Coupled neutronic thermal-hydraulic stability

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. 

Coupled neutronic thermal-hydraulic stability Decay ratio < 0.25 (damping ratio > 0.22) Decay ratio < 1.0 (damping ratio > 0)... 

Fig. 1.28 Block diagram for coupled neutronic thermal-hydraulic stability of the Super LWR. (Taken from ref. [50] and used with permission from Atomic Energy Society of Japan)...

Fig. 1.29 Gain response of closed loop transfer function of coupled neutronic thermal-hydraulic stability...

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. 

Coupled Neutronic Thermal-Hydraulic Stability Considerations... 

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. 

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 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... 

S.3.2 Coupled Neutronic Thermal-Hydraulic Stability at Partial Power Operations at 25 MPa... 

Fig. 5.52 Effect of flow rate on coupled neutronic thermal-hydraulic stability at 50% power...

From the thermal considerations in Sect. 5.3.3.2, the core power, feedwater flow rate and feedwater temperature are designed as 20 and 35% of the rated values and 280°C, respectively. The decay ratios of coupled neutronic thermal-hydraulic stability are then calculated with these conditions. The results are shown in Fig. 5.55 [10]. The stability criterion is satisfied with sufficient margin during... 

S.5.3.4 Parametric Studies of Coupled Neutronic Thermal-Hydraulic Stability... 

The coupled neutronic thermal-hydraulic stability of the Super LWR is also sensitive to various operating conditions and design parameters. Sensitivity analyses are carried out in order to predict the influences of parameter changes on the stability of the Super LWR. Here, the effects of the density coefficient, core power, mass flow rate, system pressure, core inlet temperature, and water rod flow ratio on coupled neutronic thermal-hydraulic stability of the Super LWR are investigated. 

Figures 5.57 and 5.58 [13] imply that the coupled neutronic thermal-hydrauhc stability gets worse when the power to flow rate ratio increases. The effect of the core power (or flow rate) on the constant power to flow rate ratio is also investigated. The result is shown in Fig. 5.59 [13]. The stability becomes worse as the power or flow rate become lower. This is because of the destabilizing effect of the decreases in the flow rate and moderator density feedback. The sensitivity of the coupled neutronic thermal-hydraulic stability is more than that of the thermal-hydraulic stability.

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