by opening bypass valves with the closure of steam control valves, and sending all steam to the condenser directly. It is designed so that, at the time of load rejection, the reactor power is decreased by tripping the recirculation pumps, which is the same effect as seen in the 25% bypass capacity plant, and by inserting the pre-selected control rods. The analysis results (Figure 7.2.3 (4)) show that all of the changes in the transients are mild. The reactor power decreases to about 30% and the pressure rise is small (about 7.15 MPas [gage] as the maximum) and AMCPR is also limited to about 0.02.

3. Features of abwr Transient Analysis

ABWRs have some unique characteristics in assumptions and results of the transient analysis compared with those of conventional reactor facilities due to the system and component modifications as described in Chapter 2. The main modifications are a new control rod drive mechanism and application of internal pumps.

(l)New control rod drive mechanism and operation mode

The design of the ABWR control rod drive mechanism has been changed to the motor-driven fine motion control rod drive (FMCRD) mechanism from the conventional hydro-driven one, and the gang mode control rod operation has been adopted to reduce the startup time and to improve the availability factor by rationalizing control rod operation. For conventional BWRs, only one control rod can be withdrawn at a time, but for ABWRs, a multiple number of control rods are pre-grouped and withdrawn at once in the gang mode operation.

By adoption of the gang mode operation, the amount of reactivity inserted at the "abnormal withdrawal of control rods during reactor startup" may become large. In order to avoid this, the withdrawal speed is kept slow to control the power- up rate. Figure 7.2.4 shows the analysis results of the "abnormal withdrawal of control rods during reactor startup." The increase in the neutron flux is limited by slow control rod withdrawal, the reactor period short rod block by the SRNM, and by the reactor scram.

(2) Adoption of internal pump

In the ABWR design, instead of two sets of conventional pumps with recirculation loops, ten sets of small capacity internal pumps were adopted in order to eliminate the piping that penetrated the reactor pressure vessel below the top of the effective fuel. The inertia of the internal pump is small as its capacity is small, which gives only a slight impact on the transient characteristics.

For conventional BWRs, one recirculation pump has been assumed to trip in the analysis of the "partial loss of the reactor coolant flow". On the other hand, for ABWRs, three pumps, as the maximum number of pumps that may possibly trip in a single failure mode, are assumed to trip simultaneously in the analysis. The analysis results are shown in Figure 7.2.5.

Figure 7.2.4 Transient at the time of the abnormal withdrawal of control rods during reactor startup (ABWR)

Since recirculation loops were eliminated from the ABWR design, the evaluation of "inadvertent startup of a shutdown loop of a reactor coolant system" is not necessary any more. That is, even if an inadvertent startup of the internal pump occurs, a large amount of slightly-subcooled coolant outside the shroud enters the core only. As a reactivity

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NSRA, Japan