[en] This work presents the measures of the nuclear reaction rates along of the radial direction of the fuel pellet by irradiation and posterior gamma spectrometry of a thin slice of fuel pellet of UO₂ at 4.3% enrichment. From its irradiation the rate of radioactive capture and fission are measures as a function of the radius of the pellet disk using a HPGe detector. Diverse lead collimators of changeable diameters have been used for this purpose. Simulating the fuel pellet in the pin fuel of the IPEN/MB-01 reactor, a thin disk is used, being inserted in the interior of a dismountable fuel rod. This fuel rod is then placed in the central position of the IPEN/MB-01 reactor core and irradiated during 1 hour under a neutron flux of 5.10⁸ n/cm²s. The nuclear reaction of radioactive capture occurs in the atoms of U-²³⁸ that when absorbs a neutron transmutes into U-²³⁹ of half-life of only 23 minutes. Thus, it is opted for the detection of the Np-²³⁹, radionuclide derivative of the radioactive decay of the U-²³⁹ and that has a measurable half-life (2.335 days). In gamma spectrometry 11 collimators with different diameters have been used, consequently, the gamma spectrometry is made in function of the diameter (radius) of the irradiated UO₂ fuel pellet disk, thus is possible to get the average value of the counting for each collimator in function of the specific pellet radius. These values are directly proportional to the radioactive capture nuclear reaction rates. The same way the nuclear fission rate occurs in the atoms of the U-²³⁵ that produce different fission products such as Ce-¹⁴³ with a yield fission of 5.9% and applying the same procedure the fission nuclear reaction rate is obtained. This work presents some calculated values of nuclear reaction rate of radioactive capture and fission along of the radial direction of the fuel pellet obtained by Monte Carlo methodology using the MCNP-4C code. The relative values obtained are compared with experimental values and show a good agreement between experimental and calculated values. Besides nuclear reaction rates, the spectral indices²⁸ ρ and ²⁵δ have been obtained at each different values of the radius of the fuel pellet disk. (author)