Monitoring the Burn Up History of Used Nuclear Fuels by Monitoring Ruthenium-106.
In the fast fission of plutonium, as one can learn by accessing the Brookhaven National Laboratory Data pages, fission products with a mass number 106 occur about 4.3% of the time.
After many years of thought and reading, I have convinced myself (irreversibly I believe) that the last best hope for this planet is, in fact, the fast fission of plutonium, irrespective of common public fantasies to the contrary.
The stable isotope at this mass number, 106, is palladium, but stable palladium-106 occurs in direct fission only about one in every 30 billionth fission. Before significant and salable quantities of palladium-106 can accumulate and be recovered, radioactive precursors with smaller atomic numbers with this mass number must be allowed to decay into it. The elements involved are yttrium, zirconium, molybdenum, and technetium. All of the nuclides with this mass number other than ruthenium have very short half-lives and mostly decay within the reactor, generating heat that helps drive turbines. Technetium-106, for example, ruthenium-106’s immediate precursor has a half-life of just 36 seconds. Ru-106 has a half-life of 367.6 days, short enough that it is possible to "milk" significant quantities of it's decay product, palladium-106 from it, but long enough that significant quantities remain in the used nuclear fuel after shut down of a nuclear reactor. (An intermediate in the decay of ruthenium 106 to stable palladium-106 is rhodium-106, but this has a half life of only 30 seconds and therefore exists in secular equilibrium with its parent until all of the ruthenium has decayed.)
The relatively long half-life of ruthenium-106 allows for the facile separation of the stable isotope of palladium from all other isotopes of palladium - a valuable metal, at least in the case of fast or continuous reprocessing of used nuclear fuels, something I personally favor. The way this would work would be to separate the 106 isotope as ruthenium and then to let the ruthenium decay into isotopically pure non-radioactive palladium-106 away from all other isotopes of palladium.
(Currently the only use for ruthenium-106 is in cancer treatment.)
Palladium accumulating in a reactor as palladium will invariably include the long lived radioactive isotope palladium-107, both from fissions occurring at mass number 107 and from neutron capture in palladium-106 resulting from that portion of the ruthenium-106 that decays within the reactor.
Because palladium-107 has a long half-life, 6.5 million years, the radioactivity of the pure isotope is rather low, about 0.5 millicuries per gram, activity which would be further diluted by the presence of the stable palladium isotopes also resulting as fission products, palladium-104, palladium-105 the aforementioned palladium-106, palladium-108 and palladium-110.
Still, it is likely that under most circumstances this radioactivity limit its use to closed system catalytic roles, a significant role, but of less utility than open systems. A common example of an open use of a palldium catalyst would be an automotive catalytic converter; common closed systems would be as a catalyst would be the arylation of ammonia, Suzuki couplings to form carbon carbon bonds on an industrial scale, water electrolysis, or the Fischer Tropsch synthesis of motor fuels from carbon dioxide and hydrogen.
The separation of ruthenium from the bulk of other fission products is somewhat simplified by the fact that ruthenium forms a volatile oxide, RuO4 that is a powerful oxidizing agent and is therefore easily reduced by relatively mild means. Very simple and clean chemistry can reduce it all the way to the metal. (Ruthenium oxide can be used in organic synthesis to form 1,2 diols from alkenes by oxidation, although generally its cogener's tetraoxide - also volatile - osmium tetraoxide is used for this purpose.)
In this process utilizing the volatile oxide of ruthenium to separate it from other constituents of used nuclear fuel, known as voloxidation, generally as part of an overall processing step with reduced use of solvents, the used nuclear fuel is treated with ozone to distill off the volatile oxide fission products, ruthenium, molybdenum, rhodium, technetium, tellurium and tritium, a facile and relatively simple separation. All six elements are therefore recovered for use.
(Other fission products that can be easily recovered by distillation are iodine, cesium and rubidium. Uranium, neptunium, and plutonium can all be recovered as volatile fluorides, and in principle, so can the aforementioned ruthenium and technetium be recovered as fluorides as well as oxides.)
But as I was reminded while going through my files last night, ruthenium-106, has another use, even before it is removed from used nuclear fuel, and this is to allow for understanding the operating history of a nuclear fuel rod.
I came across this paper in my files: Feasibility of 106Ru peak measurement for MOX fuel burnup analysis (Usman and Dennis, Nuclear Engineering and Design Volume 240, Issue 10, October 2010, Pages 3687-3696).
Some excerpts from the paper follow. First from the introduction:
To validate the initial computer simulation (Dennis and Usman, 2006), preliminary experimental data were collected for gamma emission from LEU test reactor fuel at Missouri University of Science and Technology (Missouri S&T). The primary goal of this effort is to determine if online burnup analysis of plutonium based mixed oxide (MOX) fuel is feasible using non-destructive gamma spectroscopy. Initial results are very encouraging and it seems feasible to develop techniques for determination of MOX fuel burnup, as well as for discrimination between MOX and uranium oxide (UO2) fuel assemblies. However, for commercial applications online,gamma spectroscopy immediately following shutdown/fuel discharge will be complicated by the extremely high activity of the irradiated fuel...
