WO2012157836A1

WO2012157836A1 - Method of controlling power and axial power distribution of nuclear reactor

Info

Publication number: WO2012157836A1
Application number: PCT/KR2012/000915
Authority: WO
Inventors: Suk Whun Sohn; In Ho Song; Jong Joo Sohn; Eun Kee Kim
Original assignee: Kepco Engineering & Construction Company
Priority date: 2011-05-17
Filing date: 2012-02-08
Publication date: 2012-11-22
Also published as: KR20120128434A; KR101224605B1

Abstract

A method of controlling power and axial power distribution of a nuclear reactor, the method including controlling power of a nuclear reactor by adjusting a concentration of a boric acid solution by using a proportional integral controller, according to a difference between a coolant reference temperature and a coolant average temperature; and controlling axial power distribution by changing insertion and withdrawal of a full strength control element assembly and a part strength control element assembly, in order to optimize the axial power distribution.

Description

METHOD OF CONTROLLING POWER AND AXIAL POWER DISTRIBUTION OF NUCLEAR REACTOR

The present invention relates to a method of controlling power and axial power distribution of a nuclear reactor of a nuclear power plant, and more particularly, to a controlling method of automatically performing a load follow operation of a nuclear reactor without any intervention of an operator.

A load follow operation refers to an operation for following the load in a nuclear reactor, in which power is reduced from 100% to specific power for several hours, is maintained constant for several hours, is restored from the specific power to 100% for several hours, and is maintained constant again for several hours. In the load follow operation, a time taken to achieve power ramping and final ramping power may be freely determined. When power is reduced, as a concentration of xenon (Xe) of which thermal neutron absorption cross section causing a nuclear reaction is several million times greater than other materials is increased with a time lag of three to four hours, negative reactivity is added to Xe. In this case, if the negative reactivity is not appropriately offset by adding positive reactivity to Xe by diluting a concentration of boron or withdrawing a control rod, a coolant average temperature and power of nuclear reactor are remarkably reduced, as shown in front circles of FIGS. 1A and 1B. When such a reduction is very serious, the unclear reactor may stop due to a low pressure of a steam generator. FIGS. 1A and 1B show a change in a coolant average temperature and a change in power of nuclear reactor, respectively, in a load follow operation that is performed during a test. On the other hand, when power is increased, as a neutron flux is increased, a concentration of Xe is gradually reduced and positive reactivity is added to a reactor core. In this case, if the positive reactivity is not appropriately offset by adding negative reactivity to Xe by increasing a concentration of boron or inserting a control rod into the nuclear reactor, a coolant average temperature and power of the nuclear reactor are remarkably increased, like in the case where the power reaches 100 % as shown in later circles of FIGS. 1A and 1B. When such an increase is very serious, the nuclear reactor may stop due to high power. Thus, an operator performs rapid reactivity compensation by inserting a control rod into the nuclear reactor so as to prevent power from increasing above the licensing power.

However, when positive or negative reactivity is added by using a control rod, axial nuclear reactor power distribution is also distorted, and thus, the axial nuclear reactor power distribution needs to be simultaneously controlled with power control for a significantly long period of time. That is, since nuclear reactor power is maintained constant, a reduction in power of an upper portion of the nuclear reactor, into which the control rod is inserted, raises power of a lower portion of the nuclear reactor, into which the control rod is not inserted, and thus, the axial power distribution has a gourd-like shape. When the distortion of axial power distribution exceeds a predetermined level, since nuclear fuel may be locally damaged in a portion of the unclear reactor of which power is excessively output, a nuclear power plant has a set safety limit for restricting axial power distribution to a predetermined range. Conventionally, since the use of the control rod has such problems, control rods of a full strength control element assembly and control rods of a part strength control element assembly are used only if the axial distribution is inclined towards an upper portion of a unclear reactor or a rapid operation is required when a concentration of a boric acid solution is adjusted, in order to change power while axial power distribution is maintained.

However, it is difficult to properly control power of a nuclear reactor since a method of controlling a concentration of boric acid has the following difficulties. First, it is difficult to obtain a correct concentration of boric acid of a reactor core due to high uncertainty of a boronometer for notifying an operator about a concentration of boric acid. Second, when an operation of adding a boric acid solution with a high concentration (4400 ppm) to increase a concentration of boric acid is referred to as Boration, and an operation of adding pure water to reduce a concentration of a boric acid solution is referred to as Dilution, it takes a long time lag to perform Boration/Dilution by a make-up water system of a chemical and volume control system to change a boric acid solution of a reactor core to a desired concentration. Third, since an amount of make-up water required for obtaining a desired concentration of boric acid is calculated by using a Boration/Dilution table generated based on a simple single-cell model, and a large amount of make-up water needs to be used in Dilution/Boration with a constant rate at one time, it is heavily impossible to finely control a concentration of boric acid of a reactor core.

