Accident Analyses at Supercritical Pressure

The calculation results are shown in Fig. 6.36. The power decreases to the decay heat level due to the reactivity feedback and reactor scram. Reverse flow occurs in the water rod channel because the buoyancy pressure drop dominates the pressure drop balance. Heat conduction to the water rods increases when the coolant temperature in the fuel channel increases. This implies that the water rods serve as a heat sink . As the coolant expands in the water rods due to heat-up, there is an increase in the flow rate downstream from the water rods, including the fuel channel inlet. Consequently, the fuel channel flow rate is maintained even though the coolant supply from the cold-leg has stopped. This is called the water source effect of the water rods. The heat sink and water source effects mitigate heat-up of the fuel rod cladding, and hence enable the AFS to have a realistic delay time. The hottest cladding temperature begins to decrease before the initiation of the AFS. The increase in the hottest cladding temperature is about 250°C while the criterion is 520°C. 

The results of sensitivity analyses are summarized in Table 6.17. The influence of the coast-down time of the RCPs is not significant because the peak temperature appears at least 5 s after the coast-down has been completed. The influence of the scram delay is not significant because the reactor power is also decreased by the reactivity feedback. It should be noted that the peak temperature does not depend on the capacity or delay of the AFS because the peak temperature appears before the initiation of the AFS. 

Coolant flow from one of the two RCPs is assumed to suddenly stop. The calculation results are shown in Fig. 6.37. The increase in the cladding temperature is below half of that in the total loss of reactor coolant flow because one of the RCPs is available. 

The CR cluster having the maximum reactivity worth is assumed to eject from the core with the velocity of 9,500 m/s as is assumed in PWRs. Reactivity feedback 

Bold characters reference case, AMCST increase in maximum cladding siuface temperature 

---

Chapter 6 covers safety topics. It includes fundamental safety principles of the Super LWR and Super FR in comparison with that of LWRs, safety features, safety system design, abnormal transient and accident analyses at supercritical pressure, analyses of loss of coolant accidents (LOCAs) and anticipated transients without scram (ATWSs) and simplified probabilistic safety assessment (PSA). 

---