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Photon Skyshine Experiment Benchmark

 1. Name of Experiment:
    Photon Skyshine Experiment Benchmark

 2. Purpose and Phenomena Tested:
    A benchmark analysis is described for an experiment performed in 1977 at the 
    Kansas State University Nuclear Engineering Shielding Facility in which 
    skyshine from 60Co photon sources was measured at distances in air up to 
    700 m. The intent of the experiment was to provide this information as 
    benchmark data against which to compare calculated data. For this reason the 
    experiment was carefully documented, and the geometry was kept simple to 
    facilitate computer modeling. Benchmark calculations for the experiment were 
    performed with the MCNP5 code using ENDF/B-VI nuclear cross-section data. 
    The simulation model was an updated and enhanced version of the model 
    described in a 1993 benchmark analysis [1].

 3. Description of the Source and Experimental Configuration:
    Photon skyshine measurements were performed using three 60Co sources with 
    nominal activities of 10.33, 229.1, and 3,804 Ci. Sources were nickel-plated 
    60Co pellets within cylindrical volumes with diameters of 0.693, 0.693, and 
    2.527 cm and lengths of 0.635, 1.905, and 2.54 cm. Each measurement was 
    performed with one of the sources placed within an annular concrete silo with 
    91.44 cm (3 ft) thick walls. Each source was contained in one of two steel 
    casks mounted on a cart. Cart mobility and slide movements in the casks 
    allowed sources to be horizontally centered within the silo and raised 2.54 cm 
    above the top of the cask to the required height of 1.98 m above grade. 
    Concrete wedges and lead bricks lining the top of the silo collimated the 
    source into a 150.5° vertical-conical beam.

    Gamma radiation exposure rates were measured using a 25.4 cm diameter, 
    high-pressure (25 atm), argon-filled ionization chamber. Measurements were made 
    at distances of 50, 100, 200, 300, 400, 500, 600, and 700 m from the source at 
    1 m above the ground. Because the height of the silo exceeded 1 m and the walls 
    were thick, only radiation that scattered from the air outside the silo 
    (skyshine), the outside walls and base of the silo, and the ground (groundshine) 
    was detected.

    Background measurements were taken. Also, spectra were measured at locations 
    along the 700 m baseline and used to generate multiplicative energy correction 
    factors for the sources. These were required because of the non-flat energy 
    response of the detector.

 4. Measurement System and Uncertainties:
    Experimental uncertainties of ~5% and ~4% were attributed to the source activity 
    and source correction factors, respectively [1]. No further experimental 
    uncertainty values were reported, although there was concern regarding the effects 
    of uncertainties in the atmospheric conditions and terrain topology. Care was taken 
    to obtain atmospheric information for the day on which the experimental data related 
    to this analysis were obtained; however, it was implicitly assumed that conditions 
    were constant throughout the course of the day. Also, measurements were performed 
    from which it was concluded that the exposure rate was not sensitive to terrain 

    Additional uncertainties related to the following factors were investigated using the 
    simulation model: material densities and composition, source spectra and location, 
    model dimensions, and variations in atmospheric conditions. It was determined that 
    uncertainties from all effects except the atmospheric conditions were small and totaled 
    ~2.5%. Uncertainties resulting from estimated variations in the atmospheric conditions 
    were estimated to be ~4%. In summary, the total combined average estimated uncertainty 
    was ~8%.

    With the exception of the measurements at 300 and 500 m, results show agreement with 
    experimental data and are within the ~8% uncertainty estimate, and a viable benchmark 
    is established for these locations (50, 100, 200, 400, 600, and 700 m). The calculated 
    results at 300 and 500 m were found to be consistent with similar earlier calculations [1], 
    which also showed disagreement with experimental values. It is concluded that there were 
    very likely measurement anomalies at these two locations that prohibited agreement between 
    experimental and simulated values within the estimated levels of uncertainty. Possible 
    anomalies include a large terrain depression, unaccounted-for fluctuations in atmospheric 
    conditions, or electronic instabilities.

 5. Description of Results and Analysis:

    Measured and calculated exposure rates are summarized and compared in [1}. 
    Corrections for background and the multiplicative correction factors have been 
    applied to the measured values.

    Each MCNP calculation was performed with 1 × 107 source histories on a Linux-cluster PC, 
    which resulted in an uncertainty (precision) of ~0.15% and required ~900 to ~1300 min.

 6. Special Features:


 7. Author/Organizer
    Edward D. Blakeman
    Oak Ridge National Laboratory
    P.O. Box 2008, MS 6172
    Oak Ridge, TN 37831

 8. Availability:


 9. References:
   [1] R. OLSHER, H. HSU, and W. HARVEY, “Benchmarking the MCNP Monte Carlo Code with a Photon 
       Skyshine Experiment,” Nucl. Sci. Eng., 114, 219–227 (1993).
   [2] “Calculation and Measurement of Direct and Scattered Gamma Radiation from LWR Nuclear Power 
        Plants,” ANSI/ANS-ANS-6.6.1-1987, American Nuclear Society (1987).

10. Data and Format:

        Filename     Size[bytes]   Content
    ---------------- ----------- -------------
  1 phot-skyshine-a.htm       10,712  This information file
  2 phot-skyshine-e.htm       35.953  Description of experiment

SINBAD Benchmark Generation Date: 12/2012
SINBAD Benchmark Last Update: 12/2012