Passive startup method for reactor approach to criticality via control rod adjustment

Abstract

The present invention relates to the technical field of nuclear reactor design, and in particular to a passive startup method for reactor approach to criticality via control rod adjustment. After fuel loading of a reactor is completed, a safety rod bank is first withdrawn from a reactor core, the boron concentration of a coolant is diluted to a theoretical critical boron concentration, and then the reactor core is brought to a critical state by gradually withdrawing a control rod; when the reactor is in a detection blind zone, the height of a single withdrawal of the control rod is strictly controlled during the gradual withdrawal of the control rod, and sufficient waiting time is allowed after each withdrawal to ensure that the reactor core remains in a safe state; and when the reactor approaches criticality and moves out of the detection blind zone, the withdrawal operation is guided by changes in the count rate of a neutron detector. In the present invention, the rate and magnitude of positive reactivity insertion during the approach to criticality are effectively controlled by an approach-to-criticality method via control rod adjustment, thereby preventing a reactor from inadvertently entering a supercritical state due to misoperation.

Description

A passive start-up and criticality approach method for a rod-controlled critical reactor Technical Field

[0001] This invention belongs to the field of nuclear reactor design technology, specifically relating to a passive start-up and criticality approach method for a rod-controlled critical reactor. Background Technology

[0002] Reactor loading and physical startup is a process of gradually introducing reactivity into the reactor core. The reactive operation process must ensure the effective neutron multiplication factor (k) of the core. eff The growth process is kept within a safe range to prevent accidental reactivity introduction that could cause a sudden change in the effective neutron multiplication factor (EVF) or even supercriticality (EVF > 1). Under conditions with an external neutron source, the reactive operations (including fuel loading, boron dilution, and control rod lifting) during fuel loading and startup to criticality (EVF = 1) are primarily guided by changes in the count rate of the source range detectors in the nuclear measurement system. The role of the external neutron source is to increase the neutron flux level within the core, ensuring that the detectors have an effective count rate signal from the initial state (detector count rate > 0.5 cps), eliminating detection blind spots, and thus guaranteeing the safe conduct of reactive introduction operations during fuel loading and startup.

[0003] During the initial fuel loading and physical startup of a reactor, two primary neutron source assemblies and two secondary neutron source assemblies are typically installed to increase the neutron flux density within the reactor core, ensuring effective monitoring of the reactor by the nuclear monitoring system and thus guaranteeing reactor safety. However, in a "passive startup" without external neutron sources, relying solely on neutrons released spontaneously from the fuel and the nuclear reaction as the reactor's neutron source results in a lower source strength and a dispersed distribution within the core. This leads to an even lower neutron flux density at the detectors of the nuclear monitoring system, making it impossible to obtain an effective signal and creating a blind spot in reactor monitoring. Furthermore, primary and secondary neutron sources require considerable expense, and primary neutron sources are highly radioactive, necessitating stringent transportation and storage requirements, increasing human and material costs. Using primary neutron sources also generates additional radioactive waste, increasing the amount of radioactive solid waste. In addition, the neutron intensity of both the primary and secondary neutron sources decays over time. If the primary neutron source is purchased in advance or the reactor shutdown time is too long, resulting in the secondary neutron source having too low an intensity, it will not be able to be used in the reactor startup process, which is an additional risk to reactor startup.

[0004] To address the startup challenges of new reactors, a reliance on high-sensitivity external detectors has emerged, with detector sensitivity >30 cps / (n / cm). 2The passive start-up technology (CUP) involves adding high-sensitivity detectors outside the reactor core to enhance neutron detection capabilities. This allows for neutron measurements at lower flux densities, reducing or even eliminating detection blind zones. While these high-sensitivity detectors can obtain effective count signals before the reactor approaches criticality, they remain in detection blind zones at deeper subcritical states (effective neutron multiplication factor < 0.9). During the passive start-up of the reactor's first cycle, the external neutron detectors may not obtain effective count rates or may have low count rates, preventing the reactor from being effectively monitored by the external nuclear measurement system throughout the process. This is particularly true during the core-to-criticality phase (the process of gradually adjusting the reactor from a subcritical to a critical state). When the core subcriticality is shallow (0.9 < effective neutron multiplication factor < 0.99), accidental or excessively rapid introduction of reactivity could lead to instantaneous supercriticality. Therefore, during passive start-up, more cautious reactivity control techniques must be employed, strictly controlling the reactivity introduction rate to prevent excessively rapid introduction of reactivity that could cause supercriticality.

