Low- and high-temperature heat capacity of metallic technetium

[NNadir]

Last edited Sat Dec 7, 2024, 12:26 PM - Edit history (2)

The paper to which I'll refer in this post is this one: J.N. Zappey, E.E. Moore, O. Beneš, J.-C. Griveau, R.J.M. Konings, Low- and high-temperature heat capacity of metallic technetium, The Journal of Chemical Thermodynamics, Volume 189, 2024, 107200.

Over the years, I've collected and read a number of papers by R.J.M. Konings, O. Beneš, who published frequently in the journal Calphad, with which, some ten or fifteen years ago, I had a fascination, that for some reason waned. It focuses on the computerized modeling of complex phase diagrams. (My son was once offered, by his master's advisor, the opportunity to go to Sweden to be trained on the Calphad program; somehow it fell through.) The program is about Chemical Thermodynamics. Public contempt for thermodynamics, one of the most important sciences in the world, a science that literally defines the world, accounts for the popularity with a general public of enthusiasm for batteries and, worse, hydrogen.

Dr. Konings has apparently moved to Delft University; for many years he was at the Karlsruhe Nuclear Research Center, which was an institute in Germany for the advancement of nuclear energy. I know nothing of him personally, but I would assume he moved since the rise of ignorance and contempt for the collapse of the climate led Germany to willfully destroy its nuclear infrastructure, fund Putin's abilities to attack other countries, and burn gas and coal instead.

I don't know that. I'm just guessing.

Anyway, about technetium, a remarkable metal with some very interesting properties, only one of which is chemical inertness, and the ability to eliminate corrosion in steels, it is the lightest element in the periodic table for which no stable isotopes exist; all of its isotopes are radioactive, and only one, ⁹⁹Tc, is available in bulk (ton) scales, where it can be isolated from used nuclear fuels.

I have recently focused some of my discussions with my son on reactor design on this metal, which has properties very similar to the rare element rhenium, and can be made to exceed supplies of rhenium via production in nuclear reactors.

From the paper's introduction:

Technetium (Tc) is a silver-grey transition metal with low natural abundance [1]. It is, however, one of the major fission product created during nuclear energy generation. Its fission yield is approximately 6 %, ranking amongst the highest of the fission products. Also its half-life ranks among one of the highest of the fission products, 2.1 × 10⁵ years. Technetium can occur in various oxidation states and the oxyanions of valence states IV, V and VI are quite stable [2], [3], and some are relatively mobile in an oxidising aqueous environment [4]. This makes technetium of major concern during both energy production as well as high-level waste storage [5].

In irradiated nuclear fuel the oxygen potential is low, and technetium remains in metallic state. It is a component of the 5-metal particles, a fission product alloy of Ru-Pd-Rh-MoTc, which are found throughout the nuclear fuel [6], [7] and are very stable, even surviving the fuel reprocessing in nitric acid. The stability of these 5-metal particles is generally modelled by computational chemical thermodynamic methods [8], which require accurate description of the lattice stability of the constituent elements. It is therefore necessary to have a accurate knowledge of both low- and high-temperature thermophysical properties. Since there is little to no information available of the technetium metal, previous assessments [9], [10], [11] have heavily relied on estimates and extrapolations of what is known. These estimates are made with sound scientific basis in mind using comparisons to other transition metals, particularly the neighboring Ru, Rh and Os metals, which have a hexagonal closepacked (A3) crystal structure, similar to technetium.

Thermophysical properties such as the heat capacity across a wide temperature range provide information on phase transitions, lattice vibrations, energy excitations as well as electronic properties. In recent years various studies have been performed on heat capacity and enthalpy increments of technetium metal. Experimental data exist between the temperature 3 and 15 K [12] and between 323 and 1500 K [13], [14]. In the range of 15 K to 323 K no experimental data exist, and data needs to be interpolated or estimated, in order to obtain the standard entropy, a key thermodynamic parameter. One of the reasons to perform experimental studies on lower temperature heat capacity is that the entropy Debye temperature model loses its validity under 100 K leading to uncertainty in the calculated values. In this study, we aim to fill this gap...

