JP2744167B2 - Reactor power distribution monitoring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a monitoring device for monitoring a neutron flux distribution in a nuclear reactor, and more particularly to a reactor power distribution monitoring device capable of quickly detecting a regional vibration.

[0002]

BACKGROUND OF THE INVENTION Reactors are extremely stable when operating at their rated operating point. However, when the flow rate becomes natural circulation due to a failure of the recirculation pump for some reason, the output also drops below the rated point. Since the temperature decreases only to about 50%, the core becomes unstable. That is, in this state, if a disturbance is introduced into the core for some reason, for example, an operation of a control rod by an operator, the output may vibrate in a cycle of 2 to 3 seconds. The vibration of this output, when the core is stable,
It rapidly attenuates and reaches a steady state, but continues in the unstable state described above, and in severe cases, a constant vibration state (limit cycle state) is realized, and the soundness of fuel may be impaired. .

At present, as a countermeasure against the above-described vibration, a value (APRM value) obtained by averaging the count values of more than 100 neutron flux measurement devices (LPRM) arranged at various points in the reactor core is used. Monitor and if this value exceeds a predetermined limit, insert all control rods (scrum)
And then shut down the reactor.

[0004]

In the conventional reactor power distribution monitoring method, since the APRM value is monitored, it is effective only when the entire power of the reactor core vibrates. The reason is that the APRM value is obtained by averaging the LPRM count values monitoring the local power of the core, so that the vibration information does not remain in that value unless the power of the entire core vibrates. is there.

However, as can be seen from recent examples of output vibrations at overseas plants, the output power of a nuclear reactor includes not only the case where the power of the entire core vibrates but also the power which vibrates in a partial region of the core. In some cases (regional vibration).
In this case, for example, when the core is viewed from directly above, a power distribution oscillation occurs such that the power increases in half of the core and decreases in the other half. The reason why such vibration occurs will be described below. The neutron flux distribution in the core is as follows (1)
It is known to satisfy the formula.

(L + A) φ ₀ = (1 / λ ₀ ) Fφ ₀ (1) where φ ₀ : neutron flux L: neutron leak cross section A: same absorption cross section F: same generation cross section λ ₀ : Critical eigenvalue Normally, a neutron flux that satisfies equation (1) is called a fundamental mode neutron flux, and a critical eigenvalue is called a fundamental mode eigenvalue. The reason is as follows. A neutron flux that satisfies the expression (1) actually exists infinitely in addition to the fundamental mode as shown below.

(L + A) φ _n = (1 / λ _n ) Fφ _n (n = 0, 1, 2,...) (2) where n: mode order φ _n : neutron flux λ _n : Same critical eigenvalue These neutron fluxes are orthogonal to each other as shown below.

[0008]

(Equation 1) Here, it is assumed that the integration is performed over the entire core, and that each neutron flux is standardized. Therefore, it is possible that an infinite mode of neutron flux exists in the core, but among these neutron fluxes, the neutron flux that can be constantly present in the core is the neutron flux of the fundamental mode (corresponding to n = 0). Only the other modes (corresponding to n = 1, 2, 3 ...,
This is because, for example, when some disturbance is applied to the reactor core, it temporarily exists but immediately attenuates.

The degree of "short-lived" can be known from the mode subcriticality. The subcriticality is defined by the difference between the critical eigenvalue of the mode and the critical eigenvalue of the fundamental mode (which is always 1.0), and the order is assigned in the order of larger critical eigenvalue (lower order of subcriticality). The greater the subcriticality of a, the more "short-lived" its mode. The subcriticality varies depending on, for example, the size and shape of the core or the loading pattern of the fuel, etc., but is also greatly influenced by the temporary operation state of the core, that is, the core power, the core flow rate, the control rod pattern, and the like. .

In general, the neutron flux when the core is in a transient state is expressed as follows. φ = Sum An _n φ _n (n = 0, 1, 2,...) (4) where φ: neutron flux during transition _An : intensity of the n-th mode The intensity of each mode in the equation (4) is It describes the so-called contribution of that mode to the neutron flux in the transient state, and is a function of subcriticality and time. In other words, the neutron flux in the core in the transient state is expressed by superposition of each mode,
The weight at that time is the mode intensity.

The intensity of this mode can be obtained as follows using the orthogonality condition between the modes given by equation (3).

