Electrochemical oxidation of 243Am(III) in nitric acid by a terpyridyl-derivatized electrode.
The paper I'll discuss in this post came out about 5 years ago, but somehow I missed it. The paper is: Electrochemical oxidation of 243Am(III) in nitric acid by a terpyridyl-derivatized electrode (Christopher J. Dares1, Alexander M. Lapides1, Bruce J. Mincher2, Thomas J. Meyer1,* Science Vol. 350, Issue 6261, pp. 652-655).
I've written several times about the element amercium, in this space, most recently about its separation from its f-series cogener, europium.
Liquid/Liquid Extraction Kinetics for the separation of Americium and Europium. The process described therein is a solvent based process, but utilizes specialized membranes. As I noted at that time, there are several drawbacks to the process.
However americium is potentially valuable fuel, particularly because of its nonproliferation value, since burning it will generate the heat generating isotope plutonium 238, (via the decay of Cm-242, formed from the decay of Am-242) which when added to other plutonium isotopes, can make them all useless in nuclear weapons.
It turns out, on further inspection, that Am-242 and its nuclear isomer Am-242m - both of which are highly fissionable - are among the best possible breeder fuels, having a high neutron multiplicity, meaning they can serve to transmute uranium into plutonium, greatly expanding the immediate availability of nuclear energy while avoiding isotope separations.
Here is the plot of the neutron multiplicity for the fission of Am-242(m):
Evaluated Nuclear Data File (ENDF) Retrieval & Plotting ("Neutron induced nubar." )
This spectrum, which is from the ENDF nuclear data files is clearly not highly resolved, to be sure, and surely represents a kind of average. It is undoubtedly difficult to obtain highly purified Am-242m for experimental verification. An interesting approach to obtaining it in high isotopic has been proposed, owing to its possible utility in making very small nuclear reactors for medical or space applications: Detailed Design of 242mAm Breeding in Pressurized Water Reactors (Leonid Golyand, Yigal Ronen & Eugene Shwageraus, Nuclear Science and Engineering, 168:1, 23-36 (2011)). To my knowledge, however, this proposal has never been reduced to practice.
There are very few nuclei that exhibit this high a multiplicity, over 3 neutrons per fission at any incident neutron energy. To my immediate knowledge, the only such nuclei is plutonium-241, and this over a fairly narrow range of incident neutron energies in the epithermal region.
I also discussed the critical masses of americium isotopes in this space: Critical Masses of the Three Accessible Americium Isotopes. In a fast nuclear spectrum, americium-241, the common americium isotope produced in the common thermal nuclear reactors that dominate the commercial nuclear industry, particularly in used fuel that has been stored for decades without reprocessing, is fissionable and thus is a potential nuclear fuel in its own right.
A "refinement" of some nuclear physics parameters relevant to the use of americium as a nuclear fuel was published about 13 years ago as of this writing, by scientists at Los Alamos: Improved Evaluations of Neutron-Induced Reactions on Americium Isotopes (P. Talou,*† T. Kawano, and P. G. Young, Nuclear Science and Engineering 155, 84–95 2007).
In a fast nuclear reactor, neutrons have an energy of 1 to 2 MeV whereas in a thermal reactor, the neutron speed is taken to be the kinetic energy of molecules in air, generally taken as 0.0253 eV, almost 100 million times lower energy. Thermalized neutrons tend to be absorbed without fission in americium-241, whereas at higher energy, they tend to fission. The following graphics from this paper show that for a region of neutrons well above the thermal region, but in the general region with the fission to capture ratio climbs:
The next graphic, from the same paper, shows that the neutron induced fission of americium-241, like its capture product, americium-242(m), has a very high neutron multiplicity, meaning that it is also an excellent breeder fuel under these conditions:
Even limited to the left most region of this graphic, which is actually the region in which "fast" reactors work, the multiplicity is shown to be above 3 neutrons/fission. The graph indicates that were the fusion people ever able to run a fusion reactor, where neutrons emerge with an incredible 14 MeV energy, and chose to use a hybrid approach fusion/fission approach to solve the unaddressed problem of heat transfer, americium could provide very high neutron fluxes to do incredible amounts of the valuable work that neutrons can do.
