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Monte Carlo Uncertainty Quantification in UF6 Cylinder Neutron Emissions

Ryan O’MaraCihangir CelikWill Wieselquist

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Outline

• Introduction – UF6 Verification• SAMPLER & MAVRIC• Model Setup• Results • Conclusions

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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

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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.”

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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

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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

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Results - Neutron Count Rates

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Results - Neutron Count Rates

Did not see a strong relationship between enrichment and neutron flux, but looking instead at mass shows better correlation.

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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%

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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

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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