JPH0360077B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention monitors the power distribution using the measured values of power range monitors in the reactor core, and in particular, monitors the necessary and sufficient power with the minimum number of monitors. This invention relates to a power distribution monitoring device for a nuclear reactor that obtains distribution monitoring accuracy.

[Background of the invention]

A plurality of power range monitors are installed in the core of a large nuclear reactor such as a power reactor. One of the purposes of this installation is to burn fuel more safely and economically by monitoring the power distribution within the reactor core.

Hereinafter, in this specification, a boiling water reactor (BWR) will be explained in detail by taking an example.

FIG. 1 is a horizontal cross-sectional view of the core. The fuel is regularly arranged in the core in the form of fuel assemblies 1. The fuel assembly 1 can be divided by a 2×2 mesh (bold line in the figure), except for a part of the outermost periphery of the core (indicated by diagonal lines in the figure). A square consisting of four fuel assemblies divided in this way is
Call it cell 2. FIG. 2 shows an enlarged view of one of the cells. Inside the cell, as described above, there are four fuel assemblies 1a to 1d, and around them there are four cross-shaped control rods 3a to 3d.
In the illustrated example, only one of the control rods 3d is partially inserted, and the other three are all pulled out. A string 4 for monitoring the in-furnace power range can be installed in the center of each cell. Inside the string 4 are two types of output area monitors. One of them is local output range monitor (LPRM) 5a to 5d.
The neutron flux is constantly measured at four locations in the height direction of the core. The other is the mobile in-core instrumentation system (TIP). The TIP is normally stored outside the core, and travels within the conduit 6 only when measuring the continuous neutron flux distribution in the height direction of the core.

Conventionally, the measured values of TIP and LPRM have been used to monitor the output distribution in the following manner (Figure 3).

(1) Travel through the TIP and measure the continuous neutron flux distribution R ^O _l,k of each string installed at the core height. Here, the subscript l is the string number, and k is the node position in the core height direction.

Normally, the reactor core is divided into 24 nodes in the height direction, with k=1 at the bottom and k=24 at the top.

(2) When monitoring the output distribution, calculate R _l,k corresponding to the TIP measurement value at that time using the following formula.

R _l,k = R ^O _l,k + ΔR ^CR _l,k + ΔR ^LPRM _l,k ……(1) Here, ΔR ^CR _l,k is the control value adjacent to the string after measuring R ^O _l,k . This is to correct the effect of rod movement, and is calculated using constants created in advance according to the amount of movement of the control rod and the distance between node k and the tip of the control rod. On the contrary,
ΔR ^LPRM _l,k is the LPRM measurement value during output distribution monitoring,
Calculate each time from the difference between the value corresponding to the LPRM measurement value calculated from (R ^O _l,k + ΔR ^CR _l,k ).

(3) From the TIP equivalent value R _l,k calculated using equation (1), calculate the four fuel assemblies j (=1 to 4) around the string l.
The output P _l,j,k of is calculated using the following formula.

P _l,j,k =F _j・R _l,k ……(2) Here, the conversion coefficient F _j is the fuel type, burnup, moderator density of fuel assembly j, and the fuel type around the string. It is a function of the combination, control rod insertion state, etc., and is calculated using constants created in advance.

(4) When the correction amounts ΔR ^CR _l,k and ΔR ^LPRM _l,k in equation (1) become large, the vehicle runs at TIP and updates R ^O _l,k .

According to this method, the output distribution monitoring error (Note) can be maintained at 4 to 5% or less by running TIP as follows.

When the control rod insertion pattern is changed.

Even if the control rod insertion pattern remains constant, the previous
From the TIP run, the average burnup of the core is IGW.
When it has advanced by about d/t.

The output distribution monitoring error σ is defined by the following equation.

Here, R ^M _l,k , R ^C _l,k : String l, TIP measurement value of node k, calculated value (average value 1), L, K : Total number of strings, number of nodes.

With monitoring errors of this level, nuclear reactors can be operated safely. However, in order to calculate the output distribution using the above method, a string containing a built-in output area monitor is essentially required for each cell. In the core shown in Figure 1, the number of strings is 204. However, in reality, as shown in Figure 4, only 52 strings are implemented (marked with ● in the figure). This is to take advantage of the fact that the core is, in principle, operated with quarter symmetry. 1/4 symmetry means that the fuel loading pattern and the control rod pattern have mirror symmetry or 1/4 rotation symmetry with the dashed line in the figure as the axis of symmetry. _In this case, as shown ⁱⁿ an example in ^FIG _. , in the case of rotational symmetry
L ₁ ^R to L ₃ ^R ) can be used as a virtual measurement value. Using this property, in the upper right quadrant of Figure 4,
When the strings implemented in the other three quadrants are virtually transferred, they become as shown by the circles in the figure. In other words, if the reactor core is operated with quarter symmetry, there will effectively be a string for every cell, and the power distribution can be accurately monitored using methods (1) to (4) above. can.

