Technetium in Use and in the Environment: Alloys, Sellafield Lobsters and Deep Eutectic Solvents.
The paper, among others, that I will discuss in this post is this one: Efficient and Selective Extraction of 99mTcO4– from Aqueous Media Using Hydrophobic Deep Eutectic Solvents (Tim E. Phelps , Nakara Bhawawet , Silvia S. Jurisson* , and Gary A. Baker,* ACS Sustainable Chem. Eng., 2018, 6 (11), pp 13656–13661)
In my late teens, I had a weekend job in a hospital pharmacy working on the distribution of IV solutions to the different hospital wings. The room in which I worked was not actually in the pharmacy, but was rather attached to the Central Supply room which was also staffed by two other teenagers, two young women - leading to all sorts of flirting but no real romance - one of whom was the niece of a prominent physician on the Hospital staff. Because of her uncle, she knew many of the doctors who worked in the hospital, including, as luck would have it, the pathologist who conducted autopsies. The pathologist, a Nisei, was an avuncular guy, and because of the connection with her uncle, that young woman was able to weasel invitations for the three of us to go to autopsies, perhaps because the pathologist was trying to stimulate interest among us kids to go into medicine or at least learn something about anatomy.
As a result, I got to watch maybe 10 or so autopsies after finishing my IV distribution work, and although I had no interest whatsoever of going into medicine, I certainly got some insight into at least one human disease, lung cancer, a disease which would later kill my father. I recall the autopsy of the lung cancer victim very well, probably much better than all the other autopsies I attended, with the possible exception of a still born baby with a three chambered heart. The pathologist was going through the lungs of the lung cancer victim and for our benefit, removed a tumor and sliced it in half to show us something very interesting, which was that at the very center of the tumor there were black particles, carbon I'd guess. Since I was worried all the time about my father - a heavy smoker who would nevertheless go on to live 20 years after these adventures of mine - I asked the pathologist if the dead man had been a smoker. "No," he replied, "smoker's lungs look much, much worse than this. This man was a teacher in New York City. This is from air pollution."
I never forgot that moment. On reflection, I think it changed my life. Thank you Dr. Araki.
Recently in this space, in an exchange with a dumb anti-nuke, if I recall correctly, I mused about the to which the idea that dangerous fossil fuels are cheap and affordable and even essential - and thus that we cannot live without them - is connected to the undeniable fact that they are routinely, without a peep, allowed to do what the nuclear industry is not allowed to do, which is to directly dump its waste products directly into the environment.
The distinction here is connected with something about which I often rail uselessly, the difference between internal and external costs. The internal cost of the dangerous fossil fuel gasoline is what you pay at the pump or at the outlet. The external cost is the cost (among others) of the people who die – often horribly, sometimes after long periods of disability - from the air pollution the combustion of your gasoline, never mind coal and gas for electricity, produces.
By contrast, the nuclear industry is held to a standard which essentially demands that no one ever be hurt or injured, much less killed by it’s by products – most commonly referred to as “nuclear waste,” although I personally don’t use that term since I insist that anything that is useful need not be “waste.”
As it happens though, the nuclear industry has deliberately released radioactive fission products in the past, most notably at the La Hague and Sellafield nuclear fuel reprocessing plants, one of which is now shut, albeit at an overall loss to humanity. The health consequences resulting from this practice would be negligible were it not for the pollution generated by people powering up their computers effectively to complain vociferously about how tragic it is that any radioactive atom exists anywhere; generally these are the same people who couldn’t care less about the 7 million people who die every year from air pollution and the fact that there are no living things on this planet that are not exposed to essentially all of the many toxic constituents of dangerous fossil fuel waste in addition to carbon dioxide.
Of course, one might argue that there are no living things on the planet which have not been exposed to fission products, for example, technetium in the form of the highly water soluble and therefore highly mobile pertechnate ion, TcO4-. This, by the way, is true. It is also true that people deliberately eat or are injected with the pertechnate ion in hopes of saving their lives, but that’s another matter on which I’ll touch below.
The processing plants at Sellafield utilize(d) the PUREX process for isolating plutonium from used nuclear fuel. This is a solvent extraction process wherein fuel rods are chopped and then largely dissolved in nitric acid. A series of solvent extractions, coupled with precise oxidation and reduction steps allow for separation of the various elements constituting fission products and actinides by selectively putting some of them in a form that makes them hydrophobic ,so that they can be extracted into the dangerous fossil fuel product kerosene containing complexing agents, the most famous of which is tributyl phosphate, although many similar complexing agents tailored for very specific target elements, for example, americium, neptunium and curium are known.
