JPH0833480B2 - Local output peaking coefficient monitoring method - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a local power peaking coefficient monitoring method for monitoring a local power peaking coefficient during a reactor operation.

[Technical background of the invention and its problems]

Generally, a linear power density operation limit is specified for safe operation of a nuclear reactor. Therefore, by constantly comparing the maximum value of the line power density obtained from the reactor power monitoring device with the line power density operation limit value, it is configured to inform the operator how close they are to this operation limit. .

However, the limit value differs depending on the fuel type, and the maximum linear power density is required for each fuel assembly segment (hereinafter referred to as segment),
Since the maximum linear power density in one segment is obtained by multiplying the output of each segment by the local output peaking coefficient of each segment, the local output peaking coefficient is the maximum fuel rod output when the segment average power is 1.0. Will be represented. Here, the output of each fuel rod in the segment is hereinafter referred to as a local output distribution.

By the way, since the local output peaking coefficient depends on the enrichment, the fuel type, the in-channel void ratio (hereinafter referred to as the void ratio), the fuel type arrangement and the control rod insertion pattern, the four fuel assembly two-dimensional diffusion calculation is currently performed. , The fitting formula was created from the result. That is, for each combination of four fuel types surrounding the neutron detector that appear in the core in advance, and four surrounding fuel types
A four-dimensional fuel assembly two-dimensional diffusion calculation is performed for several burn-ups and void fractions for each control rod insertion pattern, and a fitting formula for determining the local power peaking coefficient is created from the results. In, the local output peaking coefficient was calculated from the fitting formula. However, when the local output peaking coefficient is obtained in this way, the amount of calculation and work for creating the fitting equation becomes enormous as the number of combinations of fuel types increases due to refueling, and the fitting coefficient increases and becomes complicated. However, there is a problem that the accuracy of the local output peaking coefficient monitoring deteriorates due to the increase in the fitting error.

[Object of the Invention]

The present invention has been made in view of the above circumstances, and an object thereof is to provide a local output peaking coefficient monitoring method for monitoring a local output peaking coefficient obtained by a method that does not depend on a combination of fuel types.

[Outline of Invention]

The present invention is based on the local power distribution which is the power of each fuel rod in the fuel assembly segment obtained by the single fuel assembly nuclear constant calculation performed in advance for several burnups and in-channel void fractions. Several representative values are stored in the memory for each fuel type together with the position of the fuel rod in the segment, while the influence on the local power distribution caused by the neutron flow with the adjacent segment is determined by the single fuel assembly nuclei determination. While calculating the correction coefficient to be corrected by the ratio of the segment average thermal neutrons and fast neutrons obtained by the numerical calculation, the control rod correction coefficient for correcting the influence of the control rod insertion on the correction coefficient By obtaining for each rod position, the representative value of the local power distribution obtained by the single fuel assembly nuclear constant calculation is compared with the correction factor and the control rod correction factor. Of the local power distribution in that situation, which is obtained by interpolation of burnup and in-channel void ratio obtained from the power distribution monitoring device, is selected for each fuel rod, and the maximum value among them is selected. The present invention relates to a local output peaking coefficient monitoring method in which a local output peaking coefficient at each time point is obtained, and the local output peaking coefficient is displayed on a display device and monitored. A correction factor considering the influence of adjacent segments is generally required due to the thermal neutron flow with the adjacent segments, but the main causes of the thermal neutron flow generation between the adjacent segments are the burnup, void fraction, enrichment, etc. It is known that there are differences, and 4 for many combinations of the above 3 elements.
The result of the two-dimensional diffusion calculation of the fuel assembly is used for each position of each fuel rod. Further, in the calculation of the correction coefficient, the ratio of the segment average thermal neutrons to the fast neutrons obtained by the single fuel assembly nuclear constant calculation, so-called F infinity, is generally such that the fast neutron flux distribution does not differ significantly between the segments. In consideration of the above, the thermal neutron flux of each fuel segment in the reactor is substituted. Further, the control rod correction coefficient is the local power distribution obtained by the four-dimensional fuel assembly two-dimensional diffusion calculation for each control rod pattern and the F infinity obtained by the single fuel assembly nuclear constant calculation of the influence of the adjacent segment as described above. It is calculated in advance for each position of each fuel rod as a ratio with the local power distribution when it is corrected only by a large amount. Generally, this ratio does not depend on burnup, void ratio and fuel type It is sufficient to find out a suitable combination of burnup and void ratio. Therefore, the reason why the control rod correction coefficient is required is that if the control rod insertion segment and the segment adjacent to the segment are used to correct the local power distribution using only F infinity, which is the average constant in the entire fuel segment, the control is performed. Reflects the fact that insertion has a wider effect on all fuel rods in a segment, while in reality fuel rods closer to the control rod are more effective, and those far away are less effective. It is a thing. The representative value of the local power distribution at that time obtained by the interpolation is corrected by both the correction coefficients, and the maximum value among them is selected to obtain the local power peaking coefficient during the reactor operation. The typical value is obtained from the local power distribution obtained by the calculation of the nuclear constant of the single fuel assembly at the three void fraction points (void fraction 0%, 40%, 70% void fraction in the volume fraction inside the channel) that is usually performed. As a method of selecting several fuel rods, the fuel assembly is divided into several regions, and fuel rods which are considered to be large throughout the local power distribution of three void fraction points which is usually performed in each region are selected, or It can be said that a method of selecting a large number of large fuel rods from the maximum throughout the local output distribution of three void fraction points which is usually performed is practical.

