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PNNL Measurement Results for the 2016 Criticality Accident Dosimetry Exercise at the Nevada National Security Site (IER-148)

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PNNL Measurement Results for the 2016 Criticality Accident Dosimetry Exercise at the Nevada National Security Site (IER-148) PNNL-26497 PNNL Measurement Results for the 2016 Criticality Accident Dosimetry Exercise at the Nevada National Security Site (IER-148) May 2017 BA Rathbone SM Morley JA Stephens PNNL-26497 PNNL Measurement Results for the 2016 Criticality Accident Dosimetry Exercise at the Nevada National Security Site (IER-148) BA Rathbone SM Morley JA Stephens May 2017 Prepared for the U.S. Department of Energy under Contract DE-AC05-76RL01830 Pacific Northwest National Laboratory Richland, Washington 99352 Abstract The Pacific Northwest National Laboratory (PNNL) participated in a criticality accident dosimetry intercomparison exercise held at the Nevada National Security Site (NNSS) May 24-27, 2016. The exercise was administered by Lawrence Livermore National Laboratory (LLNL) and utilized the Godiva-IV critical assembly housed in the Device Assembly Facility (DAF) situated on the NNSS site. The exercise allowed participants to test the ability of their nuclear accident dosimeters to meet the performance criteria in ANSI/HPS N13.3-2013, Dosimetry for Criticality Accidents and to obtain new measurement data for use in revising dose calculation methods and quick sort screening methods where appropriate. PNNL participated with new prototype Personal Nuclear Accident Dosimeter (PNAD) and Fixed Nuclear Accident Dosimeter (FNAD) designs as well as the existing historical PNAD design. The new prototype designs incorporate optically stimulated luminescence (OSL) dosimeters in place of thermoluminescence dosimeters (TLDs), among other design changes, while retaining the same set of activation foils historically used. The default dose calculation methodology established decades ago for use with activation foils in PNNL PNADs and FNADs was used to calculate neutron dose results for both the existing and prototype dosimeters tested in the exercise. The results indicate that the effective cross sections and/or dose conversion factors used historically need to be updated to accurately measure the operational quantities recommended for nuclear accident dosimetry in ANSI/HPS N13.3-2013 and to ensure PNAD and FNAD performance meets the performance criteria given in the standard. The operational quantities recommended for nuclear accident dosimetry are personal absorbed dose, Dp(10), and ambient absorbed dose, D*(10). iii Summary The Pacific Northwest National Laboratory (PNNL) participated in a criticality accident dosimetry intercomparison exercise held at the Nevada National Security Site (NNSS) May 24-27, 2016. The exercise was administered by Lawrence Livermore National Laboratory (LLNL) and consisted of three exposures performed using the Godiva-IV critical assembly which is part of the Nuclear Criticality Experimental Research Center (NCERC) situated on the NNSS site. The exercise allowed participants to test the ability of their nuclear accident dosimeters to meet the performance criteria in ANSI/HPS N13.3-2013, Dosimetry for Criticality Accidents and to obtain new measurement data for use in revising dose calculation methods and quick sort screening methods where appropriate. PNNL participated with new prototype Personal Nuclear Accident Dosimeter (PNAD) and Fixed Nuclear Accident Dosimeter (FNAD) designs as well as the existing historical Hanford PNAD design. The new prototype PNNL designs incorporate optically stimulated luminescence (OSL) dosimeters in place of thermoluminescence dosimeters (TLDs), among other design changes, while retaining the same set of activation foils used historically. The prototype PNAD incorporates the Landauer InLight® OSLN four element whole body dosimeter which provides the primary gamma dose and an estimate of neutron dose to supplement the primary neutron dose information obtained from the activation foils. The prototype FNAD incorporates Landauer OSL and OSLN nanoDot dosimetrs to provide the primary gamma dose information and supplemental neutron dose information. The default dose calculation methodology established decades ago for use with activation foils in PNNL PNADs and FNADs was used to calculate neutron dose results for both the existing and prototype dosimeters tested in the exercise. Neutron absorbed dose results calculated from foil activities using this methodology were consistently low. The bias in reported personal neutron absorbed dose, Dp(10)n for PNADs exposed on phantom was -0.40. Similarly, the bias in reported ambient neutron absorbed dose, D*(10)n for FNADs exposed in air was -0.42. Even when the the under estimated neutron dose is combined with the over estimated gamma dose (bias = 1.59), nearly half of the PNADs exposed on phantom