In these extractions, most of the fission products, except the precious metal fission products, ruthenium, rhodium and palladium as well as the gaseous fission products – largely krypton and xenon along with some decay generated helium-4 and sometimes small amounts of hydrogen containing three isotopes including the radioactive isotope tritium and its decay product helium-3, – remain in the acid solution. Notable constituents of this solution are the relatively long lived radioactive isotopes of cesium, iodine – chiefly iodine-129 – strontium and the aforementioned pertechnate ion.
Historically, these aqueous solutions have been problematic, and people didn’t know exactly what to do with them. At the Hanford Site in Washington State, where during the era of cold war hysteria, huge amounts of plutonium was isolated in order to manufacture nuclear weapons - these solutions were simply placed in giant tanks. The first tanks so utilized were single shell tanks from which they ultimately leaked, causing a great deal of concern and the expenditure of huge amounts of money to clean them up. The Hanford tanks are a giant bugaboo for those interesting people who freak out every time radioactivity is mentioned without even the slightest consideration of relative risk. Of course, there is no evidence that the nearby city of Richland, Washington is at risk of being depopulated by these tank leaks; there may be some elevated but as yet undiagnosed risks there; I can’t say. Perhaps the increased level risk of living in Richland is roughly comparable to the risk might approximate the risk of commuting in one’s automobile 50 miles per day instead of walking to work around the corner. Again, I can’t say, but it seems clear to me that the risks are overstated – often vastly overstated - when nuclear materials are discussed, and largely ignored when dangerous fossil fuel materials are discussed.
For a favorite example in my own discourse, one of the most stupid remarks I’ve encountered in this space came from a correspondent, now on my “ignore list,” who announced that the collapse of a tunnel containing some old chemical reaction vessels at the Hanford site “proved” that nuclear energy was “dangerous,” – even if there is zero evidence that anyone died or will die as a result of this event. Of course, the correspondent, from what I could tell, was completely disinterested in the possibility that 70 million deaths every decade, without stop, from air pollution might have some bearing on the related question of whether fossil fuels are “dangerous.” (I routinely claim that they are just that, dangerous fossil fuels, and seldom refer to fossil fuels in any other way.)
In Britain, the equivalent of the Hanford Site is the Sellafield Site on the coast of England, in Cumbria, which borders Scotland. It is the site of the world’s first commercial scale nuclear power plant,– the Russians had built a smaller plant that they connected to the grid a bit earlier – the Calder Hall reactor, which operated from 1956 to 2003, using rather (understandably) primitive technology.
Truth (an unpopular commodity) be told, though, the “primitive technology,” the carbon dioxide working fluid might prove quite interesting in the case that humanity actually got serious about climate change – there is no evidence this will actually happen – since the key to the thermochemical splitting of carbon dioxide is, as it sounds, hot (supercritical) carbon dioxide. This suggests that one could imagine a working fluid that is a continuously shifting mixture ratio of two carbon oxides, dioxide and monoxide, with periodic separations of the two, one providing carbon for use, the other turning turbines to drastically increase thermodynamic efficiency and generating some pure oxygen on the side, which also might be useful for industrial scale carbon capture from the air.
The Calder Hall Reactor was deliberately designed to be a type of reactor that made access to weapons grade plutonium possible. Obtaining weapons grade plutonium is economically wasteful, since it requires relatively short irradiation times and low burn up in order to prevent the buildup of Pu240, which greatly complicates weapons manufacture as well as reducing the yield of weapons know colloquially as “fizzle.”
“Burn up” in a nuclear reactor may be thought of as fuel efficiency, translating “miles per gallon” into “megawatt-days per ton.” The growth in US (and other countries’) nuclear power production in the 1990’s – which lead to it having the highest capacity utilization of any type of electrical generation plant - despite the disastrous US policy of abandoning nuclear plant construction infrastructure, is, along with operating experience, a function of increased burn-up, since it reduces the number of days required for refueling shut downs. The longer the burn up time utilized in a reactor at maximum power levels, the more useless the plutonium generated in them becomes for weapons manufacture. For “breed and burn” types of reactors, accumulation of Pu238 (from the buildup and decay of Cm242) makes the use of all of the plutonium completely unusable for weapons. (Cf. A new scientific solution for preventing the misuse of reactor-grade plutonium as nuclear explosive (Kessler et al, Nuclear Engineering and Design 238 (2008) 3429–3444).)