The next question is "How much Americium is Available?"
Kessler - who writes frequently on the subject of denaturing nuclear materials to make them useless for weapons applications - addressed this issue about 12 years ago in this paper: G. Kessler (2008) Proliferation Resistance of Americium Originating from Spent Irradiated Reactor Fuel of Pressurized Water Reactors, Fast Reactors, and Accelerator-Driven Systems with Different Fuel Cycle Options, Nuclear Science and Engineering, 159:1, 56-82.
According to the IAEA calculations he referenced in this paper, as of 2005, there were about 106 MT in used nuclear fuel all around the world. During storage of used nuclear fuel, especially MOX fuel, but not limited in any sense to it, the amount of americium rises because of the decay of its precursor, plutonium-241, Pu-241, which has a half-life of 14.29 years, and which invariably accumulates as a nuclear reactor operates. Ten years after removal from an active core, about 61.6% of the Pu-241 in the fuel will still be present, with the balance having decayed to Am-241. Because of anti-nuclear fear and ignorance, which is quite literally killing the world we know and love, we now have unprocessed used nuclear fuel that has been stored for 40 years or more, without having been processed, albeit this without killing anyone. Forty year old used nuclear fuel will still contain 14.4% of the Pu-241 originally in it, but will obviously have more americium than it did when it was removed from the reactor. (The half-life of Am-241 is 432.6 years.) It seems reasonable to me that we may have more than 200 MT of americium available. This is not a lot of material, but it is significant. The energy content of this much americium, fully fissioned, is very roughly equal to about 10% to 20% of the annual energy demand of the United States. This much americium is certainly not enough to address any more than a tiny fraction of the completely unaddressed climate crisis, but on the other hand, the high neutron multiplicity can certainly accelerate the rate at which we can accumulate fissionable nuclei that are the only viable option for doing anything about climate change. (And let's be clear. Right now we are doing less than nothing to address climate change.) The amount of americium available to use is certainly enough to run 10 to 20 nuclear reactors running exclusively on americium.
Kessler's paper offered some calculations of the isotopic vectors of the Americium available in different fuel scenarios and presented them graphically. In connection with the thermal neutron spectrum graphics he writes:
...In options C, D, and E, either reenriched reprocessed uranium ~RRU! ~option C! or natural uranium or RRU both together with plutonium options D and E! are used…
...In options F and G, RRU or thorium is mixed with plutonium and MAs in order to incinerate both plutonium and MAs in PWRs.
Figure 2:
The graphic refers to a 10 year cooling period, and again, many used nuclear fuels are much older, with the result that the percentage of Am-241 in these cases would be more enriched in Am-241 with respect to the other isotopes.
Kessler offers similar analysis in various fast neutron cases which I will not discuss here. Very few of these types of reactors have operated commercially, although the advent of small modular "breed and burn" reactors, which Kessler does not address, will likely change this. Kessler also doesn't address a case which in my imagination would be superior to all solid fuel cases, specifically liquid metal fuels, a case that was abandoned in the 1960s because of the relatively primitive state of materials science in those days but is certainly worth reexamining 60 years later, given significant advances in that science.
Anyway, in writing this post, and researching it, I have convinced myself once in for all, that my previous feeling that it was a disgrace to let Pu-241 decay into Am-241 was wrong. I have changed my mind: Am-241 is more valuable than Pu-241, even accepting the high value of Pu-241.
This brings me to the Science paper referenced at the outset of this post:
Americium in its chemistry is dominated by the +3 oxidation state, and in this state, it behaves very much like the lanthanide f elements. Significant quantities of the lower lanthanide f elements are formed via nuclear fission, from lanthanum itself up to and including gadolinium.
The process described in the above referenced Science paper uses electrochemistry to make americium behave more like uranium, neptunium, and plutonium, in exhibiting oxidation states higher than +3, and thus having far different properties than the lanthanides.