[0005] Currently, nuclear power reactors introduce reactivity through two methods: boron dilution or rod lifting. In the traditional criticality process, control rods are removed from the reactor core, leaving only a small number in the core. Boron dilution is then used to bring the core close to or directly to criticality. For passive start-ups, this traditional criticality-reaching method has two problems: First, while boron dilution increases the neutron flux density within the reactor after rod lifting, the boric acid concentration in the coolant between the core and external detectors remains high, resulting in strong neutron absorption and making it difficult for external detectors to obtain effective counting signals. Second, boron dilution has a hysteresis effect, making it difficult to accurately control the boron concentration in the core, potentially causing the core to unexpectedly pass the criticality point. Summary of the Invention

[0006] The purpose of this invention is to provide a passive start-up method for a reactor that reaches criticality by adjusting the rods. This method effectively controls the rate and amount of positive reactivity introduced during the criticality process, preventing accidental entry of the reactor into a supercritical state due to misoperation.

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

[0008] A passive start-up and criticality approach method for a reactor with controlled rods involves removing the safety rod assembly from the core after fuel loading, diluting the coolant boron concentration to the theoretical critical boron concentration, and then gradually raising the control rods to bring the core to a critical state. When the reactor is in the detection blind zone, the raising height of each control rod is strictly controlled during the gradual raising process, and sufficient time is waited after each raising to ensure that the core is in a safe state. When the reactor approaches criticality and escapes the detection blind zone, the rod raising operation is guided by the change in the count rate of the neutron detector.

[0009] When the reactor is in the detection dead zone, the height of a single control rod lift satisfies the corresponding reactivity. ρ(T) represents the core positive reactivity when the reactor period is T.

[0010] Waiting time after each lift The neutron flux level required for the detector to reach its detection limit and escape the blind zone. This represents the neutron flux level at which the detector is in the initial state of core startup.

[0011] The order of gradually raising the control rods is to first raise the control rod assemblies on the periphery of the reactor core.

[0012] Neutron flux level when the detector reaches the detection limit and escapes the blind zone Not higher than 1n / cm 2 / s.

[0013] The reactor cycle T is not less than 15 seconds.

[0014] The specific steps include: Heating and Rod Removal Stage: The reactor is loaded with fuel in a cold, zero-power state with all control rods inserted and a boron concentration of 1700 ppm. The temperature is first raised to a hot, zero-power state with all control rods inserted and a boron concentration of 1700 ppm. Then, all control rods except the AO control rod group are removed. This state is defined as hot, zero-power state with a boron concentration of 1700 ppm and all control rods except the AO control rod group removed. Rapid Dilution Stage: When the boric acid concentration in the reactor coolant system exceeds the theoretical critical boric acid concentration, the core is in a deep subcritical state. Water is injected into the core at a rapid dilution rate. After rapid dilution, the boric acid concentration in the reactor coolant system reaches the theoretical critical boric acid concentration. This state is defined as hot, zero-power state with a boron concentration of 1700 ppm. 1400ppm - All control rods except the AO control rod group are pulled out, the neutron detector has left the blind zone, and an effective count rate signal is obtained; Slow rod pulling towards criticality stage: When the boric acid concentration in the reactor coolant system reaches the theoretical critical boric acid concentration state, the reactivity introduction amount of each rod pulling is set to not exceed 50pcm, ensuring that the reactor cycle is more than 30s, the differential value of the AO rod group is less than 20pcm / step, and 2.5 steps are set to be pulled out within 60s, gradually pulling out the AO rod group towards criticality. After each rod pulling, wait 10 minutes to ensure that the neutron detector count rate is stable. After ensuring that the core is in subcritical, the critical rod position of the AO rod group is predicted by reversing the count rate. Thereafter, the control rods are gradually pulled out in the above manner to make the reactor reach the critical state.

