Crucial importance of correlation between cross sections and angular distributions in nuclear data of 28Si on estimation of uncertainty of neutron dose penetrating a thick concrete
Uncertainty in neutron dose after penetrating a 3-m-thick concrete was estimated based on
three different kinds of Total Monte Carlo methods under random sampling methodology. A
thousand random nuclear data files were generated for 28Si by T6 by perturbing underlying
model parameters. In the first method, these files were used directly to yield processed library
preserving all the correlations among different physical quantities. In the second method, a
covariance file in the ENDF-6 format was generated and 1000 random files were produced
based on the covariance file. In the third method, the random files populated by T6 were used
but the angular distribution data were kept fixed to non-perturbed nominal ones. It was found
that the second and third methods gave equivalent variance of the neutron dose after deep
penetration of a concrete, and this uncertainty was larger than the uncertainty given by the
first method. It shows that the correlation between total cross section and angular distribution
of elastic scattering affects uncertainty of the calculated neutron dose. It could be concluded
that the uncertainty obtained by using the covariance files given in the ENDF-6 format may not
give correct results for the uncertainty of deep penetration calculation.
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