In re cent years, ex tended stud ies are de vel oped for in ves ti ga tion the ef fects of us ing nanofluids in NPP as cool ant. CANDU-6 re ac tors have the po ten tial to use nanofluid coolants be cause in these re ac tors, the mod er a tor sys tem is fully in de pend ent of the pri mary heat trans port sys tem. MCNPX code has been used for mod el ling and sim u la tion of a CANDU-6 re ac tor con tain ing a nanofluid as pri mary cool ant. The vari a tion of mul ti pli ca tion fac tor and to tal neu tron flux dis tri bu tion along a fuel chan nel, next to the cen tral axis, has been in ves tigated by us ing dif fer ent nanofluids. In this anal y sis, heavy wa ter-based nanofluids con tain ing var i ous vol u met ric per cent ages of Al 2 O 3 , TiO 2 , CuO, Ti, Cu, Zr, and Si nanoparticles were used. A typ i cal CANDU-6 re ac tor was se lected as ref er ence for re ac tor core mod el ling. The re sults of the neutronic anal y sis show that Al 2 O 3 nanofluids with 1% vol u met ric per cent age are the most suit able cool ant for CANDU-6 re ac tors which can in crease the cool ant heat trans fer co ef fi cient and con se quently en hance the plant ef fi ciency.