Let’s be clear as an aside: The British nuclear weapons program utilized Calder Hall weapons grade plutonium (as well as Windscale Plutonium) to make nuclear weapons. Their nuclear weapon inventory is nowhere near the scale of either the Russian or American inventory, but the recent demonstration that even in a formerly stable democracy, the control of nuclear weapons can fall into the hands of patently insane people – the existence and position of the clearly insane person Donald Trump shows that the representation of Jack T. Ripper in “Dr. Strangelove” was, and now is, not as much comedy as designed – obviates the fact that nuclear weapons must be eliminated and that no country can be trusted to possess them.
The reprocessing of used nuclear fuel for the purpose of making bomb grade plutonium with the PUREX process requires the use of far more solvents and reagents than reprocessing commercial nuclear fuel because the plutonium contained in the fuel is far more dilute. Although ultimately the British switched to the less onerous processing of commercial fuels, almost all of the initial reprocessing performed at Sellafield was for fuels being used to make weapons.
The British utilized a different approach to dealing with the aqueous raffinates containing fission products (and small residual amounts of actinides) than the American did at Hanford and the Soviets did at Mayak. After some processing, they, having a coastal plant as opposed to inland plants like Mayak and Hanford, dumped many of the fission products, along with actinide residues including plutonium, in the ocean utilizing an outfall pipe extending a few kilometers into the Irish Sea. (British Magnox fuels which were designed for simplified reprocessing, unlike American, French, and most other nations' nuclear fuel, must be reprocessed; they are not suitable for long term storage.)
For the rest of this post, I will focus on one radioactive component of this raffinate, technetium.
Technetium, which has the chemical symbol Tc, is the 43rd element in the periodic table. All of its known isotopes are radioactive. One isotope – one that is actually quite difficult to obtain – 98Tc, has a half-life of 4.2 million years, still far too short to have survived the 4.5 billion years since the accretion of the earth from the supernovae ejecta from which it formed. It is unlikely that there is even 100 grams of it on the entire planet. Any that exists has been manufactured by humanity at great expense in accelerators, or obtained in nuclear reactors at miniscule yields. The better known and more readily available isotope is 99Tc which is a major fission product when an actinide element such as uranium or plutonium, americium or curium undergoes a fission event, either spontaneously or as a result of being struck by a neutron as in a nuclear reactor. It is generally regarded as a “synthetic element,” and was, in fact, the first such element to be discovered, in 1937. That said, because uranium is a relatively common element in the earth’s crust, as common as tin, and because uranium has continuously been undergoing spontaneous fission since the formation of the earth, it is now understood that technetium does occur naturally, albeit in concentrations that are so low that it makes its detection exceedingly difficult and its isolation from natural sources nearly impossible.
The Earth’s oceans, for instance, contain – limited only by uranium’s marginal solubility in water – about 5 billion tons of uranium. The half-life of uranium’s most common isotope, 238U, is 4.468 billion years, coincidentally nearly equal to the age of the earth. The nuclear decay of uranium – which like technetium only has radioactive isotopes – usually decays by ? emission, but far more rarely it also undergoes spontaneous fission. The spontaneous fission half life of Uranium-238, as opposed to its much shorter ? decay half-life, is about 8 quadrillion years, meaning that the decay constant (=ln(2)/t½) for spontaneous fission is 2.8 X 10-24 sec^(-1). From this information, one can calculate that about 30 trillion atoms of uranium fission each second in Earth’s oceans. The fission yield of lighter isotopes for uranium-238’s spontaneous fission is given at the JENDL website and can be found by summing the fission yields of the predominant species with mass number 99 formed directly by fission, 99Rb, 99Sr, 99Y, 99Zr, 99Nb, and 99Mo, each one upon formation decaying by ?- decay into the following member of the decay series. The sum of these yields suggests that 6.154% of the time, a nucleus with mass number 99 is formed under these circumstances. With the exception of 99Mo, which has a half-life of about 64 hours, none of these highly radioactive isotopes has a half-life exceeding 2 minutes, and all of them rapidly decay ultimately to give 99Tc which has a half-life of 211,100 years. Thus 99Tc represents a “decay bottleneck,” if you will, before itself decaying into the stable ruthenium isotope 99Ru. From the decay constant of 99Tc, which is 1.04 X 10-13 sec-1, one can calculate that the steady state quantity of 99Tc – the amount of 99Tc that can accumulate before it decays at exactly the same rate at which it is formed – in earth’s oceans arising from the natural spontaneous decay of solvated uranium is about 3.4 kilograms, distributed in all the oceans in all the world.
