Posts Tagged ‘radioactivity’

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Gamma activity measurements of Tokyo-area soil samples

November 4, 2011

Three nuclear reactors melted down at the Fukushima-I Nuclear Power Plant following the Tohoku Earthquake of March 11 this year, resulting in the release of volatile fission products in what is widely regarded as the worst nuclear accident since Chernobyl.  Radionuclides were carried by air currents across eastern Japan.  Areas closer to the stricken plant suffered heavier contamination, but even densely-populated Tokyo, some 150 miles distant, received significant fallout.  Last month, I received a set of six soil samples from the Tokyo region, and, using my HPGe gamma detector, I have attempted a quantitative analysis of the two predominant gamma activities in these samples, Cs-137 and Cs-134.  I am grateful to Jamie Morris for the specimens, and to Dr. Steven Myers, Los Alamos National Laboratory, for his helpful communications about technique and analysis.

Jamie collected six soil samples of about 5 fl. ounces apiece, three from roadside gutters and three from nearby garden areas in the greater Tokyo region, and sent them to me in Ziploc baggies by regular airmail declared as “soil samples.”  He documented his collecting spots with geotagged photos (below).

Upon receipt of Jamie’s samples, I packed them into 3-oz clear plastic wide-mouth jars (Uline S-17034), weighed the contents, and Superglued the lids on to prevent spills.

It is important to control the source-detector geometry in quantitative measurements.  To that end, I lathe-turned a holder for the jars out of acrylic that fits onto the HPGe detector’s cap.  The jars press-fit into this holder until the lip of the cap thread contacts the front face of the acrylic piece.  Held thusly, the bottom of the sample jar is nominally one inch from the end of the HPGe cap.

A standard source, consisting of a known quantity of Cs-137 in a matrix and geometry approximating those of the samples as closely as possible, will be used as a reference against which to compare the activity in the samples.  Although commercially available, such sources are astronomically expensive and companies making them are reluctant to sell to individuals who just want to fool around.  So I’ll produce my own from the following supplies, using the procedure recommended on Slide 23 of this IAEA presentation:

  • Play sand (Lowe’s)
  • Liquid Cs-137 source (25µl / 0.5 µCi nominal activity, ±5%) ordered from Spectrum Techniques
  • Sealed Cs-137 disk source (0.5 µCi nominal activity, ±5%) ordered from Spectrum Techniques
  • Nitric acid
  • Beakers, syringe, stirring rod
  • Geiger counter (or scintillator)
  • An oven

Basically, the Cs-137 is mixed with sand and put in a Uline jar.  Click any photo below for a caption describing relevant details from the process.

Gamma spectra are collected from each sample and from the standard in my Canberra NIM MCA, using Mark Rivers’ open-source “mca” application for EPICS and my own LabVIEW interface.  8192 channels of memory are used, with the gain set at about 0.2 keV per channel.  I process the spectra to subtract background and find peak areas in the free evaluation version of FitzPeaks (note: does not work on 64-bit Windows 7).  Spectra for each sample are displayed below (click any image for a full-size version).

Activities are estimated by comparing net counts in the relevant peaks in the sample spectra with net counts in the 662-keV peak of the standard source.  Count rates are scaled to account for gamma emission probability of each nuclide.  A simple exponential attenuation mode is used to correct for matrix density variations; better accuracy can be expected for samples that most closely resemble the standard (i.e. the gutter debris samples).  I use only the 605-keV peak to estimate Cs-134 activity, since it lies closer to the 662-keV calibration energy and the systematic errors involved with energy and matrix density corrections will be smaller than for the 796-keV peak.  Ultimately, the values of interest—specific activities, becquerel per kilogram—are obtained, along with uncertainty propagated through the calculations.  These values are illustrated below:

Download the data and analysis spreadsheet (Excel 2010 format) here.

In conclusion: The synthetic fission products CS-137 and Cs-134 dominate the natural gamma radioactivity (K-40 and U / Th daughters) in all six samples.   Cs-137 is present at levels at least 1-2 orders of magnitude above levels expected from older atmospheric weapons tests and the Chernobyl accident in every one of these samples.  Total activity is roughly evenly divided between Cs-137 and the shorter-lived Cs-134 at this time; the Cs-134 will decay to irrelevance in the span of 5-10 years.  Together, high concentrations of Cs-137 and Cs-134 point to the recent Fukushima accident as the source of virtually all of this activity. The gutter debris sample from Chiba (#C) has the highest activity, and depending on how representative this sample is of the surrounding soil, MAY be indicative of significant enough cancer risk to human residents to encourage alternate patterns of occupancy or land use.  More information would be needed to quantify the severity of this kind of risk from external exposure and various routes of possible internal exposure.   Sample #C is also easily detected with small consumer-grade and homebrew Geiger and scintillation counters.   It should be noted that various physical / chemical mechanisms (e.g., runoff of soluble Cs into road gutters) tend to increase the activity of some of these particular samples relative to the surroundings.

