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Sat Mar 17, 2018, 04:38 PM

Forensic Analysis of One of the Earliest Weapons Grade Plutonium Samples Ever Prepared.

Recently I've been studying - because some excellent articles on the topic have been showing up in the scientific journals I routinely scan and or read - the interesting chemistry of the clean up of one of the most radioactively contaminated sites in the world, the Hanford Reservation near Richland, Washington.

Here for instance, is one such paper on this topic, on which I may comment in the future in this space: Review of the Scientific Understanding of Radioactive Waste at the U.S. DOE Hanford Site (Peterson et al, Environ. Sci. Technol., 2018, 52 (2), pp 381–396)

I am always interested in radiochemistry, since I believe understanding it represents the last best hope of the human race.

Coincidentally, I've been going through old papers that I collected years ago but never read to sort them into appropriate directories, and I came across an interesting paper relating to one of the earliest known samples of plutonium ever prepared, prepared in the early days of the Manhattan project. The paper is here: Nuclear Archeology in a Bottle: Evidence of Pre-Trinity U.S. Weapons Activities from a Waste Burial Site (Schwantes et al., Anal. Chem., 2009, 81 (4), pp 1297–1306)

The Hanford site, which is where most of the plutonium for America's nuclear weapons was made, for most of its history as a production plant, operated on what its operators considered to be "emergency" conditions, extreme conditions of races against real and putative enemies, in both hot and cold war. The mentality was not focused at all on the long term other than potential post apocalyptic scenarios in which our enemies nuked us before we could nuke them. In such a mentality, so far as radioactive fission products as well as toxic chemicals were concerned, they were handled in a way that in our more distant time we would regard as extremely cavalier. In the earliest years, nuclear by products, often referred to as "nuclear waste," were often disposed of in open trenches, to be replaced later by single shell tanks, some of which famously leaked, and then in double shell tanks. Poor records and inventories were kept, but again, the remediation of this site has some fascinating chemistry and the clean up shows as much ingenuity as the creation of these materials did.

By the way, the existence of the Hanford Site has not lead to a death toll that even remotely approximates the number of people who have been killed by the by products of combustion of dangerous fossil fuels and biomass, which approximates about 7 million people a year, every year, which is roughly the equivalent of nuking and completely wiping out a city the size of Hong Kong every year, without stop. There are people, not very bright people, who wish to represent Hanford as the worst environmental problem that has ever existed, scientifically illiterate journalists for example. The 55,000 citizens of Richland, Washington are leading useful lives - many are scientists at Pacific Northwest National Laboratories - and are not dropping dead in the streets.

But no matter.

Anyway...Nuclear Forensics and the earliest plutonium samples:

From the opening text of the paper:

The frequency of smuggling events involving radioactive materials is supply driven and is on the rise world-wide.(1, 2) While special nuclear materials from the nuclear fuel cycle have not significantly contributed to this increasing trend to date, it is likely that with the current nuclear renaissance and greater access to these materials by the public, smuggling events involving fissionable materials may rise in the near future. Perhaps the most effective tool investigators have against this type of smuggling is the successful application of nuclear forensic science.(3) Nuclear forensics is defined as the science of identifying the source, point-of-origin, and/or routes of transit of nuclear and radiological materials associated with illegal activities for ultimately contributing to the prosecution of persons responsible for those activities.(4) In many respects, the goals of nuclear archeology are identical to those of nuclear forensics, without the added constraints specifically associated with legal prosecution. As such, studies of nuclear archeology serve as an excellent means for advancing the science and demonstrating the capabilities of the nuclear forensics community. Moreover, depending upon the pedigree of the artifacts studied, fully characterized finds representing specific end members of various processes or reactors may be of direct use to forensics experts for comparative purposes against real interdicted sample materials of unknown origin.(5-7) This work provides the public a rare glimpse at a real-world example of the science behind modern-day nuclear forensics and, in doing so, uncovers a sample of historical significance.

Background

The Hanford Site in Washington became the location for U.S. plutonium production during World War II. The Pu produced at this site was used in the first Pu nuclear weapon dropped on Nagasaki, Japan, on August 10, 1945, and in Trinity, the name given to the world’s first test of a nuclear weapon on July 16, 1945. In December 2004, a safe containing several hundred milligrams of extremely low burnup Pu (a term typically associated with Pu produced as part of a weapons program) in a one gallon glass jug was unearthed by Washington Closure Hanford (WCH) personnel while excavating the 618-2 burial ground in the 300-area of Department of Energy’s Hanford site.(8, 9) The jug contained ∼400 mL of slurry characterized as a white precipitate in a clear liquid. Pictures in Figure 1 document this find. In-field γ analysis conducted on the container detected the presence of only 239Pu. The minimum 239,240Pu/238Pu and 239Pu/241Am ratios were estimated to be at least 320:1, and 1000:1, respectively, based upon the detection limits of this analytical technique, indicating the Pu was produced from extremely low exposure fuel, consistent with early military reactor operations at Hanford. The absence of γ-emitting U or fission product isotopes in the spectra also suggested the Pu had been separated and purified prior to its disposal. Considering the potential historical significance of the find, WCH personnel coordinated with staff at Pacific Northwest National Laboratory (PNNL) to conduct further analysis of the sample. All of the liquid and ∼2% of the solid from the container were repackaged into two 1 L polypropylene bottles on May 10, 2006, with one of the two bottles being transferred to PNNL. The majority of the solid material remained, caked to the walls of the original glass jug and was earmarked for disposal. We have coined the process of characterizing this sample as nuclear archeology.