The amount of technetium released at Sellafield absolutely dwarfs the amount of naturally occurring technetium in the ocean. It is estimated that the total cumulative amount of technetium as of 2006 was 1700 terabequerels. (cf. An estimate of the inventory of technetium-99 in the sub-tidal sediments of the Irish Sea (Leonard et al Journal of Environmental Radioactivity 133 (2014) 40-47).) The specific activity of technetium is 0.63355 gigabequerels per gram, meaning that the total cumulative amount of technetium released into the Irish Sea was on the order of 2.7 metric tons. Although technetium PUREX raffinates all contain technetium in the +7 oxidation state, again, as the highly soluble pertechnate ion, TcO4-, it is well known from nuclear testing, nuclear accidents, and sewage treatment plants containing technetium from the urine of patients injected with the element for imaging purposes, that the pertechnate ion can be reduced by organic matter or by organisms to the insoluble oxide in the +4 state, TcO2, technetium dioxide. The paper just cited indicates that about 2% of the Sellafield releases are found in sediments in the Irish Sea, undoubtedly largely in this form.
The quantities of technetium released at Sellafield varied from year to year and exhibited large peaks and valleys. A paper on the subject of technetium releases and incorporation of the element into the edible tissues of local lobsters and crabs reports that technetium releases peaked around 1995 after a new plant, most probably using a new variant of the PUREX process, called the EARP (Enhanced Actinide Recovery Plant) went into operation in 1994. (cf. Variability in the edible fraction content of 60Co, 99Tc, 110mAg, 137Cs and 241Am between individual crabs and lobsters from Sellafield (north eastern Irish Sea) (D.J. Swift, M.D. Nicholson, Journal of Environmental Radioactivity 54 (2001) 311-326). According to this paper, the Sellafield Tc releases rose from about 5 Terabequerels on average in preceding years to 72 TBeq (114 kg), 192 TBeq (303 kg), 155 TBeq (245 kg), 84 TBeq (133 kg), and 53 TBeq (84 kg).
The authors of this paper purchased crabs and lobsters from local fishermen who were known to be obtaining their catches near the Sellafield outfall pipe on May 25, and June 5, of 1997.
The number of crabs collected was 34, with 16 females and 18 males being collected. The mean weight of the crabs was 490 grams, and the mean weight edible tissues was 148 grams.
For lobsters, 37 were collected, 20 females and 17 males. The mean weight of these animals was 654 grams - 523 grams for females, 807 grams per male, with the edible portions of each, respectively, being 160 grams and 246 grams.
In this paper, the naturally occurring radioactive isotope K-40 that is present in all of the essential element potassium on the Earth is measured as a marker for all of the other radioisotopes measured in the experiment, Ag-110m, Cs-137, Am-241, Co-60 as well as Tc-99. In crabs, the only radisotope found to be at the same order of maginitude as postassium-40 is technetium; all the others are found at an order of magnitude lower. The mean measurement for potassium-40 is 93 Beq/kg, and 126 Beq/kg for crabs, with a rather large standard deviation which is on the order of the measurement itself. The other radioisotopes are found at levels approaching their lower limit of detection, an order of magnitude smaller than potassium-40.
For lobsters, the situation is quite different.
Lobsters seem to concentrate technetium. The mean value for lobsters is 11,300 Beq/kg, reflecting 7,131 Beq/kg for male lobsters and 14,853 Beq/kg. The mean mass of edible meat of female lobsters in this sample was 160 grams, meaning that if one ate an “average” female lobster that lived its life near the Sellafield outfall pipe in the spring of 1997, one would end up eating 2,377 Beq of Technetium, without reference to the experimental error discussed at length in this paper.
