A Comparison of Skyshine Computational Methods

ICRS10

A Comparison of Skyshine Computational Methods N. E. Hertel1, J. E. Sweezy2, J. K. Shultis3, J. K. Warkentin4, Z. J. Rose5 1Georgia

Institute of Technology 2Los Alamos National Laboratory 3Kansas State University 4Private Consultant 5NAC International

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Outline ¾ Motivation ¾ Problems ƒ Silo ƒ Trench ¾ Methods ƒ GGG-GP and QADMOD-GP Solution ƒ COHORT Monte Carlo Code ƒ NAC International Version of SKYSHINE-III Code ƒ Hybrid Method Using KSU Skyshine code • SKYCONES • MCSKY • SKYDOSE

ƒ MCNP Code ¾ Results ¾ Summary

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Motivation ¾ Skyshine problems of interest often involve estimating doses at large distances downfield ¾ A variety of approaches have been used to solve these ƒ Empirical to semi-empirical model ƒ Point kernel methods to 3D transport computations

¾ Present work compares some methods to compute skyshine dose rates for gamma ray

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The Skyshine Problem

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Silo Problem ¾ Point Isotropic Source of 1-MeV photons ƒ Centered on floor of silo ¾ Silo ƒ 1 m radius ƒ 2 m high ƒ Totally absorbing walls and floor ¾ Corresponds to a source emitting into 26.57 degree opening half angle ¾ Two parts ƒ No Roof ƒ 30-cm Concrete Roof

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Low-Level Radioactive Waste Trench ¾ 12 identical concrete canisters of ion exchange resin ƒ Resin density = 0.75 g/cm3 ƒ Concrete Density =2.35 g/cm3

¾ Two equally spaced rows at one end of trench against a vertical wall ¾ Opposite end of the trench and its sides are sloped ¾ Coordinate axis is at the center of the 12 canisters

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Concrete Canister

Dimensions in cm

Lateral or X-Direction (Dimensions in cm)

X

8

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Top View (Dimensions in cm)

y

X

10

Y-Direction of Trench

y

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Trench Canister Source Term Radionuclide Activity (GBq) Radionuclide Activity (GBq) 241 94 3 -1 Am Nb 3.806(10 ) 1.042(10 ) 243 237 4 -5 Am Np 1.481(10 ) 4.433(10 ) 137m 238 4 -1 Ba Pu 2.572(10 ) 1.629(10 ) 137 239 4 -1 Cs Pu 2.572(10 ) 1.633(10 ) 243 242 -2 -2 Cm Pu 1.169(10 ) 4.706(10 ) 244 106 -2 1 Cm Ru 4.936(10 ) 2.572(10 ) 60 125 3 1 Co Sb 5.312(10 ) 4.245(10 ) 134 234 4 Cs U 2.572(10 ) 1.491 154 235 -2 Eu U 4.266 2.405(10 ) 129 238 -5 -1 I U 6.105(10 ) 4.371(10 )

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Canister Source Term Energy Group (MeV) 0.10 - 0.20 0.20 - 0.40 0.40 - 0.50 0.50 - 0.60 0.60 - 0.70 0.70 - 0.80 0.80 - 0.90 0.90 - 1.09 1.09 - 1.25 1.25 - 1.41 1.41 - 1.80

Average Group Energy (MeV) 0.15 0.30 0.45 0.55 0.65 0.75 0.85 0.995 1.17 1.33 1.605 Total

Source Strength (Photons/sec) 8.51E+11 1.09E+10 4.13E+11 6.32E+12 6.92E+13 2.33E+13 2.12E+12 2.70E+11 5.79E+12 6.13E+12 1.87E+08 1.14E+14

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Solution Method Caveat ¾Not very careful with our definition of “dose” quantity estimated ¾Felt that people would use default values in the code.

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Point-Kernel Solutions

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GGG-GP Calculations ¾ 81,000 scattering subvolumes of air were used ƒ 300 cm to 600 m from the source ƒ From 1.5 to 87 degrees away from the vertical

¾ Silo problem is straightforward application of the code ¾ Trench problem is a twostep problem ƒ Effective sources created for use in GGG-GP

Effective Point Source Determination for PointKernel Method ¾ Two solutions were performed using GGG-GP for the Trench ƒ One effective point source at the array center ƒ 12 effective point sources at the canister centers

¾ Symmetry employed when possible ¾ Effective source determinations with QADMOD-GP ƒ Dose rate 27.4 m above the center of the array computed for the source term ƒ Effective point sources in the centers of the canisters or an effective point source in the center of the array were used to adjust the energydependent source strength until the dose rate above the array was matched.

