That was a fun title, and might have generated some interest, but probably it was the only thing that was fun about the entire entry. I lost the entry, probably because I was taking too long to prepare it. It's just as well. Take my word for it, it was a tortured entry. You really didn't want to read it.
It is true, all the same, that the wisent, the European buffalo, is one of many wild species of animals that lives in the Chernobyl exclusion zone. The wisent graze in the abandoned area where humans are more or less excluded, and the eat native radioactive grasses growing in radioactive ground that also grows radioactive trees.
After the Chernobyl reactor exploded many people, myself included, believed the area would become a desert and maybe nothing would be able to live there for a very long time. Nobody told the wild life about this though, and the wild life, being unable to read, simply paid no attention whatsoever to all the warnings and danger signs. One way or another they got into the exclusion zone where people are generally afraid to go. Some european animals that were threatened by habitat loss and other human interactions began to thrive in the Chernobyl exclusion zone, which led a number of somewhat shocked researchers to conclude that the risks to wildlife by human presence was worse than the risk of wallowing around in the entire radioactive inventory of an exploded nuclear reactor.
Now that was a surprise.
There's a movement afoot to keep humans away from Chernobyl forever, not so much to protect the humans but as to protect the animals, protect the animals not from radiation but from humans. People want to name the exclusion zone the "Chernobyl Wildlife Zone, or the "Chernobyl Preserve."
In a previous diary, entitled On Symmetry: Platonic Solids and Ugly Wastes, Lampblack, Coal and Carbon, I whined that nobody, not even the best scientists in the world, really know what's really in soot, but still soot is widely distributed, almost without discrimination, as a waste from burning fossil and biofuels. Nevertheless, I complained, as a proponent of nuclear power's rapid expansion I feel compelled - for reasons that I can only regard as the arbitrary focus of the general public - to discuss every single constituent of spent nuclear fuel, the stuff that some people call "dangerous nuclear waste," even though it has yet to be dangerous enough to have actually injured anyone.
In explainging why I am taking so long to accomplish this sisyphean task, I offer this less than reassuring defense: There are a lot of things in spent nuclear fuel, lots of different kinds of radioactive things. To discuss so called "nuclear waste," one must discuss the chemistry of a fair fraction of the elements in the periodic table, and many of the properties and the reactions. That ain't easy. On the other hand, I've spent a lot of time thinking about these things, so I have lots of stuff to say.
Some radioactive materials aren't very mobile in natural systems, but some are. I have chosen to discuss the constituents of spent nuclear fuel more or less beginning with those that find it easiest to get around, those that are without a doubt the hardest to to contain in a single place, those that are most difficult to remove from systems that are contaminated with them. I have already discussed tritium, a form of hydrogen that cannot be removed from contaminated water. And I have discussed radioactive iodine that has probably found its way into your flesh from French nuclear reactors.
Now I would like to discuss an element that is less familiar to people than either hydrogen or iodine, an element that is a large constituent of spent nuclear fuel and a potentially dangerous constituent as well, although, like I say, spent nuclear fuel from nuclear reactors has yet to kill as many people as a week's worth of air pollution in New York. This element is cesium, which is element 55 in the periodic table.
Some background on cesium: Natural cesium is a relatively rare element, 45th in crustal abundance and, owing to the high solubility of most of its compounds, is widely distributed. It is a constituent of seawater. It's chemistry is very much like the chemistry of the other members of its group in the periodic table, lithium, sodium, potassium, and rubidium. This is important in considering the dispostion of cesium in biological systems, including rare european buffalo like the wisent, some of whom, again, are eating radioactive cesium, which is all over the place in the Chernobyl exclusion zone where they live. Biologically, cesium behaves very much like potassium, an essential element for all living things. (It is ironic that all of the potassium on earth is slightly radioactive - something I will discuss further in my discussion of cesium.)
Only a few insoluble minerals containing cesium are known, and they represent the world supply for industrial applications of cesium. One of the more important minerals containing cesium is pollucite. Samples of pollucite are known from the Canadian Shield, where they are among the oldest known rocks on earth, almost unchanged since the earth's crust first solidified from the molten mass it once was.
