Archive for August, 2012

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Gamma Analysis of Chagan “Atomsite”

August 19, 2012

Lake Chagan (“Atomic Lake”) was formed in 1965 following a thermonuclear cratering explosion on the Semipalatinsk Test Site in Kazakhstan.  More photos from my recent trip to the site are here.  Merely visiting the site does not answer some of the most interesting questions about its current state, such as the isotopic origin of the significant (1-2 mR/hr) gamma radiation.

I decided to take a more scientific look at the gamma rays emitted from Chagan’s fused rock—the glassy, vesiculated slag (“atomsite” or “kharitonchiki”) that covers the ground near the shore of the lake.  A grab sample was acquired, and transported home by means other than my own return flight from Almaty (this airport’s departure lounge is guarded by a notoriously-sensitive portal scintillator made by Aspect).  I filled a 3-ounce plastic jar with the material for counting.

My method of analyzing this unique “soil sample” is HPGe gamma-ray spectrometry.  I followed the same approach discussed in my earlier analysis of Japanese soils, involving comparison of the test specimen with an identically-shaped Cs-137 sand standard.  My germanium detector is operated via a homebrew LabVIEW program built around Mark Rivers’ EPICS interface for the Canberra 556 AIM MCA and Carsten Winkler’s CA Lab; I subsequently analyze the spectra (peak fitting, background subtraction, energy calibration) with FitzPeaks.  In this experiment I collected an 8000-second count of the slag sample and a 2000-second count of the Cs-137 sand standard.  An appropriate long-duration background was subtracted from each.  The quantitative calculation of activities relies on a single major line from each nuclide, chosen (to the extent possible) to be close to 662 keV.  Corrections for detector energy response were made by calibrating the energy-dependent photopeak efficiency in FitzPeaks to a point Ra-226 source, covering the range of roughly 200-1600 keV with a power-law model.  Corrections for material attenuation, including density variations from the standard, are NOT made from a calibration but are calculated based on an exponential attenuation model that assumes the sample has the elemental composition of concrete.  It’s probably not a bad comparison, and typically results in a correction of under 20%.  However, I expect better accuracy in the quantitative analysis for peaks that are closer to 662 keV.  Finally, no corrections are made for count losses to coincidence summing.  An Excel spreadsheet of this data and analysis may be downloaded here.

Referring to the 0-1600 keV gamma spectrum below, the first major observation is that most of the lines belong to europium isotopes, Eu-154 and Eu-152.  These isotopes were produced when neutrons from the “device” were captured by the ~1ppm naturally-abundant Eu-153 and Eu-151, respectively, which have remarkably high capture cross-sections.  These activation products are also long-lived enough to persist in significant quantity to the present day.  The other major long-lived gamma-emitting activation nuclide is Co-60.  Some of this cobalt could be from metal in the bomb’s well casing, but it could also be from activation of crustal mineralization.  The remaining major activity, Cs-137, is a product of fission in the bomb’s fissionable components.

Gamma spectrum of Lake Chagan atomsite

If we examine the smaller peaks in detail (click on below thumbnails), long-lived isotopes of holmium (Ho-166m), silver (Ag-108m), and barium (Ba-133) are in evidence.  Am-241 is present at a low concentration; on the basis of its 59-keV gamma line I cannot confidently estimate its concentration using the Cs-137 reference source technique.  Am-241 is the daughter of Pu-241 produced by neutron capture on plutonium in the bomb, and thus is a reliable proxy for the presence of plutonium in the sample.  The gamma radiations from plutonium itself are too weak and swamped by the spectrum’s low-energy continuum to be observed.

The chart below presents the results of the quantitative analysis.  Gamma-emitting radionuclide activity in “Chaganite” exceeds 375 Bq / g, with Eu-152 being the most concentrated.

Nuclide concentrations, July 30 2012

Chaganite versus Trinitite: when the activities are normalized to their initial values at the time of the respective explosions (1965 and 1945), a direct comparison can be made that illustrates just how much more radioactive the Chagan slag is (see beow).  The data for Trinitite is taken from Pittauerova, Kolb, et al., “Radioactivity in Trinitite: a review and new measurements,” Proc. 3rd Eur. IRPA Conference, Helsinki, 14-16 June 2010.

