JPS64677B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a power distribution monitoring device for a nuclear reactor. For example, in boiling water reactors, traveling incore probes (TIPs) and local power range detectors (LPRMs) are regularly arranged in the reactor core.
The neutron flux distribution inside the reactor is detected using a monitor, which is converted into a power distribution to monitor the linear power density of the fuel rods inside the reactor. Figure 1 shows the relative positions of these detectors and fuel assemblies. The detectors are installed at the center of four fuel assemblies 1 (the one in front is not shown); The TIP is inserted into the reactor core as needed and used to measure the axial neutron flux distribution on the monitoring string 2, and the LPRM 3 is a fixed position detector, and multiple pieces are installed in the axial direction.
Figure 2 shows the TIP and
This figure shows the arrangement of the LPRM (hereinafter referred to as in-core detector), which monitors the local power distribution of the four fuel assemblies 1 surrounding it. Then, if the core is divided into four quadrants in the horizontal cross section, the in-core detector will have 1/4 mirror symmetry,
Or, it is placed in only one of the four locations with 1/4 rotation symmetry. The location where this in-furnace detector 4 is installed is called the real string 40, and the location where the in-furnace detector is not installed is a pseudo string with a 1/4 mirror symmetry or 1/4 rotational symmetry. 5
It is called 0. In this way, the in-furnace detector 4 of the actual string 40
Conventionally, a power distribution monitoring device for a nuclear reactor that uses only 1/4 mirror symmetry or 1/4 rotational symmetry of the reactor core is assumed, and the detected value of the in-core detector 4 of the real string 40 is directly transferred to the pseudo string 50. It was used as a detected value. In addition, considering the use of such a core monitoring method, the fuel loading pattern of conventional boiling water reactors is generally determined to have 1/4 mirror symmetry or 1/4 rotational symmetry. Ta. In other words, from the perspective of core monitoring, the fuel loading pattern is
A necessary condition was to have four-mirror symmetry or one-quarter rotational symmetry. Therefore, in such a boiling water reactor,
In order to monitor the power distribution of a core with a fuel loading pattern that does not have 1/4 mirror symmetry or 1/4 rotational symmetry, in-core detectors should also be installed at the pseudo string locations, or A method is needed to estimate the detected value of the pseudo string from the detected value of the in-core detector. The present invention is capable of accurately monitoring the power distribution of a reactor core without 1/4 mirror symmetry or 1/4 rotational symmetry without changing the number or position of conventional in-core detectors. For the purpose of providing a distribution monitoring device, when the reactor core is divided into four quadrants in the horizontal cross section, neutrons are detected only in one of the four locations corresponding to 1/4 mirror symmetry or 1/4 rotation symmetry. In a device in which a detector is arranged and the power distribution of four fuel assemblies surrounding the neutron detector is monitored using signals from the neutron detector, the neutron detector is determined from the power distribution of the entire core of the reactor. The coefficient of the function that approximates the deviation between each quadrant between the output distribution of the four fuel assemblies surrounding the quadrant and the output distribution of the other three locations located at 1/4 mirror symmetry or 1/4 rotation symmetry is a correction coefficient. means for creating a function that approximates the deviation between each quadrant using the output distribution calculated from the signal from the neutron detector and the correction coefficient stored in the storage means; The invention is characterized in that it has means for calculating an output distribution at a mirror symmetrical or 1/4 rotation symmetrical position. In other words, this output distribution monitoring device has a built-in pseudo string output distribution calculation section in the conventional device, and this pseudo string output distribution calculation section includes a correction coefficient calculated by off-line design calculation. This correction coefficient is used to calculate the value used as the detection value of the pseudo string from the detection value of the in-furnace detector of the actual string. Examples will be described below. First, a method for determining the correction coefficient will be explained. FIG. 3 is a 4-batch distributed loading pattern that does not have 1/4 mirror symmetry or 1/4 rotational symmetry. 6 in this figure indicates a 4-batch dispersion area, and a 6-batch dispersion area 7 is provided on the outer periphery. Numbers 1 to 6 written in each fuel assembly 1 indicate the number of years the fuel assembly stays in the reactor. In this fuel loading pattern, an in-core detector is surrounded by four fuel assemblies 1 having different core residence years, and these are used as unit cells to uniformly configure the entire core. The type of the four fuel assemblies surrounding the in-core detector is the same in each quadrant, but their arrangement does not have 1/4 mirror symmetry or 1/4 rotational symmetry, so the output The asymmetry of the power distribution between the actual string position and the pseudo string position increases in the core periphery where the slope increases. This asymmetry in the power distribution has a certain tendency determined by the fuel arrangement within the unit cell consisting of four fuel assemblies, and the power distribution of the pseudo string is determined by the power distribution of the real string. It can be expressed as follows using the core radial position, axial position, cycle burnup, etc. of each string as parameters. TIP _Pseudp (i, j, k, Exp) = f(i, j, k
, Exp)・TIP _Real (i, j, k, Exp)......(1) TIP _Pseudp : Power distribution at pseudo string position TIP _Real : Power distribution at real string position i, j: Each quadrant of the core divided into four radial position of the string k: core axial position Exp: cycle burnup Boiling water reactors normally undergo fuel replacement once a year. The fuel loading pattern is determined by considering the reactor shutdown margin and the core thermal characteristics of the cycle.
Furthermore, we plan control rods throughout the cycle.
The safety of operation throughout the cycle is confirmed. (1)
The correction coefficient f shown in the formula is created based on the results of these series of design calculations, taking into account the storage capacity of the core monitoring device that stores the correction coefficients. Examples of specific correction coefficients are shown below. At a certain cycle burnup, the relationship between the output distributions of the real string and the pseudo string at 1/4 mirror symmetry or 1/4 rotation symmetry can be expressed as follows. TIP ^N _P ^A _s eudo(k)=TIP ^N _R eal(k) {1+g ^NA (k)} (2) TIP ^N _P ^B _s eudo(k)=TIP ^N _R eal(k) {1+g ^NB (k) } (3) TIP ^N _P ^C _s eudo(k)=TIP ^N _R eal(k) {1+g ^NC (k)} (4) Here, TIP ^N _R ^A _e al(k)〜TIP ^N _R ^C _e al (k): String number N
Axial power distribution at actual string position TIP ^N _P ^A _s eudo(k) ~ TIP ^N _P ^C _s eudo(k): Above TIP ^N _R ^A _e al(k) ~
TIP ^N _R ^C _e Axial power distribution g(k) at a pseudo string position with 1/4 mirror symmetry or 1/4 rotation symmetry with al(k): Fitts equation for power distribution asymmetry Now, g(k) Let us take an example of a quintic equation. g(k)
is expressed as g(k)=a ₀ +a ₁ k+a ₂ k ² +a ₃ k ³ +a ₄ k ⁴ +a ₅ k ⁵ (5), and the coefficients a ₀ to a ₅ of the quintic equation are each It is calculated from the core design calculation results using the cycle burnup as a parameter for each pseudo string. The cycle burn-up is
If the cycle period is about one year, it is about 6000 Mwd/t. The asymmetry of the power distribution does not change rapidly with burnup, so it is approximately 100Mwd/
By calculating the correction coefficient every t and interpolating the coefficient during that time, it is possible to calculate the output distribution of the pseudo string with relatively high accuracy. Now, to create this correction coefficient for an 800Mwe class boiling water reactor, 6 (number of coefficients) x 93 (number of pseudo strings) x 6 (number of cycle burnup parameters) = 3348, and the correction coefficient The internal storage capacity required is approximately
It becomes 3.3Kword. This correction coefficient is stored in a pseudo string power distribution calculation unit built into the online core power distribution monitoring device. FIG. 4 shows the configuration of a core power distribution monitoring device according to the present invention, in which 8 is a nuclear reactor, 4 is an in-core detector, 9 is an actual string power distribution calculation section, 10 is a pseudo string power distribution calculation section, 11 is a fuel assembly power distribution calculation unit, 12 is a core design calculation program, 13 is an asymmetry correction coefficient creation program,
The pseudo string output distribution calculation section 10 includes a correction coefficient storage device 101 and a pseudo string output distribution calculation device 102. The flow of data during online output distribution monitoring is shown by a solid line 14, and the flow during offline design calculation is shown by a broken line 15. In the online calculation, the neutron flux distribution detected by the in-core detector 4 is converted into a power distribution by the actual string power distribution calculation unit 9. Based on the converted real string output distribution, the pseudo string output distribution calculation unit 10 imports correction coefficients from the correction coefficient storage device 101 that stores correction coefficients calculated in advance by off-line calculation, and calculates the correction coefficients from a predetermined function system ( In the above-described embodiment, the pseudo string output distribution calculation device 102 calculates the pseudo string output distributions at the other three locations at 1/4 mirror symmetry or 1/4 rotation symmetry using the quintic equation (in the above embodiment). The fuel assembly power distribution calculation unit 11 calculates the power distribution of the fuel assemblies in the entire core from the calculated power distributions of the real strings and pseudo strings. on the other hand,
For offline calculation, core design calculation program 1
Based on the three-dimensional core power distribution calculated from 2,
The asymmetry correction coefficient creation program 13 calculates an asymmetry correction coefficient using the cycle burnup as a parameter for each pseudo string. This calculated correction coefficient is stored in the correction coefficient storage device 101. FIG. 5 shows a comparison of the power distribution at the approximate string position corrected by this core power distribution monitoring device with the actual power distribution. The horizontal axis shows the axial position, and the vertical axis shows the TIP relative output. In this figure, the actual string axial power distribution at the actual string position 41 is shown by a curve 42, the pseudo string axial power distribution at the pseudo string position 51 is shown by a curve 52, and the curve 53 is the pseudo string axial power distribution obtained by this core power distribution monitoring device. String axial power distribution is shown. In the core periphery of the distributed loading pattern, there is a difference of about 10% between the actual string power distribution and the pseudo string power distribution. Therefore, if the conventional core monitoring method is adopted and the actual string power distribution is directly used as the pseudo string power distribution, the prediction error of the total core power distribution will be approximately 4 to 7% with standard deviation σ. On the other hand, when the power distribution of all cores is calculated by the core power distribution monitoring device according to the present invention, the prediction error is within 1% with a standard deviation σ, and the monitoring accuracy is greatly improved. Table 1 shows the standard deviation of the total reactor power distribution obtained by the core power distribution monitoring device of the present invention using the cycle burnup as a parameter.
It is shown in comparison with the conventional case assuming 1/4 mirror symmetry or 1/4 rotational symmetry, and it can be seen that it is significantly smaller than in the conventional case.