¾ Direct components above the trench were computed with QADMOD-GP and the volumetric source ƒ 3000 source subvolumes per canister ƒ Buildup in canister and resin included

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NAC International SKYSHINE-III Version

SKYSHINE Code Series Version SKYSHINE

Year

Features

1972 Point Isotropic 6.2 MeV Gamma Source Concrete Wall Penetration Steel Wall Penetration / Reflection Added Spatially Distributed 6.2MeV Gamma source Gamma Sources with Arbitrary Energy Spectra Neutron sources With Arbitrary Energy Spectra Secondary Gamma Sources Air Transport Calculations Only Concrete & Steel Gamma Transmission / Reflection Concrete & Steel Neutron Transmission / Reflection

1st MOD 2nd MOD

1972 1976

SKYSHINE II

1979

1st MOD

1981

SKYSHINE III

1982

SKY-PC

1988 PC Version

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SKYSHINE-III Algorithm

3

5

7 2 6 1

Source 1------Direct Beam 2------Air-Scattered 3------Wall-Attenuated Air-Scattered 4------Wall-Scattered Air-Attenuated

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Receiver

5 ------Wall-Scattered Air-Scattered 6 ------Wall-Reflected Air-Attenuated 7 ------Wall-Reflected Air-Scattered

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SKYSHINE-III (cont.) ¾ Transmission Functions ƒ Concrete ƒ Steel

¾ Line Beam Response Functions (LBRF) for infinite air ¾ Uses Monte Carlo techniques to do integration over source-shield geometry ¾ Point sources only in original version ¾ NAC Version ƒ Conical Sources ƒ Disk and Cylindrical surface sources

¾ Line beam functions computed out to 1500 m ƒ So we limit the results to that distance

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SKYSHINE-III “Building”

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SKYSHINE-III Silo Solutions ¾ Open Silo – 2 calculations ƒ Conical Source ƒ Used square building to “collimate” the source into same solid angle as the cone (blockhouse) • Roof Component only reported in results

¾ Silo with Roof ƒ SKYSHINE-III concrete transmission database • Conical • Blockhouse source

ƒ Disk source • MCNP-computed angular- and energy-dependent current on top of silo roof • 20 angular intervals

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SKYSHINE-III Trench Solution ¾ MCNP-computed current on canisters used as sources ƒ 10 angular intervals ƒ Top – disk ƒ Sides – cylindrical surface

¾ Trench sides represented as rectangular building ¾ Cylindrical array option in NAC version treats other canisters as totally absorbing ¾ Reflection off walls and floor of trench allowed

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KSU Hybrid Solution Approach ¾ Compute the spectral and angular dependence of radiations escaping the containers/shielding ƒ Ignores interaction of the radiation being transported in the air with the source containment structure a second time.

¾ Tally flux on the surface of a large segmented sphere, much larger than the source structure ƒ Centered about the source structure ƒ Vacuum between source structure and the tally surface

¾ The flow at any point on the spherical surface is essentially the fluence ƒ Bin in desired energy and cosine segmentation ƒ Can create an anisotropic point source

¾ For the Trench, no azimuthal variation around the trench was included in the effective point source ƒ No polar variation was included

¾ MCNP used to compute the effective source

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KSU SKYCONES – Silo Problem ¾ Skyshine doses from a polyenergetic point source ¾ Radiation is collimated into an upward conical annulus between 2 specified angles ¾ Source assumed to be azimuthally symmetric ¾ Gammas ƒ 0.02 to 10 MeV for distances out to 3000 meters ƒ 10 to 100 MeV for distances out to 1500 meters ¾ Photon doses ƒ Absorbed dose in air (Gy) ƒ Exposure in roentgens

¾ Methodology ƒ No effect from the containment structure ƒ Integrates LBRFs over the desired polar angle range

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KSU McSKY Code – Silo with Roof ¾ Skyshine dose for an isotropic, monoenergetic, point source of gamma rays ƒ 0.02 through 100 MeV ƒ Vertical cone

¾ Monte Carlo algorithm evaluates gamma-ray transport through the source shields coupled to a LBRF ƒ Ignores positron transport and bremsstrahlung emission ƒ Infinite horizontal slab shields assumed • 1D Monte Carlo transport

ƒ Allows collimated source on the inside of the shield

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KSU SKYDOSE code ¾ Used for trench problem ¾ Integrates the LBRF with the trench represented by the black walls of a rectangular building ¾ In this case the effective point source calculated with MCNP was represented by as being isotropic in a single conical interval

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MCNP Solution of Trench Problem

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MCNP Solution Trench Problem ¾ Version 5 ¾ Track-length tallies ¾ 100-cm thick and 100 cm high ƒ Subtended an arc of 5 degrees as viewed from the center of the cask array ƒ Constant angleÆLarger detector as distance increased

