In the search for the new and clean energy sources, nuclear energy seems to be one of the best source in the world. It has a very low carbon footprint and as the time passes the technological advances makes it safer. However, this energy source is not infinite like the fossil fuels. Therefore like the conventional energy sources the natural resources for this energy source has to be used efficiently and wisely. New fuel sources has to be found and the reactors has to be designed for these fuels also. Gas Turbine - Modular Helium Reactor (GT-MHR) is one of this type of the reactors. Because of thes coolant that is used, very high efficiencies can be achieved and new fuels can be utilized. Weapons-grade plutonium (WGrPu) can be used in a GT-MHR, which can be acquired from the old nuclear weapons. In order to increase the life of the WGrPu resources this fuel can be mixed with Thorium, which is a fertile material that can be found in the nature. In this study, utilization of the weapons-grade plutonium - thorium mixture was investigated and an optimum mixture ratio was determined. The behaviour of this mixture and the original fuel was studied by using MCNP, Monteburns 2.0 and Origen 2.2 tools. According to the calculations, a GT-MHR type reactor, which is using the original TRISO fuel particle mixture of 20% enriched uranium + natural uranium has an effective multiplication factor (keff) of 1.270. By using this keff the weapons grade plutonium - thorium oxide mixture was found as 19% - 81%, respectively. By using monteburns the operation time (time passed until the reactor reaches a keff value of 1.02) of original uranium fuel was found as 515 days, where this value is 1175 days for the weapons grade plutonium mixture. Furthermore, the burnup values for the uranium and weapons grade plutonium fuels was found as 47.69 and 119.27 GWd/MTU, respectively.