However, with this method, 1/1/2 of the total number of cells in the core
4 strings is required, and as the core becomes larger, the following problems arise.

(i) The number of strings increases, increasing installation costs. Also, it is always placed inside the reactor core.
Due to the gradual deterioration of the LPRM, the strings require periodic replacement, which also increases the cost of replacement.

(ii) TIP running is necessary to measure R ^O _l,k , but as the number of strings increases, the time required to run all the strings increases. During TIP driving, it is necessary to keep the power distribution stable, which reduces driving flexibility.

That is, it is not preferable to arrange a large number of strings as shown in FIG. 4 in order to improve economy and efficiency.

Therefore, in recent years, methods to reduce the number of strings have begun to be considered. FIG. 6 shows an example of this method, in which every other string is reduced from the conventional arrangement shown in FIG. As a result, the number of strings will be reduced to 24, less than half of the 52 in Figure 4.

However, with this placement method, if the core is operated with quarter symmetry, even if the strings are moved to one quadrant as shown by the circles in FIG. 6, cells without strings will occur. In a cell where a string does not exist and a measurement value of the output area monitor cannot be obtained, the output distribution cannot be monitored using conventional methods (1) to (4), and therefore the output distribution monitoring error becomes large.

In order to solve this problem, a method has been proposed in which a program containing a physical model of a nuclear reactor is used to calculate the power distribution. ⁽¹⁾ A physical model of a nuclear reactor (hereinafter abbreviated as a reactor model) is a physical model that uses equations that describe the nuclear characteristics and thermal-hydraulic characteristics in the reactor core, and uses equations that describe the core thermal output, core coolant flow rate,
The power distribution is calculated from data such as control rod positions. The node-coupled model FLARE ⁽²⁾ is
This is one example.

Power distribution calculation using a nuclear reactor model does not originally require measured values from a power range monitor. However, the output distribution monitoring error in that case is about 6 to 7%. This is due to the simplification of the reactor model, which requires the use of online process calculators to complete calculations within minutes.

That is, even when using a nuclear reactor model, in order to achieve the conventional power distribution monitoring accuracy, it is necessary to use the measured values of the power range monitor.

[Purpose of the invention]

An object of the present invention is to provide a power distribution monitoring device for a nuclear reactor that can reduce the number of strings in the reactor core and monitor the power distribution with accuracy comparable to conventional methods.

[Summary of the invention]

The feature of the present invention is that the four control rods are surrounded by four control rods.
In a power distribution monitoring device for a nuclear reactor, the power distribution monitoring device has a plurality of cells in the core, each of which is composed of a plurality of fuel assemblies, and in which a string containing a neutron monitor is arranged. When the cross section of the core is divided into four regions by the axis, the string placed in one region has a neutron flux at a position mirror-symmetric or 1/4 rotation symmetry with the string in the other three regions. When all the strings in the other three regions are virtually transferred to one region obtained by dividing the strings, using symmetry, the strings in the one region are The cells with the string and the cells without the string are arranged in a checkerboard pattern, and a device that calculates the output distribution using a model and a measurement value of a neutron monitor in the string are used to correct the output distribution. The reason is that the equipment is equipped. Specifically, as shown in FIG. 7, in order to improve the monitoring accuracy of the output distribution, the measured value of the output area monitor is used supplementarily as follows.

(1) During TIP driving, compare the value equivalent to the TIP measurement value obtained by converting the power distribution calculated using the reactor model with the actual TIP measurement value, and make sure that the difference between the two is minimized. Adjust model parameters.

(2) When monitoring the power distribution, compare the value equivalent to the LPRM measurement value obtained by converting the power distribution calculated using the reactor model with the actual LPRM measurement value,
The output distribution is corrected by the difference between the two.

Here, the parameters to be adjusted in the above (1) are selected to be around 5 to 6 parameters that do not affect a local portion of the reactor core, but affect the entire core. In addition, the amount of correction of the output distribution based on the LPRM measurement value is
The monitoring error of the output distribution is reduced by 2 to 3%. The reason why the number of strings can be reduced will be explained later, but the number of parameters is small,
One of the reasons why the number can be reduced is that the amount of correction of the output distribution is small.

