A method of controlling axial power shift during power operation of a VVER reactor

Abstract

This invention belongs to the field of VVER reactor control technology, specifically relating to a method for controlling axial power offset during VVER reactor power operation. When the reactor lifespan is 300-400 days, the axial power offset gradually begins to diverge. Intervention is initiated when the deviation between the peak and trough values ​​of the axial power offset calculated by the reactor monitoring / control and diagnostic system is 1. At this point, control rods are inserted when the rising axial power offset intersects with the equilibrium axial power offset to maintain full reactor power. Subsequent water injection promptly restores the rod position. When the falling axial power offset intersects with the equilibrium axial power offset, the control rods are raised. This is done when the main steam header pressure is low or after pre-injection of boron to reduce the main steam header pressure, ensuring full reactor power operation without power reduction. During this stage, the control rods are maintained at 88.2-88.6% of their rated capacity. This method will improve the safety, operational stability, and economy of nuclear power plant units.

Description

Technical Field

[0001] This invention belongs to the field of VVER reactor control technology, specifically relating to a method for controlling axial power offset during VVER reactor power operation. Background Technology

[0002] VVER units require that the axial power deviation (AO) of the reactor be kept within the allowable range during power operation to avoid exceeding the reactor volumetric power non-uniformity coefficient (Kv) and violating the technical specifications.

[0003] When reactor power changes, the altered neutron flux distribution causes changes in the 135Xe distribution, which in turn affects the neutron flux distribution within the reactor core. This recurring fluctuation in both 135Xe and neutron flux distribution is called "xenon oscillation." The neutron flux distribution changes caused by xenon oscillation can lead to excessively high local power within the reactor, affecting the integrity and safety of the nuclear fuel. Therefore, xenon oscillation needs to be avoided as much as possible during reactor operation. Simultaneously, as nuclear fuel burnup increases, the reactor's backup reactivity decreases, and the impact of 135Xe generated by power changes on the reactor core increases. If not properly controlled, this could result in an excessively high upper power peak at the end of the reactor's lifespan, causing the active power (AO) to exceed the recommended range.

[0004] During the late stages of a nuclear power unit's lifecycle, the reactor's axial free xenon oscillations are divergent, easily causing xenon oscillations that are difficult to suppress and pose risks such as distortion of core power distribution and axial power deviation (AO) exceeding permissible limits. Particularly noteworthy is the case at the Tianwan Nuclear Power Plant, where Units 1-4 repeatedly experienced excessively low safety margins for key core parameters such as the reactor volumetric power non-uniformity coefficient (Kv) during peak load regulation at the end of their lifecycle, increasing operational control pressure and posing a risk of violating operational technical specifications. Therefore, controlling axial power deviation (AO) is extremely important throughout the reactor's entire lifecycle of operation.

[0005] Based on the operational status and reactivity calculations throughout the reactor's entire lifespan, this paper presents methods for reactor control by operators throughout the reactor's lifespan, aiming to achieve safe, reliable, stable, and economical operation. Summary of the Invention

[0006] The purpose of this invention is to provide a method for controlling axial power offset during VVER reactor power operation; it enables more systematic overall AO control throughout the long life cycle and refined AO control at the end of the reactor's lifespan, achieving safe, reliable, stable, and economical operation. Even at the end of the reactor's lifespan, the AO is maintained within the recommended range, and timely intervention ensures that AO remains within the recommended range, preventing thermal and nuclear power exceedances during rod lifting, ultimately increasing the kV safety margin, avoiding fuel damage, and ensuring core safety.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for controlling axial power offset during VVER reactor power operation.

[0009] During reactor power operation, within a 300-day lifespan, the axial power offset is in a convergent state. During this stage, the control rod position is maintained at the optimal position of 88.2-88.6%.

[0010] When the reactor lifespan is 300-400 days, the axial power offset gradually begins to diverge. Intervention is needed when the deviation between the peak and trough values ​​of the axial power offset calculated by the reactor monitoring / control and diagnostic system is 1. At this time, control rods need to be inserted when the rising axial power offset intersects with the equilibrium axial power offset to maintain the reactor at full power. The rod position should be restored in time after subsequent water injection. When the falling axial power offset intersects with the equilibrium axial power offset, the control rods need to be raised. At this time, the control rods should be raised when the main steam header pressure is low or after boron injection to reduce the main steam header pressure in advance to ensure that the reactor operates at full power without reducing power. During this stage, the control rods should be controlled at 88.2-88.6%.

