JPH01193693A - Controlling of nuclear reactor - Google Patents

Abstract

PURPOSE:To improve a core reactivity and to enable an improvement of fuel economy, by inserting reactor control rods into a central and a peripheral regions of the reactor core at a beginning of an operation cycle in advance and withdrawing the reactor control rods at the peripheral region before an end of a beginning stage of the operation cycle which is less than around 500 Mwd/st of a fuel burn-up. CONSTITUTION:As shown in the figure (a), for a central region of a reactor core 1 loaded with fuels which do not require a shakedown operation, 9 reactor control rods are inserted upto the axial position (POS) 14 and for the outer most part of a peripheral region of the reactor core, 8 reactor control rods are inserted upto the position of POS 36. For reference, POSO shows a fully inserted core. Thereafter, until an end of a beginning stage of an operation cycle with around 500Mwd/st of fuel burn-up, shallowly inserted control rods at the peripheral region are withdrawn, as shown in the figure (b). And then between the operation cycle of 500-9,000Mwd/st burn-up, 5 of 9 deeply inserted control rods are withdrawn. Finally, between the operation cycle of 9,000-10,000 Mwd/st, the remaining control rods are withdrawn. In this final condition, the reactor is operated during the operation cycle 10,000-10,500Mwd/st burn-up.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a method of controlling a nuclear reactor in which the output of a boiling water reactor is controlled using reactor control rods. This invention relates to a method for controlling a nuclear reactor loaded with fuel that does not require operation.

(Prior Art) Generally, in a boiling water reactor, combustion can continue for about one year without fuel replacement. During this combustion period, the reaction changes as combustion progresses and the output fluctuates, so it is necessary to adjust the reactivity during the combustion period to maintain a constant output. This adjustment of reactivity, that is, control of output, is performed by changing the amount of thermal neutrons absorbed in the reactor core by inserting and withdrawing control rods.

In addition, this power control changes the flow rate of coolant flowing through the reactor core, that is, the core flow rate, and changes the amount of steam generated in the core, that is, the void ratio, and changes the amount of coolant that also serves as a moderator that exists in the core. This is done by adjusting the moderation effect of neutrons.

Fine adjustment of this reactivity is done by changing the core flow rate, but the range of adjustment by changing the core flow rate is not very large, so if the reactivity cannot be adjusted completely by changing the core flow rate, In order to adjust the reactivity, so-called control rod pattern changes are being carried out to change the number and amount of control rods inserted into the reactor core.

In recent years, the reactivity change is small throughout the operation cycle (and the control rod pattern is operated with a fixed control rod pattern).
(Cell Core) technology has been developed that has significantly reduced the number of such control rod pattern changes during the operating cycle.

By the way, the fuel for boiling water reactors is constructed by stacking cylindrical fuel pellets inside a zirconium alloy fuel cladding tube, but the fuel cladding tube and the fuel pellets have different coefficients of thermal expansion. Also, there are differences in temperature during operation. For this reason, for example, when the output is suddenly changed, the difference in thermal expansion between the fuel pellets and the fuel cladding causes the so-called pellet cladding interaction (pellet cladding interaction) in which the fuel pellets and the fuel cladding functionally interfere with each other.
teracti-on) (hereinafter abbreviated as PCI)
This may cause damage to the integrity of the fuel cladding.

As an operating method that does not cause PCI,
OMR (Pre-Conditioning Inte
rim Opera-atlng Management
Recommendations (break-in) are applied.

In P C1 OMR, a threshold is set for the linear power density of the fuel rods. Therefore, when operating control rods such as changing the control rod pattern, the power distribution changes significantly and the linear power density also exceeds the threshold, so the power is temporarily reduced.
After operating the control rods below the threshold, the output is gradually increased to return to the rated output. This results in a large loss in terms of plant availability.

In order to reduce this operating rate loss as much as possible, several techniques have been developed to flatten the power distribution within the furnace as much as possible and lower the linear power density.

