Keywords

1 Introduction

In the power range from 15% FP to 100% FP, the CPR1000 nuclear power plant is adaptable to the linear change rate of ±5% FP/min and to the step change of ±10% FP, without resulting in reactor trip and/or the open of pressurizer safety relief valve (PSV) and/or turbine bypass system. However, in case of large and fast load decrease, such as turbine trip and house load from 100% FP, the capacity of turbine bypass system (GCT) needs to be large enough to match the bypass steam flow. Generally the turbine bypass system capacity is set as 85% of the nominal steam flow, so as to ensure the operation of the reactor under such large and fast transients. However, as the core power decrease lags behind the turbine load, there is still the risk of reactor trip during the house load and turbine trip transients. Besides, loss of a reactor coolant pump (RCP) at full power will cause a reactor trip actuated by low-low RCP speed signal, which is not in accordance to the new UTILITY REQUIREMENT DOCUMENT (URD), requiring the unit to remain operation in case of loss of a RCP.

The Rapid Power Reduction (RPR) system in the AP1000 nuclear power plants is designed to reduce the reactor power rapidly to the capability of turbine bypass dump system in the turbine load rejection event [1]. There is no risk of reactor trip, or no opening of safety valves of pressurizer and steam generators.

In the operation of VVER-1200 which permits one or two RCPs to be switched off, the reactor control is equipped with Fast power setback (FPS) system [2]. It automatically reduces reactor power by insertion of automatic control banks and prohibits reactor power rise by prohibiting withdrawal of the control protection system rods, so that it can avoid the reactor trip and prevent violation of safety limits and conditions.

On one hand, the fast power reduction or setback system can improve the ability of the unit to deal with large and fast load transients, and improve the availability of the unit without triggering the emergency reactor trip. On the other hand, it can also reduce the design capacity of GCT system (the capacity of AP1000 GCT system is 40% of nominal steam flow), and thus save the unit cost. In order to improve the economy and flexibility of unit operation, and also to meet the new URD to enhance the advanced nature of the unit, it is necessary to introduce a fast power reduction system for CPR1000 units.

2 Function Introduction

The main function of the fast power reduction system is to realize the fast power reduction by releasing a pre-selected control rod groups, which will drop into the bottom of the core in several seconds, during the rapid and large-scale load rejection of the turbine, thus to avoid triggering the emergency reactor trip and the open of any safety valve.

In the process of releasing the pre-selected control rod groups in the fast power reduction system, an interlock logic is set to prohibit the lifting of other power control banks, which would lift in case of power reduction during normal operation. During the rapid power reduction, the average temperature control system (ACT) is assumed in an automatic state. At the same time, the RT signal of high (negative) change rate of neutron flux is blocked, so as to avoid the unexpected reactor trip during the dropping process of the pre-selected control rod groups.

After the power reduction process, the position structure of the Rod Cluster Control Assembly (RCCA) is different from that after power reduction during normal core control, which is named as calibration curve. It is necessary to adjust the RCCA position to the calibration curve to be ready for the return to full power operation afterwards. Thus, after a delay, the power control banks are operated manually to move toward the target rod position corresponding to the calibration curve, while the temperature control banks (R banks) are in automatic mode to maintain the power near the target value.

The fast power reduction system is mainly used to prevent triggering reactor trip during deviation from normal operation, and it is classified as non-safety function.

3 Rod-Drop Strategy

In order to realize the power reduction process in the fast power reduction function, the existing control rod banks are re-grouped, and the new rod-drop groups are analyzed and calculated.

3.1 RCCA Re-group Mode

The main consideration of control rod re-grouping used for fast power reduction function is the value of control rod group and the flattening of core power. Thus the control rod group releasing for the fast power reduction function is not an random combination of control rods. In order to ensure the symmetry of power distribution and avoid unacceptable power peaks after the drop of the re-grouping control rods, the re-grouped control rods used for fast power reduction function are symmetrically arranged in the core.

The control rods arranged in CPR1000 core are divided into control rod banks and shutdown rod banks [3]. The shutdown rod banks are all black rods. The control rod banks are divided into power control banks (G1, G2, N1, N2) and temperature control banks (R), among which 12 bundles of control rods such as G1 and G2 are gray rods, and the remaining 56 bundles of control rods are black rods.

As the value of shutdown rod banks is larger than power control banks, a smaller power reduction interval can be obtained by releasing power control rod groups comparing with shutdown control rod groups, which is beneficial for power control. Furthermore, the power control rods need to be adjusted to the rod position of calibration curve with corresponding power (30% FP to 60% FP for example) after a delay of the fast power reduction process, so it is easier to restore to the target rod position by using power control rod banks.

