JP2000121779A - Power distribution control method for reactor core - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the xenon oscillation of core in operation of a pressurized water reactor. SOLUTION: In operating a reactor 1, an upper half core relative power information Pt and a lower half core relative power information Pb of reactor are measured with an out-of-core neutron detectors 7 and 9 placed outside the reactor vessel, from the upper half core relative power information Pt and a lower half core relative power information Pb and a core parameter, the offset AOp of axial power distribution, the offset AOx of axial power distribution giving xenon distribution at present under equilibrium conditions and the offset AOi of axial power distribution giving iodine distribution at present under equilibrium conditions are calculated. A relative power P is multiplied to each offset AOp, AOx and AOi, and by using the product, an axial offset locus is indicated and control rods are operated so as to guide the axial offset locus to the origin of the coordinates.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for operating a nuclear reactor, and more particularly to a method for controlling a power distribution in a reactor core.

[0002]

2. Description of the Related Art In order to efficiently burn fuel loaded in a nuclear reactor and to prevent fuel damage due to a local increase in power, a power distribution in a reactor core is controlled as uniformly as possible. Preferably, the reactor is operated. From such a viewpoint, a constant axial offset control (CAOC) operation method for a pressurized water reactor has been proposed and implemented. It is known that xenon Xe and iodine I are generated as fission products by a nuclear reaction in a light water reactor, and the presence of xenon oscillations caused by the spatial distribution oscillation of xenon Xe generated by the decay of iodine I and xenon Xe as a fission product. However, the above-mentioned operation method has a specific target of not generating xenon oscillation in the axial direction.

An outline of such a CAOC operation method will be described with reference to FIG. In the figure, the relative output P is 100
It is a parameter defined as 1.0 at% output and 0.0 at 0% output, and the axial deviation or axial offset AO is
((Upper core power-lower core power) / full power), the following restrictions are provided to prevent the occurrence of xenon oscillation. 1) When the output is 90% or more, the operation in the region A is permitted. The area A is set as an operation width of ± 5% with the axial offset AO value in a stable state at the time of rated output as a center value. It is required that if the value deviates from this range, it be corrected promptly, and if it cannot be corrected, the output is reduced to 90% or less. 2) When the output is 90% or less, a deviation of one hour out of 24 hours is allowed in the area B, but when there is a deviation of one hour or more, it is required to reduce the output to 50% or less. ing. 3) When the output is 50% or less, there is no particular limitation as long as the output is within the area B. However, when the output rises to 50% or more, the deviation within the past 24 hours is required to be within 2 hours. On the other hand, after the reaction in the reactor is stopped, when the reactor is restarted, as a characteristic of the pressurized water reactor, the power distribution is biased toward the upper part of the core, and the axial offset AO tends to deviate to the positive side. In such a case, if the CAOC operation method is used, it is necessary to wait 24 hours for the output to rise at 50% output.

As described above, the above-described CAOC operation method has many restrictions, and it has been difficult to achieve efficient operation of the nuclear reactor. Such a problem is due to the fact that only the information of the power distribution is used in the CAOC driving method. To improve this, a new driving method that takes into account the xenon distribution and the iodine distribution in addition to the power distribution is used. The present inventor has previously proposed (Japanese Patent Laid-Open No.
No. 104092). This new method of operation will be referred to herein as the axial offset trajectory method, an outline of which will be described below. The axial offset locus method, the offset of the axial power distribution and AO _P, the offset of the axial power distribution to provide a xenon distribution and iodine distribution at the present time under equilibrium conditions, respectively AO _x, when the AO _i, (AO _i - AO _x, the trajectory of the _AO P -AO _x) utilizes the fact that has the following characteristics. a) The trajectory is a spiral ellipse centered on the origin, and its diameter has a constant inclination with respect to the X axis. Diameter is 1
It is in the quadrant and the third quadrant. If the xenon oscillation is stable, the trajectory will be a constant ellipse. If the xenon oscillations are divergent, the ellipse will increase with time and vice versa. b) The direction in which the ellipse is drawn is always clockwise, and makes one turn around the origin in one cycle of xenon oscillation. If the control rod moves stepwise and the output distribution changes, the trajectory behaves as follows. When c) AO _P change is negative, the trajectory moves to the negative side in parallel with the Y axis. If AO _P is moved to the positive side is the opposite. c) When the control rod stops, the trajectory draws a new spiral ellipse with the above characteristics from that point. Then, in the axial offset trajectory method, when the trajectory is monitored and the control rod is controlled to guide the trajectory to the origin, xenon oscillation is suppressed.