On the other hand, in recent years, fuels that do not require such P CI OMR have been developed and put into practical use.

In a reactor core loaded with fuel that does not require PCIOMR, an operating technology has been developed that actively increases the linear power density within a range that maintains the control value without reducing the linear power density, and as a result improves the reactivity of the fuel in the core. ing. Examples of such driving techniques include the following.

■ Local conflict peak kink conflict factor for surrounding fuel rods with high neutron flux inbortance (ratio of maximum output to average output)
By increasing the reactivity of the fuel assembly, the reactivity of the fuel assembly is improved.

■ From the beginning of the cycle to the end of the cycle, the axial power distribution is increased downward, reducing the core void fraction and accumulating plutonium. Then, at the end of the cycle, the axial power distribution peaks upward, and the accumulated plutonium is burned to obtain core reactivity.

(Problems to be Solved by the Invention) As mentioned above, although it is possible to increase the operating rate in a core loaded with fuel that does not require PC1OMR, There is. An example of a nuclear reactor control method is shown in FIG. 5, in which a fixed control pattern is used in most of the combustion sections during the operation cycle (cycle burnup up to approximately 9.000 Mvd/sL). In FIG. 5, reactor control rods are inserted into the reactor core 1 at predetermined axial positions indicated by numeral 14, for example.

The reason why the linear power density is high at the beginning of the operating cycle is that, as shown in FIG. 6, the local peaking factor reaches its maximum at the beginning of the operating cycle and decreases significantly with combustion.

Therefore, in order to keep the linear power density at the beginning of the driving cycle within predetermined limits, an upper limit on the local peaking factor is necessarily established.

FIG. 7 shows the relationship between local peaking factor and infinite multiplication factor. That is, if the local peaking factor decreases, the infinite multiplication factor also decreases, which reduces the core reactivity and impairs economic benefits.

On the other hand, in a boiling water reactor, the following has been found in a control method using reactor control rods.

In other words, shallowly inserted control rods (reactor control rods inserted to within about 1/3 of the bottom of the core) are effective in suppressing the power distribution below the core, but they hardly contribute to reactivity. .

In addition, deep-inserted control rods (reactor control rods that are inserted deeply to about 1/3 from the top of the reactor core) make a small contribution to the power distribution of the entire reactor core, but they cannot adjust the reactivity. Contribute greatly.

Reactor control rods in the outermost part of the core have approximately half the power distribution suppression and reactivity adjustment compared to reactor control rods in the central region.

Furthermore, when withdrawing a control rod in the middle of the operation cycle, it is easier to pull out if the reactivity of the fuel around the pullout is low, but if this is not the case, the longer the insertion period, the more likely an output peak will occur after pullout. Become.

In such a case, for example, a shallowly inserted control rod as described above,
It would be advantageous to implement a control method that can improve fuel economy by effectively combining deep insertion control rods and outermost control rods.

The present invention has been made in consideration of these points,
The present invention aims to provide a nuclear reactor control method capable of optimizing the linear power density at the beginning of the operating cycle and improving fuel economy.

[Structure of the invention]

(Means for Solving the Problems) The present invention is a nuclear reactor control method in which reactor control rods are gradually withdrawn from the core of a boiling water reactor loaded with fuel that does not require PCIOMR to control output. , the reactor control rods are inserted in advance into the central region and the peripheral region of the reactor core at the start of the operation cycle, and the reactor control rods in the peripheral region are pulled out by the end of the initial operation cycle up to about 500 Mvd/st, and then the The feature is that the output is controlled only by the reactor control rods in the area.

(Function) According to the present invention, the maximum linear power density can be reduced by suppressing the power peak in the lower part of the core at the initial stage of the operation cycle up to approximately 500 Mvd/st.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing a first embodiment of the nuclear reactor control method according to the present invention. In Figure 1, nine reactor control rods are placed in advance at axial positions (hereinafter referred to as PO8) in the central region of the reactor core 1 loaded with fuel that does not require PCI OMR.
) Insert it up to position 14. At the same time, eight reactor control rods are inserted into the outermost circumference of the peripheral region of the reactor core 1 up to the PO 336 position (FIG. 1(a)).

