JPH0762714B2 - Reactor operation method - Google Patents

Description

TECHNICAL FIELD The present invention relates to an operating method for changing a control rod pattern in a boiling water reactor.

[Prior Art] In a boiling water type (BWR type) nuclear power plant, usually about 6 to 8 fuel rods per pair of fuel assemblies are made to contain a burnable poison such as gadolinia. Gadolinia has the ability to absorb neutrons, but this ability decreases as it is exposed to neutrons. Therefore, during the output operation of the BWR nuclear power plant, the excess reactivity of the core first increases at the initial stage of combustion due to the combustion of combustible poisons such as gadolinia. On the other hand, in the latter half of the operating cycle where the burnup has advanced to some extent, the loss of fissile material has an effect, and the excess reactivity of the core gradually decreases.

In a BWR type nuclear power plant, it is necessary to operate in order to keep the output constant in accordance with such a temporal change in the excess reactivity. First, during the rising period of the excess reactivity, the core flow rate is reduced in proportion to the increase of the excess reactivity of the core while keeping the control rod pattern constant. This keeps the output constant. If the excess reactivity of the core increases even when the core flow rate is reduced to the specified lower limit,
Control rods are inserted between the fuel assemblies to provide reactivity compensation.

Next, in the latter half of the operation cycle in which the excess reactivity decreases, the core flow rate is increased in accordance with the decrease in the excess reactivity of the core to keep the output constant. When the core flow rate reaches the specified upper limit value, the control rod is pulled out from this point and the power is adjusted.

By the way, the control rod pattern is changed when performing the excess reactivity compensation during the output operation. This change is roughly classified into a pattern exchange and a pattern adjustment. Here, the pattern exchange is
It refers to shifting to a control rod pattern on a sequence different from the control rod drawing sequence to which the current control rod pattern belongs. Next, the pattern adjustment means that the changed pattern is on the same control rod drawing sequence as the current control rod pattern, but the control rod that is left in the fuel assembly at the end by pulling or inserting differs. Say. These pattern exchanges and pattern adjustments are mainly performed to adjust the excess reactivity of the nuclear reactor, but may be performed because it is necessary to adjust the combustion distribution of fuel. That is, if the combustion with the same control rod pattern is continued, the combustion of the fuel assembly around the inserted control rod is bad, and the combustion of the fuel assembly without the control rod is advanced, resulting in a so-called one-sided combustion state. Become. This is not economical in terms of fuel combustion efficiency. Therefore, in such a case, pattern replacement or pattern adjustment is performed to adjust the combustion distribution. In the following part of the specification, these pattern exchange and pattern adjustment will be collectively referred to as pattern change.

In BWR type reactors, for example, when performing work that involves changing the core power distribution, such as pattern replacement and pattern adjustment, the relationship between core heat output and core flow rate is regulated. Therefore, the soundness of nuclear fuel and various equipment is maintained. That is, the operation of the nuclear reactor needs to be performed within the specified limit range.

Similarly, from the viewpoint of maintaining the integrity of nuclear fuel, it is allowed to freely change the power distribution only within the power distribution (PC envelope) at the time of pre-running operation. In addition, when increasing the output exceeding the PC envelope, it is necessary to suppress the output so as to satisfy the next output increase rate.

(I) Under normal conditions, it should be 0.10KW / ft / hr (kilowatt / foot / hour) or less.

(Ii) The maximum output increase rate for any four hours is at most 0.
11KW / ft / hr or less.

(Iii) One step of changing the output distribution is 0.2 KW / ft / hr or less.

However, in the conventional control rod operation performed conventionally, the control rod can be operated only in units of 2 notches at the full stroke 48 notches, and the above conditions cannot be satisfied at the rated output of the reactor. In many cases.
Therefore, conventionally, the core power is once reduced so that the linear power density does not exceed the PC envelope, and then the control rod pattern is normally exchanged.