The basic procedure of the output distribution monitoring method has been explained above. Next, a method for arranging strings will be explained.

When focusing on one quarter of the core, the strings are arranged in a checkered pattern as shown by the circles a or b in Figure 8. In both cases, the following two
It is designed to satisfy two rules.

The density is one string for every two cells.

The strings are evenly spaced.

These two rules are necessarily determined in response to physical phenomena for the following reasons.

The rules for string placement are based on the principle that the contribution from each fuel assembly is reflected in the power range monitor in at least one string. To meet this principle,
Fast neutrons generated in a fuel assembly are slowed down to become thermal neutrons, and the average range before they are absorbed and used for the next fission is longer than the distance from that fuel assembly to the nearest string. The density of the string can be determined as follows. Based on diffusion theory, the following equation approximately holds true in a thermal neutron reactor. ^{(3) 2} = 6・M ₂ ...(3) Here, ² : Average square of the straight-line distance from the neutron generation point to the absorption point M ² : Moving area In BWR, M ² = 100 to 150 [cm ² ], and when this is substituted into equation (3), ² = 600 to 900 [cm ² ]. Therefore, the average range is √ ² = 25 to 30 [cm]. On the other hand, in BWR, one side of the cell is
It is 1ft (approximately 30cm). In other words, if the strings are arranged as shown in a or b in Figure 8, all fuel assemblies will be within the average range of neutrons from the nearest string (as shown in a partially enlarged view in Figure 9). Enter the circle with the dashed line inside.

The reason why the arrangement of strings as described above is sufficient is to improve the method of monitoring the output distribution, as explained earlier. In other words, unlike the conventional method based on the measured values of the power range monitor, first, the power distribution is directly calculated using the reactor model without using the measured values of the power range monitor.
In order to correct this output distribution, a method is adopted in which the measured value of the output area monitor is used as an auxiliary measure. Therefore, if there is one string that reflects the influence of each fuel assembly, the output distribution of that fuel assembly can be corrected correctly.

Next, the reason for establishing the rules regarding the arrangement of strings is to rationally correct the output distribution. That is, first of all, if the fuel assemblies are arranged evenly as shown in FIG. 8, the amount of correction of the output distribution of each fuel assembly becomes approximately the same. This means that the calculation accuracy of the power distribution for each fuel assembly is approximately the same. Locally uniform calculation accuracy not only facilitates monitoring but also improves safety. Second, by arranging them evenly, the output distribution can be corrected using the same procedure at all locations. Since the correction process is performed by a program stored in a computer, the program can be simplified and the calculation time can be shortened.

[Embodiments of the invention]

EMBODIMENT OF THE INVENTION Hereinafter, the present invention will be described in detail based on the drawings.

First, an example of a method for monitoring output distribution will be described. Among the steps shown in FIG. 7, the method of the TIP running example can be used as described in Japanese Patent Publication No. 53-22639 ``Nuclear Reactor Power Distribution Monitoring Device'', so a detailed explanation will be omitted here. In this method, FLARE is used as a nuclear reactor model, and there are six adjustment parameters (two mixing parameters for neutron transport nuclei and four albedo). When the power distribution is calculated using a reactor model with these parameters adjusted, the monitoring error is about 7%.

In order to reduce this monitoring error, the output distribution monitoring method of the present invention further corrects the output distribution using the LPRM measurement value. An embodiment of this correction method is shown in FIG. The procedure is as follows.

(1) TIP the power distribution P ^M _l,j,k calculated using the reactor model
Convert to equivalent value R ^M _l,k and LPRM equivalent value L ^M _l,o .
This transformation is the inverse transformation of equation (2).

(2) For each string l, LPRM position n (=1 ~
In 4), the difference ΔL _l,o between the LPRM measurement value and the calculated value is calculated using the following formula.

ΔL _l,o = L _l,o −L ^M _l,o ...(4) Here, L _l,o is the LPRM measurement value.

(3) For each string l, linearly interpolate ΔL _l,o at four points in the core height direction, and calculate the difference ΔR ^LPRM _l,k between the 24 nodes equivalent to TIP.

(4) Fuel assembly j (=1
Regarding ~4), the output distribution is corrected using the following equation.

P _l,j,k =P ^M _l,j,k・(1+C _l,j,k ) ……(5) Here, C _l,j,k =ΔR ^LPRM / _l,k /R ^M / _{l, k} P _l,j,k is the output distribution after correction.

