Mechanism of Coupled Neutronic Thermal-Hydraulic Instability

1 Mechanism of Coupled Neutronic Thermal-Hydraulic Instability 

The interaction between thermal-hydraulics and neutronics may bring about instability in the Super LWR and this instability also needs to be considered in the Super LWR design. These two processes are coupled through the heat transfer from the fuel rod to the coolant and moderator and through the reactivity feedback effects due to changes in temperature and changes in density of the coolant and the moderator. 

Since the power generation is affected by the reactivity feedback, it depends on the average fuel temperature and average water density in the core. In the forward loop, when a small reactivity oscillation occurs in the reactor system, the reactor power oscillates, and it leads to average fuel temperature perturbations and average water density perturbations through the fuel dynamics and channel thermal-hydraulics. In the feedback loop, as a consequence of these average fuel 

The neutronic feedback involves the neutron kinetics, the fuel dynamics, the core thermal-hydraulics, and the reactivity feedback dynamics. The neutron kinetics affects and is affected by the power generation in the fuel, and is directly responsible for the power perturbations. The fuel dynamics affects and is affected by the fuel surface heat flux, and is responsible for the time delays between power production and the response of coolant flow heating. The core thermal-hydraulics affects the power production and the response of the water density perturbations to fuel surface heat flux perturbations. Finally, the reactivity feedback dynamics is responsible for the feedback reactivity due to water density perturbations and fuel temperature perturbations, and is affected by neutron kinetics. 

The time delay of the heat transfer to the coolant and moderator water is an important factor in the mechanism of coupled neutronic thermal-hydraulic instability. The Super LWR is a reactor system with a positive density coefficient of reactivity and a large time-delay constant. If there is no time delay, a decrease in density would cause a decrease in power generation, which suppresses further decrease in density, stabilizing the system. However, if there is a large time delay, it causes a decrease in the gain of the density-reactivity transfer function and reduces the effect of density reactivity feedback, making the system less stable. 

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