¾ Weight windows used ƒ Conical shells to partition the geometry

¾ Less than 5 % error for all detectors

Silo Results

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Open Silo 1-MeV Photons

-16

10

-18

Dose (µGy/hr)

10

Skyshine III Conical Source Skyshine III Square Building SKYDOSE MCNP GGG-GP COHORT

-20

10

-22

10

-24

10

1

10

2

10

Distance (meters)

3

10

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Silo With Roof, 1-MeV Photons -16

10

-18

Dose (µGy/hr)

10

-20

10

SKYSHINE III - Square Bldg MCSKY MCNP

-22

10

GGG-GP COHORT SKYSHINE III - Disk Option

-24

10

SKYSHINE III - Conical Source

-26

10

1

10

2

10

Distance (meters)

3

10

Distance (m)

d

SKYSHINE-III

GGG-GP Multiple Sources

MCNPa

MCNP Error

Hybrid Method

COHORT

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Y c

50 c 100 c 150 c 200 300 400 500 -50 -100 -150 -200 -300 -400 -500 50 100 150 200 300 400 500

X

a. b. c. d.

Direct Scattered 286.14 165.3 45.6 36.48 13.68 12.54 5.7 9.12 1.482 0.54 0.21 14.25 4.86 2.09 0.99 0.25 0.07 0.02 28.16 9.28 4.15 2.06 0.60 0.19 0.07

Direct 230 41.0 12.9 5.13

Scattered 62.2 19.3 8.50 4.30 1.21 0.419 0.160 4.48 1.47 0.618 0.285 0.068 0.017 0.005 15.1 4.89 2.26 1.17 0.359 0.122 0.0433

293 54.8 17.9 7.44 1.03 0.29 0.09 22.3 6.40 2.37 0.97 0.19 0.04 0.01 41.04 11.51 4.42 1.90 0.42 0.11 0.03

(0.79%) (0.98%) (1.28%) (1.38%) (1.22%) (1.08%) (1.35%) (1.07%) (1.14%) (1.23%) (1.29%) (1.82%) (1.98%) (4.24%) (0.45%) (0.64%) (0.66%) (0.7%) (1.97%) (1.1%) (2.25%)

b

46.512 b 15.162 b 6.1788 b 2.5764 0.62 0.18 0.05 15.16 4.13 1.48 0.60 0.12 0.03 0.01 31.69 8.87 3.40 1.47 0.34 0.09 0.03

Direct Scattered 230 108 41.0 21.9 12.9 7.26 5.13 2.84 0.473 0.163 0.047 6.56 1.65 0.501 0.23 0.041 0.009 0.002 20.2 5.07 2.32 0.86 0.17 0.053 0.016

MCNP calculations include both direct and scattered components. Only air scattered kerma rate, no direct component is included. Dose point inside x and y boundaries of the trench. These values are tissue absorbed dose rates. No attempt has been made to convert them to air kerma.

Summary Open Silo Using MCNP as the reference: ¾ Point kernel (tissue absorbed dose) is a factor of 3 higher at 10 m and is within a few percent at 700 m ¾ COHORT is 50% higher in close but drops to 10’s of percent at longer distances. ¾ SKYDOSE is initially a factor of 1.2 higher but drops to about 40% of the MCNP results at 700m ¾ SKYSHINE results are about 30-35% higher up to 100 m dropping to about 1% at 400m then increase increasing to 25% higher at 700 m

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Summary Silo with Roof Using MCNP as the reference: ¾ Point kernel results (tissue absorbed dose) are a factor of 1.6 – 2 higher ¾ MCSKY ranges between 13% higher and 10% lower ¾ COHORT ranges from about 50% higher dropping to within a few percent ¾ SKYSHINE-III ƒ Two solutions using the embedded concrete transmission data are initially a factor of 8.5 higher and drop to within a factor of 1.6 ƒ Disk source option is about 5% higher at 10m and drops to 710% lower

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Summary Trench Problem Using MCNP as the reference: ¾ KSU hybrid results do not have direct component ƒ About 60% lower where there is no direct component – could be improved by treating the source with azimuthal dependence and some polar dependence

¾ The rest are a mixed bag but in usually higher in the +y direction

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Conclusions ¾ Depending on the application and accuracy, one can obtain reasonable values of the skyshine for large, complex source problems without resorting to highly sophisticated techniques. ¾ For design studies, the speed with which one can set up and execute some of the less complicated methods still make them valuable ¾ Given the fact that the source is usually not as well defined as the desired statistical uncertainty in Monte Carlo results (