 First, the principle of the present invention will be described.

The present invention does not use the fitting formula as described above, but uses a part of the local power distribution obtained by the calculation of the nuclear constant of the single fuel assembly, and considers and corrects the influence of the adjacent segment and the control rod. By doing so, the local output peaking coefficient is obtained.

Therefore, a method of obtaining the correction coefficient for correcting the influence from the adjacent segment will be described.

The change in the local power distribution due to the influence from the adjacent segment is due to the inflow and outflow of neutrons caused by the difference in burnup, void fraction, or the fuel type itself.
Thermal neutron flux and fast neutron flux can be obtained by the single fuel assembly nuclear constant calculation. The ratio of the thermal neutron flux to the fast neutron flux (hereinafter referred to as F infinity) is mainly determined by the characteristics of each segment such as burnup, void fraction and enrichment.

By the way, the fast neutrons in each segment in the nuclear reactor generally do not largely depend on the burnup, void fraction, enrichment, etc., and there is no great difference in almost all the segments. Therefore, the F infinity calculated by the single fuel assembly is considered to be proportional to the thermal neutrons of each fuel segment in the core, and the difference in the thermal neutron flux (hereinafter referred to as thermal neutron mismatch) Since the thermal neutron flow between the fuel segments is generated and the local power distribution calculated by the single fuel assembly is changed, the local power distribution considering the adjacent adjacent segments is expressed by the following equation (1).

▲ P ¹ _ij ▼ = ▲ P ⁰ _ij ▼ (1 + Δ ▲ P ^f _ij ▼ (1) where Δ ▲ P ^f _ij ▼: Correction by thermal neutron flux mismatch ▲ P ⁰ _ij ▼: Single fuel assembly calculation Local output distribution of P ¹ _ij ▼: Local output distribution in which the influence from the adjacent segment is corrected. Further, the term indicating the correction due to the thermal neutron flux mismatch is expressed by the following equation (2).

Where, 0: Subscript indicating the fuel assembly of interest x1: Subscript indicating the left assembly of the fuel assembly of interest x2: Subscript indicating the right assembly of the fuel assembly of interest y1: Fuel assembly of interest Subscript y2 indicating the lower assembly of the above: Subscript indicating the upper assembly of the fuel assembly of interest f ^∞ : F infinity i, j: Fuel rod position (indicates the x and y directions, respectively) M: Fuel Rod arrangement Σ: Total cross-sectional area of heat group D: Diffusion coefficient of heat group p: Fuel rod pitch a, b: Each coefficient In the above equation, each coefficient is the enrichment, void fraction and The four-dimensional fuel assembly two-dimensional diffusion calculation is performed for a large number of combinations having different enrichments, and the results are used to obtain it in advance.

In addition, F infinity at the time of calculating the single fuel assembly nuclear constant of each fuel segment is stored for each burnup and void ratio for each fuel type as in the case of the representative value of the local power distribution. It is calculated and used by interpolation of the burnup and void fraction of the segment obtained by the device.

 Next, how to obtain the control rod correction coefficient will be described.

As described above, the present invention corrects the influence of the adjacent fuel assemblies by the F infinity mismatch of each fuel assembly. However, in general, the F infinity of the fuel in which the control rods are inserted is the absorption of thermal neutrons by the control rods. Is a very small value due to
Further, in the present invention, the coefficients a and b are similarly used for all the fuel rods as shown in the equation (2), and the effect that the closer the control rods are, the greater the influence thereof is not considered at all. ing. Therefore, in the present invention, FIG.
The ratio of the local output distribution by the four-fuel-assembly two-dimensional diffusion calculation calculated for each control rod insertion pattern shown in (e) to the local output distribution when the local output distribution is corrected from F infinity is It is decided to use it by obtaining it for each position of the fuel rod as in the equation (3).