From the paper's introduction:
Partitioning of Am from the lanthanides is arguably the most difficult separation in radiochemistry. The stable oxidation state of Am in aqueous, acidic solutions is Am(III). With its ionic radius comparable to the radii of the trivalent lanthanide ions, its coordination chemistry is similar, leaving few options for separation. One approach is the use of soft donor ligands that exploit the slightly more diffuse nature of the actinide 5f-orbitals over the harder lanthanide 4f-orbitals. This provides a stronger, more covalent bond between actinides and N-donor ligands. Notable progress has been made in complexation-based strategies, but considerable challenges have been encountered when attempting to adapt their narrow pH range requirements for process scale-up, stimulating efforts to find alternatives. Another approach is oxidation and separation using the higher oxidation states of Am (5, 6). Unlike the lanthanides, with the exception of the ceric cation, Am(III) can be oxidized and forms [AmVO2]+ and [AmVIO2]2+ complex ions in acidic media. The high oxidation state ions could be separated from the lanthanides by virtue of their distinct charge densities using well-developed solvent extraction methods (4)...
Although solvent extraction method are in fact, well developed, it does not follow that they are the best approach to separations of the elements in used nuclear fuel. If I were to look at electrochemical oxidation of americium, I would also look at electrochemical based separation, including an analogue to the separation of nucleic acids and proteins, electrophoretic migration through gels, or liquid/liquid, gel, or solid based ion selective membranes.
The beauty of electrochemistry is that it avoids the use of reagents. the wide use of primitive solvent extraction methods in the mid to late 20th century led to problematic situations like the Hanford tanks about which very stupid anti-nukes carry on endlessly even as they completely ignore, with contempt for humanity, the millions upon millions of people killed every damned year, at a rate if about 19,000 human beings per day, from dangerous fossil fuel waste.
More excerpts of text in the paper:
...Oxidizing Am(III) in noncomplexing media is hampered by the high potential for the intermediate Am(IV/III) couple (Fig. 1) (15). Only a limited number of chemical oxidants, including persulfate and bismuthate, have been explored for this purpose (16). Oxidation by persulfate gives sulfate as a by-product, which complicates subsequent vitrification of the waste (17). Bismuthate suffers from very low solubility, necessitating a filtration step that complicates its removal (16). Adnet and co-workers have patented a method for the electrochemical generation of high-oxidation-state Am in nitric acid solutions (18), based on earlier results by Milyukova et al. (19, 20), who demonstrated Am(III) oxidation to Am(VI) in acidic persulfate solutions with Ag(I) added as an electron transfer mediator with Eo[Ag(II/I)] = 1.98 V versus SCE...
The fact that the authors are embracing the cultural mentality of so called "nuclear waste," rather than the recovery of valuable nuclear materials, has no bearing on the quality of their science. I just have to say that.
The redox potentials of americium:
The caption:
The authors proposed the use of a new type of electrode, an organic/indium tin oxide electrode, to oxidize americium. Here is a a cartoon representation of this electrode:
The caption:
(Left) A simple molecular illustration. (Right) A density functional theory–optimized p-tpy structure with pyridine rings oriented to show the potential tridentate bonding motif, ideally placed on a surface.
A word on indium tin oxide: This compound is found in pretty much every touch screen in the world. It is also widely used in some types of solar cells. In the past, at DU, I used to engage a dumb "renewables will save us" anti-nuke in discussion of the fact - facts matter - that the world supply of indium is limited, and therefore any so called "renewable energy" technology based on it is, um, well, not in fact "renewable." Ultimately this person bored me to death, and I put him or her on my lovely "ignore" list, since there is no value in engaging people who simply repeat distortions and outright lies over and over, in a Trumpian fashion. The fact remains supplies of indium are limited. It is considered a "critical material" of concern to people who study such things. The world does risk running out of it. The difference, of course, between cell phones and solar cells and nuclear fuel reprocessing plants do not require large amounts of mass, since the environmental superiority of nuclear fuel to all other energy forms derives entirely from its enormous energy to mass ratio. In fact, indium is a minor fission product found in used nuclear fuel, and more than enough can easily be recovered from such a fuel to make these electrodes. I am not endorsing these particular types of electrodes by the way; I'm not sure that nitric acid solutions are in fact the best approach to nuclear fuel reprocessing; there are strong arguments that they are not. The point is, that in comparison to the requirements of solar cells and cell phones, the requirement for indium in this case, were it to go commercial - and it never may do so - is trivial.
