Efforts have been made recently to study the possibility of core conversion of the 10MW MTR type research reactor from the HEU to LEU fuels due to the proliferation issue. Since the inventory of the reactor core is a required parameter to study the atmospheric dispersion calculation for a postulated accidental airborne radionuclide release from the reactor, the inventory of the radioactive nuclides accumulated in the UAl x –Al fuels: HEU (93% 235 U), MEU (45% 235 U) and LEU (20% 235 U) after 200days of the reactor operating time was calculated using the GETERA code. The result showed, after 200days of the reactor operation time (35% burnup), that the total consumptions of 235 U and 238 U for the HEU, MEU and LEU fuels were: 2360, 2334 and 2320g for the 235 U and 13, 105 and 238g for the 238 U, respectively. The amounts of 239 Pu produced in the core were: 67.07, 157.86g for the MEU and LEU fuels, respectively, compared with 7.95g. The total core radioactivity after 200days for the MEU and LEU cores were: 8.84×10 16 and 9.31×10 16 Bq, respectively, compared with 8.63×10 16 Bq for the HEU core.