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 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 α emission, but far more rarely it also undergoes spontaneous fission. The spontaneous fission half life of Uranium-238, as opposed to its much shorter α 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 β– 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. (See Figure 1 below) 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.
Figure 1: The Fission Yield Distribution for the Spontaneous Fission of 238U.
Actually the amount of 99Tc that is found in the oceans today is much larger than 3.4 kg, owing to historical open air nuclear weapons testing and because of the nuclear fuel reprocessing operations conducted at La Hague in France and at Sellafield in the UK, where, historically technetium in its anionic form TcO4-1 was disposed of by dumping it in the ocean.
If you think that this historic dumping of this radioisotope at Sellafield and La Hague was tragic, I agree with you, but it’s quite possible that that your reasons for thinking so are very different than my reasons for thinking so.
Irrespective of what I expect would be the common perception I would argue that the risk to human (or for that matter fish) health from Sellafield/La Hague technetium is extraordinarily – even vanishingly – small, as can be seen when one recognizes that 99Mo is often manufactured in accelerators for medical purposes, in order to provide a source of 99mTc, a nuclear isomer of 99Tc (to which 99mTc decays with a 6 hour half-life) so that people can eat technetium (or be injected with it) for medical imagining and medical treatment purposes, and the isotope is regarded as the “workhorse” of nuclear medicine. The amount of technetium that one could obtain by eating 100’s of metric tons of North Atlantic fish – or drinking tons of seawater – is dwarfed by the amount might eat for a typical medical test or treatment, probably dwarfed by the radioactivity in a typical banana. I very much doubt that anyone has ever died anywhere because they’ve ingested Sellafield or La Hague technetium. If I am wrong, and someone has died from Sellafield or La Hague technetium there is no way that the number of deaths associated with technetium even remotely approaches, within six or seven orders of magnitude, the number of deaths that would have been caused if a fossil fuel had been used to generate the same electricity that was generated using the process by which technetium forms, since the fossil fuel plants cause health problems and death – on a large scale – whenever they operate, with or without treatment of dangerous fossil fuel waste, with or without accidental events.
No, the reason that I find the practice of dumping technetium to be tragic is that occurs to me – and I can make a similar case for just about every radioactive or nonradioactive element found in that wonderful mishmash called “used nuclear fuel” or less accurately “high level nuclear waste” – that technetium is potentially a very, very, very, very useful material.
The idea that technetium might well be useful, possibly even essential, is not a new idea, nor does it originate with me. An old book I happen to have in my personal library – from a series on the analytical chemistry of the elements prepared by Soviet scientists and translated into English by Israeli scientists1 – contains the following text:
As it happens, in the forty years since this remark was translated into English, humanity has accumulated large amounts of technetium, a crude estimate on my part estimates that the amount should be roughly on the order of 100 MT of the stuff, mostly suspended in the matrices of used nuclear fuel.
I am always interested in the chemistry of this wonderful element, and whenever I happen to be scanning the title pages of a scientific journal or monograph, my eye is always drawn to a title in which it is mentioned. As it happens, today the scientific literature relating to technetium is dominated either by articles about the geochemistry of the element, primarily its anionic chemistry in the form of the water soluble TcO41-ion, often in equilibrium with the insoluble compound TcO2, or about the medicinal nuclear chemistry of technetium, which represents, right now, the only common technological application that the element enjoys.
The interest in the geochemistry is of course, related to the interest in the still absurd quest to dispose of the element for eternity, or worse, to destroy it outright via transmutation, even though its health effects are trivial when compared to humanity’s less urgent quest to dispose of, for instance, again, air pollutants or, for that matter, fecal matter, each of which is responsible, unlike technetium, for many millions of deaths each year.