In advanced countries nuclear have developed several technologies for the load follow operation in order to overcome these difficulties. Constant axial offset control (CAOC) refers to a nuclear reactor operating method developed by US Westinghouse, wherein a power change by using a control rod and a change in a concentration of boron while controlling distribution of Xe by maintaining an axial offset (AO) value, which indicates the axial power distribution during a power change of the nuclear reactor, are permitted in a predetermined range. The AO value refers to an axial power distribution deviation with respect to a power change in a nuclear reactor.

Power distribution is adjusted by using an AO value in order to previously prevent a change in axial power distribution of Xe causing an excessive local power change. A Mode-A operating method is a kind of a CAOC operating method for maintaining AO within a predetermined band in order to prevent unstability of a nuclear reactor due to Xe. In this case, in the presence of an excessive power change, a controlling power of a nuclear reactor is delayed. In addition, it is difficult to reduce the dilution capability of boron at the end of life of the reactor core, and thus, it is possible to perform a load follow operation up to 80% to 85% only of the lifetime of a reactor core. In order to improve the operational flexibility, Westinghouse has developed a relaxed axial offset (RAOC) control, mechanical shim (MSHIM), and the like to reinforce the load follow capability of a nuclear reactor. The MSHIM rod is similar to a gray rod from France and includes a light shim and a heavy shim with relatively large reactivity. Since the light shim can be deeply inserted into a reactor core in order to perform power distribution compensation, the light shim is designed to perform a load follow operation during the entire life of the reactor core, but the light shim has not been used in reality.

France has used the Mode-A operating method developed by Westinghouse as a load follow operation. After 1975, France has developed and used a Mode-G operating method to perform frequency-control as well as load-follow. In the Mode-G operating method, a portion of a control rod is formed as a gray rod by using steel with a small absorption cross section, and the gray rod is used to control power distribution so that radial and axial power distributions are reduced by inserting a control rod in the nuclear reactor. Thus, the Mode-G operating method has higher operational tolerance than the Mode-A operating method. Germany has chosen an overlapping control bank (OCB) mode operating method that was developed by KWU in 1973. The OCB mode is configured to perform a load follow operation by using a SPNR method up to 100% of the life of the reactor core and to perform the load follow operation without any change in a concentration of boric acid by using a D-bank with low reactivity and an L-bank with high reactivity. Boration and Dilution are used only to compensate for slow reactivity change by using Xe. Korea has actively conducted research into a load follow operation using Mode-K as the next generation technologies for nuclear reactors, but has not reflected the results into systematic design.

The present invention provides a method of controlling power and axial power distribution of a nuclear reactor, by which the power of the nuclear reactor is changed according to load by automatically controlling a concentration of boric acid solution, and the axial power distribution is controlled by changing insertion and withdrawal of a full strength control element assembly and a part strength control element assembly.

According to an aspect of the present invention, there is provided a method of controlling power and axial power distribution of a nuclear reactor, the method including controlling power of a nuclear reactor by adjusting a concentration of a boric acid solution by using a proportional integral controller, according to a difference between a coolant reference temperature and a coolant average temperature; and controlling axial power distribution by changing insertion and withdrawal of a full strength control element assembly and a part strength control element assembly in order to optimize the axial power distribution.

The controlling of the power of the nuclear reactor may include reducing overshoot and undershoot of the coolant average temperature which occur at a starting point and an ending point of a change in turbine power by using a value obtained by filtering an error between a calculated axial shape index (ASI) and a measured ASI through a high pass filter.

The controlling of the axial power distribution may include setting initial positions of the full strength control element assembly and the part strength control element assembly; calculating a difference of axial shape index (DASI) corresponding to a target ASI and a calculated ASI as a position of the part strength control element assembly is change; determining whether the part strength control element assembly reaches a positional limit; when the part strength control element assembly reaches the positional limit, detecting a position of the part strength control element assembly, at which a sign of the DASI is reversed; determining whether the full strength control element assembly reaches a positional limit; when the full strength control element assembly reaches the positional limit, determining whether the difference between the coolant reference temperature and the coolant average temperature is equal to a predetermined temperature or less, or whether a burnup of the nuclear reactor is equal to a predetermined burnup or less; when the difference is equal to the predetermined temperature or less, or when the burnup is equal to the predetermined burnup or more, determining whether there are positional combinations of the full strength control element assembly and the part strength control element assembly which satisfy DASI=0; when there are positional combinations of the full strength control element assembly and the part strength control element assembly which satisfy DASI=0, detecting a position combination by which moving distances of the full strength control element assembly and the part strength control element assembly are minimized is detected from among the position combinations; when there is no position combination of the full strength control element assembly and the part strength control element assembly which satisfies DASI=0, detecting positions of the full strength control element assembly and the part strength control element assembly by which DASI is minimized; and controlling the axial power distribution according to results of the detecting, wherein, when the difference is equal to the predetermined temperature or more, and the burnup is equal to the predetermined burnup or less, a logic for controlling positions of the full and part strength control element assemblies is terminated.