A nice description of the history and procedure of using plutonium in commercial thermal reactors is included:
While the U.S. has not reprocessed and researched MOX fuel since the mid-70s, other countries have proceeded with extensive research and deployment efforts. European nations including France, Germany, and the United Kingdom have found that only replacing a fraction of a light water reactor (LWR) core with MOX provides the best neutronics and safety characteristics. In fact, France limits its cores to 30% MOX and different plutonium enrichments within a given assembly to flatten power peaking (Cochran and Tsoulfanidis, 1990b). Ultimately, the MOX fuel behaves differently based on multiple fissile plutonium isotopes, varying concentrations based on the level of recycling, how long the plutonium has been stored allowing the 241Pu decay product and poison 241Amto build-in, and neutron parameters such as absorption cross sections (Cochran and Tsoulfanidis, 1990b). As a result, along with limiting the amount of MOX in a LWR core, it is prudent to locate MOXaway from boron control rods due to reduced reactivity worth concerns and using the fuel as soon as possible to limit growth of 241Am.
The paper's stated purpose however, is to evaluate "NDA," Non-Destructive Analysis of MOX fuel, by using gamma spectroscopy to detect the "burn-up" of commercial MOX fuel. In general, "burn up," usually measured in terms like GWd/THM (gigawatt-day per ton of heavy metal) is a measure of mass to energy fuel efficiency, very much like "miles per gallon" in the automotive parlance. The higher the burn-up, the less fuel that has to be handled and the longer a reactor can run without refueling. MOX fuel is particularly desirable because it utilizes the mass more efficiently, requiring less mining and processing of uranium, although it can be shown that uranium supplies are inexhaustible, particularly in the case of the widespread use of fast fission of plutonium.
As the authors plainly confess, their research reactor at the Missouri University of Science and Technology does not produce fuel that is radioactive enough to properly evaluate the utility of the detecting of Ru-106 gamma radiation peaks to determine the burn up of commercial MOX fuel, which will be, especially on high burn up, far more radioactive than a research reactor can possibly provide.
Nevertheless the paper is an interesting read on some fairly technical grounds, in particular describing the criteria by which fission product spectra can differentiate in situ the fission totals of differing fissionable and fertile actinide nuclides. I will not go into these technical details any further; they're not appropriate for such a post.
After decades of studying nuclear technology, I have convinced myself that nuclear fuels having a mixture of actinides with a wide distribution of isotopes as is possible for each of them is the ideal approach to utilizing nuclear energy. The modeling of such use is highly complex of course, and perhaps at the dawn of the nuclear energy age would have been prohibitive, but now, at the end of 2017, with our great advances in both computational and materials science as well as in the science of thermodynamics, much more is possible than was ever possible before.
From my perspective, over the long term, I believe non-destructive testing may not be necessary. I favor liquid phase nuclear fuels, not necessary those diluted with salts such as FLIBE (to which I've learned to have certain objections) nor FLINAK. I suspect liquid metals will prove superior, if only for their tendency to offer spontaneous separations of particular use, both in phase separation and high temperature distillation. Liquid phases provide for facile removal and chemical separations in fuel, and to the extent one needed access to palladium that was non-radioactive, voloxidation to separate radioactive ruthenium-106 for decay into stable palladium-106 would be an option.
I do not know however, about the distribution coefficients of palladium and ruthenium in, for example, liquid plutonium or liquid neptunium relative to a barium/strontium/rubidium/cesium phase.
I do know that in the oxide fuels now utilized in most of the world's 400+ operating nuclear reactors, the solid metals plate out as what is know as ?-metal, an alloy of the metals palladium, rhodium, ruthenium, technetium, and molybdenum. They are typically resistant to attack by the nitric acid used in the old but still utilized PUREX process, making their separation somewhat easier than other separations, but unfortunately, the idea of what to do with them is still to throw them away, bury them.
This is absurd.
As I pointed out above, palladium that is obtained from used nuclear fuels - except for that which is obtained by separating ruthenium from palladium and allowing the Ru-106 in it to decay - will be radioactive, but as only one of the isotopes is radioactive and because it has a fairly long half life, such radioactivity is hardly unmanageable.
In ores on the planet, palladium is relatively rare and expensive. The price found on the internet today is a little higher than $900/troy ounce or around $28,000/kg.
Besides its use as a catalytst in closed systems, where low levels of radioactivity would not matter, it might also be possible to use this slightly radioactive palladium as a constituent of certain alloys that might be used in reactors, superalloys that in silico modeling suggests might well prove superior to the widely used nickel based superalloys of the Inconel and Hastelloy types. While much of the world's technology from jet engines to combined cycle power plants to certain kinds of chemical reactors depend on the utilization of nickel based superalloys, one has to very careful to prevent them from melting by utilizing, for example, zirconium or hafnium based ceramic oxides as thermal barrier coatings. The failure of these coatings can be catastrophic, and one of the interesting things found in the following paper is that palladium based superalloys, were they available have considerably higher melting points.
Effects of alloying elements such as Ti, Zr and Hf on the mechanical and thermodynamic properties of Pd-Base superalloy (Feng et al, Journal of Alloys and Compounds Volume 710, 5 July 2017, Pages 589-599)
It can be shown that in a theoretical and cleaner world where all of humanity's energy was derived from the fast fission of plutonium, say on a scale of 600 exajoules of energy per year, that the accumulation of palladium would annually be about 500 to 600 metric tons per year, certainly enough material to construct superalloy based industrial sized devices such as dual pyroprocessing/reactors that would operate at very high temperatures, be highly thermally efficient, and be suitable for the preparation of liquid fuels for use in devices where they might always be required, such as farm machinery or remote generators. It would not matter if the alloys in such pyroprocessing/reactors were radioactive, and in fact, the neutron fluxes might well prove to reduce their radioactivity over the long term.
But that particular technology is for discussion another day.
Being a little ill on a Sunday afternoon, I'm rambling a bit.
Anyway.
Have a nice Sunday evening.
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