Comparison of “Chaganite” with Trinitite

The Chagan slag contained almost an order of magnitude more Cs-137 at the time of formation, but it is the rather staggering ratios of the activation nuclides that surprises me the most: 400 times as much Eu-154 in Chaganite versus Trinitite.  70 times as much Eu-152.  And 370 times as much Co-60.  Why?  One fairly obvious explanation is found in the facts that Chagan was a more powerful bomb, detonated in closer proximity to the crustal rock that its neutrons activated since it was underground.  Some further considerations may also be relevant.  According to Carey Sublette’s Nuclear Weapons Archive, Chagan “was reported to be a low-fission design, which had a pure thermonuclear secondary driven by a fission primary with a yield of about 5-7 kt.”  In contrast, the Trinity bomb was a pure fission core surrounded by a uranium tamper.  Thus, escaping neutrons with a hard DT fusion spectrum probably carried a significantly higher fraction of Chagan’s energy yield relative to Trinity’s.

There is not a statistically-different concentration of Ba-133 between the two slags.  I think most of Trinity’s Ba-133 came from the bomb’s explosives, while Chagan’s probably came from crustal concentrations of barium.

Finally, if the Trinity bomb had a fission yield more than three times larger than Chagan, why is the latter’s concentration of Cs-137 higher?  The best reason I can suggest is Chagan’s better underground containment of volatile fission products.  In a surface explosion, volatile Cs and its beta-decaying precursors exist as gases for a long time, enabling atmospheric dispersal.  In an underground explosion, volatiles are condensed rapidly near where they were formed.

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Visit to the Semipalatinsk Nuclear Test Site

August 13, 2012

Soviet Ground Zero

At 7:00 on the morning of August 29, 1949, a nuclear fireball lit up the skies over a desolate expanse of steppe about 100 km from Semipalatinsk in the Kazakh Soviet Socialist Republic.  This explosion—the culmination of a research effort personally supervised by fearsome NKVD chief Lavrenty Beria—earned the Soviet Union status as a nuclear-armed superpower to rival the United States.  Over the course of the next 50 years, 615 more nuclear explosions, as well as numerous subcritical, radiological, and reactor-based experiments, occurred on the same New-Jersey-sized reservation—the Semipalatinsk Test Site.  The STS was largely abandoned in 1991 in the turbulent prelude to Kazakhstan’s independence.

This July I had the good fortune to visit the STS and its formerly-secret support city, Kurchatov.  Physical access to the STS is minimally controlled, but given the Kazakhstani police behaviors we observed, foreigners would arouse decidedly too much suspicion traveling to the area without official sanction for their trip.  Some reactors remain operational and some testing grounds (particularly Degelen) contain proliferation-sensitive debris.  I recommend contracting with a registered adventure tour company (I hired Nomadic Travel) to handle permissions, lodging, and appropriate transportation.  Roads on the Test Site are impassible in wet weather, merely brutal when dry, and I don’t exaggerate in the judgment that some of them would be faster on horseback!

Photo selections below include Kurchatov; Soviet “Ground Zero;” the aerial bombing target for the first Soviet staged thermonuclear bomb; the Degelen Mountain underground test site; a borehole on the Balapan underground site which experienced an “emergency situation;” and finally, the radioactive crater known as Lake Chagan.  The photos provided below are all captioned with additional detail.

Links to photo galleries (or scroll down):

  1. Kurchatov, the Secret City
  2. Soviet “Ground Zero”
  3. The RDS-37 Site
  4. The Degelen Mountains
  5. Borehole 1007: “Emergency Situation”
  6. Lake Chagan, the “Atomic Lake”
  7. Reactor Facilities at STS

Kurchatov, the Secret City

Kurchatov appeared on no maps and had no name (except for a cryptic post office number) for most of its existence. It was built hastily by GULAG labor and hosted many famous (and infamous) people of importance to the Soviet nuclear weapons project. Now it has a new life as a peaceful nuclear city, with a satellite campus of Kazakhstan’s National Nuclear Center occupying new buildings in town. Meanwhile, historic structures are crumbling and the town is clearly a shadow of its former self.