The controlling of the axial power distribution may include extracting as a bias an ASI calculation error between a reactor core model and an actual core of a nuclear plant so as to control insertion and withdrawal of the full and part strength control element assemblies for controlling the axial power distribution.

The controlling of the axial power distribution may include, when the difference is equal to a predetermined temperature or higher, and the burnup is equal to a predetermined value or less, prioritizing a temperature deviation signal, wherein, when the temperature deviation signal is prioritized, the logic for adjusting positions of the full and part strength control element assemblies is bypassed.

The controlling of the axial power distribution may include, when the burnup of the nuclear reactor is equal to a predetermined level or higher, controlling insertion and withdrawal of the full and part strength control element assemblies for controlling the axial power distribution by using a feedback effect of a coolant having a negative reactivity coefficient.

In order to control the axial power distribution, the full and part strength control element assemblies may be inserted and withdrawn at a low speed.

Accordingly, a method of controlling power and axial power distribution of a nuclear reactor can automatically perform a load follow operation of a nuclear reactor without any intervention of an operator.

The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:

FIGS. 1A and 1B are graphs showing a change in a coolant average temperature and a change in power of a nuclear reactor, respectively, in a test of a load follow operation of a nuclear power plant;

FIG. 2 is a flowchart of a method of controlling power and power distribution of a nuclear reactor, according to an embodiment of the present invention;

FIG. 3 is a reference diagram for describing a logic for adjusting a boric acid solution used to control power of a nuclear reactor, according to an embodiment of the present invention;

FIG. 4 is a reference diagram for describing a logic for adjusting a boric acid solution used to control power of a nuclear reactor, according to another embodiment of the present invention;

FIG. 5 is a reference diagram for describing a method of adjusting positions of a full strength control element assembly and a part strength control element assembly, which are used to control axial power distribution, according to an embodiment of the present invention;

FIG. 6 is a flowchart of a logic for adjusting positions of a full strength control element assembly and a part strength control element assembly which are used to control axial power distribution, according to an embodiment of the present invention; and

FIG. 7 is a reference diagram for describing an overall method of controlling power and axial power distribution of a nuclear reactor, according to an embodiment of the present invention.

Hereinafter, a method of controlling power and power distribution of a nuclear reactor will be described with regard to exemplary embodiments of the invention with reference to FIGS. 2 through 7.

FIG. 2 is a flowchart of a method of controlling power and power distribution of a nuclear reactor, according to an embodiment of the present invention.

First, power of a nuclear reactor is controlled by adjusting a concentration of a boric acid solution by using a proportional integral controller, according to a difference between a coolant reference temperature and a coolant average temperature (operation 100).

FIG. 3 is a reference diagram for describing a logic for adjusting a boric acid solution used to control power of a nuclear reactor, according to an embodiment of the present invention. A temperature variation signal between a reference signal Tref of a nuclear reactor coolant and an average signal Tavg of the nuclear reactor coolant is transmitted through a proportional integral controller so as to drive a Dilution control valve and Boration control valve, thereby controlling a concentration of boric acid solution of a reactor core. In this case, a gain signal KBC of a proportional integral controller may vary according to a burnup of the reactor core. That is, as the reactor core burns, an effect of boron dilution operation deteriorates. Thus, when a burnup reaches a predetermined burnup, a gain value of the proportional integral controller is increased to facilitate the boron dilution operation at the end of life of the reactor core.

When the power of the nuclear reactor is controlled by adjusting the concentration of the boric acid solution, a coolant average temperature at a starting point and an ending point of power of a reactor is offset by using a value obtained by filtering an error between a calculated axial shape index (ASI) and a measured ASI. An ASI value has the same absolute value as an axial offset (AO) and has an opposite sign to the AO.

