Amir Karimi Jafari - Department of Faculty of Engineering Science and Research branch, Islamic Azad University Tehran – Iran
Farrokh Khoshahval - Reactor and Nuclear Safety Research School Nuclear Science and Technology Research Institute, AEOI Tehran - Iran

Numerous burnable -absorber (BA) type have been used in commercial nuclear fuel assembly designs to suppress initial reactivity. In this study neutronic calculation results of PWR 17 17 fuel assembly with gadolinium isotopic content has been shown by using the DRAGON code simulation. These BAs have varied gadolinium enrichment and number in fuel assembly. Seven gadolinium isotopes naturally exist on earth. From these, only two isotopes, the Gd-155 and Gd-157 have extremely high thermal neutron absorption cross sections. In order to decrease this parasitic absorption in the end and emphasize the absorption in beginning of the life it is possible to use gadolinium enriched in Gd-155 or Gd-157. For all fuels containing gadolinia the reactivity peak is observed after some days of irradiation and not at zero burnup condition and these peaks decreasing in value with increasing the number of gadolinia rods.