Besides papers on the geochemistry or medicinal chemistry of technetium, it is far rarer to find papers about the metallurgy of the element, in particular, the properties of alloys and compounds. To the extent that technetium metal and its alloys are discussed at all, it is usually the single technetium/ruthenium alloy which is discussed in the context of transmuting technetium via the bombardment with neutrons – into that second, albeit valuable element. When 99Tc captures a neutron in a nuclear reactor, it becomes 100Tc, which has a 15 second half-life and decays into 100Ru, a stable naturally occurring isotope of ruthenium: Thus any technetium metal in a neutron flux will automatically be transformed into a ruthenium alloy. Although ruthenium is a valuable metal in its own right, I think it an unfortunate idea to attempt to deliberately convert technetium into it. For one thing, there are only seven metals – they are tantalum, tungsten, rhenium, molybdenum, niobium, ruthenium and osmium – and eight elements if one includes carbon, that have a higher melting point than technetium. (The melting point of technetium is 2430 degrees Kelvin.) Although ruthenium has a higher melting point, it also has a different chemistry, particularly with respect to oxygen that make it less than ideal in certain circumstances, and in any case, plenty of ruthenium is available in used nuclear fuels even without transmuting technetium into it.
Rhenium, to focus on one such higher melting element listed, is exceedingly rare. Rhenium is so rare that it was the last stable element in the periodic table to be discovered, this in 1923 by Ida Noddack, who also claimed to have discovered rhenium’s cogener, technetium, which she named “masurium.” (Although the masurium claim was long discounted, in recent years there are some people who claim that Noddack did in fact isolate technetium resulting from the spontaneous fission of uranium found in rocks – we will probably never know – but her unambiguous discovery of rhenium alone defines her as a brilliant chemist.)
Despite its rarity and very high cost, rhenium is now regarded as an extremely important strategic industrial material because of its use in the preparation of what are known as “superalloys” – metallic alloys that exhibit high strength at very high temperatures, refractory alloys. (These alloys also, depending on their design, can show very high resistance to chemical attack.) All of the world’s jet engines, as well as the turbines in many power plants – especially those found in so called “combined cycle” power plants –utilize superalloys, including some that contain significant amounts of rhenium. It is not too much to say that the role of superalloys is of paramount importance to almost any attempt address large scale improvements in both energy efficiency and sustainability. Superalloys are also used in spacecraft and potentially would be of use in certain classes of nuclear reactors, should humanity come to its senses and fully embrace that form of energy, even if it is probably too late to do so.
The base metal for almost all superalloys is nickel, which has a melting point significantly lower than either technetium or rhenium, and the purpose of putting other elements – many elements are involved – in these alloys is to improve upon the properties of pure nickel to optimize their use in specific applications, particularly those involve extreme conditions of temperature, chemistry and mechanical wear.
I will focus on the existing use of rhenium in current commercial and experimental superalloys, since it turns out, the chemistry of rhenium and technetium, as well as their metallurgical properties are very similar: Chemically they are congeners. They are so similar in fact that if technetium had isotopes long lived enough to form ores, rhenium would almost always be an impurity in them, as is the case with ores of niobium and tantalum, of zirconium and hafnium, and of tungsten and molybdenum.
(A little appreciated fact is that one of the industrial triumphs of the Manhattan project, besides the separation of uranium isotopes and Glenn Seaborg’s scale up of the separation of plutonium from uranium and fission products from a picogram scale to a kilogram scale, was to develop an industrial process to separate zirconium from hafnium. Although zirconium and hafnium are almost identical in a chemical and metallurgic sense, hafnium has very different nuclear properties than zirconium and poisons nuclear chain reactions: To this day hafnium free zirconium is an important nuclear material.)
Anyway, about the rhenium containing superalloys: The properties of some superalloys – not all, but many, of those containing rhenium – derive from their being polycrystalline, from is having atomic scale inhomogeneities. One important atomic property that has bearing on the nature of these inhomogeneities is actually the size of the atoms involved in forming a structure, specifically the atomic radius of the atoms in thecrystals involved, in this case metallic crystals.2
The chemistry and physics of metals is elucidated by their positions in the periodic table. It is thus worth noting that because of an effect, a quirk, in the periodic table relating to quantum mechanical effects in what are called “f elements” that is known as the “lanthanide contraction,” the atomic radius of technetium (136 pm) is not very different than its cogener rhenium (137 pm) even though rhenium is nearly twice as massive. This results, within some limits, in the expectation that in many cases rhenium and technetium might be almost interchangeable, that one might substitute one for the other without much sacrifice in the usefulness of materials. This fact, I argue, makes the current worldwide policy in dealing with technetium stockpiles, the policy of seeking to get rid of it, to put it out of our sight – somewhat dubious in my view.