FIG. 4 is a reference diagram for describing a logic for adjusting a boric acid solution that is used to control power of a nuclear reactor, according to another embodiment of the present invention. That is, FIG. 4 is a reference diagram for describing a logic for reducing the overshoot and undershoot of a coolant average temperature, which occur at a starting point and an ending point of a change in turbine power. An error between a calculated ASI and a measured ASI is filtered through a high pass filter and is added in the form of feed-forward to a temperature deviation of the form of feedback so as to alleviate a large amount of overshoot and undershoot of the coolant average temperature, which occur at a starting point and an ending point of a change in turbine power.

After operation 100, in order to optimize axial power distribution, the axial power distribution is controlled by changing the insertion and withdrawal of a full strength control element assembly and a part strength control element assembly (operation 102).

FIG. 5 is a reference diagram for describing a method of adjusting positions of a full strength control element assembly (FSCEA) and a part strength control element assembly (PSCEA) which are used to control axial power distribution, according to an embodiment of the present invention. Referring to FIG. 5, a full strength control element assembly position-signal (FSCEA Position Demand) and a part strength control element assembly position-signal (PSCEA Position Demand), which are calculated through a logic for adjusting positions of the full strength control element assembly (FSCEA) and the part strength control element assembly (PSCEA), may be compared with a measured full strength control element assembly position-signal (Measured FSCEA Position) and a measured part strength control element assembly position-signal (Measured PSCEA Position), so as to generate a signal for inserting or withdrawing a control rod. That is, the full strength control element assembly position-signal (FSCEA Position Demand) that is calculated through the logic is compared with the measured full strength control element assembly position-signal (Measured FSCEA Position) so as to generate the signal for inserting or withdrawing the control rod. In addition, the part strength control element assembly position-signal (PSCEA Position Demand) that is calculated through the logic is compared with the measured part strength control element assembly position-signal (Measured PSCEA Position) so as to generate the signal for inserting or withdrawing the control rod.

FIG. 6 is a flowchart of a logic for adjusting positions of a full strength control element assembly and a part strength control element assembly which are used to control axial power distribution, according to an embodiment of the present invention.

First, initial positions of the full strength control element assembly and the part strength control element assembly are set (operation 200).

After operation 200, as the position of the part strength control element assembly is changed, an axial shape index (ASI) is obtained for each respective position. A difference of ASI (DASI) is calculated based on an ASI bias value (ASI_BIAS) that is calculated based on the obtained ASI, a target ASI (EASI), and an ASI calculation error between a reactor core model and an actual core of a nuclear plant (operation 202).

After operation 202, whether the part strength control element assembly reaches a positional limit is determined (operation 204).

If the part strength control element assembly reaches the positional limit in operation 204, a position of the part strength control element assembly at which a sign of the DASI is reversed is detected (operation 206). On the other hand, if the part strength control element assembly does not reach the positional limit in operation 204, the method proceeds back to operation 202, and operations 202 to 204 are performed again.

After operation 206, whether the full strength control element assembly reaches the positional limit is determined (operation 208).

If the full strength control element assembly does not reach the positional limit in operation 208, the position of the full strength control element assembly is changed(operation 210). After operation 210, the method proceeds back to operation 202, and operations 202 to 208 are performed again.

If the full strength control element assembly reaches the positional limit, ASI calculation is completely performed on all limit positions of the full strength control element assembly and the part strength control element assembly.

After operation 208, if the full strength control element assembly reaches the positional limit, whether a difference between a coolant reference temperature and a coolant average temperature is a predetermined value or less is determined or whether a burnup of the reactor core is a predetermined value or more is determined (operation 212). If the difference between the coolant reference temperature and the coolant average temperature is a predetermined value or more, and the burnup of the reactor core is a predetermined value or less, a temperature deviation signal is prioritized, and thus, the logic is terminated.

On the other hand, if the difference between the coolant reference temperature and the coolant average temperature is equal to a predetermined value or less, or the burnup of the reactor core is equal to a predetermined value or higher in operation 212, whether there are positional combinations of the full strength control element assembly and the part strength control element assembly which satisfy DASI=0 is determined (operation 214).

After operation 214, if there are positional combinations of the full strength control element assembly and the part strength control element assembly which satisfy DASI=0, a position combination where moving distances of the full strength control element assembly and the part strength control element assembly are minimized is detected from among the position combinations (operation 216).

On the other hand, if there is no positional combination of the full strength control element assembly and the part strength control element assembly which satisfies DASI=0, positions of the full strength control element assembly and the part strength control element assembly by which an absolute of DASI is minimized is detected (operation 218).