[0011] When the reactor lifespan is 400-450 days, the axial power offset divergence becomes significant. Intervention is needed when the deviation between the peak and trough values ​​of the axial power offset calculated by the reactor monitoring / control and diagnostic system is 0.8. At this time, control rods need to be inserted when the axial power offset rises and intersects with the equilibrium axial power offset to maintain the reactor at full power. Subsequent water injection should restore the rod position in a timely manner. When the axial power offset falls and intersects with the equilibrium axial power offset, the control rods need to be raised. At this time, the control rods should be raised when the main steam header pressure is low or after boron injection to reduce the main steam header pressure. Before raising the control rods, the nuclear power needs to be reduced to 99.5%, and the power should be restored after raising the control rods. During this stage, the control rods should be controlled at 88.2%.

[0012] When the reactor life exceeds 450 days, the axial power offset divergence is significant, reducing the rod position to 85-86.1%.

[0013] During the 300-day lifespan, it is crucial to maintain the control rod position at the optimal level of 88.2-88.6% to ensure uniform core burnup and prevent excessively high lower power peaks and burnup, which could lead to excessively high upper power peaks at the end of the lifespan.

[0014] With a lifespan of 300 days, axial power offset and control rod position can be controlled by injecting boron and water.

[0015] When the reactor life exceeds 450 days, the axial power offset diverges significantly, approaching refueling. The axial power offset is controlled by inserting control rods when the rising phase of the axial power offset intersects with the equilibrium axial power offset, ultimately reducing the rod position to 85-86.1%.

[0016] If the control rods must be upgraded when the reactor life exceeds 450 days, the nuclear power must be reduced to 99% before the upgrade can be carried out to prevent the nuclear power from exceeding the limit.

[0017] The beneficial effects achieved by this invention are as follows:

[0018] Wide applicability: Currently, the Tianwan Nuclear Power Plant Units 1-4, Units 7 and 8, and the Xudabao Nuclear Power Plant Units 3 and 4 all use the same VVER type reactor design. This control method can be widely used in the nuclear power industry to improve the overall operational stability and economy of China's nuclear power units.

[0019] Economic benefits: Throughout the control process, the overall fuel consumption level of the reactor can be improved, enhancing the unit's economic efficiency. This method can reduce the number of control rod interventions at the beginning of the reactor's lifespan and reduce the number of power reductions and control rod activations at the end of the lifespan, further improving the unit's economic efficiency. Based on a one-lifespan calculation for Units 3 and 4 of the Tianwan Nuclear Power Plant, this method can guarantee a reduction of at least 30 power reductions, with an estimated economic benefit of 20,000 kW × 0.5 h × 30 × 0.4 = 120,000 yuan; the improvement in fuel consumption efficiency is expected to reach 1 day, resulting in a benefit of 10 million yuan.

[0020] Safety Enhancement: This method enables more stable and reasonable AO control, ensuring the accuracy and timeliness of AO intervention throughout the entire lifespan, thereby improving unit safety; it also makes core fuel burnup more uniform, avoiding fuel damage, ensuring Kv margin, and improving unit safety; at the same time, this method can ensure a reasonable and slight reduction in reactor power during the end-of-life rod lifting process, preventing NFME nuclear power from exceeding the limit, avoiding reactor power limiting system operation, and improving unit safety and reliability.

[0021] This method will improve the safety, operational stability, and economy of the Tianwan Nuclear Power Plant and other nuclear power plants in the same region. Detailed Implementation

[0022] The present invention will now be described in detail with reference to specific embodiments.

[0023] Utilizing existing reactor monitoring / control and diagnostic systems (MCDS) and reactor control and protection systems (GICS) for AO and K VThe method involves monitoring and controlling the control rods, determining their positions during stable reactor operation. At the end of the reactor's lifespan, if the reactor's aerodynamic control (AO) diverges, the AO is controlled by raising or lowering the control rods. However, raising the rods at the end of the lifespan necessitates power reduction, impacting economic efficiency. This method, through refined process control throughout the entire lifespan and optimized control rod control at the end of the lifespan, ultimately forms a reactor operation control method for the entire lifespan, improving reactor safety and economic efficiency. This calculation method has been well validated in Tianwan Units 3 and 4, achieving the expected results.