Let me elaborate a little on what rhenium impurities in superalloys do: The most important role that rhenium plays in superalloys is to provide creep resistance.3 What is creep resistance? Metals have, as most people know from experience, the property of retaining their shape under strain, and what’s more, if being deformed or bent by a force, the property of returning to their original shape once the applied force is removed, that is, spring behavior. We say that they obey Hooke’s law. Under some circumstances, depending on the metal or alloy, however this property is not observed and the force results in a permanent change of shape or structure: This is called creep. One might observe creep if one has purchased cheap metal chairs at Walmart and makes the mistake of inviting an obese person, me for instance, to sit on them, whereupon the chairs legs are permanently bent or even broken. (Another familiar example of creep is a dent in a metal.)
In turbines, the centripetal forces that superalloys must exert in large massive – and hot – turbines to keep them from flying apart are enormous, meaning that according to Newton’s first law, enormous strains are, in turn, exerted upon them. Moreover the requirements that the turbines be efficient, places considerable restrictions on the mechanical spatial tolerances allowed to them. From these considerations we see that creep is not just undesirable, it is unacceptable. These very expensive devices, ideally designed to function at high rotational velocities and temperatures for appreciable fractions – or, potentially, even appreciable numbers – of centuries could fail spectacularly if creep causes them to radially elongate so much as to collide with their housings and self-destruct. Moreover, turbines often have very thin ceramic thermal barrier coatings that allow even higher temperatures than the actual alloy’s melting point to be employed: Being ceramics, and thus being more brittle than the metal they protect thermal barriers can break, crack or otherwise fail if their metal base to which they are bonded is deformed. For these reasons, a great many rhenium containing superalloys have been developed experimentally or commercially. Some publically disclosed examples include CM-186LC, CMSX-3, CMSX-4, CMSX-681, RR-2072, TMS-82, UM-F13 and TRW-NASA.
All this is wonderful, except that, as I have noted, rhenium is a very rare element. To the extent that we are able to address the urgent fact that our existing energy infrastructure unacceptably threatens not only civilization but perhaps much more, it is very clear, to me at least, the accessibility of high technology materials such as superalloys are of critical importance in any effort to make clean energy. Yet worldwide production of rhenium amounts to only about 50 MT per year, and the supply is certainly subject to exhaustion – the only known place where supplies are being regenerated, this at a scale of a few tens of kilograms per year, is a volcano in the Russian Kuriles that spews a rhenium mineral that is unknown elsewhere in the world. The US Geological Survey estimates that the world reserves of rhenium amount to about 2,500 tons, a fifty year supply assuming that humanity does nothing more than whine about climate change, probably a good bet, albeit not a satisfying one. By contrast, the supply of technetium, as the fission product of actinide elements, is theoretically much larger.
How much larger?
My position is that nuclear energy represents the form of energy that has the lowest possible external costs for any form of energy capable of being expanded to a power level of 100’s of exajoules per year. As events like Chernobyl and Fukushima demonstrate, nuclear energy is not risk free, but a half a century of large scale industrial practice shows it to be risk minimized, which in a rational world would be enough. In nuclear circles, lots of noise is made about the thorium cycle. I have nothing against the thorium cycle per se, but in order for nuclear energy to be able to provide energy at this level for millennia – because of the geochemistry of uranium as opposed to thorium – the uranium/plutonium cycle using fast neutrons will be required, since recoverable uranium may prove to be inexhaustible. In the following discussion I will suppose the 238U/239Pu (fast) nuclear fuel cycle.