[Table] The 4-batch distributed loading pattern shown in Figure 2 is
This method adopts the reactor fuel exchange method proposed in JP-A-54-1786. According to this replacement method, it is possible to reduce fuel shuffling during fuel replacement, which was conventionally necessary, and shorten the regular inspection period. Figure 6 shows the relationship between cycle burn-up and maximum linear power density in comparison with a conventional core. The horizontal axis shows the cycle burn-up (Gwd/
t), the maximum linear power density is shown on the vertical axis, and 61
indicates a 4-batch distributed loading core, 62 indicates a conventional core, and 63 indicates a limit value for reactor operation. As shown in this figure, the maximum linear power density is
Approximately 10% reduction compared to conventional cores. The above effects improve plant utilization efficiency. The effects of this distributed liquid loading method can be maximized by accurately monitoring the power distribution with the core power distribution monitoring device of the present invention. In this way, the core power distribution monitoring device of the example can measure the power distribution of a core that does not have 1/4 mirror symmetry or 1/4 rotational symmetry using the same number of in-core detectors and the same arrangement as conventional ones. It becomes possible to monitor accurately. Furthermore, by using this core power distribution monitoring device, there is no need to significantly expand the storage capacity of currently used core monitoring devices, and the degree of freedom in loading patterns increases, making it possible to achieve the same results as with distributed loading patterns. Improvements in core characteristics can be expected. As described above, the core power distribution monitoring device of the present invention can accurately measure the power distribution of a core without 1/4 mirror symmetry or 1/4 rotational symmetry without changing the number or position of conventional in-core detectors. This makes it possible to monitor the situation, and has great industrial effects.