The experimental handling of the metal is interesting:

The technetium metal was taken from the batch that was produced by Spirlet and coworkers, as described in [15]. The metal was obtained by reducing ammonium pertechnetate in Ar/H2 in a resistance furnace at 1073 K. The metallic powder was remelted several times into small buttons by arc melting to improve the purity of the material by evaporating the impurities. Finally the metal was casted into rods that were cut to cylinders of required size. The microstructure of the rods consisted of large grains often extending from the centre to the rim of the rods [15]. Three analysis performed on the final ingots produced at that time show a purity of 99.819 % to 99.982 %, with main impurities W (?1000 ppm), Ce (?800 ppm), Re (?450 ppm), Si (?220 ppm) and (Pu + U) (?210 ppm). The oxygen content was found between 44 and 187 ppm.

There's a nice description of the Setaram calorimeter instrument in the paper and a bunch of discussions of the Einstein and Debye theory of heat capacity.

Some graphics showing the results of the experimental work:



The caption:

Figure 2. Measured low-temperature heat capacity for the normal state of technetium metal from 2.1 ? 293 K in a magnetic field of B = 1 T. The solid line show the extrapolation to T = 298.15 K.

The caption:

Figure 4. The high-temperature heat capacity of technetium metal; the red line shows the fitted equation, the blue line the recommendation from the OECDNEA review of 1999 [11], [37]. Also shown are the recommended curves for rhenium (light green), ruthenium (yellow) and osmium (dark green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The caption:

Figure 5. The low-temperature heat capacity of the metals in group (7) (top) and of the Mo-Tc-Ru suite in the second d-block series (bottom) of the periodic table of elements. The blue circles represent the results from the current study, the small magenta circles those of Trainor and Brodsky [27]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

These heat capacities have implications as a structural material for nuclear reactors which may smooth temperature changes in cases where nuclear energy is used in thermal applications beyond merely producing electricity. This would only be possible in a sane world, which is not the world in which we live. It's feasible, but again, given the rising enthusiasm for lies, stupidity, etc., not likely to be realized.

It's a very interesting metal though, only available from used nuclear fuels, and I object personally to efforts to dump it in waste dumps.

Have a wonderful weekend.

[drray23]

1. On a related topic, we were asked a few years ago

To work on exploring means of producing Tc-99m for medical purposes. This isotope is mainly produced in nuclear reactors via Mo-99m decay.

Tc-99m is valuable for medicine because it has a short half life ( 6 hours).

There is currently a shortage of facilities where the Mo-99m can be produced to yield Tc-99m.

This prompted the DOE to look at alternative means such as using an Accelerator ( e.g a cyclotron) to produce it. The idea was that if we could make a compact source, we would be able to locate them near major hospitals and produce Tc-99m on the fly for medical diagnostics.
I briefly worked on this but we could not get a funding grant to further pursue it since, meanwhile the Canadian reactors who were the main source restarted.

Of course, we are in the same situation again, as most European reactors specializing in this are going offline.

https://snmmi.org/Web/News/Articles/Imminent-Mo-99-Tc-99m-Shortage-Due-to-European-Reactor-Restart-Delay.aspx


One of my friends is running the isotope program at DOE, I expect we will get a call soon.

[NNadir]

2. Yes, this is a serious problem. It appears that a hospital accelerator using proton bombardment of...

...⁹⁸Mo might give the ⁹⁹Tc nuclear isomer, to which ^99mTc decays, so to use an accelerator, one would need to use it as a spallation source, which is why, I guess, ⁹⁹Mo fission product sources are used.

(My son did a summer internship at a spallation facility at Oak Ridge, but I think it was pretty energetic. I'm not sure that hospital accelerators qualify in this setting.)

It is notable that during imaging, the ^99mTc decays to ⁹⁹Tc, not ⁹⁹Ru and one has to piss away the longer lived isotope, which remains in the body for several days. The ^99mTc imaging technology saves lives.

⁹⁹Tc (ground state) is not much of a health hazard, and it does call to mind all the hysteria about Sellafield and La Hague releasing ⁹⁹Tc, into the ocean where the concentration was incredibly lower than in a typical medical test.

This was a bad idea I think, not because of risk, but because the metal, which has marvelous properties - including the ability to catalyze the reduction of NO₃^- to N₂ and N₂O with hydrazine - as well as remarkable metallic properties, high strength, chemical inertness, and a high melting point, is extremely valuable in my opinion. We definitely need to retain technetium for use, particularly because rhenium supplies are limited.

The property of preventing the corrosion of steel via the formation of a TcO₂ passivation layer is, I think, an important application that should not be ignored.