Impact of Initial MCNP Spectrum Guess on Experiment-Based Neutron Spectrum Determination at Missouri S&T Reactor


The energy spectrum of the prompt-neutron flux at the Missouri University of Science and Technology Reactor (MSTR) was obtained using an activation method. Foils were irradiated at the bare rabbit tube (BRT) of the reactor core (which has a 120 W configuration). The neutron spectrum was determined using an unfolding method implemented in SAND-II code. The Monte Carlo N-particle (MCNP) model was used to calculate the spectrum at the bare rabbit tube, which was used as the initial guess for input into the SAND-II code. Various MCNP spectra with 620-, 143-, 89-, 50-, 22-, and 12-energy groups were used as the initial guess for input into SAND-II. Seventeen different foils were irradiated at 100 kW. The foil set covers energies from 0.025 eV to 13 MeV for a broad spectrum analysis. The photon counts for the activated foils were obtained using a high-purity germanium (HPGe) detector. The count results were input into the SAND-II code to predict the neutron flux spectrum for the MSTR. The spectra were collapsed into three groups: thermal, epithermal, and fast flux. With the 620-group initial guess, the thermal, epithermal, and fast neutron fluxes were 1.43 x 1012 ± 2.82 x 1011 n/cm2s, 4.51 x 1011 ± 2.85 x 1010 n/cm2s, and 5.38 x 1011 ± 4.85 x 1009 n/cm2s, respectively, giving a total flux 2.42 x 1012 ± 3.02 x 1011 n/cm2s. Disparities were noted in the distribution of the thermal and epithermal flux predictions, as the number of groups in the initial spectrum guess changed. The 59%/19% distribution of the thermal/epithermal flux, as predicted with the 620-group guess, is inconsistent with the 49%/26% distribution predicted with the 89- and 143-group guesses. The predictions based on the 89- and 143-group guesses are fairly consistent with the predictions obtained with the 22- and 50-group guesses. The 12-group initial guess resulted in the prediction of a fairly even distribution between the thermal (38%) and epithermal (36%) fluxes. Regardless of the number of groups in the initial guess, the SAND-II prediction is fairly consistent in the fast energy range. The fast neutron flux was found to range between 22% and 26%.


Mining and Nuclear Engineering

Keywords and Phrases

Foil activation; Initial guess; MCNP; MSTR; Neutron flux spectrum; SAND-II

International Standard Serial Number (ISSN)

0306-4549; 1873-2100

Document Type

Article - Journal

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