[0015] The rapid dilution rate refers to the maximum dilution flow rate allowed by the flushing pump in the chemical volumetric system.

[0016] A passive start-up and criticality-approaching method for a Hualong reactor after rod-adjustment criticality includes the following steps: Heating and Rod Removal Stage: The reactor is loaded with fuel at a cold zero-power state, with all control rods inserted and boron concentration A. The temperature is first raised to a hot zero-power state, with all control rods inserted and boron concentration A, then all control rods except for the AO control rod group are removed; Rapid Dilution Stage: When the boric acid concentration in the reactor coolant system exceeds the theoretical critical boric acid concentration, the core is in a deep subcritical state. Clean water is injected into the core at a rapid dilution rate; after rapid dilution, the boric acid concentration in the reactor coolant system reaches... Reaching the theoretical critical boric acid concentration, this state is defined as hot zero power - boron concentration B - all control rods except the AO control rod group have been pulled out, the neutron detector has left the blind zone, and an effective count rate signal has been obtained; Slow rod pulling towards criticality stage: when the boric acid concentration in the reactor coolant system reaches the theoretical critical boric acid concentration state, the AO rod group is gradually pulled out towards criticality. After each rod pulling, a wait is made to ensure that the neutron detector count rate is stable. After ensuring that the core is in subcritical condition, the critical rod position of the AO rod group is predicted by using the reciprocal count rate. Thereafter, the control rods are gradually pulled out in the above manner to make the reactor reach the critical state.

[0017] The rapid dilution rate refers to the maximum dilution flow rate allowed by the flushing pump in the chemical volumetric system.

[0018] Once the boric acid concentration in the reactor coolant system reaches the theoretical critical boric acid concentration, the amount of reactivity introduced in each rod lifting operation is set to not exceed 50 pcm, ensuring that the reactor cycle is more than 30 seconds and the differential value of the AO rod group is less than 20 pcm / step. The operation is set to lift 2.5 steps within 60 seconds, gradually lifting the AO rod group towards criticality. After each rod lifting, a 10-minute wait is required to ensure the stability of the neutron detector count rate.

[0019] Boron concentration A is greater than boron concentration B.

[0020] The boron concentration A is 1700 ppm.

[0021] The boron concentration B is 1400 ppm.

[0022] The beneficial effects achieved by this invention are as follows:

[0023] This invention enables safe reactor startup without an external neutron source. Addressing the risk of instantaneous supercriticality in reactors with shallow subcriticality due to accidental or excessively rapid reactivity introduction, this invention utilizes a passive startup-to-criticality technique to strictly control the reactivity introduction rate and amount. This ensures that the detector reaches effective counting under acceptable core positive reactivity conditions and neutron flux growth rates, thus avoiding the risk of instantaneous supercriticality. Simultaneously, it saves significant neutron source procurement costs, eliminates potential risks associated with using external neutron sources, and reduces the generation of radioactive solid waste. Attached Figure Description

[0024] Figure 1 is a schematic diagram showing the change in core reactivity of the Hualong reactor during the first cycle with the doubling cycle. Detailed Implementation

[0025] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0026] A passive start-up and criticality-approaching method for a reactor with controlled control rods: After reactor fuel loading, the safety rods are first removed from the core. Then, the coolant boron concentration is diluted to the theoretical critical boron concentration. The core is then brought to criticality by gradually raising the control rods. When the reactor is in the detection blind zone, the raising height of each control rod must be strictly controlled during the gradual raising process, and sufficient time must be allowed after each raising to ensure the core is in a safe state. When the reactor approaches criticality and escapes the detection blind zone, the rod raising operation is guided by changes in the neutron detector count rate.