I plainly confess that for about 1/3 of my adult life, I was an anti-nuke, sometimes rabid, sometimes passive, and as such, was completely divorced from critical thinking about nuclear issues and in fact I knew nothing at all, effectively, about nuclear science but nonetheless felt myself qualified occasionally to be outraged by the existence of things like the Sellafield plant. I can easily therefore imagine how a person dumb enough to say, think that the collapse of a tunnel holding old radiologically contaminated chemical vessels at Hanford suggests that nuclear power is “dangerous” on a planet where 7 million people die every year from air pollution, might view the idea of 2,377 Beq of technetium in the meat of a lobster growing up near the outfall pipe of the Sellafield nuclear reprocessing plant. One can be quite sure as well that the ship of fools belonging to that benighted organization Greenpeace, the Rainbow Warrior, cruised all around the North Sea burning diesel fuel carrying functional idiots flashing signs and banners with inane content all because of Sellafield.
The means of detection of technetium in lobster meat says nothing at all about the chemical speciation of the technetium in it, whether it is present as soluble pertechnate – which apparently behaves much like iodine biochemically – or as insoluble technetium dioxide. In the latter case, it’s not clear that any technetium ate by a lobster lover would be bioavailable and thus absorbed into the blood stream of the feasting person. But let’s consider opposite case, the pertechnate case, and assume really without any justification that all of the technetium found in a consumed female lobster is absorbed by the person eating the lobster. The biological half-life of technetium, the rate at which it is excreted, or put in cruder terms, pissed away, is well known because of its extreme technological importance in medical imagining and in cancer treatment. It is about 1 day. The reason it is so well known is that about 20 million people per year are deliberately injected with solutions containing technetium complexes. The reason this is so is not for the purpose of comparing them to Sellafield lobsters of course, but is rather intended to save their lives, or perhaps just to improve them dramatically. The technetium so injected is far more radioactive than Sellafield technetium, since it is largely a nuclear isomer of technetium-99, technetium-99m, which decays via an isomeric transition with the release of a gamma ray into the same technetium-99 at the outfall pipes of Sellafield. The radiological half-life of the decay of Tc-99m into Tc-99 is about 6 hours, and it is obtained from shipments of its parent nuclide, Mo-99, with a half life of 64 hours, prepared in research reactors.
According to the Wikipedia page for Technetium-99m, the radiological dose that is typical of at least one procedure, a bone scan, ranges from 700 million Beq to 1,110 million Beq, which the reader is invited to compare with eating a female Sellafield lobster, which would provide a dose that is 2 to 3 orders of magnitude smaller assuming complete absorption of all of the technetium the animal contains. Please note that the number of people who have eaten such lobsters is vanishingly small.
The conceit of anti-nukes who actually regard Greenpeace as an environmental organization, egged on in this belief by a credulous scientifically illiterate news media, as opposed to how I regard them, as an organization roughly comparable to an organization of anti-vaxxers, or creationists (at best), is that if one eats even one radioactive atom that has found its way into the environment as a result of the operations of nuclear energy related plants, tragedy will inevitably result.
This is a Trumpian scale misrepresentation of reality, in other words, a bald faced lie. The toxicology of any agent of environmental insult is inherently statistical. It is simply not true, for example, that everyone who is exposed to air pollution will die from it. Statistical analysis suggest that 7 million people die each year from it, but billions of people who have been exposed to it will die from something else, like say automobile accidents, gun fire, excess consumption of fried foods, malnutrition, etc, "the thousand natural shocks that flesh is heir to."
I opened this long post with a description of the dissection of lung tissue that so obviated the relationship between air pollution and fatal disease that even a teenaged kid could discern the relationship visually. No such relationship between eating lobsters captured near the outfall pipes of the Sellafield Nuclear reprocessing facility is likely to exist, and if there are any people on this planet whose cancers developed from technetium released at either Sellafield or La Hague, there is simply no way that they exceed the cancers and deaths that would have resulted if the Magnox fuel had never existed, if Britain continued to rely on coal as it did at the time that the Calder Hall reactor was built.
To wit:
One of the best evocations of critical thinking about nuclear energy was written by the late Nobel Laureate (and Stanford Professor of Physics) Burton Richter, commenting on the paper written by the anti-nuke fellow Stanford Professor Mark Z. Jacobson, who I personally regard as a useless idiot. Writing in a comment in the journal Energy and Environmental Science, Richter wrote:
For context, in December of 1952, a few years before Calder Hall went on line, about 4000 people died as a result of a serious coal pollution event, the London Smog event, resulting from cold temperatures and low wind speeds.
