Fuel comprised of thorium has been explored since the early development of nuclear energy in the 1960s. In the last decade, there has been a renewed interest in thorium fuel and it has now become a primary focus in studies and proposed in next-generation nuclear reactors. This has been prompted by a limited supply of uranium in the foreseeable future and an abundance of thorium resources. Additionally, when compared to natural uranium (NU), thoria (ThO$_2$) produces substantially less long-lived radioactive waste and the fissile content can be reprocessed for additional fuel cycles. The CANDU 6 reactor has a unique ability to harvest thorium fuels because of its superior neutron economy. Thorium requires a driving isotope to sustain neutron fission until the long absorption chain produces viable amounts of U-233. Previous studies have investigated many different practical fissile isotopes and core modeling techniques that would make thorium feasible in a CANDU 6 reactor. This thesis focuses on a safety evaluation of thorium fuel compared to NU fuel in a lattice cell and full core configuration. \\ DRAGON 3.06 and SERPENT 2 are used to examine the infinite lattice cell containing NU and homogeneous thorium fuel enriched with 2.0\% U-235, emphasizing the relationship between multiple nuclear libraries. This configuration is used to determine the enrichment concentration, temperature coefficient, coolant void reactivity, and the power relationship. Thorium fuel exhibits a higher negative temperature coefficient, a lower coolant void reactivity, and a greater reactivity change when simulated at different powers. If the lattice cell is simulated at 75\% nominal power there is an 11 mk adjustment for thorium fuel, whereas the adjustment is only three mk for NU fuel. This is related to the extensive cross section of Th-232 and the long fertile absorption chain results in a sizeable inventory of the intermediate isotope Pa-233. The fissile content of the fuel bundle after exiting the reactor will continue to accumulate U-233 and should be monitored and properly stored. \\ A full core evaluation in a CANDU 6 reactor is performed in DONJON 4. Thorium fuel has an inferior reactivity worth for the control mechanism than does NU fuel in an operating CANDU 6 reactor. The reactivity worth of leakage and absorption in the reactor is estimated to be slightly lower for the thorium fuel. \\ This thesis presents a new computational model for analyzing full core power transients built upon previous results. The approximation model utilizes many assumptions to develop an expeditious code for analyzing the infinite square lattice retaining the isotopic densities. This model has demonstrated the ability to accurately emulate the reactivity of a lattice cell at different powers and power transients formed in DRAGON. The model is coupled with a point kinetic code to perform power transients in a CANDU 6 reactor. \\ Load following operations are performed in cycles of 24 hours examined at 80\%, 60\%, and 40\% full power. Power adjustments are performed in increments of 10 minutes, two hour, or four hour periods with a constant reactivity input. The power adjustment time has minimal effect on the reactivity perturbations and only influences the rate of reactivity. Thorium fuel has enhanced load following capabilities compared to conventional NU fuel.\\ The long-lasting effects of Pa-233 introduces safety concern when reducing power or reactor shutdown scenarios. Reactivity transformation within the first two days of immediate power reduction will yield similar results for both fuels. Excess reactivity in the thorium fuel will continue to accumulate and eventually double the reactivity peak of NU fuel in the following 90 to 120 days. A shutdown simulation is performed in incremental power reduction steps of 20\% for a range of different days. It is found that NU fuel can adequately control the additional reactivity in this simulation. Thorium fuel maintains a disconcerting amount of excess reactivity that will need to be addressed accordingly. The protactinium transient highlights the need to adequately monitor the buildup of Pa-233 for thorium-based fuels in a reactor. / Thesis / Master of Applied Science (MASc)