[0027] Reactivity corresponding to a single rod lifting height within the detection blind zone Where ρ(T) represents the core positive reactivity when the reactor period is T (in seconds).

[0028] Waiting time after each lift in The neutron flux level required for the detector to reach its detection limit and escape the blind zone. This represents the neutron flux level of the detector at the initial stage of core startup.

[0029] The high-sensitivity detectors added outside the reactor during the first cycle can overcome the blind zone and obtain effective counts while the reactor core still has a certain degree of subcriticality. Therefore, when the subcriticality is deep and the detectors are in the blind zone, we rely on theoretical analysis and do not rely on the detector count rate to guide the rod lifting operation; only after the high-sensitivity detectors overcome the blind zone near criticality will we rely on the change in the high-sensitivity detector count rate to guide the rod lifting process.

[0030] In this embodiment, the Hualong downstream reactor is used as the object of description to introduce this method. This invention utilizes only the fuel assembly... 235 U and 238 The reactor is started using neutrons generated by spontaneous nuclear fission. The specific implementation method is as follows:

[0031] In the first cycle of a reactor without an external neutron source, when using the rod-lifting approach to reach criticality, in order to effectively control the rate and amount of positive reactivity introduced during the rod-lifting process and prevent accidental entry of the reactor into a supercritical state due to misoperation, the reactivity operation steps for reaching criticality after core loading are as follows:

[0032] During the heating and rod removal phase, the reactor is loaded in the "CZP-ARI-1700" state (cold state, zero power, control rods fully inserted, boron concentration 1700ppm). It is first heated to the "HZP-ARI-1700" state (hot state, zero power, control rods fully inserted, boron concentration 1700ppm), and then all control rods except the AO control rod group are removed. This state is defined as HZP-1700-AO (hot state, zero power, boron concentration 1700ppm, all control rods except the AO control rod group are removed).

[0033] During the rapid dilution phase, when the boric acid concentration in the reactor coolant system exceeds the theoretical critical boric acid concentration, the reactor core is in a deep subcritical state. Clean water can be injected into the core at a rapid dilution rate (the maximum dilution flow rate allowed by the chemical and volumetric system's flushing pump). After rapid dilution, the boric acid concentration in the reactor coolant system reaches the theoretical critical boric acid concentration. This state is defined as HZP-1400-AO (hot zero power - boron concentration 1400ppm - all control rods except the AO control rod group are removed). The high-sensitivity detector has left the blind zone and can obtain an effective count rate signal.

[0034] During the slow rod lifting towards criticality stage, once the boric acid concentration in the reactor coolant system reaches the theoretical critical boric acid concentration, i.e., the HZP-1400-AO state is reached, the reactivity introduction amount for each rod lifting is set to not exceed 50 pcm, ensuring the reactor cycle is above 30 seconds (see Figure 1). The differential value of the AO rod group is approximately less than 20 pcm / step. A setting is made to lift 2.5 steps within 60 seconds, gradually lifting the AO rod group towards criticality. After each rod lifting, a 10-minute wait is allowed to ensure the high-sensitivity detector count rate remains stable. After ensuring the core is in a subcritical state, the inverse count rate is used to deduce the predicted critical rod position (A). c Subsequently, the control rods were gradually removed in the manner described above to bring the reactor to criticality.