(cf. Richter, Opinion on “Worldwide health effects of the Fukushima Daiichi nuclear accident” by J. E. Ten Hoeve and M. Z. Jacobson, Energy Environ. Sci., 2012,, Energy Environ. Sci., 2012, 5, 8758.
The essence of the paper is that nuclear energy need not be completely free of risk in order to save lives on balance.
Balance.
From all of the above, a fair assessment of what I have written would suggest that I am perfectly OK with people dumping technetium in the ocean. This, however, is not true.
Rhenium, the cogener of technetium is a very rare, but very valuable strategic metal, the chief use of which is as an alloying agent to make “superalloys” – alloys displaying remarkable chemical resistance and structural integrity at high temperatures. Because of the lanthanide contraction, the atomic radii of technetium and rhenium are quite nearly identical, and thus the alloy properties are very similar. Rhenium is subject to depletion, and, since the world supply of uranium can be shown to be effectively infinite, it is quite possible that the world supply of technetium might well be made to outstrip the supply of rhenium, and alloy for its use in closed systems such as high temperature turbines used in combined cycle power plants, almost all of which in existence today are dangerous natural gas plants, although it is quite possible to imagine cleaner and safer nuclear plants that would utilize this same combined cycle approach to improving thermodynamic efficiency.
There are only small differences between the chemistry and metallurgy of rhenium and technetium, these differences resulting from the shielding by filled f orbitals in rhenium that are not present in technetium. The melting point of rhenium (3186 C) is higher than technetium (2157 C) and rhenium forms a volatile heptafluoride whereas technetium does not form a heptafluoride at all. Otherwise the two elements are nearly identical.
Prevented from growing as much as it would in a sane world by appeals to fear and ignorance delivered by Greenpeace types, nuclear energy has been stuck at providing consistently about 28 exajoules for more than 3 decades. While this is nearly three times as much as the combined solar, wind, geothermal and tidal so called “renewable energy” schemes have managed despite nearly half a century of mindless cheering, it is clearly not enough, since the use of dangerous fossil fuels is rising, not falling, this as the obvious dire effects of climate change are becoming more obvious, and as the accumulation of the dangerous fossil fuel waste carbon dioxide is rising at the fastest rate ever observed.
An estimate of the total accumulated technetium available in used nuclear fuels has been made and is available here:
Determination of technetium-99 in environmental samples: A review (Hou et al Analytica Chimica Acta 709 (2012) 1– 20). In this publication the authors state that up to 2006 about 140 Petabequerels of technetium had accumulated and that the growth rate of technetium is roughly 5.8 terabequerels per GWy of thermal (primary) nuclear energy production. From these figures, given the yearly average primary nuclear energy production of 28.4 exajoules, one can estimate that nuclear power plants have generated about 303 metric tons of technetium.
Of more interest to me personally than superalloys is the effect of alloying with technetium has on tungsten.
In the early 1960's, based on designs developed in the 1950's, a small test nuclear reactor having liquid plutonium fuel - actually a plutonium/iron eutectic - ran for a few years. This was the LAMPRE reactor. Since I first learned about this reactor, it's intrigued me. At the time of its development, only two metals were known that could contain liquid plutonium, since the liquid metal is quite corrosive to many metals since it dissolves them. One such metal was tantalum; the other was tungsten, both of which are high melting metals. For the actual reactor that was build and operated, tantalum was chosen because tungsten is a poorly machinable metal, particularly because it is brittle and lacks ductility, and the LAMPRE design included capsules featured the use of capsules that could only be made using a machinable metal subject to welding.
Tantalum is now understood to be a conflict metal, which means that it is mined under appalling social conditions, which sometimes include effective slavery for children. (The chief use of this rare metal is in cell phones, where it is useful to make small compact supercapacitors.) In an ethical world, its use would be constrained as much as is possible. For this reason, if one were to build reactors designed to exploit the many powerful features that liquid plutonium and/or its known binary and ternary eutectics – the ability spontaneously to separate fission products on line with in situ extractions, distillations and/or phase separations as well as the ability to instantaneously denature weapons grade plutonium to make it useless for use in weapons – one could not ethically do so using tantalum capsules.