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Inside Chernobyl Nuclear Power Plant 2011, Part III: Dosimetry Control Room

August 9, 2011

With a decade-plus lead on the rest of the RBMK fleet in confronting the uncertainties of the decommissioning process, which involves fuel movements and the continuing generation of radioactive wastes, Chernobyl Nuclear Power Plant must continue to remain particularly vigilant on matters of radiation safety.  There is a well-maintained underground bunker at the ABK-1 administrative building that serves as a modern emergency operations center, for example.  This year we visited the Phase I dosimetry control room as part of our tour, where a dosimetrist monitors radiation levels and aerosol levels in the rooms of the Unit 1 and Unit 2 complex, and monitors discharge of radioactivity from the operational VT-1 ventilation stack.  The dosimetry control room is accessed from the +10-meter deaerator corridor, between Unit 1 and Unit 2 reactor buildings.

Click any photo below for a larger version with my description; click again for a full-size file.

For this summer’s other photos from ChNPP, see this post and this post.

To compare and contrast facilities at Chernobyl with those at an operational RBMK-1000 plant, please see jencha’s wonderful recent photodiary from the Kursk NPP (the 12th photo shows the dosimetry control room there with obviously more modern equipment than ChNPP).

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Inside Chernobyl Nuclear Power Plant 2011, Part II: Deaerator Corridor and Unit 1 Control Room

August 8, 2011

More interior photographs from the Chernobyl Nuclear Power Plant, this selection focusing on highlights of the Unit 1 control room and the building’s perhaps most distinguishing interior feature, its 600-meter-long “Gold Corridor.” Right-click any photo and select “open in new window” (or equivalent) for a larger version with my caption.

For this summer’s photos of the Unit 3 end of the power plant, see this post.

Our photos from ChNPP last year are displayed at this site.

The floor plan below is compiled from an official plant safety document and is meant to help illustrate the geography of the power plant on the +10-meter elevation, near Units 1-2.

+10-meter floor plan, Phase I, Chernobyl Nuclear Power Plant

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Albuquerque, Ground Zero

January 16, 2010

May 27, 1957. N34.99°, W106.57°.  A lone steer was grazing this windswept expanse of mesa five miles south of the Albuquerque airport under the noonday sun.  Overhead, a B-36 “Peacemaker” churned toward the runway, ferrying a Mark 17 nuclear bomb from El Paso to Kirtland Air Force Base for service.  Such routine Cold War traffic would not normally be sufficient to jar the animal from his bucolic reverie.  But on this day, owing to a freak accident (the cause of which officially remains unknown), our bovine was about to receive airmail of a highly disruptive nature.

The 42,000-pound two-stage H-bomb–historically the largest nuclear weapon in the American arsenal–was dropped by mistake as the B-36 descended through 1700 feet.  Though the plutonium pit was not on board for safety reasons, the bomb did contain its fissile second-stage “spark plug” made from either plutonium or enriched uranium, as well as the tamper (probably uranium).  It plunged nose-first into the cow-populated mesa, whereupon the shock wave from 300 pounds of detonating high explosive puréed the unfortunately-situated ruminant with inconceivable violence†.  Thunder pealed off the distant hills; Burqueños gaped in awe at the fireball rising in the southern sky.

The acrid fog of charred cow pulp had barely settled when the crack AFSWP (Armed Forces Special Weapons Project) team from Kirtland arrived to discreetly liquidate the consequences of the “broken arrow.” They encountered a 25-foot crater with gamma exposure readings of 0.5 mR / hr at the rim.  Although the Army filled in the crater and recovered most of the weapon, to make a clean sweep of the several square miles peppered with debris would have been a Herculean task.  They did a job that was good enough for government work–in other words, plenty of radioactive H-bomb components still litter the desert for the interested public to collect.  That’s the good news, and I’ll discuss my collection of bomb chunks shortly.

There’s some bad news, however. The inexorable tide of urban sprawl has engulfed just about anything resembling a “windswept expanse of mesa” in the Albuquerque vicinity, and such is the imminent fate of this one.  Forest City Covington NM, LLC has begun marketing the land as a master-planned development called “Mesa del Sol.”  Now it would be a crying shame if this unique venue for radioactive  material collectors got overrun by banal New-Urbanist homes, schools, and shoppes.  Let me make a plea to you, dear reader: If you respect the history of this place, and believe that the wonderful actinide-laden goodies in the topsoil ought to remain accessible to the collecting public rather than gumming up lawnmowers in the front yards of yuppie-stuffed townehomes, please send your thoughts to the developers by clicking here.

