Reactivity control system and method for boron-free reactor core

Abstract

A reactivity control system for a boron-free reactor core, according to an embodiment of the present invention, comprises: a control rod drive device for driving a nuclear reactor by lifting a control rod assembly and performing a step drive while controlling the output of the nuclear reactor through insertion and withdrawal of control rods; and a microstep drive device for driving the insertion and withdrawal of the control rods in microsteps that are obtained by dividing the step drive by an integer multiple, in order to control the overall reactivity of the boron-free reactor core, wherein the boron-free reactor core comprises: an individual central control rod that is disposed at the center and is individually controlled; and a negative moderator temperature coefficient (MTC) control device using an MTC for reaching the final reactivity of the boron-free core. Therefore, excess reactivity in the core of the nuclear reactor can be stably controlled and a safe reactivity control method for a boron-free reactor core is provided, thereby enabling safe operation of the boron-free reactor core.

Description

Boron-free core criticality approach system and method

[0001] The present invention relates to a boron-free core criticality approach system and method, and more specifically, to a boron-free core criticality approach system and method that controls control rods in a microstep operation mode for criticality approach of a boron-free core, uses individual control rods in the center of the core, and utilizes a final criticality reaching MTC.

[0002] In a typical nuclear reactor control system, "criticality" refers to a state in which a nuclear fission chain reaction is maintained at a constant rate.

[0003] When a single uranium-235 nucleus splits, an average of 2.5 new fast neutrons are produced, and if there is no separate control device for the countless new neutrons, they immediately participate in a chain reaction.

[0004] Therefore, systems to thoroughly manage the number of neutrons generated in the reactor must be included to maintain a critical state during normal operation.

[0005] The US-NRC (Nuclear Regulatory Commission) is an agency that sets safety standards and regulates industries related to nuclear power.

[0006] Looking at the US-NRC design standards, two independently different types of reactivity control systems are required. Generally, in the case of pressurized water reactors, control rods are used as the primary reactivity control system, and a method of injecting boric acid solution into the moderator is used as the secondary reactivity control system.

[0007] However, the method of using boric acid solution as a secondary reactivity control system has the following problems.

[0008] When boric acid solution is used, there is a possibility of corrosion of the reactor coolant system due to boric acid and pressure boundary corrosion and failure due to boric acid leakage, and there is a problem that a significantly large additional space is required due to the need for complex boric acid injection and recovery facilities for the Chemical and Volume Control System (CVCS).

[0009] Therefore, the need for boric acid-free operation is emerging, and in order to apply boric acid-free operation, a new second-order reactivity control system to replace boric acid is required.

[0010] In addition, referring to Fig. 1, the conventional secondary reactivity control system is designed to operate the control system through manual manipulation during both normal operation and in the event of an accident. However, this method has a problem in that if the control system cannot be operated due to an error in judgment by the operator in a situation where power is lost due to an accident and the reactor must be shut down quickly, it can lead to a serious accident.

[0011] Conventional nuclear power plants use water-soluble boron, combustible poison rods, and control rods to control excess reactivity within the reactor core. In particular, the means available to plant operators to maintain critical or subcritical states are limited to controlling the concentration of water-soluble boric acid and inserting / withdrawing control rods. Initial criticality is stipulated to be approached slowly through boron dilution.

[0012] However, since nuclear power plants with boron-free cores cannot use conventional boron, other methods must be devised.

[0013] The present invention provides a boron-free core criticality access system and method that can control excess reactivity within the reactor core of a nuclear power plant with a boron-free core that cannot use conventional boron, and provides a safe criticality method for the boron-free core to safely operate the boron-free core.

