Coupled Neutronic Thermal-Hydraulic Stability Analyses

3 Coupled Neutronic Thermal-Hydraulic Stability Analyses 

The average power channel is analyzed to study coupled neutronic thermal-hydraulic stability of the Super LWR. A block diagram is shown in Fig. 5.45 [10, 13]. The neutronic model is used to find the forward transfer function G s), i.e., the transfer function from the reactivity perturbations to the power perturbations. The thermal-hydraulic, heat transfer, and excore models are used to determine the feedback transfer function H s) which is the transfer function from the power perturbations to the feedback reactivity perturbations through the neutronic effect. 

The Doppler reactivity coefficient and the density reactivity coefficient of the present design of the Super LWR are shown in Figs. 5.46 and 5.47. Under normal operation, the Doppler coefficient and density coefficient are —1.2 x 10 dk/k/°C and 0.2 dk/k/(g/cm ), respectively. Compared to other LWRs, the Doppler 

The frequency response of the closed loop transfer function for coupled neutronic thermal-hydraulic stability of the Super LWR for the 100% average power channel is shown in Figs. 5.48 and 5.49. It can be observed that the presence of 

1 Coupled Neutronic Thermal-Hydraulic Stability at Full Power Normal Operation 

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Coupled neutronics/thermal hydraulics stability analyses of the STAR reactor at these plant equilibrium states at full and partial load will be required. Such analyses have been conducted already for the STAR-LM which shares the neutronics and thermal hydraulics properties of STAR-H2 reactor, - and stability has been demonstrated. 

Moreover, coupled neutronics/thermal-hydraulics stability analyses would be required for the ending equilibrium states from the passive accommodation of ATWS initiators. Work for the STAR-LM suggests that these states are indeed stable ones. 

Fig. 1.32 Coupled neutronic thermal-hydraulic stability analysis result at power increase phase...

T. T. Yi, S. Koshizuka, Y. Oka, A Linear Stability Analysis of Supercritical Water Reactors, (II) Coupled Neutronic Thermal-Hydraulic Stability, Journal of Nuclear Science and Technology, Vol. 41(12), 1176-1186 (2004)... 

Power oscillations may also occur like BWRs because there is a time delay from the change in the thermal power (or neutron flux) to the change in the coolant or moderator density through heat conduction and heat transfer. In Sect. 5.5, coupled neutronic thermal-hydraulic stability of the Super LWR is analyzed with the frequency domain approach. The analysis includes both supercritical and subcritical pressure conditions. 

The Super LWR employs separate large square water rods as neutron moderators. The time delay of the heat transfer to the water rod is much larger than that of the heat transfer to the coolant. Thus, the reactor system becomes less stable when a water rod model is included than when no water rod model is used. The descending water rods will have a significant effect on the coupled neutronic thermal-hydraulic stability because of the moderator density reactivity feedback from the large square water rods, and it needs to be considered in stability analysis of the Super LWR. 

Chapters 3-5 treat the plant system and behaviors. They include system components and configuration, plant heat balance, the methods of plant control system design, plant dynamics, plant startup schemes, methods of stability analysis, thermal-hydraulic analyses, and coupled neutronic and thermal-hydraulic stability analyses. 

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