## 4 Discussion
The two behaviors are interlinked: the amount of cooling (single-side heat transfer) achieved in the primary fuel salt depends on the velocity of the system, and the heat transferred from the primary fuel salt to the secondary coolant depends on the heat transfer coefficient. The velocity of the fuel salt determines the residence time of the salt in the heat exchanger, and as a result, the heat transfer coefficient's correlation with the velocity of the system increases. Specific heat, thermal expansion, and heat transfer coefficients have the biggest impact on temperature and velocity of the system and only these parameters are taken into consideration in this study. The minimum and maximum temperatures inside the core occur along the centerline and these temperatures are impacted by the specific heat of the molten salt. As a result, there is a strong correlation between temperature and specific heat observed along the centerline of the core (Figure 17). Since this is a natural circulation-driven flow, the temperature differences inside the core drive the fuel-salt flow and specific heat contributes towards these temperature differences within the core; as a result, a strong correlation between specific heat and velocity of the fuel salt can be observed. The thermal expansion coefficient has a similar effect on the temperature of the salt as specific heat and is strongly correlated near the centerline of the core. As soon as the pump is deactivated, the system becomes density-driven and the thermal expansion coefficient determines the magnitude by which the density change occurs, thereby impacting the fuel-salt velocity. The amount of cooling (single-side heat transfer) achieved in the primary fuel salt depends on the velocity of the system, and the heat transferred from the primary fuel salt to the secondary coolant depends on the heat transfer coefficient. The velocity of the fuel salt determines the residence time of the salt in the heat exchanger, and as a result, a strong correlation between heat transfer coefficient and velocity occurs. As the hot fuel salt enters the HEX, the temperature of the fuel salt depends primarily on the heat transfer coefficient input parameter, but as the fuel salt moves through the core, other input parameters such as the thermal expansion coefficient, specific heat, and density come into play, and the uncertainties propagated through these parameters start to add up, thereby leading to higher uncertainties just before the entrance to the HEX. With regards to velocity, the maximum uncertainty is observed near the curved wall of the reactor where the velocities are very close to zero, whereas in other regions, the standard deviation is relatively lower by many orders of magnitude. As heat is transferred to and from the system, the temperature of the fuel salt changes, and this temperature change depends on two factors: the number of molecules in the system and the heat capacity of the system. Since the volume of the system is fixed, the fuel-salt temperature strongly depends on the specific heat capacity of the salt. The fuel-salt temperature shows the expected negative correlation with specific heat capacity. Upon initiating the ULOF transient, as the temperature of the system increases, the thermal expansion of fluid causes an increase in the density difference between the cold and hot sections which leads to the increase in driving head, thereby increasing fuel-salt velocity. Since it is now a density-driven system, it impacts the maximum temperature of the core as shown in Figure 18. The SRCC of maximum temperature and average velocity are very similar to the average temperature and maximum velocity, and for brevity, they are not discussed in detail.