(5) For cells without strings, use the correction coefficients C _l1,j,k to _{C l4,j,k} for the surrounding strings l ₁ to l ₄ as shown in Figure 11, and use the following formula. Find a virtual correction coefficient C _lc,j,k .

C _lc,j,k = 1/4 _l4 〓 ^l=l1 C _l,j,k ……(6) If the output distribution is corrected using the LPRM measurement value using the above procedure, the monitoring error of the output distribution will be further reduced by 2 ~
This is an improvement of 3% and is below the target of 4-5%. In addition, in FIG. 8, for the cells at the outermost periphery of the core where no strings exist, FIG. 11 and formula (6)
The correction coefficient cannot be calculated using the method shown below. In such a case, the average value of the correction coefficients of two or three surrounding strings may be used.

Next, an example of a method for arranging strings will be described. An example in which the core is operated with quarter symmetry is shown in FIGS. 12 and 13. If we use symmetry to virtually transfer all the strings to the upper right quadrant, from Figure 12 we can see Figure 8a.
However, FIG. 8b is obtained from FIG. 13. That is, both embodiments satisfy the above-mentioned rules regarding the arrangement of strings.

Furthermore, in the embodiments shown in FIGS. 12 and 13, when looking at the arrangement of the strings in the entire core, there is no local deviation and the strings are distributed almost uniformly in the entire core. In addition to monitoring the power distribution, LPRM measurements are used as inputs to reactor protection systems such as the average power range monitor (APRM) or the control rod withdrawal block monitor (RBM). Therefore,
Uniform distribution throughout the entire reactor core means that no matter where an abnormality occurs, the reactor protection system can be activated immediately.

In addition to the embodiments described above, the following embodiments can also be realized without changing the basic concept of the output distribution monitoring method and string arrangement method of the present invention.

(i) Output is low at the outer periphery of the core, and even if the monitoring accuracy of the power distribution deteriorates a little, it will not have a major impact on safety or economics. Therefore, in Figure 8, It is also possible to delete existing strings. In this way, the number of strings can be reduced by 4 in (a) and 6 in (b).

(ii) As a method of determining the correction coefficient for a cell in which no string exists, it is also possible to smoothly interpolate the correction coefficients of surrounding strings, instead of simply taking the average as in equation (6).

(iii) If the core is operated with one-eighth symmetry,
As shown in the example in FIG. 14, strings may be placed at the positions marked _{l and k} on ^{the LPRM} . When gathered in one area, they form a checkered pattern as shown by the ○ marks.

〔Effect of the invention〕

As explained above, by adopting the string arrangement method and output distribution monitoring method according to the present invention, the number of strings can be reduced by half, economical efficiency can be improved, and output distribution can be monitored with less error. , which has the effect of improving efficiency.

[Brief explanation of drawings]

Figure 1 is a horizontal sectional view of the core, Figure 2 is a diagram showing the cell configuration, Figure 3 is a diagram showing the conventional power distribution monitoring method, Figure 4 is a diagram showing the conventional string arrangement, and Figure 5 is a diagram showing the conventional power distribution monitoring method. is a diagram showing the symmetrical relationship of strings, Figure 6 is a diagram showing an example of reducing the number of strings, and Figure 7 is a diagram showing an example of reducing the number of strings.
The figure shows the output distribution monitoring method of the present invention, Figure 8 shows the string arrangement method of the present invention, Figure 9 is an enlarged view of a part of Figure 8, and Figures 10 and 11.
Figure 12 shows an embodiment of the output distribution monitoring method.
Figures 13 and 14 are diagrams showing examples of string arrangement methods. 1... Fuel assembly, 2... Cell, 3... Control rod, 4... String, 5... LPRM, 6...
TIP conduit.

Claims (1)

[Claims] 1 Power distribution of a nuclear reactor that has a plurality of cells in the core that are composed of four fuel assemblies surrounded by four control rods, and in which a string containing a neutron monitor is arranged. In a monitoring device, when the cross section of the core is divided into four regions by two axes that pass through the center of the core and are perpendicular to each other, the strings placed in one region are mirror-symmetrical to the strings in the other three regions. or 1/4
When all the strings in the other three regions are virtually transferred to one region obtained by division, which is arranged so as to detect the neutron flux at a rotationally symmetrical position, using symmetry, In one region, the cells with the string and the cells without the string are arranged in a checkerboard pattern, and a device that calculates the output distribution using a model and a device that calculates the measured value of the neutron monitor in the string are provided. A power distribution monitoring device for a nuclear reactor, comprising: a device for correcting the power distribution using the device.