[0014] A boron-free core criticality approach system according to one embodiment of the present invention includes a control rod drive device that starts the reactor by lifting a control rod assembly and controls the reactor output by inserting and withdrawing the control rods, while performing step operation, and a microstep drive device that controls the insertion and withdrawal of the control rods in microsteps divided by integer multiples of the step operation to control the overall criticality of the boron-free core, wherein the boron-free core includes a central individual control rod that is placed at the center and individually controlled, and includes a negative MTC control device that utilizes the final criticality-reaching moderator temperature coefficient (MTC) of the boron-free core, thereby enabling stable control of excess reactivity within the reactor core and providing a safe criticality method for the boron-free core, thereby enabling safe operation of the boron-free core.

[0015] According to the boron-free core criticality approach system and method of one embodiment of the present invention, the excess reactivity within the reactor core of a nuclear power plant with a boron-free core that cannot use conventional boron can be stably controlled, and a safe criticality method for the boron-free core can be provided, thereby enabling the safe operation of the boron-free core.

[0016] FIG. 1 is a configuration diagram of a boron-free core critical access system according to one embodiment of the present invention,

[0017] FIG. 2 is a diagram illustrating the microstep operation mode of the control rod of FIG. 1.

[0018] FIG. 3 is a diagram illustrating the use of individual control rods in the center of the core of FIG. 1. and

[0019] FIG. 4 is a flowchart illustrating a boron-free core critical approach method according to one embodiment of the present invention.

[0020] Embodiments of the present invention are described below in detail with reference to the attached drawings so that those skilled in the art can easily implement them. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein. Furthermore, in order to clearly explain the present invention in the drawings, parts unrelated to the explanation have been omitted, and similar parts throughout the specification are denoted by similar reference numerals.

[0021] Throughout the specification of the present invention, when a part is described as being "connected" to another part, this includes not only cases where they are "directly connected," but also cases where they are "electrically connected" with other elements interposed between them.

[0022] Throughout the specification of the present invention, when it is stated that a member is located "on" another member, this includes not only cases where a member is in contact with another member, but also cases where another member exists between the two members.

[0023] Throughout the specification of the present invention, when a part is described as "comprising" a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but may include additional components. Throughout the specification of the present invention, terms such as "approximately" and "substantially" are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the stated meaning, and are used to prevent unscrupulous infringers from unfairly exploiting disclosures in which precise or absolute values ​​are mentioned to aid in understanding the present invention.

[0024] The terms “step” or “step of” used throughout the specification of the present invention do not mean “step for”.

[0025] Throughout the specification of the present invention, the term “combination thereof” included in the Markush-style expression means one or more mixtures or combinations selected from the group consisting of the components described in the Markush-style expression, and means including one or more selected from the group consisting of said components.

[0026] Now, with reference to FIGS. 1 to 4, a boron-free core critical access system and method according to one embodiment of the present invention will be described.

[0027] FIG. 1 is a configuration diagram of a boron-free core critical approach system according to an embodiment of the present invention. FIG. 2 is a diagram explaining the microstep operation mode of the control rods of FIG. 1. FIG. 3 is a diagram explaining the use of individual control rods in the center of the core of FIG. 1. FIG. 4 is a flowchart explaining a boron-free core critical approach method according to an embodiment of the present invention.

[0028] First, as illustrated in FIG. 1, the boron-free core critical approach system (100) according to one embodiment of the present invention primarily uses nuclear fission reaction heat to control the boron-free core (110) and to heat the coolant during reactor startup. Therefore, during initial reactor startup, the control rod drive device (150) lifts the control rod assembly (130) of the control rod (120) inserted into the core (110) to start the reactor, and the reactor output is also controlled by the insertion and withdrawal of the control rod (120).

[0029] To this end, the control rod control device (180) communicates with the control rod position sensor (140) to detect the actual position of the control rod (120) and drives the control rod drive device (150) to move the control rod (120) submerged in the primary coolant at high temperature and high pressure.

[0030] The above control rod drive device (150) may further include a microstep drive device (160), wherein the control rod drive device (150) or the microstep drive device (160) may be a rotary stepper motor, and the control rod position sensor (140) may determine the minimum travel distance of the control rod (120) when the rotation axis rotates by an angle corresponding to one step, and the rotary stepper motor of the control rod drive device (150) or the microstep drive device (160) may be configured to include a bevet gear and determine the minimum travel distance according to the bevet gear ratio and the pitch of the ball screw.