Also, an appropriate maximum value of linear power density (MLHGR) during normal operation is defined. This is to prevent excessive strain on the cladding tube due to expansion of the fuel pellets, and to prevent the linear power density from exceeding the transient limit value even if an abnormal transient change occurs during operation. is there.

In order to change the pattern while satisfying the above-mentioned various restrictions, provisional operational management (PCIOMR; Pre-conditi
oning Interium Operation Management Recomendation
s) Adoption of a plan is recommended.

FIG. 3 shows an example of changes in reactor power when the conventional reactor operation method is performed based on the PCIOMR scheme. In this figure, the output of the reactor is 100% until the day when the pattern change is started (the number of days on the horizontal axis is 0). The core flow rate is decreased from the point a where the pattern change is started,
When the point b is reached, the control rod of the pattern B is inserted in order to change from a certain pattern A to a different pattern B. This further reduces the output to point C. After that, a part of the control rods forming the pattern A is pulled out, and the output rises to the point d. After that, the core flow rate is increased so that the output rises within the above-mentioned limit range.

After this, the flow rate is controlled again from the point e, and the output is changed to the point f.
To lower. Then, the remaining control rods forming the pattern A are pulled out. The output thereby rises to point g and the control rod pattern is B. After that, the core flow rate is increased again so that the power output rises within the limited range. The adjustment is completed at the point h at which the core flow returns to 100%, and the output is maintained thereafter.

[Problems to be Solved by the Invention] Such pattern exchange is performed several times during one cycle operation of the nuclear reactor, but when the operation by the PCIOMR scheme as shown in FIG. 3 is performed, the pattern exchange is performed. Occasionally, there is a problem that the output decreases and the operating rate deteriorates.

In view of such a situation, the present invention can perform pattern exchange while suppressing a decrease in output to the minimum. It is an object of the present invention to provide a nuclear reactor operation method.

[Means for Solving Problems] The present invention provides a fine adjustment control rod drive mechanism (FMCRD; Fine Motion Control Rod) in operation of a boiling water nuclear power plant.
Drive) to change the control rod pattern while maintaining the reactor power at approximately the rated level, the maximum line power density in the control rod operation from the current control rod pattern to the target control rod pattern is contoured. This is a reactor operation method in which a map of the operation limit value represented by is created, a route satisfying the constraint condition of the operation limit value is searched from this map, and the target control rod pattern is shifted along this route.

Therefore, by using a fine adjustment type control rod drive mechanism
The output increase rate is suppressed within a predetermined limit range in the area exceeding the PC envelope. By operating the control rods along the optimum route, the control rods of the boiling water nuclear power plant are taken in and out while keeping the output reduction to a minimum. As a result, it is possible to operate the reactor satisfying various controls when changing the pattern of the control rod.

[Examples] The present invention will be described in detail below with reference to Examples. First, an example of control rod pattern exchange will be described.

FIG. 1 shows control rod patterns from pattern A to pattern B.
MLHGR when core flow rate is fixed when replacing
This is a map. In the figure, the upper left corner shows the state before the pattern change. That is, all the control rods forming the pattern A have been inserted, and no control rod forming the pattern B has been inserted yet. On the other hand, the lower right corner of the figure shows the state after the pattern exchange. Here, all the control rods forming the pattern A are pulled out, and all the control rods forming the pattern B are inserted. In this figure, the control rods forming the pattern B are inserted from the top to the bottom of the diagram, and the control rods forming the pattern A are pulled out from the left to the right.

The contour lines shown on the map are created by connecting points having the same MLHGR (maximum line power density) corresponding to each control rod pattern by the core three-dimensional nuclear characteristic code and connecting them. Of these, the meshed area is the area where MLHGR exceeds the limit value (13.4 KW / ft) for ensuring fuel integrity.