The R control banks are not only used for regulating average temperature, but also used for axial power distribution control. Besides the R banks position will change during the plant normal operation, so R control banks are not considered for the fast power reduction function.

The general principle for re-grouping is that two RCCAs symmetrically distributed on the diagonals are classified as a new group. As the value of G1 banks is small, there is no need to divide G1 banks. The original G2 banks are divided into four groups, which are G2a, G2b, G2c and G2d respectively. The original N1 banks are divided into four groups, of which two groups are in the outer ring named as N1a and N1b respectively, and two groups in the inner ring named as N1c and N1d respectively. The original N2 banks does not need to be grouped. The re-grouping mode of power control banks is shown in Fig. 1.

Fig. 1.
figure 1

Re-group Mode of Power Control RCCAs

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3.2 Rod-Drop Group Pre-selection

The value of control rods varies with the burnup and cycle. In the algorithm of fast power reduction system, it is necessary to calculate the value of control rods at different burnups, so as to select the control rod group to be released. Taking the equilibrium cycle of a CPR1000 unit as an example, the control rod groups to be released for fast power reduction system are analyzed. Both the beginning of cycle (BOL) and the end of life (EOL) are calculated.

The calculation results of the power control rod groups to be released for the fast power reduction function are summarized in Table 1. The power control rod groups are assumed to be released in the order of G1→G2a→G2b→G2c→G2d→N1a→N1b→N1c→N1d→N2. In BOL, the power can be reduced to 47% FP when power control rod groups from G1 to G2c are released, and 33% FP when G1 to G2d are released. As can be seen in Table 1, the nuclear enthalpy rise hot channel factor (FΔH) after the drop of some control rod groups exceeds the limit slightly. However, as the fast power reduction is a short-term process, and the peak linear power density and the maximum outlet temperature decrease monotonously with the power reduction, the safety of the core will not be threatened. In EOL, when power control rod groups G1 to G2d are released, the power decreases only to 60% FP, which is due to the large feedback coefficient (in absolute value) in EOL. More positive reactivity is introduced when the power is reduced, so more power control rod groups are needed to be released comparing with that in BOL. When power control rod groups G1 to N1a are released, the power can be reduced to 49% FP, and 36% FP when G1 to N1b are released. The outer ring N1a and N1b control rod groups was selected instead of the inner ring N1c and N1d because the power peak factor was lower when N1a and N1b are released.

Table 1. Main Parameters for the Selected Power Control Groups during Fast Power Reduction Process
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To sum up, it is recommended to select power control rod groups to perform the fast power reduction function. Taking equilibrium cycle as an example, the pre-selected rod-drop groups for fast power reduction are G1G2aG2bG2c with target power at 50% FP and G1G2aG2bG2cG2d with target power at 30% FP at BOL, and G1G2aG2bG2cG2dN1a with target power at 50% FP and G1G2aG2bG2cG2dN1aN1b with target power at 30% FP at EOL, respectively.

4 Nuclear Design Analysis

The effects of fast power reduction on nuclear design parameters are analyzed, including xenon toxicity, radial power distribution, axial power distribution, and FΔH. Two typical burn-ups, BLX and EOL, are analyzed. The rod-drop groups are selected according to the analysis in Sect. 3.

  1. (1)

    The negative reactivity induced by xenon accumulation can be compensated by boron concentration regulation or power control banks lifting. In BOL, due to the high boron concentration, the boron concentration regulation is completely able to compensate the negative reactivity induced by xenon accumulation. In EOL, as the boron concentration is nearly zero, the power control banks are considered to compensate the negative reactivity, which will produce a penalizing axial power distribution. Furthermore, this will also impact the power lifting hereafter. In order to facilitate control and subsequent power lifting, it is suggested to limit the core burnup for implementation of fast power reduction function.

  2. (2)

    In the process of fast power reduction, the radial power distribution of the core will be affected when the rods are dropped. Taking BLX of the equilibrium cycle as an example, the radial power distribution during the implementation of fast power reduction is analyzed, and the calculation results are shown in Fig. 2. The radial power distribution is very uniform before the rods are dropped. The radial power distribution deteriorates after the pre-selected rod groups drop. The maximum relative power is 1.158, and the minimum relative power is 0.860. When the power control rods are fully lifted and restored to full power, the radial power distribution is further improved, and the maximum relative power is 1.014 and the minimum relative power is 0.989. The maximum deviation of radial power distribution is only 1.4%, which is basically uniform.