The industrial uses for cesium for which pollucite is mined include making powder for gloves, scavengers for air in vaccuum tubes, and as a material that makes light beam detection devices, such as are used in automatic door openers and security devices that detect intruders.
Some people also say that cesium is the very best possible material possible for making ion propulsion engines that can propel spacecraft very far distances if operated beyond the earth's atmosphere. This is because cesium is a relatively heavy element that is very easy to ionize. This is not really an industrial application, but it could be someday, if spacecraft continue to operate in the future, especially if we want heavy spacecraft travel long distances, like say, to other planets. Natural cesium, which is 100% cesium-133, the non-radioactive isotope, is only the second best such material for this purpose. The very best possible material is the slightly radioactive isotope, cesium-135, which is a constituent of so called "dangerous nuclear waste." Cesium-137 would be even slightly better than Cs-135 in theory, but it is probably too radioactive, when compared to Cs-135 or non-radioactive Cs-133, to be used in this way.
So what about cesium and so called "nuclear waste?"
Cesium is a relatively common fission product in nuclear reactors, representing, depending on circumstances, more than ten percent of the elements formed when uranium or plutonium splits in a nuclear reactor. Cesium-137, which forms in 6.27% of the nuclear fissions of U-235 by thermal neutrons, is considered a potentially dangerous substance. It has a half-life of 30.07 years. If someone handed you a kilo of that stuff, and you stayed with it for only a very short time, maybe only a few seconds even, you would be killed, no ifs ands or buts. Your death would be painful, nasty and cruel.
Wow, that's scary.
Another radioactive isotope of cesium that is formed in nuclear reactors is Cs-135. The situation with its formation is somewhat more complex. Like cesium-137, it is not actually formed directly, but is formed in a series of decays from elements like iodine and xenon that are formed directly. Cesium-135 is a very long lived radioactive isotope. It has a half-life of 2.3 million years. Half-life is inversely proportional to radioactivity, and you could probably spend a fair amount of time in the presence of cesium-135 without being injured, especially if you stood a meter or so away from it. Cesium-135 is what is called a pure beta emitter, which more or less means that the radiation from it doesn't travel very far before being stopped. It would only be really dangerous if you swallowed it, and a fair amount of it too.
Here's some advice: Don't swallow it.
The amount of cesium-135 that is formed in a nuclear reactor depends on a lot of factors. I hate to get overly technical, but I'll try to explain it by first appealing to a mathematical formula - don't glaze over - called the Bateman Equations. If you have followed this link you see all kinds of mathematical symbols with which you may or may not be familiar. If you are unfamiliar with such symbols all you need to know is this: The term dN/dt is a measure of how rapidly the amount of a particular isotope, say cesium-135, is changing. On the right side of the equation there are two terms. One has a negative sign and one has a positive sign. If all of the stuff that is negative is exactly balanced by all of the stuff that is positive then dN/dt, the change in the amount of isotope present can in fact be zero. When dN/dt is zero the isotope is said to be in equilibrium. It is being destroyed as fast as it is being formed.
This effect very much involves the amount of cesium-135 formed in nuclear reactors, since it forms in the reactor from xenon-135 by beta decay. If you understood the Bateman equations, you would know that a lot of xenon-135 is destroyed in the reactor before it can form cesium-135, especially if the reactor is running at full power. This is because xenon-135 has the highest ability of any known substance to absorb neutrons. When it is formed in a reactor, it tends to slow the reactor down. Nuclear engineers have to engage in all sorts of tricks to get around this matter, but it's pretty routine now. The salient point is that the higher the level of power that you run a reactor, the less long lived cesium-135 you will see. (But be assured you will see some.)
Once the nuclear fuel is removed from the reactor, all of the xenon-135 in it will decay in a few days to form cesium-135. Meanwhile all of the cesium-137 will begin to decay and no new cesium-137 will be formed in the fuel. The two isotopes will also be mixed with some non-radioactive cesium-133, which is also formed in the reactor, and small amounts of two other isotopes, cesium-134 and cesium-136. The bateman equation will still apply to the removed fuel though, but certain terms having to do with the presence of neutrons will drop out. Radioactive cesium-isotopes will still be formed in the new nuclear fuel, but at the same time they will be decaying in the old fuel without being formed.