In the nuclear reactor operating method of the present embodiment, ideally, pattern exchange is performed without reducing the output of the nuclear reactor. To do this, a route from the point of pattern A to the point of pattern B that does not pass through the over-limit region of MLHGR may be obtained. For pattern exchange in the state where the output of the reactor is maintained at 100%, for example, while pulling out one control rod belonging to pattern A, insert one control rod belonging to pattern B, and perform such a procedure one after another. It can be achieved by repeating.

The paths P ₁ and P ₂ shown by the two solid lines in FIG. 2 represent examples of such paths when the core flow rate is constant.
Of course, other paths may be realized by changing the pulling-out order of the control rods of the pattern A and the insertion order of the control rods of the pattern B. By such route selection, the restriction of MLHGR at the time of pattern exchange can be satisfied first.

By the way, although the core flow rate is fixed in FIG. 1, if the flow rate is changed, there is a wide range of route selection. The shaded area in FIG. 2 represents the case where the core flow rate is controlled within the allowable range in the control rod operation map shown in FIG. The allowable range is shown for two routes P ₁ and P ₂ .

In this way, there is a width in the path of pattern exchange,
If turned upside down, it means that the route for pattern exchange can be set in combination with the control of the core flow rate even if an appropriate route does not exist under the conditions shown in FIG. That is, in Figure 1 MLHG
At a position corresponding to the R control value excess region, the core flow rate can be appropriately reduced to reduce the reactor output, and the excess over the limit value can be prevented.

Generally, when the control rod is to be taken in and out with the output kept constant, the control rod should be taken in and out with the eigenvalue kept constant as shown by paths P ₁ and P ₂ shown by the solid line in FIG. There is a need to do. Therefore, if the ideal path such as the paths P ₁ and P ₂ cannot be found, the rated output cannot be maintained completely. However, by controlling the flow rate, it is possible to ensure efficient operation of the reactor while suppressing the decrease in output to the minimum.

Next, the restriction of the linear power density change rate in the case of adopting the nuclear reactor operating method in this embodiment will be described. Also in this embodiment, when the PC envelope is exceeded, the rate of change of the linear power density must be below a predetermined value. However, in the past, the control rod was inserted or withdrawn in units of two notches. Therefore, in consideration of the impact of the change in the output distribution due to this, the pattern exchange was performed with the output lowered in advance. However, in the present embodiment, this is done in several mm units instead of the conventional movement of control rods in several cm units. Such control rod operation has already been proposed for other purposes as a fine control rod drive mechanism (FMCRD).

In the control rod operation by the FMCRD method, the control rod can be moved much slower than the conventional one, and the output increase in the state of exceeding the PC envelope can be kept below the increase rate already explained.

The example of pattern exchange of the control rod has been described above,
The present invention can be similarly applied to the control rod pattern adjustment.

[Effects of the Invention] As described above, according to the present invention, the limit of the PC envelope, which is the limit in the conventional reactor operation method, and the change rate of the linear power density when the power is increased beyond the PC envelope, It is possible to satisfy the restriction, the maximum linear power density, and the restriction caused by the relationship between the core heat output and the core heat quantity, and perform the pattern exchange at the rated output or a slightly lowered output.

As a result, the operating rate of the nuclear reactor can be significantly improved, and the economic efficiency can be improved.

[Brief description of drawings]

FIG. 1 is an explanatory view showing an MLHGR map at the time of pattern exchange of a control rod in one embodiment of the present invention, FIG. 2 is an explanatory view showing an eigenvalue map at the time of pattern exchange in this embodiment, and FIG. It is explanatory drawing showing the conventional reactor operating method.

Claims (1)

[Claims] 1. When a boiling water nuclear power plant is operated, a control rod drive mechanism is used to change the control rod pattern while maintaining the output of the reactor at a substantially rated level. Create a map of the operation limit value that represents the maximum line power density in the control rod operation from the pattern to the target control rod pattern with contour lines, search for a route that satisfies the constraint condition of the operation limit value from this map, and A method for operating a nuclear reactor, characterized in that the target control rod pattern is transferred along with it.