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Wed Dec 27, 2017, 12:43 AM

Oxygen Isotope Ratios at the Trinity Nuclear Test Site and Laser Fluorination Extraction.

Recently I have been interested in the properties of actinide nitrides - which feature very high melting points - and their use in multiphasic nuclear reactors featuring low melting plutonium/neptunium eutectic liquid fuels, reactors about which I've been musing for some time. It seems to me that these types of reactors could be designed to run, without refueling, for more periods well longer than half a century in a "breed and burn" scenario, where the actinide nitride is UN, uranium nitride consisting of unreacted uranium in used nuclear fuels or depleted uranium, thus eliminating, ultimately, the need for mining anything, oil, gas, coal and, um, uranium for a very long time, for generations.

Nitrogen consists of two natural isotopes, N-14 and N-15 which represent respectively, 99.6% and 0.4% of the content of natural nitrogen. Interestingly Nitrogen-14, the most common isotope by far is, parenthetically, the only stable nuclide in the entire table of nuclides to feature both an odd number of neutrons and an odd number of protons.

In a neutron flux, N-14 undergoes a 14N[n,p]14C reaction during which it is converted into radioactive carbon-14. Carbon-14, as I recently confirmed in a search of the literature, itself has a very low neutron capture cross section which means that it would be extremely useful in actinide carbide fuels which also have extremely high melting points, as well as in known carbide refractories such as silicon carbide, titanium carbide and carbon containing MAX phases. Low capture cross section elements in materials increase neutron efficiency and thus breeding ratios.

(Any attempt to reverse the oxidation of carbon to fight climate change - a formidable engineering task to be sure - will require access to extremely refractory materials to provide high temperatures to produce hydrogen from water thermochemically and carbon monoxide from carbon dioxide, also thermochemically.)

Most of the nuclear scientists whose lectures I have the opportunity to attend are fusion people. They are interested in various cross sections of the light elements, whereas generally, I am not, except as described above, in the case of there presence in fusion fuels. (While their work is fun, none of it will become practical in any time remotely capable of addressing climate change; there are far too many practical issues to address for which they only have a loose approach to addressing.)

I should pay more attention though to the light elements though; their nuclear behavior is important in both fusion and fission systems.

A few years back, in some of my desultory wanderings in the scientific literature, I came across an interesting paper relating to the capture cross sections of light elements. Specifically, the material being tested was "trinitite," the glass material that was first observed after the first nuclear weapons test at the Trinity test site in 1945.

That paper is here: Oxygen Isotope Composition of Trinitite Postdetonation Materials (Koeman et al, Anal. Chem., 2013, 85 (24), pp 1191311919)

To my personal surprise, they find that the oxygen isotopic ratios at the Trinity test site are not significantly altered, indicating that there was very little neutron capture resulting from the high neutron flux associated with the test, which was, after all, a ground test. The oxygen isotopic ratios were consistent with the natural isotopic fractionation that goes on during the formation of various minerals at the site, which consists of arkosic sands.

(A marker for the neutron flux was the radioisotope 152Eu, which formed from neutron capture in natural 151Europium in the sands. This isotope has a half-life of 13.54 years, and a little more than 2% is still present today at the test site.)

What is also very interesting is the analytical technique that was utilized in determining these oxygen isotope ratios, a beautiful extraction technique called "laser fluorination." This technique is described in the following paper authored by French scientists: IR Laser Extraction Technique Applied to Oxygen Isotope Analysis of Small Biogenic Silica Samples (Alexandre et al Anal. Chem., 2008, 80 (7), pp 23722378).

In this technique oxygen is liberated from a sample by oxidation by the powerful oxidant BrF5.

A description from the paper:

Oxygen Extraction Using the IR Laser-Heating Fluorination Technique and δ18O Measurement.

Molecular O2 was extracted from silica in a laser extraction line close to the one described by Sharp.35 A Merchanteck 30 W CO2 IR laser was used. The nickel sample holder was loaded in the sample chamber, prefluorinated with 50 mbar of BrF5 for 1 h and pumped for several hours. In an atmosphere of 100 mbar of BrF5, samples were preheated for 20 s with a 2000 μm diameter laser beam, increasing the power of the laser beam until the particles start moving:  from 0% to 3.6% of the laser full power for fine quartz, 2.2% for diatoms, and 2.6% for phytoliths. The laser emission was stopped after 30 s. Quartz grains and phytoliths were then heated with a 2000 μm diameter laser beam at 30−35% of its full power (11−12 W), starting at the center and slowly moving the laser beam following concentric circles until a bowl of liquid silica formed. Diatom samples were heated with a 2000 μm diameter laser beam, starting at the edge and progressively increasing the laser power from 0% up to 30−35% of its full power (11−12 W), slowly moving the laser beam following concentric circles. For all samples, when a bowl of liquid silica formed, the laser beam was then focused at 1000 μm of diameter until the liquid disappeared. The remaining particles were heated with a focused 200 μm of diameter laser beam. Laser emission was stopped when no more reaction to the laser beam occurred. Some residues remained for the diatom subsamples. They decreased from KYO 40 to KYO 90. These protocols prevented ejecta.

The liberated oxygen was then purified and trapped by adsorption in a microvolume filled with 13X molecular sieve and cooled in liquid nitrogen. The oxygen gas was then heated at 100 C and directly sent to the sample bellow of the dual-inlet mass spectrometer (ThermoQuest Finnigan Delta Plus).

In order to get a sufficient 34/32 signal (2−3 V), the oxygen from 0.3 mg aliquots was concentrated in the mass spectrometer in an autocooled 800 μL microvolume filled with silica gel and directly connected to the dual-inlet system.

Cool I think, at least if you think a certain way. Analytical inorganic chemistry can be very beautiful.

I wish you a happy and prosperous New Year.

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Reply Oxygen Isotope Ratios at the Trinity Nuclear Test Site and Laser Fluorination Extraction. (Original post)
NNadir Dec 2017 OP
PoindexterOglethorpe Dec 2017 #1
NNadir Dec 2017 #2
PoindexterOglethorpe Dec 2017 #3

Response to NNadir (Original post)

Wed Dec 27, 2017, 12:50 AM

1. You may be interested to know that the Trinity site is open to visitors

on the first Saturday of April and the first Saturday of October every year. Go, if you can. Absolutely fascinating.

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Response to PoindexterOglethorpe (Reply #1)

Wed Dec 27, 2017, 10:10 AM

2. Thanks. I'd go if I were in the area for some reason. I looked at the online visitor's guide...

...which covers the site pretty well.

Trinity Site Open House

Interesting, I'm sure, but for me probably not worth a special trip. I know more than the average citizen, I think, about the history of nuclear testing and if I were to able to make and afford a special trip to a nuclear test site, a cruise to Bikini would be definitely worth it.

There were a lot of huge nuclear tests there, and being in the South Pacific, there's a lot of biological specimens at the site.


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Response to NNadir (Reply #2)

Wed Dec 27, 2017, 12:15 PM

3. To be honest, you won't learn a great deal by actually going,

other than to see how isolated it was. Still is to a large extent.

It's right up there with seeing someone well known in person as compared to seeing them on a video. But I do understand that it depends on where you live if going there would we worth it.

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