By access to the BNL Nuclear Data Tables one can learn that the fission yield of isotopes accumulating to give 99Tc is, for 2 MeV (fast) neutron irradiated 239Pu, is 6.12%. There are physical limits – as discussed above for naturally occurring oceanic technetium – to how much of a radioactive isotope can accumulate before it reaches a point at which it is decaying as the same rate as it is being formed, the rate of formation in the nuclear reactor case being dependent on the overall power being produced by nuclear energy. Suppose that humanity was able to stabilize its annual energy consumption at 500 exajoules per year, slightly less than it now uses from all energy sources combined. It can be shown by solving a form of the Bateman Equation – simplified to account for the fact that for most of its lifetime technetium would not be in the presence of a neutron flux after being removed from nuclear fuel – that for a world powered by 500 exajoules of nuclear energy per year – that the maximal amount of technetium that could accumulate is on the order of 50 million tons, approached asymptotically although it would take almost a million years of continuous operation – just about 6,400 years to reach a million tons – of nuclear power plants operating at this power level to do so. For the first several hundred years, the accumulations would occur at a rate of slightly less 158 tons per year, three times greater than the current annual supply of rhenium, albeit without being subject to depletion when ores ran out.
Moreover, technetium has the advantage of being lighter than rhenium, further increasing its utility as this would tend to reduce the energy requirements of turbines in which it was used, and the atomic weight difference would require smaller masses to be added to the alloys.
One might object, of course, that any technetium containing superalloys would perforce be radioactive, and this is true, but the health impacts to humanity of such radioactivity, given that in power plants turbines for use in very high temperature systems – precisely those that exhibit the highest thermodynamic efficiencies – are also perforce isolated from the environment.
How radioactive would they be?
In the calculations whose results I will report here, I will discuss a class of 4th generation superalloys that were under development in 2006 at the University of Michigan4 by Dr. Q.Feng – who now works at the Key State Laboratory for Advanced Metals and Materials in Beijing – and coworkers. I have chosen these alloys – developed to have enhanced liquidus (higher melting point) properties as well as enhanced creep resistance – because they contain, besides rhenium, considerable amounts of ruthenium, also a fission product, into which technetium slowly decays, albeit in trivial amounts in ordinary human lifetimes. I discuss these particular alloys for informational, representative, purposes only. I imported table 1 from reference 4 – which lists the composition of 22 superalloys, ranging between 3.9 and 7.5 weight percent rhenium – into Excel in order to do some calculations reflecting the amounts of technetium that would be required to substitute for the rhenium they contain. Converting weight percent into mole fraction, substituting technetium, and then reconverting back into weight percentages, the new theoretical alloys would contain between 1.3% to 2.6% technetium. The mass – and thus their densities since the volume would be expected to vary little from the rhenium analogues owing to the lanthanide contraction discussed above – would vary from 2.5% less to 5.0% less than the rhenium analogues, which might not sound like much until one considers the energy consumption of a turbine turning for half a century or more. I have calculated the specific activity per kg for the technetium containing alloys: They range from 224 millicuries (mCi) to 445 mCi per kg. Given that the alloy – containing a large array of heavy elements – would be self-shielding, and considering the housings and other structures and materials would also be shielding this radioactivity is almost certainly trivial from a health risk standpoint, unless one were to eat kilograms of superalloys, something I trust no one is inspired to do. Moreover whatever technetium was contained in these alloys would be monitored not as waste, but as a valuable and important piece of infrastructure, there would be an added impetus to continually monitoring the environmental fate of the alloys.
Besides the advantage of weight, there are other ways that technetium superalloys might be superior to rhenium alloys. In the design of superalloys, an important parameter involves subtle interactions with oxygen. One of the first things to be discovered about technetium is that the element has remarkable properties as a corrosion inhibitor for steel in oxygenated water, in the form of pertechnate. Although I am uncertain as to whether the chemical mechanism explaining this property is known, it is quite possible that it also might operate with other metals, nickel for instance. Although superalloys are generally resistant to chemical attack, it does seem possible that technetium might offer further improvements, and longer life times for technetium containing superalloys as opposed to the rhenium analogues.
How much might technetium cost?