[0031] The above control rod drive device (150) operates the control rod (120) in a step operation manner. Domestic standard nuclear power plants operate with 1,905 cm per step and a total of 200 steps, while Westinghouse nuclear power plants operate with 1,625 cm per step and a total of 225 steps. Since existing nuclear power plants using boric acid control the overall reactivity with boric acid, the change in core reactivity caused by the step operation of the control rod is not significant.

[0032] However, the boron-free core (110) controls excess reactivity locally with combustible poison rods and control rods (120) within the core, so the step operation of the local control rods has a large reactivity, and as a result of actual calculations, the control rod (120) control capability of the boron-free core (110) is much greater than that of the boron core.

[0033] Accordingly, the control rod drive device (150) drives the control rod (120) in a general step operation mode of 1.905 cm per step, and separately, the microstep drive device (160) enables the control rod (120) to be driven in microsteps in a microstep operation mode for critical approach, i.e., 0.381 cm per step, for a total of 1,000 steps.

[0034] In this way, by using the microstep drive device (160) together with the control rod drive device (150), the boron-free core (110) can perform a safe critical approach.

[0035] As shown in FIG. 2, the microstep drive device (160) can be used in the same way as the general control rod driving mode of the control rod drive device (150) by setting it to 1,000 steps for 200 steps and 900 steps for 225 steps, thereby driving the control rod (120) in microsteps by 0.381 cm per step or 0.406 cm per step.

[0036] Meanwhile, as illustrated in FIG. 3, a boron-free core critical approach system (100) according to one embodiment of the present invention is characterized by using a central individual control rod (170) that is individually controlled in the center of a boron-free core (110).

[0037] Generally, since the output at the center of the core is high, a single control rod can be used to regulate the criticality of the entire core, but this is a method not used in existing nuclear power plants.

[0038]

[0039] In a boron-free core criticality approach system (100) according to one embodiment of the present invention, the overall criticality and output distribution of the boron-free core (110) can be controlled using a central individual control rod (170) installed in the center of the core and individually controlled.

[0040] For the above central individual control rod (170), a general control rod operation mode or a microstep operation mode for the center of the boron-free core (110) can be driven in microstep mode using the control rod drive device (150) or the microstep drive device (160).

[0041] A boron-free core critical approach system (100) according to one embodiment of the present invention further includes a negative MTC control device (190) to utilize the final critical approach MTC (Moderator Temperature Coefficient) of the boron-free core (110).

[0042] As illustrated in FIG. 4, a boron-free core criticality approach system (100) according to one embodiment of the present invention operates the microstep driving device (160) to perform a microstep driving mode of the local control rod (120) (S110), microstep drives the boron-free core central individual control rod (170) using the control rod driving device (150) or the microstep driving device (160) to control the criticality of the entire boron-free core (110) (S120), determines whether the criticality within the boron-free core (110) of the reactor is approximately 10 to 30 pcm subcritical through the subcritical determination unit (191) of the negative MTC control device (190) (S130), and determines the subcritical of the negative MTC control device (190) so that the criticality within the boron-free core (110) of the reactor becomes approximately 10 to 30 pcm subcritical. The judgment unit (191) controls the local control rod in a microstep driving mode (S131) ​​in conjunction with the control rod driving device (150) or the microstep driving device (160), and operates the central individual control rod (170) (S133) to adjust the criticality in the boron-free core (110) of the reactor to a subcritical level of about 10 to 30 pcm.

[0043] When the subcriticality determination unit (191) of the above negative MTC control device (190) determines that the criticality within the boron-free core (110) of the reactor has been adjusted to a subcritical level of about 10 to 30 pcm (S135), the temperature of the cooling water within the boron-free core (110) is adjusted using the cooling water temperature control unit (193) of the above negative MTC control device (190) to use the negative MTC to adjust the overall criticality within the boron-free core (110) of the reactor and reach an initial criticality (S140).