[0035] A passive startup and criticality-reaching method for a reactor is disclosed. This method adjusts the reactor control rod assembly to control the rate and amount of positive reactivity introduced during the criticality process, thereby bringing the reactor to criticality. No external neutron source is required during startup. There are no external primary or secondary neutron sources; the startup relies solely on neutrons spontaneously released from the fuel. A high-sensitivity detector is used to monitor the neutron flux rate during startup. After fuel loading, the safety rods are first removed from the core. Then, the coolant boron concentration is diluted to the theoretical critical boron concentration. The core then reaches criticality by gradually raising the control rods. When the reactor is in the detection blind zone, the height of each control rod lift must be strictly controlled, and sufficient time must be allowed after each lift to ensure the core is in a safe state. When the reactor approaches criticality and escapes the detection blind zone, the rod lifting operation is guided by changes in the neutron detector count rate. Within the detection blind zone, the reactivity corresponding to a single rod lift height is... Where ρ(T) represents the core positive reactivity at a reactor cycle of T (in seconds). The waiting time after each rod lifting is also considered. in The neutron flux level required for the detector to reach its detection limit and escape the blind zone. This represents the neutron flux level of the detector during the initial startup state of the reactor core. The reactor period used is T (in seconds) of not less than 15 seconds. The neutron flux level at which the detector reaches its detection limit and escapes the blind zone is also considered. Not higher than 1n / cm 2 / s. The sequence for gradually raising the control rods is to first raise the control rod assemblies on the periphery of the reactor core.

[0036] This invention proposes a passive start-up and criticality-approaching method for a reactor with controlled-rod criticality. This method controls the reactivity introduction rate and amount, enabling the detector to reach effective counts under acceptable core positive reactivity conditions and neutron flux growth rates. This guides subsequent reactivity operations, ensuring the safe implementation of reactivity introduction operations during fuel loading and start-up. Compared to external neutron source start-up methods, this method eliminates the risks associated with neutron source use, reduces the generation of solid radioactive waste, and significantly lowers economic costs.

Claims

1. A method of passively starting up a reactor to criticality with a control rod bank, characterized by: After the reactor is loaded, the safety rod group is first withdrawn from the core, then the boron concentration of the coolant is diluted to the theoretical critical boron concentration, and then the core is brought to the critical state by gradually lifting the control rods; when the reactor is in the detection blind area, the lifting height of the single control rod is strictly controlled during the gradual lifting of the control rods, and sufficient time is waited after each lifting to determine that the core is in a safe state, and when the reactor approaches the critical state and escapes the detection blind area, the lifting operation is guided by the change of the counting rate of the neutron detector.

2. The reactor passively start-up-to-critical method of rod-criticality as defined in claim 1 wherein: When the reactor is in the detection blind area, the single control rod lifting height meets the corresponding reactivity ρ(T) is the corresponding core positive reactivity when the reactor period is T.

3. The reactor passively start-up-to-critical method of rod-criticality of claim 2, wherein: Wait time after each rod withdrawal The neutron flux level at which the detector escapes the blind area at the lower detection limit, The neutron flux level at which the detector is at the initial state of the core start-up.

4. The reactor passively starts up to criticality method of rod worth criticality as defined in claim 1, wherein: The sequence of gradually lifting the control rods is to first lift the control rod assemblies at the periphery of the core.

5. The reactor passively starts up to criticality method of rod worth criticality as defined in claim 3 wherein: Neutron flux level at which the detector reaches the lower detection limit out of the blind area Not higher than 1 n / cm 2 / s.

6. The reactor passively starts up to criticality method of rod worth criticality as defined in claim 2 wherein: The reactor period T is not less than 15s.