The “ore” for technetium is, of course, used nuclear fuel. The quantity of technetium generated in a nuclear reactor is a function of many parameters in nuclear technology. The startup composition of the fuel, the degree of isotopic enrichment, the design of the reactor, the burn-up (the amount of energy extracted from the fuel before it is removed from the reactor), the neutron spectra, the fuel cycling, etc. The overwhelming majority of used nuclear fuel resources available today was generated using thermal reactors burning235U at start up and smaller amounts of 239Pu that formed during operations. However, for the purposes of illustration, I will use some representative data as calculated in a textbook on reactor physics,5 as calculated by the burn-up code ORIGEN, with sufficient cooling time, 180 days for thermal fuel and 30 days for fast breeder reactor fuel to allow essentially all of the accumulated 99Mo to decay to form 99Tc. The fractional value for the former suggests that the concentration factor is 1.94 X 10-3 and 8.4 X 10-4for the latter.
Recently in connection some unpublished writing I was doing on the subject of the feasibility, technology, and economics of capture of carbon dioxide from the air, I was reading a paper by House, Herzog et al in PNAS6a which, in the context of the dilution of carbon dioxide in air attempts an estimation of the cost of removing – and dumping – it, appealed to the concept of a “Sherwood Plot,” first advanced by the great chemical engineer Thomas Sherwood in 1959.6b The version used by House and Herzog, reproduced from another source,6c is here:
Figure 2: A modern version of a Sherwood Plot6a,c
Appeal to this graphic suggests that the cost of technetium should be something on the order of $50/kg. That said, the graphic also suggests that the cost of plutonium should be between $5/kg and $10/kg since it is present in its “ore” – also used nuclear fuel – is present at roughly 1% (10-2). The figures I’ve seen suggest that the price of plutonium is a few hundred times larger, roughly $1000/kg.
If one looks at Sherwood’s original plot6b one will recognize that it has a different geometry than that produced here. Like all economic laws, as opposed to most physical laws, whatever function the Sherwood plot represents – a log/log plot and thus exponential in form – is fuzzy and empirical rather than determinative. The price of rhenium has ranged in recent times, according to the USGS, between $3,400 and $4,300 per kilogram, but the price reflects not just demand or process, but the fact that the element is available as a side product of the copper and molybdenum mining industries. This helps to lower its cost because some of the isolation is derived at obtaining other, in fact main, products. Obviously process technology also plays a role in the cost of materials, and improvements during the golden age of chemistry have continuously made materials available that were not available before. Industrial plutonium isolation chemistry relies on 1950’s and 1960’s technology – Purex solvent extraction technology – and the artificial requirement that nuclear technologies meet standards that no other energy technology can meet – that no one anywhere on earth face any risk whatsoever, for eternity, from its practice – has fostered, coupled with public fear and ignorance expressed as NIMBY, artificially high disposal costs for supposed “waste” products. In this social climate, it will always be impossible to construct something like Yucca Mountain, which was probably a bad idea in any case for reasons that have nothing to do with risk or health.
Since the 1950’s many brilliant insights to actinide and lanthanide chemistry – lanthanides also are important fission products – have emerged especially as the lanthanides became critical strategic materials in their own right. I argue – recognizing that this idea is hardly mainstream yet – that there is no such thing as “nuclear waste,” since I can make arguments much like this one for technetium for almost every other constituent of used nuclear fuel. With this recognized, with modern materials, solvent, other chemical and robotic sciences in mind, I believe that the actual price of plutonium and technetium might approach the values suggested by the Sherwood plot, thus eliminating, again in a rational world, the requirement for any energy related mining – coal mining, gas fields, oil fields and even, given stocks already mined, uranium and thorium mining – for many centuries to come.
This is not to say that I believe that this is what will happen, but only that this is what might have been possible were it not already too late. (What I believe will happen is that, as been the case in much of human history, is that fear and ignorance will prevail and we will literally and figuratively choke to death on dangerous fossil fuel waste.)