P _ij = P ¹ (1 + Δ ▲ P ^c _ij ▼) (3) Where, ▲ P ^c _ij ▼: Correction by inserting control rod ▲ P ¹ _ij ▼: F Local output distribution P _ij corrected by infinity only P _ij : Local Output Distribution Correcting the Influence of Control Rod Insertion Here, the method of obtaining this correction coefficient will be described in detail. The rate of change in the local output distribution due to the influence of the control rod is the same for each control rod insertion pattern. The four-dimensional fuel assembly two-dimensional diffusion calculation is performed to obtain the local output distribution.
Equal numbers among numbers 1 to 11 given to the respective segments shown in FIGS. 1A to 1E indicate that the influence of the control rod 2 is the same. Next, the local output distribution corrected from only F infinity is calculated for each of the control rod insertion patterns, and the ratio is calculated for each segment numbered 1 to 11, and this is used as a control rod correction coefficient for fuel. It is calculated for each position of the rod. This correction coefficient does not generally depend on the burnup, void rate and fuel type of the control rod, so it is calculated with several burnups and void rates for typical fuel types, and the average is obtained at each fuel rod position. It's enough.

From the above, the local output distribution in which the effects of the adjacent segment and the control rod are corrected can be calculated using the following equation (4).

P _ij = ▲ P ⁰ _ij ▼ (1 + Δ ▲ P ^f _ij ▼ + Δ ▲ P ^c _ij ▼) ...
(4) Therefore, in order to obtain the local output peaking coefficient, the above equation (4) is calculated for the stored fuel rods and the maximum value among them is obtained.

Example of Invention

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of an embodiment of the present invention. In FIG. 1, reference numeral 26 denotes a nuclear reactor, and an output x _s from a local power range monitor (not shown) installed in the nuclear reactor 26 is It is input to the output distribution monitoring device 22. The information x ₃ regarding the burnup and void ratio for each segment calculated by the output distribution monitoring device 22 is input to the calculation device 23. 20 is a storage device,
In this storage device 20, the number of fuel rods required for correction calculation, for example, the representative value from the local power distribution obtained from the single fuel assembly nuclear constant calculation at three void fraction points which is usually performed for each fuel type, is calculated. The main selection, its output and position in the fuel assembly are stored. In other words, a single fuel assembly calculation is performed for several burnups and void fractions for each fuel type, a representative fuel rod is selected from the outputs, and the output at each burnup and void fraction is calculated for that fuel assembly. It is stored as data x _{1 in the} storage device 20 together with the position inside the body. The method of selecting the representative fuel rod is to divide the fuel assembly into several regions and select a fuel rod that is considered to be large throughout the local output distribution of the three void fraction points normally performed in that region, or It can be said that a method of selecting several large fuel rods from the highest rank throughout the local output distribution of the three void fraction points is practical.

The calculation device 23 uses the storage device 20 for each fuel type and burnup to provide the local output at the three void fraction points of the representative fuel rod with the burnup corresponding to each segment obtained by the output distribution monitoring device 22. By determining the degree and interpolating with the void rate, the representative value of the local output distribution at that time x ₄
Then, the representative value x ₄ of this local output distribution is input to the calculation device 25. The coefficient x ₂ for correcting the control rod effect obtained by the calculator 21 and the coefficient x ₆ for correcting the influence of the adjacent segment obtained by the calculator 24 are input to the calculator 25, and both correction coefficients x ₂ , Multiply x ₆ to obtain a representative value of the local output distribution at that time, with all effects corrected.

Further, the calculation device 25 compares the representative values of the local output distributions and obtains the target local output peaking coefficient x ₇ with the maximum value.

The calculation device 24 uses each segment burnup and void fraction obtained by the output distribution monitor 22 and interpolates F infinity by a prebuilt-in single fuel assembly calculation with the burnup and void fraction. The correction coefficient for correcting the influence of the adjacent segment is calculated using the obtained F infinity and the correction formula.

Further, the control rod correction coefficient is a local output distribution obtained by four-dimensional fuel assembly two-dimensional diffusion calculation for each control rod insertion pattern for an appropriate fuel type, and adjacent segments, which is a method used in the present invention for each system. Is obtained for each fuel rod position as a ratio with the local output distribution obtained as a result of correction by the difference of F infinity, which is stored in the correction coefficient storage device 4 as a control rod correction coefficient x ₂ . It is a thing.

