There have been a number of major fusion reactor design studies especially during the last 35 years. One of them is ARIES-ST fusion reactor, a 1000 MWel fusion reactor power plant [1-10]. Its design is based on the spherical tokamak concept [1-10]. Furthermore, it has an advantage of having compact power core [1-10]. In ARIES-ST fusion reactor, the coolant of LiPb is considered to be used as both energy carrier and tritium breeder [1-10]. The current study investigates the main neutronic parameters of the ARIES-ST fusion reactor utilizing the molten salts with fissile isotopes: Flibe + Weapon Grade (WG) PuF4 or Flibe + Spent Fuel Grade (SFG) PuF4. The neutronic calculations in the fusion blanket were carried out with the help of Scale 5 for various coolant compositions. Numerical results showed that the best neutronic performance in the reactor was attained with the coolant of Flibe + 1% WG PuF4. In addition, a significant improvement in the energy multiplication of the reactor was obtained in comparison to the pure ARIES-ST fusion reactor. (C) 2010 Elsevier Ltd. All rights reserved.