Fast neutron computed tomography for phase distribution visualization

Many nuclear safety analyses rely on correlations developed from experimental thermalhydraulic. Such experiments are often expensive and difficult or impossible to perform at full-scale conditions and thus the correlations are subject to uncertainties which ultimately impact the accuracy of the analysis predictions. Globally there is a shift towards using more physically-based methods which can predict transient thermalhydraulic behavior with less reliance on full-scale empirical information. High-resolution measurements for full-scale geometries are in high demand in the nuclear safety field, in particular for the validation of multi-phase computational fluid dynamics (CFD) models. Computed tomography (CT) using penetrating radiation is a non-intrusive, non-destructive testing (NDT), measurement technique growing in popularity in recent years. In this paper proposal and relevant theory for a fast neutron computed tomography (FNCT) system is outlined and used to demonstrate the feasibility of imaging void distribution at the subchannel level. Serpent 2 simulations indicate that phase distribution information in a CANDU channel can be obtained with high contrast and resolution using FNCT (on the order of 1 mm). Key factors governing performance are incident neutron flux and direction, scintillation efficiency and geometry, detector response and noise. Since count-rates are projected to be low due to the relatively low incident neutron flux from a portable neutron generator, the development has been structured in consultation with medical imaging experts (e.g., single-photon emission computed tomography (SPECT) systems also involve very low count rates) to overcome this constraint. Ongoing development efforts and early stage results will be presented.