Now, the calculation error of the local output peaking coefficient obtained by the present invention will be described with reference to FIGS. 2 (a) to (c).

FIG. 2 (a) shows that the control rod 2 and the fuel assembly 1 have the positional relationship as shown in the figure, and the local output peaking coefficient of the segment at each position is to be obtained. Fig. 2 (a)
Is composed of two fuel types A and B. With respect to these two fuel types, the local output peaking coefficient was calculated for each segment by the local output peaking coefficient monitoring device according to the present invention when the control rod of the fuel type A was inserted or not, and FIG. ),
In (c), a white square is shown. 4 in addition to these figures
The correct answer of the local power peaking coefficient obtained by the fuel assembly two-dimensional diffusion calculation is shown by a black square, and the one obtained by the single fuel assembly nuclear constant calculation is shown by an open circle.
In conclusion, the local output peaking coefficient obtained by the present invention agrees with the correct solution obtained by the two-dimensional diffusion calculation of four fuel assemblies within an error range of 1%, and it can be said that there is no problem in practice.

〔The invention's effect〕

As described above, according to the present invention, there is provided a method of monitoring a local output peaking coefficient that does not depend on a fitting formula based on a combination of fuel types, and several burnups and void fractions are calculated for each fuel type. The representative value of the local output distribution is corrected by a correction coefficient that considers the influence of adjacent segments and a control rod correction coefficient that considers the influence of the control rod on the correction coefficient. Since the output peaking coefficient is used, the influence from the adjacent segment can be corrected online by using the correction formula and the correction coefficient which are built in advance. Therefore, it is not necessary to consider the fuel type arrangement in advance, and the two-dimensional diffusion calculation that has been conventionally performed for each combination of four fuel assemblies is unnecessary. 4 Fuel assembly 2
Although the dimensional diffusion calculation is still unnecessary when calculating the control rod correction factors, it is sufficient to calculate this for the appropriate fuel type as well, as the control rod correction factors are generally largely independent of fuel type. To obtain the desired local output peaking coefficient in this way,
The amount of calculation and work required for the fitting formula work is completely eliminated, and even if the number of fuel type combinations increases due to refueling, there is no need to create a new fitting formula and maintenance is easy.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, FIGS. 2 (a) to 2 (c) are diagrams for explaining an error of a local output peaking coefficient obtained by the present invention, and FIG. 3 (a). ~
(E) is a figure which shows the insertion pattern of the control rod adjacent to 4 fuel assemblies. 1 ...... Fuel assembly 2 ...... Control rod 20 ...... Single fuel assembly local output distribution representative value storage device 21 ...... Control rod correction coefficient calculation device 22 ...... Output distribution monitoring device 23 ...... Calculation device 24 ...... Adjacent segment F infinity correction coefficient device 25 …… Calculator 26 …… Reactor

Claims (3)

[Claims] 1. A local power distribution, which is the power of each fuel rod in a fuel assembly segment, obtained by a single fuel assembly nuclear constant calculation performed in advance for several burnups and in-channel void fractions. Are stored in the storage device for each fuel type together with the position of the fuel rod in the segment, while the effect on the local power distribution caused by the neutron flow with the adjacent segment is stored in a single fuel assembly. While obtaining the correction coefficient to be corrected by the ratio of the segment average thermal neutrons and fast neutrons obtained by the nuclear constant calculation, further control rod correction coefficient for correcting the influence of the control rod insertion to the correction coefficient for each control rod insertion pattern By obtaining for each fuel rod position, the correction coefficient and the control rod correction coefficient can be used to obtain a representative value of the local output distribution obtained by the single fuel assembly nuclear constant calculation. Then, the representative value of the local power distribution in that situation, which is obtained by the interpolation of burnup and in-channel void ratio obtained from the power distribution monitor, is corrected for each fuel rod, and the maximum value among them is selected. A local output peaking coefficient monitoring method is characterized in that the local output peaking coefficient at each point of time is obtained by displaying the local output peaking coefficient on a display device. 2. Several representative values of the local power distribution calculated by the single fuel assembly nuclear constant calculation are obtained by dividing the single fuel assembly into several regions and selecting the fuel rod having the maximum value in each region. The local output peaking coefficient monitoring method according to claim 1. 3. The local output according to claim 1, wherein several representative values of the local output distribution obtained by calculation of the nuclear constant of a single fuel assembly are selected from the maximum of the local output distribution. Peaking coefficient monitoring method.