Here, PO8 is a value indicating the axial position of the reactor control rod in the reactor core, pos o indicates the fully inserted state,
PO348 shows the fully withdrawn state.The nine reactor control rods in the central region are at the POS 14 position, so they are deeply inserted control rods. In addition, the eight reactor control rods in the surrounding area are PO3
Since it is at position 36, it is a shallow insertion control rod.

Next, by the end of the initial operation cycle of 500 Mvd/st, the shallowly inserted control rods in the surrounding area should be pulled out (first
Figures (a) (b)).

Subsequently, operation cycles 500 to 9. 000Mvd/s
During the period t, all five of the nine deeply inserted control rods in the central region are withdrawn (Fig. 1(b)). Pull out all the remaining four deep insertion control rods in the area (first
Figure (C)). In this state, all reactor control rods are fully withdrawn, and in this state, the operating cycle is 10,000 to 10,5.
The operation is carried out for 00Mwd/st (Fig. 1(d)
).

Next, the effects of the first embodiment of the present invention will be explained.

FIG. 2 is a diagram showing the relationship between the axial nodes of the reactor core and the relative power, where the relationship in the first embodiment is indicated by a, and the relationship in the conventional example is indicated by C. Note that the reference numeral indicates the second embodiment.

As shown in FIG. 2, according to this embodiment, the power peak in the lower part of the core can be suppressed compared to the conventional example.

In addition, Fig. 3 shows the relationship between cycle burnup (Gvd/st) and maximum linear power density (Kv/('t). In Fig. 3, the first embodiment is indicated by a, and the conventional example is indicated by a. code C
are shown respectively.

As shown in Figure 3, compared to the conventional example, the
The maximum linear power density can be kept low at the beginning of the operation cycle of Vd/st (0 to 0, 5 Gvd/st). In the case of the conventional example, the maximum linear power density is high at the beginning of the driving cycle because the local peaking factor is 1.0 at the start of the driving cycle, as shown in FIG.
This is due to the fact that it is as high as 3.

On the other hand, in the case of this embodiment, the reason why the maximum linear power density decreases at the beginning of the operation cycle is because the lower peak of the axial power distribution is suppressed, as shown in FIG.

As described above, according to this embodiment, the maximum linear power density can be reduced at the beginning of the operation cycle. Therefore, the maximum linear power density is the operating limit value (13,4Kv/I't)
The local peaking factor can be increased within a range not exceeding , thereby increasing the infinite multiplication factor and core reactivity as shown in FIG.

Additionally, by suppressing the relative power below the core at the beginning of the operating cycle, combustion below the core can be delayed. The lower part of the reactor core burns out in the latter half of the operating cycle, making output peaks more likely to occur. Therefore, as shown in FIG. 3, it is possible to maintain a higher maximum linear power density than before during the driving cycle, especially in the latter half of the driving cycle, and it is also possible to increase the maximum linear power density in the latter half of the driving cycle.

If the period during which the relative power below the core is suppressed is extended in the early stage of the operating cycle, the peak power in the lower part of the core will become larger in the latter half of the operating cycle.

The period at the beginning of the operation cycle that does not significantly reduce the maximum linear power density in the early stage of the operation cycle and does not adversely affect the maximum linear power density in the latter half of the operation cycle is determined by the applicant's three-dimensional simulation from 0 to 500 M.
It has been found that wd/st is desirable.

Furthermore, since the reactor is loaded with fuel that does not require PCIOMR, the shallowly inserted control rod can be withdrawn at approximately the rated output at the beginning of the operation cycle.

Therefore, there is no loss in operating rate.

Next, a second embodiment of the present invention will be described with reference to FIG.

