Abstract This work aims to characterize the neutron spectrum in the beam going out of the university research reactor (VR-1) using tubular-type of the nuclear fuel, version 4M (IRT-4M) fuel. Thanks to its variability, the core is often rearranged to fulfill different research tasks. Measurements with new core configuration have been carried out to confirm the spectrum shape in the neutron beam of the radial channel remains unchanged even though the core has been rearranged. Based on this finding, the VR-1 can be considered as a mockup for other IRT-4M fueled reactors, even with higher power. The neutron spectrum stability has been proven by measurement and by comparison of measurements done on the C12 and C13 cores. Fast neutron spectrum in the channel has been evaluated by means of neutron spectrometry by scintillation detector and activation materials (Au, Co, Ni, Al, Fe, and NaI). If the neutron field stability is proven, the radial channel beam can be used for evaluation of spectrum weighted cross section disregarding changes in the core configuration. Assuming reactions with higher threshold, their rates can be compared with rates obtained in the pure prompt fission neutron spectrum (PFNS), since earlier measurements have shown that the neutron spectrum in the light water reactor cavity is equal to the PFNS above 6 MeV threshold. Result 1.1831 mb for 127I(n,2n) reaction evaluated from the VR-1 activation measurement demonstrates good agreement of the measured reaction rate with tabulated rate averaged in 235U PFNS, confirming the neutron spectrum stability and equality to the PFNS.