7. The reactor passively starts up to criticality method of rod worth criticality as defined in claim 1 wherein: The specific steps include: warming up and lifting the rods: the reactor is loaded in the cold zero power-control rod full insertion-boron concentration 1700ppm state, first warmed up to the hot zero power-control rod full insertion-boron concentration 1700ppm state, then all the control rod groups except the AO control rod group are withdrawn, and the state is defined as hot zero power-boron concentration 1700ppm-all the control rod groups except the AO control rod group are withdrawn; rapid dilution stage: when the boron acid concentration in the reactor coolant system is greater than the theoretical critical boron acid concentration, the core is in a deep subcritical state, and clean water is injected into the core at a rapid dilution rate; after rapid dilution, the boron acid concentration in the reactor coolant system reaches the theoretical critical boron acid concentration, and the state is defined as hot zero power-boron concentration 1400ppm-all the control rod groups except the AO control rod group are withdrawn, the neutron detector has exited the blind area and obtained an effective counting rate signal; slow rod lifting and approaching critical stage: after the boron acid concentration in the reactor coolant system reaches the theoretical critical boron acid concentration state, the AO rod group is gradually withdrawn to approach critical by setting the reaction introduced by each rod lifting not more than 50pcm, ensuring that the reactor period is above 30s, the AO rod group differential value is less than 20pcm / step, setting 2.5 steps within 60s, and gradually withdrawing the AO rod group, waiting for 10 minutes after each rod lifting to ensure that the neutron detector counting rate is stable, and ensuring that the core is in a subcritical state, the AO rod group is predicted to be critical by inverse counting rate, and then the control rods are gradually withdrawn in the above-mentioned manner to make the reactor reach the critical state.

8. The reactor passively starts up to criticality method of rod worth criticality as defined in claim 7, wherein: The rapid dilution rate refers to the maximum dilution flow allowed by the surge pump of the containment system.

9. A method for passively starting a Hualong follow-up reactor to approach criticality by adjusting a rod criticality, characterized in that: The specific steps include: warming up, rod extraction phase: the reactor is loaded in a cold zero power state with all control rods inserted and boron concentration A, then warmed up to a hot zero power state with all control rods inserted and boron concentration A, and then all control rods except the AO control rod group are extracted; rapid dilution phase: when the boron acid concentration in the reactor coolant system is greater than the theoretical critical boron acid concentration, the reactor core is in a deep subcritical state, and clean water is injected into the reactor core at a rapid dilution rate; after rapid dilution is completed, the boron acid concentration in the reactor coolant system reaches the theoretical critical boron acid concentration, and this state is defined as a hot zero power-boron concentration B-all the remaining control rods except the AO control rod group are extracted, the neutron probe has exited the blind area, and an effective count rate signal is obtained; slow rod extraction approach critical phase: after the boron acid concentration in the reactor coolant system reaches the theoretical critical boron acid concentration, the AO rod group is gradually extracted to approach criticality, and after each rod extraction, waiting is performed to ensure that the neutron probe count rate is stable, and after the reactor core is in a subcritical state, the AO rod group is predicted to be critical by using the inverse count rate, and then the control rods are gradually extracted in the above manner to make the reactor reach a critical state.

10. The method of claim 9, wherein the method is a passive start-to-criticality method for a HWR follow-on reactor with a control rod bank criticality. The rapid dilution rate refers to the maximum dilution flow allowed by the surge pump of the containment system.

11. The method of claim 9, wherein the method is a passive start-to-criticality method for a HWR follow-on reactor with a control rod bank criticality. After the boron acid concentration in the reactor coolant system reaches the theoretical critical boron acid concentration, by setting the reaction reactivity introduced by each rod extraction to be no more than 50pcm, the reactor period is ensured to be above 30s, the AO rod group differential value is less than 20pcm / step, 2.5 steps are extracted within 60s, the AO rod group is gradually extracted to approach criticality, and after each rod extraction, 10 minutes of waiting is performed to ensure that the neutron probe count rate is stable.

12. The method of claim 9, wherein the method is a passive start-to-criticality method for a HWR follow-on reactor with a control rod bank criticality. The boron concentration A is greater than the boron concentration B.

13. The method of claim 12, wherein the method is a passive start-to-criticality method for a HWR follow-on reactor with a control rod bank criticality. The boron concentration A is 1700ppm.

14. The method of claim 12, wherein the method is a passive start-to-criticality method for a HWR follow-on reactor with a control rod bank criticality. The boron concentration B is 1400ppm.

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