The machinability of tungsten can be greatly improved by alloying it rhenium, but significantly more rhenium is required than is the case with commercial superalloys found in jet engines and gas turbines. Since rhenium is rare and expensive and easily subject to depletion, this does not represent a viable approach to reviving the LAMPRE concept on the industrial scale that would be required for any serious effort to both replace dangerous fossil fuels while cleansing the atmosphere of dangerous fossil fuel wastes, particularly the most dangerous of all, given the implications of climate change, carbon dioxide.
Completed in 1968, about six years after the LAMPRE project was defunded and the reactor shut down, at the Pacific Northwest National Laboratory (PNNL) where plenty of technetium was available since the laboratory is adjacent to the Hanford reservation, experiments were conducted to consider whether technetium offered the same advantages to tungsten alloys that rhenium did. The report on these experiments is here: Concluding Progress Report: A Study of Tungsten-Technetium Alloys (Nelson and O'Keefe, BNWL-865, 1968). It was found that the ductilization of tungsten by alloying with technetium had a transition temperature comparable to that of alloying with rhenium in the range of 3% to 25% technetium.
The phase diagram for the tungsten/technetium system is available on the ASTM database, and I have it in my files. The melting point of pure tungsten, 3422C, falls only to 3000C with the addition of 20% technetium.
The maximum temperature requirement for splitting carbon dioxide to get carbon monoxide and oxygen in one known system (cerium oxide catalysis) is considerably lower than this temperature, around 1400C for the oxygen generating step. The boiling point of strontium metal is 1377C, and of course, lower at reduced pressure. These facts make the properties of the tungsten-technetium alloy intriguing, and suggest that the quantities of technetium available currently – around 300 MT – are huge only if one is considering the metal as “nuclear waste” but small if one is considering the metal as a valuable alloying agent.
When I contemplate potential LAMPRE based reactor designs, I do so in imagining a “breed and burn system, reactors designed to run without refueling for significant fractions of a century. It is interesting to note, as an aside – without going into significant detail about how a tungsten technetium alloy might fit into this system – that a tungsten alloy under neutron irradiation for 7 or 8 decades would result in the transmutation of relatively inexpensive tungsten into the extremely valuable and rare metals including the aforementioned rhenium, the very valuable catalytic metal iridium, as well as osmium. Under the same circumstances, technetium would be transmuted into ruthenium and rhodium, also very valuable metals.
All of the above suggests that there are far more important things to do with technetium than to dump into the Irish Sea, thus causing some whining and crying from say, Norwegians, about detectable technetium in the coastal seas where they drill for oil and gas, the waste products of which end up in the flesh of every living thing on this planet while completely destabilizing the planetary climate.
(Norwegian caterwauling about Sellafield technetium, funded by the Norwegian ministry of fisheries, can be found here: Technetium-99 Contamination in the North Sea and in Norwegian Coastal Areas 1996 and 1997 (Brown et al StrålevernRapport 1998:3))
It is difficult to say what the energy demand of a world that was both sustainable and ethical might be. Current world energy demand, as of 2017, according to the most recent World Energy Outlook published by the International Energy Agency a few weeks ago was 584 exajoules. For a wild guess, let’s say that a world in which the dual goals of eliminating world poverty, eliminating the use of dangerous fossil fuels, and cleaning up the planetary atmosphere by removing much of the dangerous fossil fuel waste carbon dioxide from it – all these goals are synergistic – world energy demand might be on the order of 750 exajoules/year. To achieve these goals, in my view, all of the world’s energy would need to be obtained from nuclear energy, not necessarily via an electricity intermediate, but rather in a very highly thermodynamic efficient manner, wherein electricity might only be a side product from the use of primary nuclear energy to drive chemical reactions and separations.
It is certainly possible to estimate where the secular equilibrium between the formation of technetium and its rate of decay and/or transmutation as a reactor material might lie. (The secular equilibrium point is the point at which technetium would be decaying and/or transmuting at exactly the same rate it is being formed; all radioactive nuclei undergoing formation have such a point, representing the maximal amount that can accumulate.) I have neither the time nor the resources accurately and conveniently to do this estimation, but no matter. This said, using the calculations of the type above, and the Beq/GWy conversion factor, we can estimate that at 750 exajoules/year produced by nuclear fission using U-238 as a fuel in “breed and burn” reactors, roughly 220 tons could accumulate in a single year, and be available for use. The long half-life of Tc-99 suggests that it would be possible to obtain considerable quantities of this valuable metal, surely enough to displace rhenium demand for almost all closed systems demanding its alloys.
