph, Zr-93 and I-129 are minor problems to begin with. (However, as iodine will tend to stay in the tyroid, I-129 is potentially more harmful than those numbers indicate, hence the desire to destroy it through transmutation, to which it is very well suited and which I’ll talk about next.)

    Because of this, the main problems are Plutonium and Americium.

    There are very realistic plans to deal with nuclear waste, without having to bury it as it comes out of the reactor (which is what has been implicitly assumed in the poll here). Instead plans call for the following

    1st step: Reprocess the fuel to seperate it into different fractions and treat each of those fractions according to their characteristics. This is currently implemented by separating Uranium and Plutonium from the waste, the remaining fraction is vitrified and prepared for geological storage. This fraction dominated by Am-241 as the longest-lived harmful isotope. It takes only about 10.000 years for this fraction of the waste to decay to the point when the harmful effect of its radioactivity is at the level of the uranium ore that was mined before it was turned into fuel. In other words: After 10.000 years, the stuff you bury in the ground has become less harmful than the stuff you took out of the ground to begin with.

    2nd step: Separate the Americium off from the remaining fraction (currently not being done). What is left takes between 500 and 1500 years to decay to levels similar to uranium ore. The exact time depends on the purity of the separation process (98% or better), but 1000 years seem to be no less arbitrary than the 10,000y, 100.000y or 1,000,000 years typically talked about.

    3rd step: The Plutonium and Uranium is reused as fuel for nuclear reactors. Am-241 can be added to the fuel. Given sufficient concentrations, more Am-241 will be fissioned than produced in the process. Some of the remaining isotopes, especially Tc-99 and I-129, can also be separated from the waste and introduced to a reactor and transmuted to non-radioactive isotopes, using surplus neutrons from the chain reaction.

    This can only be done in non-moderated reactors. Such reactors were called fast breeder reactors, when the focus was on using additional neutron to breed more plutonium. But this is just one possible use of the neutrons such reactors offer. The neutrons don’t care if they are being used to breed Plutonium, split Americium or transmute Technetium.

    In case you think that such reactors are highly experimental or don’t work, there are several such reactors that have been build and were used for several decades (e.g. EBR-II, FFTF, Phoenix, Bn-350, Bn-600). Given that there will be centuries to sort out the engineering, it would be unreasonable to discard this possibility.

    And now, finally, to answer your question what we should do with radioactive waste:

    Process it as described above, reuse it in nuclear reactors. Meanwhile, store waste meant to be transmuted or reused. Either in above ground facilities or shallow repositories. For the USA, Yucca mountain would be very suitable for this task. The remaining fraction, that cannot be dealt with, should either be stored as well (There is room to improve the technology for transmutation and reduce radioactivity further.) or disposed of in a place that would be stable for at least 1000 years. Such places should be much easier to find.

    Given that the pyramids in Egypt are over 4000 years old, simply constructing a permanent storage building could be a feasible option and may very well be the best one.

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      Thanks for your comments. I disagree with your first point about the graph however. The purpose of the graph is to illustrate the relative half lives and decay times of the common isotopes contained in fuel waste. While your point about the effect of these isotopes is true and interesting it does not apply to the situation I am discussing here, which is the need to isolate waste over geologic time scales. The graph uses Bq’s because it is a simple way to look at the change in activity of the total waste and compare these isotopes to one another as they decay over time. Looking at each isotopes risks based on energies and decay series is far more complicated.

      Your second point adds a nice nuance to the discussion. Reprocessing, transmutation and breeder reactors are not novel ways of dealing with waste and are great. However, as you point out they are not 100% efficient and some fraction of the waste must be stored geologically or in some other manner. I am all in favour of these technologies as they certainly make disposal simpler and allow us to get the most energy possible out of the fuel. Ultimately, though some the of the fuel must be stored. I appreciate your answer and your well reasoned comment.

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        You can find more suitable graphs using “radiotoxicity” as the keyword. (I should have mentioned that right away, sorry.) This is using sieverts, with the understanding that this would be the absolute maximum exposure to radioactivity. (In order to reach it, you would essentially need to inject every last ounce of nuclear waste into peoples blood.)

        Just googling, there is an example of such a graph in here:

        https://eddiehonorato.wordpress.com/tag/nuclear-waste/
        Specifically:
        https://eddiehonorato.files.wordpress.com/2011/09/radiotoxicity1.jpg

        The main differences are the time frame and a much better reference level. Using U-238 as a reference level for radioactivity really doesn’t give the matter its due. U-238 decays in a long decay chain, unlike the fission products we’re concerned with in nuclear waste, which decay in one or two steps. U-238 decays in 8 alpha and 6 beta decays until it becomes stable lead-206 and we have to compare the residual waste with that level of radiotoxicity.

        While I agree with your point that geologic isolation is still necessary for the residual waste, it doesn’t have to be isolated to the same degree. The difference are several orders of magnitude both in terms of time scales and potential risks, should there be some fault in the assessment of the repository. That’s not advocating for doing sloppy work, but it certainly ensures feasibility beyond reasonable doubt.

        Furthermore, the time scales concerned are no longer geologic, but historic (several centuries to several millenia):

        The town where I was born is well over 1000 years old. The first written records coincide roughly with the time when they started building a cathedral there, which is still around. Georg Argicola was buried there some 500 years ago. The remains of one of the Lords of the castle that was build around the cathedral, have been kept there for over 300 years, along with those of his wife and several of his children. (Not because he build it, but because he turned it into a baroque Schloss.) 200 years ago Napoleon stood next to the same place, talking to his generals on his way to Russia. (Don’t ask. Somebody thought this was remarkable enough to commemorate it on a stone.)

        There is a stronghold in a nearby village that is at least 800 years old, but hasn’t been of any military use for the last 500 years – it has been kept up quite well by the villagers and really, it doesn’t take all that much work.

        Those are timescales we are well acquainted with, though certainly not in all parts of the world. (And yes, of course I’m shaking my finger towards the USA when I say that. On the other hand, people here in Germany fail to get that point even though they are living right here in the middle of it.)

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