[en] Beryllium is known to have a very low threshold for the (n, 2n) reaction and as a result can be and has been used as a neutron multiplier in the military and the industry, for example, for the enhancement of fast neutron flux in reactor. Flux measurements have been performed for a number of Beryllium assemblies and calculations have been made using the ^well-known^and complicated MCNP code. It has been shown that, in general, some discrepancies between the experiment and the theory exist which could be caused by the uncertainties in the ⁹Be cross-sections