[en] The new generation of rare event searches experiments requires a new level of very high radiopurity. Main impurities come from the natural long-lived radionuclides Th, U, Ra, Ac and K. In the frame of this field purification of Se polluted with those radioimpurities was carried out taking special care of radiopurity of any reagent used. An instrumental neutron activation analysis of bidistillated water, deionized water and nitric acid was performed. These reagents were used during the process of selenium purification from impurities of Th, U, Ra, Ac and K using the reverse method. Distribution coefficients of ²³⁴Th, ²³⁰U, ²²³Ra, ²²⁵Ac on the cation-exchange resin DOWEX 50W-X8 in different concentrations of selenium and nitric acid were experimentally measured. The technique of selenium purification from Th, U, Ra, Ac and K was developed. The developed technique of selenium purification has a simple technology of execution and minimal number of reagents (water, nitric acid) which possess a high degree of cleanliness. Operations applied during purification (elution, evaporation) practically exclude losses of a target product (chemical yields of Se > 99%). Based on this technique test purification of 100 g selenium was carried out. The efficiency of this technique was confirmed by low background spectrometry of purified selenium sample