†Note: some creative license has been taken with this description of cow’s demise

Links to further historical information:

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Collecting nuclear weapon parts is fun and suitable for the whole family.  Both radioactive and non-radioactive components of the Mark 17 bomb may be obtained on the site (which is publicly accessible on dirt roads with a high-clearance vehicle, or by passenger car with some caution).  I am greatly indebted to Taylor Wilson for bringing my attention to this location.  He has a very nice summary of his findings at his website.  The mesa is devoid of large vegetation, so prepare for wind and weather.  Bring water.  Do not enter the Sandia shooting range to the north of the  bomb site, or approach the Sandia fenceline on the east.   Activity is almost entirely alpha and beta radiation; charged-particle spectroscopy is pending to identify the nuclides responsible.  A pancake Geiger counter is my preferred field instrument.  Shown here is a 15-pound sheet of lead with a surface reading of about 1300 cpm.

Example components of the bomb (click thumbnail for numbered image), relative to calipers set at 2 inches for scale.  Some pieces have identifiable function, others are more mysterious.  Details about the Mark 17 construction remain classified.  Any readers with a better technical eye for these components please feel free to correct my guesswork in the comments, and I will update the list accordingly:

  1. Laminated cork composite from bomb liner.  Cork is very abundant, but never radioactive.
  2. White solid plastic resembling polyethylene, perhaps from interstage.  Most shows signs of melting and charring.  Frequently radioactive.
  3. Black plastic or composite.  Brittle, unlike #2 material.  Never radioactive.
  4. Aluminum casing components, still retaining the greenish-yellow exterior paint.  Never radioactive.
  5. Part of a wiring harness, containing remnants of wires.  Only example found.  Not radioactive.
  6. Fabric sheath / strap material.  Only example found.  Radioactive.
  7. Gear (not radioactive).
  8. Aluminum sheet (no exterior paint).  Not radioactive.
  9. Steel.  Rarely radioactive.
  10. Lead metal, probably from the bomb’s radiation reflector.  Sometimes radioactive.

My spiciest findings are shown at left.  The most radioactive is a small piece of lead, one surface of which registers 13,000 CPM on a 2″ pancake Geiger counter.  The most radioactive plastic piece registers about 7,000 CPM, apparently due to a small embedded object.

My heaviest finding is a contorted piece of lead tipping the scales at almost 30 pounds.  (Sadly, the behemoth is not radioactive.)

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Nuclear Collection (Part III)

April 1, 2009

Radioactive chemical reagents (and a bit of non-radioactive fake yellowcake) constitute this instalment of my Nuclear Collection feature.

u_metal_lazarDepleted uranium metal from United Nuclear. These two rough-hewn triangular slabs weigh in at about 13 g apiece.  No idea what Bob Lazar cut up to put these on the market, but they’re not a bad deal while they last. They sport very rough, sharp edges and have to be stored under oil because of the risk of pyrophoric ignition.  Uranium fires are a bummer, especially when they occur in your living room.

conquista_uFake yellowcake memento from the Conquista Project.  About 20 cm3 of a canaryyellow, non-radioactive powder that resembles a diuranate salt is contained in a small vial embedded in this commemorative plastic paperweight.  The Conoco-Pioneer strip mining and milling operations in Karnes County, Texas commenced in 1971, for a time producing most of that state’s uranium.

u308_timkoethReal yellowcake, or actually a chemically-pure grade of depleted U3O8 in a 1-lb reagent bottle from Research Organic /Inorganic Chemical Corp.  After calcining, this is indeed what most modern “yellowcakes” resemble both in chemistry and appearance.  This bottle is a gift from another amateur scientist.

uranyl_acetate_2Uranyl acetate reagent bottles.  Uranyl acetate is still widely available as an electron-microscopy stain.  It’s a beautiful color, and like most uranyl salts, exhibits striking UV fluorescence.  The yellow crystals have a faint odor of vinegar.  Likely they taste accordingly (although taking uranium internally is generally frowned upon).

c14_vial_2Vial of urea labeled with 50 microcuries of carbon-14.  C-14 is a weak beta emitter that is best known for its role in carbon dating.  Because of the importance of carbon in biological processes (durrrh!), C-14 is also useful as a tracer in research, which is the suspected purpose of this product from New England Nuclear.  The label says “Use only as authorized by Atomic Energy Commission,” effectively dating this carbon to 1974 or earlier.  Activity is only detectable by removing the lid and holding a Geiger tube over the opening.

bi_210Calibrated bismuth-210 beta sources. Bi-210, or archaically “radium E”, appears in the uranium decay series.  The sources actually contain lead-210 (radium D) with a half-life of 22 years in secular equilibrium with the 5-day Bi-210 daughter.  The weak betas from Pb-210 are absorbed in the source, while the 1.2-MeV betas from Bi-210 are free to escape.  The set is incomplete; present are four sources ranging in activity from 7.73 nCi to 0.364 μCi (measured in 1962).

thorium_bottle_2Quarter pound of thorium nitrate. This bottle of Baker ACS-grade reagent is still sealed, preventing radon from escaping and allowing the delicious thorium decay chain to build up.  The penultimate thorium decay product, thallium-208, is responsible for one of the most energetic gamma rays found in nature: 2.62 MeV.  I use this bottle as a source of 2.62-MeV gamma radiation to calibrate the high end of the energy scale in scintillation spectrometry.

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