Fig. 2.
figure 2

Radial Power Distribution Change

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  1. (3)

    The action of control rods will lead to the change of ΔI of the core. The point of worse axial power distribution appears in the rod dropping process at EOL. It can be seen in Table 2 that, if R banks are placed in the position out of core (ARO) or in the middle of regulating belt (RMBM) before rod dropping, ΔI will exceed the right boundary of operation diagram after rod dropping. If the R rod is placed at 190 insertion steps in advance, ΔI can be controlled in the operation diagram, but the implementation of rapid power reduction will be delayed. Therefore, it is necessary to limit the burnup of rapid power reduction.

Table 2. ΔI after Fast Power Reduction at EOL
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  1. (4)

    The action of control rods will also lead to the increase of FΔH. Taking BOL as an example, Fig. 3 shows that FΔH is worse in the process of rod dropping, and the value for some low power level exceeds the limit. So it is necessary to analyze and perform the safety evaluation under the new FΔH limit, which will be discussed in Sect. 6.

    Fig. 3.
    figure 3

    FΔH during Fast Power Reduction for BOL

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    Thus it is found that the impact of fast power reduction on nuclear design mainly comes from rod-dropping process, and all parameters will be improved after the power control rod banks are adjusted back to the corresponding position of the calibration curve corresponding to the target power level.

5 Normal Transients Analysis

The fast power reduction function is mainly used for rapid and large-scale load decrease transients. House load from 100% FP to auxiliary power supply is one of the typical transients of this kind, which will trigger the fast power reduction function [4].

Fig. 4.
figure 4

House Load from 100% FP

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Taking BOL of equilibrium cycle as an example, the normal transient house load from 100% FP is analyzed considering the fast power reduction function. The target power level after fast power reduction function is considered as 30% FP and 50% FP respectively.

Figure 4 shows the changes of key parameters in the transient process of house load from 100% FP. It can be seen that after the introduction of the fast power reduction function, the power balance time of the primary and secondary circuits is shortened (Fig. 4a). The over-temperature ΔT margin means the difference between the real ΔT and the over-temperature ΔT reactor trip channel setpoint. The results in Fig. 4b shows that the over-temperature ΔT margin is significantly improved after the introduction of the fast power reduction function, which means the operational margin is improved. At the same time, the total capacity requirement of GCT can be reduced to 60% of the nominal steam flow (Fig. 4c), which can improve the flexibility and economy of the unit.

6 Safety Analysis

The function of fast power reduction system belongs to non-safety class, so its mitigation effect on accidents is not considered in the analysis. However, during rod-drop process of fast power reduction system, the FΔH will exceed the limit under some low power levels. Therefore a new FΔH limit is established when the power is lower than full power level (100% FP). The affected events or accidents are evaluated with the new FΔH limit [5].

For incidents of moderate frequency, which is defined as condition II events, the events affected by the new FΔH limit for low power levels include loss of off-site power, partial loss of forced reactor coolant flow and rod drop. The affected infrequent incident (condition III event) is the total loss of forced coolant flow accident. The affected postulated incident (condition IV event) is the RCP shaft seizure (locked rotor) accident.

For the rod drop event, the thermal power at the time of minimum DNBR in the transient process is close to 100% FP, and the new envelope has little influence on the results. The analysis results show that the minimum DNBR does not change in the transient process with the new FΔH limit.

Among other affected condition II events, the minimum DNBR appears in the loss of off-site power supply event. For the loss of power supply, the minimum thermal power at the time of minimum DNBR is 96.2% FP. If the new FΔH limit is adopted, the DNBR margin in the transient process will be reduced from 27.7% to 26.3%, which still meets the safety criteria.

For total loss of forced coolant flow accident, if the new FΔH envelope is adopted, the minimum DNBR margin in transient process is reduced to 11.0%, which still meets the limit requirements. For the RCP shaft seizure (locked rotor) accident, the new FΔH envelope has no effect on the result.

Table 3. Safety Evaluation with the New FΔH limit
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To sum up, with the new FΔH limit (see Table 3), the DNBR margin decreases, but the consequences of all the events still meet the safety criteria.

7 Conclusion

The function of fast power reduction can be realized for CPR1000 unit by regrouping the existing power control rod banks. At the same time, the effect of implementing fast power reduction system is analyzed from the aspects of nuclear design, normal transient operation, and safety analysis. The analysis results show that the fast power reduction system can improve the unit ability to cope with large and fast load drop, and improve the availability of the unit. Meantime it can also reduce the design capacity of GCT system and save the cost of the unit.