Uncertainty Quantification for Safeguards Measurements

Transcription

1 Uncertainty Quantification for Safeguards Measurements Stephen Croft1, Tom Burr2, Ken Jarman3, Robert McElroy1, Andrew Nicholson1, and Adam Shephard1 1Oak Ridge National Laboratory, 2Los Alamos National Laboratory, 3Pacific Northwest National Laboratory Acknowledgement of funding from the United States Department of Energy National Nuclear Security Administration Office of Defense Nuclear Nonproliferation Research and Development ORNL is managed by UT-Battelle for the US Department of Energy

2 Uncertainty Quantification for Safeguards Measurements - Objectives Physical Non-Destructive Assay (NDA) measurements are the bedrock of nuclear accountancy and verification agreements. Understanding measurement and model bias is vital to understand trends material balances. Goal: Advance the quality of Uncertainty Quantification and reporting for NDA measurements and to help define a unified approach across the field. The progress of one case study is presented in this report: Uncertainty Quantification using the 235 U Enrichment Meter Method. 2 Uncertainty Quantification

3 Method Objective: Investigate uncertainties in 235 U enrichment meter measurements. Example: Attended HPGe measurements on UF 6 cylinder enrichment verification. Many historical measurements and is still used today. Relatively simple to describe mathematically. Can be modeled using simple models and analytical expressions. B. R. McGinnis, et. al. INMM Proceedings (2009). 3 Uncertainty Quantification

4 Method Method Overview Enrichment is determined from the number of counts in the 235 U 186 kev gamma-ray peak. 186 kev gamma-rays are strongly attenuated by the material inside of the container and the container walls. The measured material is determined by the geometry of the experiment, namely the collimator, detector position and the mean free path of 186 kev gamma-rays in the composition of the container. If the depth of the measured material is much larger than the mean free path of the 186 kev gamma rays, we have reached the Infinite Thickness regime. 4 Uncertainty Quantification

5 Case Study: 235U Enrichment Meter Method - Overview Create a comprehensive database for uncertainty quantification of NDA measurements on a variety of certified standards and stainless steel attenuators used as surrogates for UF6 cylinders. Certified Enrichments: 0.31, 0.71, 1.94, 2.95, 4.46, 20.11, 52.49, and weight percentage.1,2 SS304 Stainless Steel Attenuators: None, 8, 13 and 16 mm used as surrogates for bare source, 48G, 30B and 48Y UF6 cylinders, respectively. [1] National Bureau of Standards Certificate, Standard Reference Material 969, Uranium Isotopic Standard Reference Material for Gamma Spectrometry Measurements (1985), SRM 969. [2] New Brunswick Laboratory Certified Reference Material, Certificate of Analysis, CRM 146, Uranium Isotopic Standard for Gamma Spectrometry Measurements (1999), NBL CRM Uncertainty Quantification Certified U308 Cans

6 Method Experimental Setup For each can enrichment/ss304 stainless steel thickness combination: High resolution HPGe gamma-ray spectra recorded: Twenty Field Measurement Inspired Measurements (300 seconds) One High Quality Measurements (10 5 counts in 186 kev peak) 300 second Background Measurements (Routinely Recorded) Long Background Measurements (overnight) 70 mm Measured Material Pb Bricks for Background Shielding U308 Collimator HPGe SS304 Attenuators 6 Uncertainty Quantification

7 Method Recorded Data Total of 849 gamma-ray spectra recorded, over a period of two months for each attenuator/enrichment combination. 235 U peak count rate determined via a region of interest methods and peak fitting. 186 kev 7 Uncertainty Quantification Difference: / cnts/sec

8 Method Data Verification Measurements Validation: Effective SS304 stainless steel linear attenuation coefficients (µ) for each material. Uncertainty is generated from goodness of fit to Field Measurement inspired data. SS304 Linear Attenuation Coefficients (Literature): ρ = 7.88 (g/cc), µ = (1/mm) 1 ρ = 8.00 (g/cc), µ = (1/mm) 2 [1] G. M. Krüger et. al. EUR EN (1997). [2] NIST XCOM: Photon Cross Sections Database ( SS304 Composition: 0.8% C, 2.0% Mn, 0.75% Si, 0.045% P, 0.03%S, 19% Cr, 9% Ni, 0.1% N, % Fe 8 Uncertainty Quantification

9 Future Work Investigate other methods of determining 235 U 186 kev peak area (i.e. peak fitting). Use collected data of experimental results to test against standard safeguards codes. Look into possible systematic contributors to uncertainty: Look for variations in SS304 stainless steel sample thickness and material composition using ultrasonic thickness gauge measurements. Build a comprehensive error model including total measurement uncertainty for enrichment meter measurements. Provide a comprehensive guide to enrichment meter uncertainty measurements. Workshop On Uncertainty Quantification Where: Oak Ridge National Laboratory When: To Be Announced Why: Attract, encourage, and motivate new ideas on Uncertainty Quantification and to broadcast these ideas to the larger community. 9 Uncertainty Quantification