So what is known about technetium alloys and their properties? Not as much as one might hope. Much of the work on them took place in the 1950’s and 1960’s. Technetium, with its relatively high superconductivity transition point was studied in alloys with niobium for building devices like MRI, NMR etc, but other options were explored and developed. Technetium additions to tungsten greatly improve the machinability and ductility of that element. Iron and zirconium alloys have been explored as “waste forms” or as targets for transmutation devices, but personally I approve of neither. In a wiser world, we would know far more about this remarkable element’s alloy and other chemical properties.
Other uses for the element besides as a superalloy constituent suggest themselves.
The evolution of materials science has resulted in the development of many interesting intermetallic compounds, carbides among them: One I’ve been studying recently is the remarkable compound Ti3SiC2 and its zirconium and hafnium analogues, known as MAX phases. It is possible that technetium carbides might be superior to even these remarkable materials. Recently Chinese scientists have written two papers on the theoretical properties of technetium carbide. 7,8 These papers are based on computational analysis – an increasingly important approach to materials design. Both predict that certain structures of technetium carbide (and its rhenium analog) are thermodynamically stable, and that they are not only extremely hard, approaching and maybe even exceeding the hardness of diamond, and what’s more, unlike pure diamond, they have metallic properties, including but not limited to the ability to conduct electricity. One can easily imagine circumstances in which these properties might be extremely useful, for instance in the machining of MAX phase materials, or in the construction of certain types of electrodes designed to be stable for use in fluorine/fluoride based or other types of extreme chemical systems, where one would expect a silicon based MAX phase to corrode. It is an open question, to a certain extent, whether TcC has been synthesized, if so, it has not been unambiguously characterized.9
This suggests many potential applications of interest to energy technology, especially in the field of nuclear technology and the conversion of nuclear energy to chemical energy for residential or industrial use.
Personally, I am happy to a certain extent about all the dithering that has gone on – albeit much of it deriving from fear, ignorance and selective attention – about the over the question of “what do we do with”… (so called)… “nuclear waste.” The dithering has not lead to the same loss of life that the inattention to dangerous fossil fuel wastes has caused; it is hard to argue that so called “nuclear waste” has ever killed anyone in this country or in many other countries: Despite much rhetoric to the contrary, the storage of used nuclear fuel has proved remarkably safe for more than half a century. One of the things accomplished by said dithering has involved the preservation of technetium, technetium that might have led to the destruction of this potentially invaluable resource, had the unfortunate waste dump mentality been allowed to prevail.
References and Notes:
1. Lavrukhina and Pozdnyakov, “The Analytical Chemistry of Technetium, Promethium, Astatine and Francium” Translated by R Kondor, Ann Arbor-Humphrey Science Publishers (1970) pp 7-8
2. Geddes, Leon, Huang, Superalloys, Alloying and Performance, Copyright 2010ASM International, pp 123-124
3. Ibid. See pp 80-81 for a discussion of the role or rhenium in superalloy performance.
4. Q. Feng, L.J. Carroll, and T.M., Pollock Metallurg.Mat.Trans.A.37A.1949-1962 (2006).
5. William Stacy, Nuclear Reactor Physics, (2001) John Wiley and Sons, NY. The data is taken from table 6.8, page 225. The data is given as Curies (Ci) per ton, and has been converted into mass by calculation from the specific activity of 99Tc.
6. (a) Kurt Zenz House, Antonio C. Baclig, Manya Ranjan, Ernst A. van Nierop, Jennifer Wilcox, and Howard J. Herzog Proceedings of the National Academy of Science PNAS December 20, 2011 vol. 108 no. 51 20428–20433 (b) Thomas K. Sherwood, Mass Transfer Between Phases, 35th Annual Priestly Award Lecture, Pennsylvania State University, (1959). (c) Grübler A (1998) Technology and Global Change (Cambridge Univ Press, Cambridge, (UK).
7. Wang, Physica.Status.Solidi. Rapid Research Letters, Vol 2, No. 3, 126–128 (2008) “Ultraincompressible and hard technetium carbide and rhenium carbide: First-principles prediction”
8. Liang, Li, Guo and Zhang, Physical Review B 79, 024111 (2009).
9. For one report of the synthesis of TcC see Berens and Rheinhart, The Journal of Physical Chemistry, 83 (15) pp 2051-2053 (1979).