Some graphics on the electrochemical results obtained in working with the electrode:
The caption:
(Left) Am speciation measured by visible spectroscopy in a 1-cm cuvette at an applied potential of 1.8 V versus SCE. The appearance of Am(V), concurrent loss of Am(III), and overall mass balance are plotted. (Right) Electrochemical Am oxidation scheme involving a p-tpy–derivatized electrode.
The authors remark on the effect of radiation on these solutions:
Radiolysis of water by Am generates one-electron reducing agents such as H atoms and two-electron reducing agents such as hydrogen peroxide, as well as other redox transients (35). The concentration of radiolysis products varies linearly with total Am concentration, with zero-order reduction kinetics observed for the appearance or disappearance of Am species. Under these conditions, rate constants for these Am species during autoreduction can therefore be derived from the slopes of concentration-time plots (36, 37).
Radiolytically produced one-electron and two-electron reductants provide independent pathways for Am(VI) reduction, with an overall rate constant for Am(VI) loss of 23.4 × 10?6 s?1 (fig. S10). The Am(IV) produced from the two-electron reduction of Am(VI) by radiolytic intermediate or intermediates, presumably H2O2, rapidly disproportionates to Am(V) and Am(III). The reduction of Am(V) to Am(IV) or Am(III) is slow on this time scale (fig. S10).
It seems to me, perhaps naively, that this issue might be addressed by continuous separation, by methods to which I alluded above, to continuous separations, driving the equilibrium. Part of the problem might be addressed by putting beta emitting species - which in real life would be present anyway - in the solution, but no matter.
Some more commentary:
Figure 4:
The caption:
(Left) Am speciation as measured by visible spectroscopy in a 50-cm waveguide. (Right) Visible spectra of species before controlled potential electrolysis and after 13 hours of electrolysis with highlighted speciation changes.
The authors conclude:
This is an interesting little paper. I very much enjoyed going through it as well as thinking a little more deeply about the entire subject of americium, which might prove a valuable tool in saving the world via the use of nuclear energy.
While considering all of this, I have to say what's always on my mind:
To my way of thinking, opposition to nuclear energy - even though such opposition appears often on my end of the political spectrum - is criminally insane.
Right now, hurricanes are marching, year after year, all over the United States and other parts of the world. Vast forests, farms and homes have burned all over the Western United States this year, droughts are destroying crops as are things like derechos. We do not record the number of people killed by extreme heat events, but the scientific literature suggests these numbers are rather large, and we have seen the most extreme temperatures ever recorded in 2020. Insects that spread infections are appearing at ever higher latitudes...
The list of the scale of what we are experiencing from climate change goes on and on and on...
And yet...and yet...and yet...I hear from people that "nuclear power is too dangerous." Compared to what?
Let's be clear on something, OK? Opposition to nuclear energy is as ridiculous and as absurd as refusing to wear masks in the Covid-19 epidemic because of political orthodoxy; similarly it kills people. Many of us on the left should examine and confront our own anti-science political orthodoxy, much as old white men like myself need to examine and confront our suppressed racist rationalizations.
Questioning ourselves is something we all must do; because we cannot hope to struggle to achieve the status of being moral human beings without doing so.
Nuclear energy need not be entirely risk free to be vastly superior to everything else; it only needs to be vastly superior to everything else, which it is. It is immoral to oppose nuclear energy.
I trust that you will have a safe, yet rewarding weekend.