[0024] 1. During reactor power operation, within a 300-day lifespan, the reactor core burnup (AO) is in a convergent state. During this stage, minimal control of the AO using control rods is unnecessary. However, it's crucial to maintain the control rods at the optimal position of 88.2-88.6% to ensure uniform core burnup and prevent excessively high lower power peaks due to excessive burnup, which could lead to excessively high upper power peaks at the end of the lifespan. During this stage, AO control via control rods is unnecessary, and the reactor power remains at full capacity. However, it's essential to maintain the control rods at 88.2-88.6% burnup, which can be achieved through boron injection and water injection to control the AO and control rod positions.

[0025] 2. When the reactor life is between 300-400 days, the atomization (AO) gradually begins to diverge. Intervention is needed when the deviation between the AO peak and trough values ​​calculated by MCDS is 1. At this time, control rods need to be inserted when the AO rises and intersects with the equilibrium AO* to maintain full power. The rod position should be restored promptly after subsequent water injection. When the AO falls and intersects with the equilibrium AO*, the control rods need to be raised. At this time, raising the control rods when the main steam header pressure is low or after pre-injecting boron to reduce the main steam header pressure can ensure full power operation of the reactor without reducing power. During this stage, the control rods should also be kept at 88.2-88.6% of their maximum strength.

[0026] 3. During reactor lifespan of 400-450 days, significant AO divergence occurs. Intervention is needed when the deviation between the AO peak and trough values ​​calculated by MCDS reaches 0.8. At this stage, control rods should be lowered when the AO rises and intersects with the equilibrium AO*, maintaining full reactor power. Subsequent water injection should promptly restore the rod positions. During the AO decline stage, when the control rods intersect with the equilibrium AO*, they need to be raised. This should be done when the main steam header pressure is low or after pre-injecting boron to reduce the main steam header pressure. Before raising the control rods, nuclear power needs to be reduced to 99.5%, and power restored after raising them. During this stage, the control rods should also be maintained at 88.2% in a timely manner.

[0027] 4. When the reactor life is over 450 days, AO (Anaerobic Reactor) diverges significantly during this stage, but refueling is imminent. During this process, AO should be controlled as much as possible by lowering the control rods when they intersect with the equilibrium AO* (Anaerobic Reactor). Ultimately, the rod position can be lowered to 85-86.1%, thus maximizing power generation. If it is necessary to raise the control rods, the nuclear power output must be reduced to 99% before raising them to prevent exceeding the nuclear power limit.

[0028] Calculation process for improving the reactivity balance of the control rod (taking 87% rod position as an example):

[0029] At any given moment, regardless of its state, the reactivity of a reactor is always in a state of equilibrium, i.e.: Δρ 反应堆 =△ρ 功率 +△ρ 控制棒 +△ρ 氙 +△ρ 钐 +△ρ 硼 +△ρ 燃耗

[0030] At the instant the control rods are raised, the boron concentration does not change, nor do the amounts of xenon and samarium in the reactor. Changes in burnup are also negligible at that instant. Therefore, Δρ 氙 ≈0, △ρ 钐 ≈0, △ρ 硼 ≈0, △ρ 燃耗 ≈0. During the instantaneous change of the control rod: Δρ 功率 =-△ρ 控制棒

[0031] According to the neutron physics manual for the fourth cycle of Unit 3, considering that the position of the tenth control rod is generally between 85-90% during normal unit operation, the power change caused by raising the tenth control rod by one step when it is at 87% position under different lifespans is as follows: Initial lifespan: Balance calculation:

[0032] △ρ 控制棒 =1.39*10 -3 *2 = 2.78 * 10 -3 ,

[0033] That is, 0.20%.

[0034] EFPD=100:

[0035] △ρ 控制棒 =1.79*10 -3 *2 = 3.58 * 10 -3 ,

[0036] That is, 0.24%.

[0037] EFPD=200:

[0038] △ρ 控制棒 =2.08*10 -3 *2 = 4.16 * 10 -3 ,

[0039] That is, 0.26%.

[0040] EFPD=300:

[0041] △ρ 控制棒 =2.26*10 -3 *2 = 4.52 * 10 -3 ,

[0042] That is, 0.26%.