Soviet Ground Zero

60 kilometers southwest of Kurchatov is the 20-km-diameter “Experimental Field” (Опытное Поле), dotted with strange and dilapidated structures, radioactive slag, and swampy craters.  Its P-1 site, shown in all the photos below, was Ground Zero for “Joe-1″, RDS-6S (the first Soviet thermonuclear bomb, named for a delightful Russian pastry), and two other successful bombs.  All bombs tested at this spot were positioned on 15-30m towers.  At least two dozen more surface tests took place elsewhere on the Experimental Field.  To watch a video of “Joe-1,” click here.  To watch a video of the “sloika,” click here.

The RDS-37 Site

On Nov. 22 1955, the Soviet Union’s first multi-stage hydrogen bomb (embodying what is known as the “Teller-Ulam” configuration in the US, credited as Andrei Sakharov’s “Third Idea” in the Soviet Union) was dropped from an airplane toward a target designated by an 800-meter-diameter chalk circle on the Experimental Field about 3 km southwest of the P-1 site.  The bomb detonated at an altitude of 1.6 km with an unexpectedly-high yield of 1.5 megatons, killing a number of people in the region (including a 3-year-old girl). What remains today are faint traces of the target markings. Like the Nazca Lines, these are easier seen from space (see the Google satellite pic). Radiation levels at the site are modest, no more than about twice regional background.  There is no notable “atomsite” slag on the surface of this site.  Watch video of the RDS-37 blast here, which shows some footage of the event as seen from Kurchatov at the end.

The Degelen Mountains

“There’s plutonium in them thar hills!”  The Degelen Mountains were used for hundreds of underground nuclear tests carried out in horizontal adits in the rock. These adits are now “prohibited areas” because many tests were subcritical and chunks of plutonium remain in the residues that the Soviet Union neglected to clean up.  According to William Tobey’s sources, “hundreds of pounds of weapons-grade fissile material was ‘readily recoverable’ in the tunnels” at Degelen, enough to make quite a number of bombs.  The mountains themselves are hauntingly beautiful, and the surrounding foothills dotted with military ruins.

Borehole 1007: “Emergency Situation”

Borehole 1007 at the Balapan site was supposed to contain a routine underground nuclear test in February of 1972. But the bomb was a little too feisty, and ended up blowing the top off the well. A piece of the well casing (quite radioactive, I should mention) is now displayed in the STS Museum in Kurchatov. The rest of the well, and all its radioactive ejecta, is right here where we found it on the steppe.

Lake Chagan, the “Atomic Lake”

An idyllic and suspiciously-round lake of some 10 million cubic meters capacity graces the left bank of the Chagan River. It owes its existence to a 140-kiloton “peaceful” nuclear explosion carried out on January 15, 1965. The stated objective was to experiment with changing the course of rivers. Chagan was a filthy test, heavily contaminating the surroundings with radioactive byproducts. Like the American Operation Plowshare, bomb developers found that these peaceful uses worked after a fashion, but resulted in contamination that tended to preclude practical use. Lake Chagan would make a great picnic spot, but we were not able to enjoy some nourishment here ourselves because we were required to wear respirators over our pie-holes. The banks of Lake Chagan are strewn with this bomb’s unique slag, a sort of foamy, pumice-like rock.  Hottest spots on the bank now seem to be about 2 mrem / hour.  Click here to watch a video of Lake Chagan’s creation, including footage of swimmers in the water.

Reactor facilities

The Semipalatinsk Test Site contains more than just old nuclear weapons tests; it is also home to some working nuclear facilities that are quite fascinating. We didn’t make it inside the Baikal and IGR complexes, but I grabbed some photos in their general direction.

For more photos, including photos from the Tien Shan Mountains, Astana, Almaty, and other cities in Kazakhstan, please see my Facebook page.

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