SINBAD ABSTRACT NEA-1517/91
Photon Skyshine Experiment Benchmark
1. Name of Experiment:
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Photon Skyshine Experiment Benchmark
2. Purpose and Phenomena Tested:
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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:
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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:
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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
features.
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:
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None
7. Author/Organizer
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Edward D. Blakeman
Oak Ridge National Laboratory
P.O. Box 2008, MS 6172
Oak Ridge, TN 37831
USA
8. Availability:
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Unrestricted
9. References:
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[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:
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DETAILED FILE DESCRIPTIONS
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Filename Size[bytes] Content
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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