Uncertainty Quantification of a Novel Method of Void Fraction Assay in Molten Salt Reactors

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Schreiber, Ian Scott
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Abstract
In loop-type molten-salt reactors, gaseous fission products may become entrained in the primary loop. Additionally, sparging and cover gases, such as helium, may also become entrained. The fraction, by volume, of the loop occupied by gas is named the void fraction. Void fraction affects reactivity and thereby the power dynamics of the reactor. Indirect interrogation by activity analysis through gamma spectroscopy may be a viable method of assay. An in-silico study of such a solution utilizing MCNP was executed. Uncertainty quantification was conducted in order to estimate the accuracy and precision of such a system. A validation phase, with no variation in the composition of the salt, and an uncertainty quantification phase, with variation in the composition of the salt, were executed. The simulated system was strongly validated, with an absolute error in the reconstructed void fraction no greater than 0.01 and a 95% confidence interval no wider than 0.001. Uncertainty decreased monotonically with increasing void fraction. In the uncertainty quantification phase, uncertainty increased across the entire range of void fractions by a factor of about 100. Additionally, bias increased, by a factor ranging between 50 near void fractions of zero to about 2, due to artifacts in the reconstruction algorithm. In this case, error and uncertainty both decrease monotonically with increasing void fraction. Overall, the method is theoretically valid, but more work is needed to functionalize gamma spectroscopy-based void fraction reconstruction routines.
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2024-04-15
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