Audit of NIOSH Assessment of External Doses from Plutonium Fuel Pellets

Robert Anigstein, SC&A

December 4, 2018
Ambient Dose Equivalent Rates from External Exposure to (U,Pu)C Pellets (mrem/h)

Distance SC&A NIOSH Differencea

Photons Neutrons Photons Neutrons Photons Neutrons

1 ft 6.318 0.202 9.507 0.313 50.5% 54.7%

1 m 0.624 0.026 1.209 0.038 93.7% 47.3%

a

 NIOSH ÷ SC&A – 1

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MCNP (Monte Carlo N-Particle) Radiation Transport Code

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CONVERSION COEFFICIENTS FOR USE IN RADIOLOGICAL PROTECTION

Table A.1. Conversion coefficients for air kerma

per unit fluence, Ka/Φ, of monoenergetic photons

Photon energy

(MeV)

Ka/Φ

(pGy/cm2

)

0.010 7.43

0.015 3.12

0.020 1.68

0.030 0.721

0.040 0.429

0.050 0.323

0.060 0.289

0.080 0.307

0.100 0.371

0.150 0.599

0.200 0.856

0.300 1.38

0.400 1.89

0.500 2.38

0.600 2.84

0.800 3.69

1.000 4.47

1.500 6.14

2.000 7.55

3.000 9.96

4.000 12.1

5.000 14.1

6.000 16.1

8.000 20.1

10.000 24.0

Data from ICRU Report 47 (ICRU, 1992a) using data from

Hubbell (1982).

Table A.21. Conversion coefficientsa

 for the ambient dose

equivalent, H*(10) from photon fluence and air kerma free-in-air

Photon energy

(MeV)

H*(10)/Ka

(Sv/Gy)

Ka/Φ (pGy/cm2

) H*(10)/Φ

(pSv cm2

)

0.010 0.008 7.60 0.061

0.015 0.26 3.21 0.83

0.020 0.61 1.73 1.05

0.030 1.10 0.739 0.81

0.040 1.47 0.438 0.64

0.050 1.67 0.328 0.55

0.060 1.74 0.292 0.51

0.080 1.72 0.308 0.53

0.100 1.65 0.372 0.61

0.150 1.49 Q.600 0.89

0.200 1.40 0.856 1.20

0.300 1.31 1.38 1.80

0.400 1.26 1.89 2.38

0.500 1.23 2.38 2.93

0.600 1.21 2.84 3.44

0.800 1.19 3.69 4.38

1 1.17 4.47 5.20

1.5 1.15 6.12 6.90

2 1.14 7.51 8.60

3 1.13 9.89 11.1

4 1.12 12.0 13.4

5 1.11 13.9 15.5

6 1.11 15.8 17.6

8 1.11 19.5 21.6

10 1.10 23.2 25.6 a Data compiled from ICRU Report 47 (1992a) using Hubbell and Seltzer

(1995). The Ka/Φ data are slightly different from those used for the protection

quantities (see Table A.1) which used earlier data from Hubbell (1982).

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Incorrect H*(10) conversion coefficients 

Finding 1: NIOSH used H*(10) conversion coefficients from photon fluence, based on outdated

data, that resulted in a reduction of approximately 2% in the H*(10) doses from 241Am.

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Portion of Work Area of Plutonium Research Facility Showing Glovebox

Arrangement

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Glovebox Geometry Used in NIOSH MCNP Analysis (scales in cm)

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MCNP6 version 1.0 Known Issues, LA-UR-13-22900

Selection of Low Priority Bugs in MCNP6:

14) Incorrect source biasing with -31 function. Artf23313.

Bug Fixes for MCNP Version 6.2, LA-UR-18-20808

Tracking Number Category Description

artf23313 Source Incorrect source biasing if using the '-31' special function on the SB card

Finding 2: NIOSH used incorrect source biasing in the MCNP dose analyses.

Ambient Photon Dose Equivalent Rates from External Exposure to (U,Pu)C Pellets

Recalculated Without Source Bias (mrem/h)

Distance SC&A NIOSH-recalc Differencea

1 ft 6.318 3.458 -45%

1 m 0.624 0.474 -24%

 a NIOSH ÷ SC&A – 1

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Finding 3: The simulated dosimeters in the glovebox geometry modeled by

NIOSH are partially shielded by the floor of the glovebox, which reduces the

calculated doses.

Ambient Photon Dose Equivalent Rates from External Exposure to (U,Pu)C Pellets

Source Elevated 24 cm Above Glovebox Floor (mrem/h)

Distance SC&A NIOSH-recalc Differencea

1 ft 6.318 6.316 -0.03%

1 m 0.624 0.598 -4.17%

 a NIOSH ÷ SC&A – 1

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