Here's a photograph of the safe and the bottle in it in which the plutonium was found:



The caption:

Figure 1. Pictures of (a) excavated safe and contents and (b) glass bottle containing several hundred milligrams of Pu.


Apparently the process utilized to isolate the plutonium used a lanthanum fluoride carrier. It must have been the case that there was very little plutonium available at the time of the isolation, which is not surprising. In 1944, a chemist named Don Mastick broke a test tube in such a way as to end up eating what was then the world supply of the element; and many years later, as an old man was interviewed on the subject before dying in 2007 at the age of 87.

The scheme for analyzing the contents of the bottle is shown in the following graphic, also from the paper:



One of the interesting things about this paper which surprised me - this after more than 3 decades of reading about nuclear science - was that there was enough Na-22, a radioactive isotope of sodium in the sample to use it as a kind of tracer of the history of the bottle. As it is a radioactive isotope that is neutron deficient, as opposed to neutron rich, it's not an isotope I ever bothered thinking much about. It arises from the interaction of fluorine, a monoisotopic element with a mass number of 19, with alpha particles:

GEA revealed the presence of the relatively short-lived 22Na isotope within the sample. The mechanism for the formation of 22Na (t1/2 = 2.6 years) within fluoride matrixes in the presence of α-emitting actinides has been well documented in the literature(18-23) following the reaction pathway of 19F(α,n)22Na. The production rate for this reaction is a function of the physical characteristics of the fluoride matrix, the production rate and energy of the α particles, and the proximity of the α particles to the 19F atoms. Equation 5 provides a mathematical model for the production of 22Na within simple actinide fluoride solids.




You learn something every day. This might be of interest to all those people working on MSRs (Molten Salt Reactors) utilizing the "FLIBE" or "FLINAK" salts. It's probably not a serious drawback, but one probably requiring some attention.

Some additional comment from the authors on the role of Na-22 in their analysis:

Isotopes like 22Na that are produced from secondary nuclear reactions involving radioactive material may be useful to investigators when a sample of unknown history containing such material is discovered. With the use of the Pu jug as an example, the 22Na activity becomes an easy to detect (γ energy, 1275 keV; branching ratio, 99.4%) signature for 239Pu under steady-state conditions (regions 2 and 4 of Figure 4). In addition, with the assistance of an accurate production model for 22Na, the total Pu within the sample prior to repackaging can be estimated prior to reaching steady-state conditions (i.e., within region 1) if it is known a priori when 22Na production began. Alternatively, the time since 22Na production began may be estimated during the in-growth period (region 1) if the amount of Pu within the sample (prior to repackaging) is known. However, it is region 3 of Figure 4 that is of most interest to the nuclear forensics community. Here the Pu jug after repackaging (2006) resembles what might be expected from an interdicted sample that, unknown to the investigator, had been separated from the majority of the Pu prior to confiscation. In such a case, a decrease in the 22Na activity with time would suggest the confiscated sample may have been portioned off from a greater amount of Pu that had escaped interdiction


Figure 4:



The caption:

Figure 4. Predicted and measured 22Na content with time within the Pu sample from 1945−2038.


Further elaboration is in the text of the original paper about how to use isotopes like 22Na (or similar secondary nuclear reactions) to determine whether the same contains all of the plutonium originally available from its source, or only a fraction of it.

By the way, the plutonium in this sample was almost pure Pu-239, a grade of plutonium that today would be considered an extreme weapons grade material. This is unsurprising, since the Manhattan project had no way of knowing the effect of plutonium-240 would have on their weapons, and probably went to great lengths to avoid its accumulation. This requirement, regrettably greatly increased the volume of waste in order to isolate it, and this remained an issue, even after it was understood that weapons grade plutonium could tolerate more Pu-240 than was realized. Weapons grade plutonium is still not at all like reactor grade plutonium.



The caption:

Figure 7. Comparison of measured Pu isotopic ratios from the sample (solid red squares) with predicted ratios within spent fuel from X-10 reactor at 3.6 and 3.7 MWd/MTU (solid and dashed blue lines, respectively) and B-reactor operations (dashed grey line) in the 1940s. The model line for the B-reactor represents ratios that would have been produced at the lowest recorded power level (17.2 MWd/MTU) for that reactor. The area above the B-reactor model line represents the possible range of isotopic ratios that could have been produced at power levels above the lowest recorded value. The value of the 242/239 ratio for the sample was found to be below the limit of detection (identified by the open red square) of the analytical technique used.


Using the ratio, the authors determined that the source of the plutonium was not Hanford's more famous B-reactor, but rather the X-reactor, which was not located at Hanford, but rather at Oak Ridge.

This is explicated in the full text.

Interesting stuff, I think.

I wish you a pleasant rest of the weekend.





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