After

operation

216 or 218, axial power distribution is controlled by adjusting the positions of the full strength control element assembly and the part strength control element assembly according to the logic described with reference to FIG. 5 by using the detecting results (operation 220).

FIG. 7 is a reference diagram for describing an overall method of controlling power and axial power distribution of a nuclear reactor, according to an embodiment of the present invention. FIG. 7 is obtained by adding to a combination of FIGS. 3, 4, and 5 the logic of FIG. 6 by which the temperature deviation signal is prioritized when a temperature deviation is a predetermined reference value or more and a burnup is a predetermined value or less. When the temperature deviation signal is prioritized, the logic for adjusting positions of the full and part strength control element assemblies is bypassed.

In the method of controlling axial power distribution, when a burnup of a reactor core is equal to a predetermined level or more, insertion and withdrawal of a full strength control element assembly and a part strength control element assembly are controlled in order to control the axial power distribution by using a feedback effect of a coolant having a negative reactivity coefficient. In addition, in order to control the axial power distribution, the full and part strength control element assemblies are inserted and withdrawn at a low speed.

The embodiments of the present invention can be written as computer programs and can be implemented in general-use digital computers that execute the programs using a computer readable recording medium. Examples of the computer readable recording medium include magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.), and storage media such as optical recording media (e.g., CD-ROMs, or DVDs).

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Claims

A method of controlling power and axial power distribution of a nuclear reactor, the method comprising:

controlling power of a nuclear reactor by adjusting a concentration of a boric acid solution by using a proportional integral controller, according to a difference between a coolant reference temperature and a coolant average temperature; and

controlling axial power distribution by changing insertion and withdrawal of a full strength control element assembly and a part strength control element assembly in order to optimize the axial power distribution.
The method of claim 1, wherein the controlling of the power of the nuclear reactor comprises reducing overshoot and undershoot of the coolant average temperature which occur at a starting point and an ending point of a change in turbine power by using a value obtained by filtering an error between a calculated axial shape index (ASI) and a measured ASI through a high pass filter.
The method of claim 1, wherein the controlling of the axial power distribution comprises:

setting initial positions of the full strength control element assembly and the part strength control element assembly;

calculating a difference of axial shape index (DASI) corresponding to a target ASI and a calculated ASI as a position of the part strength control element assembly is change;

determining whether the part strength control element assembly reaches a positional limit;

when the part strength control element assembly reaches the positional limit, detecting a position of the part strength control element assembly, at which a sign of the DASI is reversed;

determining whether the full strength control element assembly reaches a positional limit;

when the full strength control element assembly reaches the positional limit, determining whether the difference between the coolant reference temperature and the coolant average temperature is equal to a predetermined temperature or less, or whether a burnup of the nuclear reactor is equal to a predetermined burnup or less;

when the difference is equal to the predetermined temperature or less, or when the burnup is equal to the predetermined burnup or more, determining whether there are positional combinations of the full strength control element assembly and the part strength control element assembly which satisfy DASI=0;

when there are positional combinations of the full strength control element assembly and the part strength control element assembly which satisfy DASI=0, detecting a position combination by which moving distances of the full strength control element assembly and the part strength control element assembly are minimized;

when there is no position combination of the full strength control element assembly and the part strength control element assembly which satisfies DASI=0, detecting positions of the full strength control element assembly and the part strength control element assembly by which DASI is minimized; and

controlling the axial power distribution according to results of the detecting,

wherein, when the difference is equal to the predetermined temperature or more, and the burnup is equal to the predetermined burnup or less, a logic for controlling positions of the full and part strength control element assemblies is terminated.
The method of claim 1, wherein the controlling of the axial power distribution comprises:

extracting as a bias an ASI calculation error between a reactor core model and an actual core of a nuclear plant so as to control insertion and withdrawal of the full and part strength control element assemblies for controlling the axial power distribution.
The method of claim 1, wherein the controlling of the axial power distribution comprises:

when the difference is equal to a predetermined temperature or higher, and the burnup is equal to a predetermined value or less, prioritizing a temperature deviation signal,

wherein, when the temperature deviation signal is prioritized, the logic for adjusting positions of the full and part strength control element assemblies is bypassed.
The method of claim 5, wherein the controlling of the axial power distribution comprises:

when the burnup of the nuclear reactor is equal to a predetermined level or higher, controlling insertion and withdrawal of the full and part strength control element assemblies for controlling the axial power distribution by using a feedback effect of a coolant having a negative reactivity coefficient.
The method of claim 3, wherein, in order to control the axial power distribution, the full and part strength control element assemblies are inserted and withdrawn at a low speed.