In Figure 4, five reactor control rods are placed in advance at the position of PO316 in the central region of the reactor core 1 loaded with fuel that does not require PCIOMR, and four reactor control rods are placed at the position of PO318. Insert until. At the same time, four reactor control rods are inserted into the peripheral area of the reactor core 1 up to the PO 318 position (FIG. 4(a)). The reactor control rods inserted into these central and peripheral regions all become deeply inserted control rods.

Subsequently, by the end of the initial operation cycle of 500 Mwd/st, 4 was inserted into the surrounding area to the position of PO318.
While pulling out the deep insertion control rod in the book, gradually pull out the deep insertion control rod in the central area (Figure 4 (a) (b))
.

In this state, nine deep-inserted control rods are located in the central region of P
It has been inserted up to the O314 position.

Subsequently, operation cycles 500 to 9. 000Mvd/s
During time t, five deep insertion control rods out of nine deep insertion control rods in the central region are withdrawn (FIG. 4(b)).

Subsequently, during the operation cycle of 9,000 to 10,000 Mwd/st, all the remaining four deeply inserted control rods in the central region are withdrawn (fourth
Figure (C)). In this state, all the reactor control rods are fully withdrawn, and in this state, the operating cycle is 10.000 to 10.5.
The operation is carried out for 00Mwd/st (Fig. 1(d)
).

Next, the effects of the second implementation 'PI of the present invention will be explained.

As shown in FIG. 2, compared to the conventional example (marked C), the second embodiment (marked b) can suppress the power peak at the lower part of the core. In addition, as shown in Fig. 3, the conventional example (
0 in the case of the second embodiment (code b) compared to code C)
~500 Mvd/5t (0~0, 5 Gwd/s
At the beginning of the operation cycle of t), the maximum linear power density can be kept low.

〔Effect of the invention〕

According to the present invention, compared to conventional control methods, the power peak in the lower part of the core can be suppressed and the maximum linear power density can be lowered. Therefore, the local peaking factor can be increased within a range in which the maximum linear power density does not exceed the operating limit value, thereby increasing core reactivity and improving fuel economy.

[Brief explanation of the drawing]

FIGS. 1(a) to (d) are diagrams showing a first embodiment of the nuclear reactor control method according to the present invention, and FIG. 2 is a diagram showing the relationship between axial nodes and relative outputs showing the effect of the present invention. Figure 3 showing
The figure shows the relationship between cycle burn-up and maximum linear power density, which shows the effect of the present invention. Figures 4 (a) to (d) illustrate the second embodiment of the present invention. Figure 5 shows the relationship between cycle burn-up and maximum linear power density. Figure 6 shows the conventional nuclear reactor control method, showing burnup and local conflict peaking.
FIG. 7 is a diagram showing the relationship between the local peaking factor and the infinite multiplication factor. 1...The core of a nuclear reactor. Applicant's agent Sato -O 0-500 MWd/sf gg22! ! Sade S shigekikyo zero 8 (a) N: 9 w e9 ta 8 sutsukyo zero 8 (C) pm 1 500-,. 9000MWd/s meeting (b) 10000-10500MWd/s↑ 889! 8 (d) - Fig. 48 engraving output Fig. 2 Saiful san, t-t/f (GWd/sfl 小 3 Fig., smoke pickle (GWd/s↑) Fig. 6 Local beakinku゛・Factor Geiso Figure 88c!!! 8 Kato Raikyo Zero 8 (C) 1st 10000~105105O0/s↑ cgg2:a! Sahi 8 S Enjo Zero 8 (d) Figure 0~500MWd, /5f N: 22 ae δ 8 Manga Masha 8 (C) Easy 5: 500 to 9000 MWd/s↑ 382"!N"f4g; z yen 9 zero g (d) Figure

Claims (1)

[Claims] In a reactor control method in which power control is performed by gradually withdrawing reactor control rods from the core of a boiling water reactor loaded with fuel that does not require PCIOMR, a The reactor control rods are inserted, and the reactor control rods in the peripheral area are pulled out by the end of the initial operation cycle of approximately 500 Mwd/st, and then the output is controlled only by the reactor control rods in the central area. How to control a nuclear reactor.