To achieve this goal, it is important that as little technetium as is possible be lost to waste, either in seawater or in waste dumps of any kind.
This brings me finally to the paper cited at the beginning of this post, the paper on the extraction of technetium from aqueous solutions using deep eutectic solvents, a capability that, along with separations of other radionuclides, might have well eliminated the need for the wasteful and unpopular Sellafield outfall pipe. (Tim E. Phelps , Nakara Bhawawet , Silvia S. Jurisson* , and Gary A. Baker,* ACS Sustainable Chem. Eng., 2018, 6 (11), pp 13656–13661, cited at the outset.)
For the uninitiated, the authors describe, in their opening paragraph, what a deep eutectic solvent is:
They continue:
A picture from the paper showing the structures of these deep eutectic solvents might clarify any difficulty associated with the chemical names:
The caption:
The use of these solvents is investigated for two purposes: One is to recover technetium from aqueous processing solutions and the other is to remove it from contaminated environmental matrices, for example groundwater. For this reason, the authors investigate its use in the presence of many common anions, chloride, phosphate, nitrate, carbonate, etc. They also add perrhenate to the equation to examine their utility in separating these two closely related species.
The measure the extraction efficiency and distribution coefficients (measures of the selectivity of the extractions) they use very dilute solutions, and utilize Tc-99m, not Tc-99, because of its higher activity, and thus ease of detection, in very dilute solutions.
The caption:
The distribution ratios are also very high:
The caption:
The effect of volume ratios and time of extraction is examined.
Volume:
The caption:
Time:
The caption:
However back extraction, recovery of the technetium from the deep eutectic solvent using another solvent solution is somewhat problematic, at least for the few solvents explored:
The caption:
In their conclusion the authors refer to this problem and propose, but do not claim to have explored alternative solutions to this problem:
do not impede 99mTcO4 ? extraction. Unsurprisingly, the ReO4 ? anion suppresses 99mTcO4 ? extraction when present in stoichiometric amounts relative to the DES. Attempts at back extraction showed limited success, although a number of avenues (e.g., Zn reduction, electrodeposition) can be considered for sequestering 99Tc from the spent DES in the future.
Given their favorable properties and low extraction volumes required, the current results have important ramifications for emerging applications using hydrophobic DESs for the extraction and separation of important tetra-oxo anions and radionuclides listed as priority pollutants by the U.S. Environmental Protection Agency, particularly for removing low levels of TcO4 ? from contaminated groundwater and potentially for remediating other metalate pollutants such as perchlorate as well...
In their conclusion they also discuss certain modified PUREX like solvent extraction procedures, and the potential utility of their system for cleaning up solvents utilized in it.
I should say that personally, I'm not a solvent extraction (PUREX, UREX, TRUEX…) kind of guy in general, and prefer the development pyroprocessing electrochemical approaches given recent improvements in the electrochemical reduction of metal cations to the metals. My ideas about these processes, involving esoteric molten salts of various types, however may include liquid membranes as separation tools, and thus the existence of ionic liquids, which these deep eutectic solvents are a subset, demonstrating immiscibility with aqueous solutions are always of interest to my ruminations on this topic.
I realize that this post is fairly technical and long, and that few people will read it, and fewer will derive any value from it. My real purpose in writing it was not to convince anyone of anything - where nuclear issues are concerned, most people regrettably have closed minds, much to the detriment of humanity, and, or course, the environment - but rather to clarify and expand on some old ideas in my own mind.
A few paragraphs in this post have come from the saved text of another post I posted elsewhere on the internet, but which has disappeared, probably because the website on which I posted it (Energy Collective) has been acquired and archives have been deleted. That post focused primarily on the use of technetium in superalloys, since I was not even aware of issues in the LAMPRE reactor at that time, and had given no thought at all to the machinability of tungsten.
I personally had a wonderful weekend. Both of my sons came home to visit us and I got to spend lots of time with them and with my wife and extended family.
I trust and hope yours was as wonderful as mine.