[0005]

According to the axial offset trajectory method proposed above, the operation output for calculating the trajectory is obtained by back calculation from the xenon concentration. In some cases, the intended purpose could not be achieved. In addition, when the output deviates from the limit of the axial offset when the power is increased after the core is stopped, it is necessary to wait for the output to be increased at 50% output in accordance with the requirement of the CAOC operation method. Accordingly, an object of the present invention is to provide an improved power distribution control method for a nuclear reactor core, which has less operation restrictions at the time of power increase and can suppress xenon oscillation.

[0006]

In order to achieve the above object, in the power distribution control method for a reactor core according to the present invention, the relative power output in the upper half of the core is determined by an external neutron detector installed outside the reactor vessel. information P _t and core lower half measures the relative output information P _b, offset AO _P of the axial power distribution and a core upper half relative output information P _t and core lower half relative output information P _b and core parameters , Offset AO of axial power distribution giving the current xenon distribution under equilibrium conditions
_x and the offset AO _i of the axial power distribution giving the current iodine distribution under equilibrium conditions are calculated, and each offset A
O _P, multiplied by the relative output P in AO _x, AO _i, to display the axial offset locus using this product, to operate the control rods so as to guide the axial offset trajectory at the origin of the coordinate axes.

[0007]

Embodiments of the present invention will be described below. First, in order to explain the analytical basis of the method of the present invention, the specifications are defined as follows, and the variation of the xenon concentration will be described. Relative output and the rated output is 1.0: relative power in the P core upper half: relative power in P _t core lower half: P _b axial power distribution offset: AO _P iodine, generated by the fission xenon Rate: y _i , y _x Decay constant of iodine and xenon: λ _i , λ _x Macroscopic fission cross section: _{ｆ f} Microscopic absorption cross section of xenon: σ _a Effective neutron flux at rated output power: φ Above core halves and iodine concentration of the reactor core lower half: I _t, I _b core upper half and xenon concentration of the reactor core lower half: X _t, the use of X _b or more symbols, variation of the xenon concentration in the following formula expressed.

(Equation 1)

(Equation 2)

(Equation 3)

(Equation 4)

(Equation 5)

(Equation 6) Since the offsets AO _i and AO _x are defined as the offsets of the axial power distribution giving the current distribution of iodine and xenon under equilibrium conditions, they can be calculated as follows from the relational expression between the equilibrium concentration and the power. . The relational expression between the equilibrium concentration and the output is

(Equation 7)

(Equation 8)

(Equation 9)

(Equation 10) It is. Given the upper and lower iodine concentrations of the core, the corresponding power levels P ′ _t and P ′ _b
Can be calculated from equations (7) and (8). Thus offset AO _i is given by the following equation.

[Equation 11] Also, (9) using the formula and (10), likewise offset AO _x can be calculated by the following equation.

(Equation 12)

Since the xenon concentration change does not have linearity with respect to the output of each region of the reactor core, this method cannot be applied to xenon oscillation suppression at the time of load fluctuation. This is because in the equations (5) and (6), the outputs P _t and P _b and the xenon concentration X
_{Due to} the product term of _t and _Xb , the xenon concentration becomes non-linear with respect to the output. For this reason, as can be seen from the relational expression between the xenon concentration and the output (equations (9) and (10)),
As the power increases, the equilibrium xenon concentration becomes (y _i + y _x )
/ Σ Asymptotically to _f . If the xenon concentration approaches this asymptotic value, the output back calculated from this value can be infinite. Such a xenon concentration can appear, for example, if the output is reduced from rated output operation to partial output operation. That is, AO _x obtained by using the equation (12) becomes very large, and may greatly deviate from the value obtained under actual operating conditions. In such a case, the trajectory diverges. This problem can be solved by defining AO _x as an offset in the axial visual power distribution that gives the current offset of the xenon distribution at equilibrium conditions at the output during operation.

First, the xenon offset is defined by the following equation.

(Equation 13) Leads to a new AO _x in the following manner by using the _{X r.}
By substituting equations (9) and (10) into equation (13),

[Equation 14]

By definition, _Pt and _Pb are represented by the following equations.

(Equation 15)

(Equation 16) This P is the actual operation output described above, and is obtained as a measured value. Substituting equations (15) and (16) gives

[Equation 17] That is,

(Equation 18) Since AO _x and _Xr have the same sign, AO _x is represented by the following equation.

[Equation 19] here,

(Equation 20)

(Equation 21)

(Equation 22) When X _r is zero, AO _x is also zero. By doing so, the above-described xenon vibration control can be performed in the entire operation range.

