Analysis of different variance reduction techniques in research reactor beam tube calculations

One of the main applications of research reactors is the use of the available neutron flux for medical or industrial utilizations. Beam tube facilities provide the necessary tool in order to use the generated neutron flux in the reactor core. Calculation of the neutron flux distribution as a function of the energy spectrum at the beam tube ends is necessary for the evaluation of the beam tube efficiency and use. In this paper, analysis is performed for a sample beam tube. Specifically, the spectra unfolding using the MCNP code are applied to determine the neutron flux in the beam tube. Initially, the geometry splitting method of the MCNP code was used to obtain the neutron flux. However, the obtained errors were unacceptable. After the application of variance methods, The weight window (W-W) method was found to be the most suitable method where the resulting errors were reasonable. It is noted that the geometry splitting method can be applied and provide reasonable results when the number of neutron group is small (about 3).

► Neutron flux dis. As function of energy is necessary for evaluating efficiency.
► The G.S. method of the MCNP (to obtain the neutron flux) produce unacceptable error.
► The weight window (W-W) method was found to be the most acceptable method.
► The geometry splitting method is useful when the number of neutron group is low.