[0044] Since the boron-free core (110) has a larger negative MTC than a conventional boron core, the cooling water temperature control unit (193) of the negative MTC control device (190) can inject sufficient reactivity into the reactor even if it only slightly adjusts the cooling water temperature.

[0045] By using a negative MTC control device (190) to reach the final criticality of the reactor's boron-free core (110), it is possible to approach criticality while minimizing the gradient of the output distribution by injecting positive reactivity into the entire core rather than a local criticality approach.

[0046] As shown again in FIG. 1, the cooling water temperature control unit (193) of the negative MTC control device (190) can adopt methods generally used in commercial nuclear power plants, such as controlling the main steam bypass valve and controlling the main feedwater flow rate.

[0047] According to the boron-free core criticality approach system and method of one embodiment of the present invention, the excess reactivity within the reactor core of a nuclear power plant with a boron-free core that cannot use conventional boron can be stably controlled, and a safe criticality method for the boron-free core can be provided, thereby enabling the safe operation of the boron-free core.

[0048] According to the boron-free core criticality approach system and method of one embodiment of the present invention, the excess reactivity within the reactor core of a nuclear power plant with a boron-free core that cannot use conventional boron can be stably controlled, and a safe criticality method for the boron-free core can be provided, thereby enabling the safe operation of the boron-free core.

Claims

1. A control rod drive device that starts the reactor by lifting the control rod assembly and controls the reactor output by inserting and withdrawing the control rods, while performing step operation; and To control the overall criticality of the boron-free core, the microstep drive unit operates the insertion and withdrawal of the control rods in microsteps divided by integer multiples of the step operation. The above boron-free core critical approach system includes a centrally located and individually controlled central individual control rod.

2. In Paragraph 1, A boron-free core critical approach system comprising a negative MTC control device utilizing the final criticality-reaching moderator temperature coefficient (MTC) of the boron-free core.

3. In Paragraph 2, The above negative MTC control device is a boron-free core criticality approach system comprising a subcriticality determination unit that determines whether the criticality in the boron-free core is subcritical with a criticality of about 10 to 30 pcm, and a cooling water temperature control unit that controls the temperature of the cooling water in the boron-free core to make it negative MTC.

4. A step of controlling the insertion and withdrawal of the control rod assembly of the boron-free core by operating a microstep drive to a microstep operation mode that divides the normal step mode into integer multiples; and A method for critical approach to a boron-free core, comprising the step of controlling the criticality of the entire boron-free core by controlling the insertion and withdrawal of a central individual control rod disposed in the center of the boron-free core in a microstep operation mode.

5. In Paragraph 4, A boron-free core critical approach method in which the microstep drive device is used in the same way as the general control rod operation mode, by setting the general control rod operation mode to 1,000 steps when the general control rod operation mode is 200 steps and to 900 steps when the general control rod operation mode is 225 steps, thereby driving the control rod in microsteps by 0.381 cm per step or 0.406 cm per step.

6. In Paragraph 4, A step of determining whether the criticality in the above boron-free core is approximately 10 to 30 pcm subcritical, and A method for approaching criticality of a boron-free core, comprising the step of reaching an initial criticality by adjusting the temperature of the cooling water in the boron-free core using a negative MTC when the criticality in the boron-free core is determined to be approximately 10 to 30 pcm subcritical.

7. In Paragraph 6, A method for approaching criticality in a boron-free core, comprising the step of adjusting the criticality in the boron-free core to a subcritical level of approximately 10 to 30 pcm by using the steps of inserting and withdrawing the control rod assembly in a microstep operation mode and inserting and withdrawing the central individual control rod when it is determined that the criticality in the boron-free core is not subcritical of approximately 10 to 30 pcm.

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