failed to meet the ANSI/HPS N13.3-2013 ± 25% accuracy requirement for total gamma+neutron dose. These results indicate that the effective cross sections and dose conversion factors used historically need to be updated to accurately measure the operational quantities recommended for nuclear accident dosimetry in ANSI/HPS N13.3-2013 and to ensure PNAD and FNAD performance meets the performance criteria given in the standard. The operational quantities recommended for nuclear accident dosimetry are personal absorbed dose, Dp(10), and ambient absorbed dose, D*(10). Current dose calculation methodology is based on measurement of tissue kerma which is typically 70% - 80% of the absorbed dose value. The foil activity measurements documented in this report have acceptable uncertainty and are consistent across measurement locations exhibiting relatively low variability. Combined with the reference dosimetry for the exercise provided by LLNL, and the fluence/spectrum characterization work performed by AWE, the activity measurements documented in this report should be suitable for use in validating and/or revising the activity-to-fluence conversion factors, and the fluence-to-dose conversion factors for unmoderated criticality events. The revised dose calculation methodology will also need to to accommodate the neutron energy spectrum expected in a solution type criticality event at PNNL. The OSL/OSLN response data obtained during the exercise indicate that OSL gamma dose calculations need to be revised to correct for the influence of neutrons on gamma dose response. The response data also indicate that OSL/OSLN nanoDot pairs in the FNAD can provide accurate D*(10)n dose results and the InLight OSLN dosimeter in the PNAD can potentially provide accurate gamma dose results and unbiased estimates of neutron dose with proper calibration to a specific source spectrum. v vi Acknowledgments The authors would like to thank PNNL Radiation Protection Division Manager, Richard Pierson, for making funding and other resources available for participation in the exercise, as well as his technical input. In addition, the authors are indebted to Larry Greenwood and Andrey Mozhayev for their modeling calculations and expert advice on a number of technical issues related to gamma spectroscopy and neutron activation. Randy Berg, Brian Glasgow, Jamie Davies, and Elliot Dutcher are recognized for their invaluable assistance in preparing, calibrating and shipping equipment that was used to make the measurements at NNSS which are the subject of this report. Finally, the authors would like to thank David Heinrichs and David Hickman at LLNL for organizing and hosting the exercise, and ultimately making the measurements possible. vii Acronyms and Abbreviations ABS acrylonitrile butadiene styrene ANSI American National Standards Institute AWE Atomic Weapons Establishment (U.K.) BOMAB BOttle MAnnikin Absorber DAF device assembly facility EOB end of burst EPD electronic personal dosimeter FNAD fixed nuclear accident dosimeter FWHM full width at half maximum GM Geiger-Mueller tube HD high dose HPGe high purity germanium HPRR Health Physics Research Reactor HPS Health Physics Society IAEA International Atomic Energy Agency ICRP International Commission on Radiation Protection ICP-MS inductively coupled plasma-mass spectrometry LANL Los Alamos National Laboratory LCD liquid crystal display LD low dose LED light emitting diode LLNL Lawrence Livermore National Laboratory MCA multi-channel analyzer mil one thousandth of an inch NAD nuclear accident dosimeter NCERC Nuclear Criticality Experimental Research Center NNSS National Nuclear Security Site NRC Nuclear Regulatory Commission (U.S.) OSL optically stimulated luminescence OSLN optically stimulated luminescence (neutron sensitive) PMMA polymethylmethacrylate PNAD personal nuclear accident dosimeter PSSS passive single sphere spectrometer TLD thermoluminescence dosimeter XRF X-ray flurorescence ix x Contents Abstract iii Summary v Acknowledgments vii Acronyms and Abbreviations ix 1.0 Introduction 1 2.0 Dosimeter Descriptions 3 2.1 Hanford PNAD 3 2.2 PNNL PNAD 6 2.3 PNNL FNAD 7 2.4 Electronic Personal Dosimeters 11 3.0 Experimental Design 12 4.0 Activity Measurements 14 4.1 Sulfur Pellets (32P) 15 4.2 Copper Foils (64Cu) 19 4.3 Indium Foils (115mIn, 116mIn) 21 4.4 Gold Foils (198Au) 22 4.5 Specific Activity per Unit Fluence 22 5.0 Methodology for Calculating Neutron Dose from Foil Activity 23 5.1 Neutron Fluence Calculation 23 5.2 Neutron Dose Calculation 26 6.0 Neutron Dose Results Calculated from Foil Activity 27 7.0 InLight® OSL/OSLN Measurements 29 7.1 microStar® Reader Calibration 30 7.2 InLight® Dose Calculation 31 8.0 nanoDot OSL/OSLN Measurements 33 8.1 nanoDot Gamma Dose Measurement in the Hanford PNAD 33 8.2 nanoDot Gamma and Neutron Dose Measurement in the FNAD 34 9.0 Dosimeter Performance Evaluation 35 10.0 Electronic Personal Dosimeter Measurements
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