It's nice to see papers about industrial technetium to start out 2022.
Starting out the New Year with the first issue of the new volume of one of my favorite journals, I was pleased to see this paper appear in the table of contents: Distribution Coefficient Model for Zirconium and Technetium Extraction from Nitric Acid Solution (Hongyan Chen, Megan Jobson, Robin J. Taylor, Dave A. Woodhead, Andrew J. Masters, and Clint A. Sharrad Industrial & Engineering Chemistry Research 2022 61 (1), 786-804)
Technetium is a remarkable metal in many ways, although it does not occur on Earth except in extremely low concentration from spontaneous fission in uranium ores; all of its isotopes are radioactive. It is however, a prominent component among fission products found in used nuclear fuel.
It's value derives from the fact that owing to the lanthanide contraction effect between the 5th and 6th periods of the periodic table, technetium's atomic radius and valence electron configuration are nearly identical with its valuable congener rhenium, a very, very rare and expensive metal with outstanding alloying and catalytic properties.
Many of the "superalloys" which have extreme resistance to corrosion and temperatures contain rhenium. Rhenium at 3180°C is the second highest melting metal element after tungsten ( 3407°C) Technetium is the eighth highest melting metal at 2200°C, but melts higher than metals like titanium, zirconium, chromium, vanadium, iron and nickel, the latter being the base metal of many superalloys utilized in high temperature turbines (albeit with thermal barrier coatings). Since the supply of rhenium is limited to ores subject to depletion, and technetium can be accumulated indefinitely - at least to the point of a Bateman equilibrium - it is worth considering the element as a rhenium substitute in superalloys, particularly those in closed systems such as power plant turbines. As technetium is a pure beta emitter and a reasonably long half-life, and in many applications would be contained in self-shielding in alloys, one can imagine many valuable structural uses for it. (It's main use today is to utilize its nuclear isomer 99mTc in medical imaging; basically people drink technetium solutions.) Because of the high price of rhenium, the quantities of rhenium in superalloys is small
The current mentality is, however to throw it away, which is tragic to my thinking.
But here's the really cool thing about this paper: The fact that the consideration it involves is taking place at all. It's evidence that the world is waking up and understanding that we need nuclear energy if we are to survive. It's that simple. It's would be nice to have, as a side product, to have this metal for the applications it can preform.
This paper is all about Zr/Tc separations, a pretty good thing to understand if we are to have a sustainable future.
The most widely used process for recycling used nuclear fuels is the Purex process and variants; these are solvent extraction processes. The work like this: These processes rely on chopping up the cladding of the fuel rods and dissolving everything in concentrated nitric acid. Complexing agents are added to solvents that do not dissolve in aqueous acid, the presence of the complexing agents allows them to become hydrophobic and thus they are extracted into the organic layer. These types have been in industrial use for more than half a century. The down side is that they generate a significant amount of chemical waste, some of which is radioactive.
I personally think there are better processes, many of which are pyroprocessing or electrochemical processing or hybrid processing.
This is a paper about the old ways though, but comes with the benefit of stripping radioactive materials, the metals zirconium and technetium, from the solvents and acids, thus allowing for their recovery. It also contains one interesting note reading between the lines.
From the paper's introduction:
Zirconium is used as a proven structural material for nuclear fuel cladding in light-water reactors. Zirconium isotopes, such as the high-energy ?-radiation emitter 95Zr (9) and the long-lived pure ?-particle-emitter 93Zr, (10) are generated by nuclear fission of U and Pu. (11) As zirconium can significantly affect the extraction of technetium in spent nuclear fuel reprocessing processes, (5,12?16) its distribution in separation processes can significantly affect the routing of technetium...
The bold is mine. What these authors are discussing is apparently recycling fresh nuclear fuel shortly after removal from the reactor. This is obviated by the fact that the half-life of 95Zr is on the order of 64 days. This means that used fuels that are five years only 2.6 billionths of the radioactivity from 95Zr would remain, the rest having decayed to stable Molybdenum. Since they are concerned with the properties of this isotope, this means they have been thinking about the recycling of hot fuels. By contrast, many people think only of reprocessing fuels that have experienced long cooling periods; there is an impressive inventory of these long cooled fuels and admittedly, recovering the valuable materials in them is simpler than it would be for hot fuels. Reprocessing cooled fuels offers some advantages, but there are also good reasons, I think, for recycling hot fuels, but that's a topic for another day.
Nevertheless, to repeat, this subtle point strikes me as good news, an implication that we need to consider recycling fuels while still hot.
Again, I think there are better approaches for approaching fuel recycling, especially for hot fuels, but solvent extraction has a long industrial history and is well characterized. To the extent we rely on used nuclear fuels, the less mining will be required. Uranium mining companies might not like this, but to the extent we use our already mined uranium stocks, we offer hope to future generations.
My only point is this: It's good to see the chemistry of technetium processing discussed in the current fresh literature, because it is a sign post that the scientific community is beginning to focus on the value of used nuclear fuels as we enter 2022. I am not shy about stating that I regard the use of the materials found in used nuclear fuels as essential to human survival.
Have a pleasant Sunday.