Next, an embodiment for implementing a power distribution control method for a nuclear reactor core based on the above-described principle, that is, a method for suppressing xenon oscillation, will be described. Referring to FIG. 1, a reactor 1 has a built-in reactor core 3 composed of a fuel assembly, and a nuclear reaction in the reactor core 3 is controlled by operating a control rod driving device 5 provided at an upper part of the reactor vessel. Is done. Outside the reactor 1, a neutron detector 7 outside the upper reactor corresponding to the position of the core 3 is provided.
And a lower off-reactor neutron detector 9, which communicates with a calculator 11 and further with a display unit 13.
Generally, the out-of-core neutron detectors 7 and 9 are arranged in each quadrant in the plan view, and detect the out-of-core neutron flux corresponding to the output of the upper half and the lower half of the core 3. That is, the calculator 11 calculates the relative output information _P t, a _{P b} on the basis of the detected value of the ex-core neutron detector 7 and 9. Then, using the calculated value to calculate the axial offset _{AO P, AO i, AO x} on the basis of the above-mentioned operation principles. Based on these values, and displays an axial offset trajectory _{(AO i -AO x, AO P} -AO x) on the display device. However, the relative output P is set to each axial offset A so that the comparison can be made in the same manner as at 100% output in the entire output range.
O _{P, AO} i, using a value obtained by multiplying the AO _x.

Next, FIG. 2 shows an example of an axial offset trajectory during a driving simulation. FIG. 3 shows the core behavior at the time of this simulation. In this state, as can be seen from FIG. 3, at the end of the core life when the xenon oscillation becomes unstable, the xenon accumulation is restarted 8 hours after shutting down the furnace near the maximum value, and after the xenon reaches 100% output, It is a plot of the time elapsed. The output rise rate is 3% / hour. In this example, the operation is out of the permissible range at 50% output or less. (Goal O
A ± 5%) to increase the output. As shown in FIG. 2, the trajectory is maintained near the origin, and xenon oscillation does not occur even after the output rises. After the output reaches 100%, it is not necessary to control the output distribution by operating the control rod. The above simulation was performed using the PWR design code. As input conditions, required initial conditions, in this case, the rated output equilibrium conditions at the end of the core life, are given, and the subsequent output history is given, and the calculation is sequentially performed over time. The axial offset trajectory is calculated from the iodine and xenon concentrations obtained from this calculation and the axial offset of the output, and the position of the control rod is appropriately selected while observing the values.

In the graph showing the control rod positions in FIG. 3, a solid line indicates the control rod bank D and a broken line indicates the control rod bank C. This means that: That is, the control rod group (bank) for control of the PWR includes A, B,
There are four types, C and D, and insertion is performed in the order of D, C, B, and A. During normal operation, the bank D is mainly used, and the rest is in the fully pulled-out state. Therefore, as shown in FIG. 3, the bank D is used, and the bank C maintains its position.

[0014]

As described above, according to the present invention,
The output can be increased without being restricted by the CAOA operation limitation and without causing xenon oscillation.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing a system of an apparatus for carrying out a method of the present invention.

FIG. 2 is a graph showing a result of an operation simulation according to the present invention.

FIG. 3 is a graph showing changes in core parameters at the time of the simulation.

FIG. 4 is a graph showing restrictions in a conventional driving method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Nuclear reactor 3 Core 7 and 9 Neutron detector outside the reactor 11 Computing unit 13 Display device

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yoichiro Shimazu 1-1-1, Wadasaki-cho, Hyogo-ku, Kobe-shi, Hyogo F-term in Mitsubishi Heavy Industries, Ltd. Kobe Shipyard (reference) 2G075 AA05 BA03 CA08 CA39 DA01 DA09 EA01 EA08 FA19 FB15 FC04 GA31

Claims (1)

[Claims] 1. A nuclear reactor by ex-core neutron detector placed in a container outside measured and the core upper half relative output information of the reactor P _t and core lower half relative output information P _b, the core upper half from the relative output information P _t and core lower half relative output information P _b and core parameters of axial power distribution offset AO _P,
Calculating an offset AO _i of the axial power distribution which gives an offset AO _x and iodine distribution of the current axial power distribution that gives xenon distribution at the present time under equilibrium conditions under equilibrium conditions, the respective offset AO _P, AO _x, A power distribution control method for a nuclear reactor core, comprising multiplying AO _i by a relative power P, displaying an axial offset locus using the product, and operating a control rod according to the axial offset locus.