[0043] EFPD=350:

[0044] △ρ 控制棒 =2.82*10 -3 *2 = 5.64 * 10 -3 ,

[0045] That is, 0.30%.

[0046] EFPD=400:

[0047] △ρ 控制棒 =3.38*10 -3 *2 = 6.76 * 10 -3 ,

[0048] That is, 0.33%.

[0049] End of life:

[0050] △ρ 控制棒 =3.81*10 -3 *2 = 7.62 * 10 -3 ,

[0051] That is, 0.334%.

[0052] According to theoretical calculations, when the position of the tenth control rod is 87% at the end of its service life, raising it by one step will introduce a maximum power change of 0.334%.

[0053] (The above calculation data are all from the Neutron Physics Handbook for the Fourth Cycle of Unit 3 Operation)

[0054] The above calculations are theoretical values, representing the change in reaction power after equilibrium. It can be seen that, under all circumstances, raising the control rods at full power will not significantly affect the thermal power. However, in actual operation, due to the influence of the AO peak value, the upper power peak is high, and the control rod raising is transient. This primarily affects the nuclear power. In the nuclear non-nuclear measurement system (NFME), the rapid change in neutron flux can cause the nuclear power to momentarily exceed the limit, with the maximum fluctuation exceeding 1.3%, as shown in Table 1 below, causing the reactor limitation system to activate. Therefore, under the premise of conservatively ensuring nuclear safety, the reactor nuclear power needs to be reduced when raising the control rods at the end of the reactor's lifespan. Refined control is described in steps 1-4.

[0055] Table 1. Changes in nuclear power during actual rod lifting process.

[0056]

Claims

1. A method for controlling axial power offset during VVER reactor power operation, characterized in that: During reactor power operation, within a 300-day lifespan, the axial power offset is in a convergent state. During this stage, the control rod position is kept at the optimal position of 88.2-88.6%. When the reactor lifespan is 300-400 days, the axial power offset gradually begins to diverge. Intervention is needed when the deviation between the peak and trough values ​​of the axial power offset calculated by the reactor monitoring / control and diagnostic system is 1. At this time, control rods need to be inserted when the axial power offset rises and intersects with the equilibrium axial power offset to maintain the reactor at full power. The rod position should be restored in time after subsequent water injection. When the axial power offset falls and intersects with the equilibrium axial power offset, the control rods need to be raised. At this time, the control rods should be raised when the main steam header pressure is low or after boron injection to reduce the main steam header pressure in advance to ensure that the reactor operates at full power without reducing power. During this stage, the control rods should be controlled at 88.2-88.6%. When the reactor lifespan is 400-450 days, the axial power offset divergence becomes significant. Intervention is needed when the deviation between the peak and trough values ​​of the axial power offset calculated by the reactor monitoring / control and diagnostic system is 0.

8. At this time, control rods need to be inserted when the axial power offset rises and intersects with the equilibrium axial power offset to maintain the reactor at full power. The rod position should be restored promptly after subsequent water injection. When the axial power offset falls and intersects with the equilibrium axial power offset, the control rods need to be raised. At this time, the control rods should be raised when the main steam header pressure is low or after boron injection to reduce the main steam header pressure. Before raising the control rods, the nuclear power needs to be reduced to 99.5%, and the power should be restored after raising the control rods. During this stage, the control rods should be maintained at 88.2%. When the reactor life exceeds 450 days, the axial power offset diverges significantly, reducing the rod position to 85-86.1%.

2. The method of controlling axial power shift during power operation of a VVER reactor claimed in claim 1, characterized by the fact that: With a lifespan of 300 days, axial power offset and control rod position can be controlled by injecting boron and water.

3. The method of controlling axial power shift during power operation of a VVER reactor claimed in claim 1, characterized by the fact that: When the reactor life exceeds 450 days, the axial power offset diverges significantly, approaching refueling. The axial power offset is controlled by inserting control rods when the rising phase of the axial power offset intersects with the equilibrium axial power offset, ultimately reducing the rod position to 85-86.1%.

4. The method for controlling axial power offset during VVER reactor power operation according to claim 1, characterized in that: If the control rods must be upgraded when the reactor life exceeds 450 days, the nuclear power must be reduced to 99% before the upgrade can be carried out to prevent the nuclear power from exceeding the limit.

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