Abstract
In this work, we describe a method for converging nonlinear feedback during the convergence of the neutron fission source in a Monte Carlo reactor simulation. This approach involves updating feedback physics during discard batches in the Monte Carlo simulation rather than fully (or partially) converging the neutronics prior to the nonlinear update. This approach is demonstrated for a single PWR pin with thermal feedback and with both thermal and xenon feedback. Converging these feedbacks inline with the fission source is shown to have the benefit of reducing numerical instability by effectively underrelaxing the tallied quantities in the Monte Carlo simulation and improving computational performance by converging feedback within (or near to) the number of discard batches required to converge the fission source even without any feedback.
Highlights
There has been significant progress in the development of Monte Carlo tools capable of performing large-scale reactor analysis over the past decade
In addition to solving the neutron transport problem, Monte Carlo reactor analysis tools must account for feedback mechanisms such as thermal feedback, xenon feedback, or core depletion
To assess the convergence behavior of the inline method we generated numerical reference solutions for thermal feedback only (TF) and for the thermal and xenon feedback (TF-XE) case. These solutions were generated using 20 Picard iterations for the thermal feedback with an underrelaxation factor of 0.7 applied to local power passed to the thermal solver and the inline approach for xenon feedback with α = 10 and β = 25
Summary
There has been significant progress in the development of Monte Carlo tools capable of performing large-scale reactor analysis over the past decade. As the capability of the analysis tools themselves has advanced, work into the iterative methods used to perform feedback calculations has been an active area of research [1] Research in this area is motivated both by observed numerical instabilities in power and temperature distributions when solving coupled Monte Carlo and thermal-hydraulic (T-H) problems and the desire to reduce the computational expense of these coupled calculations. In this work we will look at performing thermal feedback iterations in this manner, which we will refer to as “inline“ on, but consider for the first time the simultaneous convergence of thermal feedback, equilibrium xenon, and the fission source distribution. A similar sensitivity study will be performed with the thermal and xenon distributions being simultaneously converged with additional consideration given to the relative frequencies of the thermal and xenon updates
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