Frequency domain Monte Carlo simulations of void velocity measurements in an actual experimental setup using a neutron noise technique

ABSTRACT Monte Carlo simulations of the void velocity measurement in a light-water moderated critical assembly CROCUS are performed using a continuous energy Monte Carlo code that has a special function to conduct frequency domain calculations using complex-valued weights of neutron noise. In the measurements that are simulated in this paper, two vertically aligned fission chambers detected the neutron noise caused by the upward moving void in a rectangular channel. The neutron noise transport calculations in the frequency domain are performed for the measurement simulations. Using the cross power spectral density between two chambers, the void velocities are calculated at frequencies from 0.1 to 12 Hz. The void velocity is overestimated at lower frequencies, and accurately predicted at higher frequencies. This frequency dependence is explained using a simple model that takes neutron noise spatial attenuation and source intensity distribution into consideration.

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