[en] The IEA-R1 reactor has undergone the modernization to increase its operating power to 5 MW, in order to allow a more efficient production of the ⁹⁹ Mo radioisotope. An irradiation element made of Be was acquired for the reactor, and studies were initiated for determining its performance when compared to other irradiation elements currently in use in the reactor. The results show some advantages of the Be irradiation element for producing ⁹⁹ Mo: the epithermal neutron flux in the Be irradiation element approximately 15 % greater than those in the water and graphite irradiation elements; the negative reactivity introduced in the reactor by the Be irradiation element is substantially smaller than those introduced by the other elements: 1636 pcm for the Be element, - 2568 pcm for graphite element and - 2977 pcm for the water element. It is concluded that the production of the ⁹⁹ Mo radioisotope with the Be irradiation element can be increased by 15% in the IEA-R1m reactor. It also requires less fuel for the reactor operation due to the smaller negative reactivity introduced in the reactor core. (author)