What does this mean? This means that radioactive isotopes in spent fuel cannot, particularly those with short half-lives - accumulate indefinitely. They reach a maxium amount, after which nothing that anyone does, other than changing the total amount of power being generated nuclear power, can more nuclear fission products than a certain maximum. This is very different from say, coal waste, where - if the waste in question is say, mercury, where every time one burns more coal one gets a fixed portion of more waste. The exact amount of the maximum that can accumulate for a particular radioactive fission product is subject to a lot of special circumstances, including the rate of transmutation whereby one element, which may have either a short or long half-life, is converted to another which has a different half-life. Generally one can accumulate lots of material that has a long half-life, and not all that much if the half-life is relatively short.
Some people, looking at the long half-life of cesium-135, have thought about transmuting it into barium. This is conceivable, but probably not practical, I think, because cesium-135 is always mixed with non-radioactive cesium-133, which wastes neutrons and can slow the rate of transmutation, as well as cesium-137, which is highly radioactive. It sounds cute, but I don't think it's worth the expense or effort.
I don't think the problem with cesium is as dangerous as advertised.
A technical mathematical interlude that you can skip over, if you'd like: The bateman equation is a type of equation known as a differential equation, and some of these have very complex solutions. However to a first approximation, considering that long lived nuclei will mostly decay outside of the reactor, where it will not be subject to transmutation, the process whereby nuclei decay will have a relatively simple form, in which the "parent" of the fission product is the fission process itself. Then, the accumulation of a fission product has the form shown in Equation 7.
The maximum is never reached, but is approached asymptotically so that the first year you are accumulating cesium-137, hundreds of tons of cesium-137 for instance, but if you keep at it for a few hundred years, you will only accumulate a few kilos of new cesium-137.
The number N(0) is a constant that depends on how much power you are producing by nuclear means and its half-life, and its fission yield. This constant represents the maximum amount amount of a particular constituent of so called "nuclear waste" that can accumulate.
I always give people an opportunity to assert that I am liar in the poll, and I will do so here, but I have built a somewhat crude spread sheet that calculates the maximum amount of cesium-137 that could accumulate if the world generated all of it's energy, now about 470 exajoules per year, via nuclear means. You can either take my word for it, or you can call me a liar.
It is actually not possible, with current technology, to make all of the world's energy via nuclear means, but these numbers representing some simplification, will suggest the general idea, assuming that I am not a liar.
For cesium-137 the maximum quantity one could accumulate by producing 470 exajoules of energy per year, is about 9,500 metric tons. Once one had this amount, so long as one continued to produce 470 exajoules of power, that is all one would have. At that point it would be decaying exactly at the same rate at which it is formed.
One would not have this much immediately, but would take many centuries to approach this number, each year adding a smaller and smaller amount of new material to the inventory. For example, the first year one would accumlate 218 MT of cesium-137, but also not a single kg of carbon dioxide, coal ash, nitrogen oxides, or any of the other filth associated with burning coal, oil and natural gas. (One might even be able to see the milky way at night, just as in olden times.) The 100 th year, one would have a total of 8,500 MT of cesium-137, but only 22 tons of this would be new material, the difference representing the amount that decays to non-radiaoctive barium-137 as the new material is formed. At the same time one would be still avoiding billions of tons of noxious fossil fuel wastes that nobody knows with what to do. In year 200, one would only need to find a place for 2 MT tons of cesium-137 than one needed to find before.
It's really not quite that simple as I am representing, since the cesium-137 will be mixed with other cesium isotopes, but this is the general idea on which I will expand in due course.
So how dangerous and deadly is 8,500 MT of cesium-137? It depends on what you mean by "dangerous." If you mean how many people could it kill if they all ate it, the answer could be stated as "very dangerous." But I will argue that one could create circumstances underwhich 8,500MT of cesium-137 would not mean very much at all. In fact, I will argue that the situation on some level has already been observed on earth and may even have something to do with why we exist at all. I will further argue that 8,500MT of cesium-137 accumulated over several centuries will probably never be as bad as the fossil fuel wastes that will accumulate over the next year. Maybe you'll believe me, and maybe you won't.
But this is a long diary entry, and I am running out of time right now. I will write more about cesium in the future, since there's a lot to say. I will also talk about other so called "dangerous" nuclear wastes in the future besides cesium.