[Brief explanation of the drawing]

Figure 1 shows TIP and
A perspective view showing the relative position of the LPRM and the fuel assembly,
Figure 2 is an explanatory diagram showing the position of the in-core detectors as seen in the horizontal cross-section of the core, and Figure 3 is a 4-batch distributed loading pattern that does not have 1/4 mirror symmetry or 1/4 rotational symmetry. FIG. 4 is a block diagram of an embodiment of the power distribution monitoring device for a nuclear reactor according to the present invention, and FIGS. FIG. 4...In-reactor detector, 8...Reactor, 9...Actual string power distribution calculation unit, 10...Pseudo string power distribution calculation unit, 11...Fuel assembly power distribution calculation unit, 12...Reactor core design calculation program, 13
...Asymmetry correction coefficient creation program, 101...
. . . Correction coefficient storage device, 102 . . . Pseudo string output distribution calculation device.

Claims (1)

[Claims] 1 When the core of a nuclear reactor is divided into four quadrants in the horizontal cross section, a neutron detector is placed in only one of the four locations that are 1/4 mirror symmetric or 1/4 rotation symmetric, and the neutron detector In the device for monitoring the power distribution of four fuel assemblies surrounding the neutron detector using signals from the four fuel assemblies surrounding the neutron detector, the power distribution of the four fuel assemblies surrounding the neutron detector is determined from the total core power distribution of the reactor. means for storing, as correction coefficients, coefficients of a function that approximates the deviation between each quadrant between the output distribution of the body and the output distributions at three other locations located at 1/4 mirror symmetric or 1/4 rotation symmetric positions; means for creating a function that approximates the deviation between the respective quadrants using the output distribution calculated from the signal from the detector and the correction coefficient stored in the storage means;
1. A power distribution monitoring device for a nuclear reactor, comprising means for calculating a power distribution at a 1/4 mirror symmetry or a 1/4 rotation symmetry position.