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Monte Carlo Uncertainty Quantification in UF6 Cylinder Neutron Emissions
Ryan O’MaraCihangir CelikWill Wieselquist
2 2019 SCALE User’s Group Meeting
Outline
• Introduction – UF6 Verification• SAMPLER & MAVRIC• Model Setup• Results • Conclusions
3 2019 SCALE User’s Group Meeting
UF6 Verification
• NDA of UF6 in storage or transport containers represents a unique safeguards challenge
• IAEA has expressed interest in developing verification technologies– Measure U235 enrichment,
U235 mass and/or total U
• Measurements should have minimum practical impact on operations
4 2019 SCALE User’s Group Meeting
Challenges UF6 Verification Measurements
• Proposed systems measure neutron and gamma signatures
• Measurable signals sensitive to a number of unknowns:– Cylinder history– Material distribution (filling
profile)– Cylinder wall thickness– Material age
Figure: Simplified UF6 filling profiles. Detector position
affects “view.”
5 2019 SCALE User’s Group Meeting
SAMPLER & MAVRIC
• Sampler and Mavric are both components of the SCALE code system
• Sampler can perturb or adjust a given input variable in any SCALE modules or sequences and creates statistical analysis of desired output variable
• Mavric performs fixed-source shielding simulations using powerful variance reduction methods (CADIS & FW-CADIS)
• Combined, parametric searches for verification of UF6 measurements become relatively easy
6 2019 SCALE User’s Group Meeting
Model Setup
• Goal: test the effect of mixed U enrichments and filling profiles on measured signals
• Combined SAMPLER, Origen and MAVRIC
• Four material phases– Each with independent
enrichments and masses
• Only constrained total mass and mass of heels
Run MC simulation
Calculate source term
DetermineFilling profile Enrichment
Sampler Origen
Mavric
(above) Simple UF6 cylinder model
simulation work flow
8 2019 SCALE User’s Group Meeting
Results - Neutron Count Rates
Did not see a strong relationship between enrichment and neutron flux, but looking instead at mass shows better correlation.
9 2019 SCALE User’s Group Meeting
Implications
• Large inherent variability in possible neutron count rates decreases discriminatory capability
• Compare:– Case 1: All enrichments
between 0.5 and 5%– Case 2: Liquid phase only
between 18 and 22%– Case 3: Heels between 18 and
22%
10 2019 SCALE User’s Group Meeting
External Photon Flux
• m.f.p of gammas only ~0.2cm in UF6 à detector can only “see” 1-2cm into cylinder
• Calculated photon spectra for 2 cases:– All UF6 with 3% enrichment– 20% enrichment in the heels– 20% enrichment in the liquid
11 2019 SCALE User’s Group Meeting
Conclusion
• Uncertainties in the filling profile (material distribution) of the UF6 a significant source of variability in measured count rates– Reality likely worse where uniformly distributed materials
are unlikely
• Propagated uncertainty in neutron count rates on the order of 30% based on 300 samples
• Need a way to account for or better characterize total propagated uncertainties in analysis of measurements