[0012]

(Equation 2) When the subcriticality of the higher-order mode is large, the intensity of this mode decreases with time, and in the steady state, as described above, Equation (4) becomes as follows. φ = A ₀ φ ₀ (6) However, if the subcriticality of the higher-order mode is small for some reason (for example, a fuel loading pattern in which the neutron distribution in the steady state is close to the distribution of the higher-order mode is selected) ), The decay of the higher-order modes is slow, and the neutron flux in the reactor core is tentatively expressed as the sum of the fundamental mode and the first-order mode with the lowest subcriticality among the higher-order modes as follows: become.

Φ = A ₀ φ ₀ + A ₁ φ ₁ (7) In this state, when any disturbance that excites the primary mode is applied, the primary mode becomes the fundamental mode. It may change in shape and may cause vibration if the core is unstable. FIGS. 4A, 4B, and 4C show higher-order modes up to the second order together with the fundamental mode in a core state of a 1.1 million kwe class reactor. The vertical axis of the figure indicates the size of the neutron flux (the unit is arbitrary), and the two horizontal axes indicate the position of the fuel assembly. In this figure, the neutron flux distribution is averaged in the core axis direction. As shown in the figure, in the first mode, the neutron flux decreases in the upper half of the core and increases in the lower half. In the second mode, the distribution of the first mode is rotated by 90 degrees in the circumferential direction of the core. It can be seen that the distribution is as follows. When such vibration occurs, the output of the entire core does not vibrate because the fundamental mode does not change, but the power distribution vibrates in a primary mode distribution form.

[0014] When the above-mentioned vibration occurs, the operator cannot detect the vibration because the power of the whole core does not greatly change because only the APRM value is monitored at present, and the vibration grows. If the amplitude is large, even the health of the fuel may be impaired.

The present invention has been made in view of the above points, and provides a reactor power distribution monitoring device capable of quickly detecting and notifying an operator when a regional vibration occurs. The purpose is to do.

[0016]

According to the present invention, as a means for achieving the above object, there are provided a plurality of neutron flux measuring instruments arranged in a reactor core, and LPs from each of the neutron flux measuring instruments.
And neutron flux distribution calculating means for calculating the neutron flux distribution based on the RM counts S ^M, and the higher mode calculating means for calculating a higher mode of the neutron flux on the basis of the calculation result of the neutron flux distribution calculating means, the higher-order mode LP calculated from neutron flux
Based on the RM calculated value S _n ^C and the LPRM count value SM,

(Equation 2) And an output means for outputting a calculation result of the intensity change calculation means.

[0017]

In the reactor power distribution monitoring apparatus according to the present invention, a neutron flux distribution is calculated based on the LPRM count value, and a higher mode of the neutron flux is calculated based on the neutron flux distribution. Based on the count value,
A change in the intensity of each mode is calculated. Then, this calculation result is output and notified to the operator. For this reason, AP
Unlike the conventional method using the RM value, when a region vibration occurs, it can be detected quickly.

[0018]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of a reactor power distribution monitoring apparatus according to the present invention. In the figure, reference numeral 1 denotes a nuclear reactor in which a core 2 is housed. LPRM3 is disposed, and inside the reactor 1, the total flow rate of the coolant, the core inlet / outlet temperature of the coolant,
A core current data measuring device 4 for measuring the current data of the core such as the control rod position and the like is arranged. And each L
The count value in the PRM 3 and the measurement value in the core current state data measuring device 4 are sampled periodically by the data sampler 5 or based on the request of the operator, and sent to the process control computer 6 and the higher mode calculator 7 described later. Is to be entered. These calculation results are output via the input / output device 8 and are notified to the operator.

FIG. 2 shows an arrangement state of the LPRM 3. In the figure, reference numeral 10 denotes a fuel assembly, and reference numeral 11 denotes a control rod. The LPRM 3 is a fuel assembly 10 constituting the core 2.
It is located in the so-called corner gap surrounded by
If the arrangement of the fuel in the reactor core 2 and the position of the control rods 11 maintain 1/4 symmetry, the arrangement is the same as that in all corner gaps.

The process control computer 6 is started when the data sampler 5 shown in FIG. 1 samples data, and calculates a neutron flux distribution (basic mode) in the core 2 at the time of the start. This corresponds to a monitoring function among the functions of the process control computer 6 in the sense that the neutron flux distribution in the current core 2 is calculated.

The process control computer 6 also has a prediction function in addition to the monitoring function. In the prediction function, the core state specified by the operator is calculated based on the result of the latest monitoring function. Flux distribution (fundamental mode)
Is calculated. In this case, the process control computer 6 is started at the request of the operator.

The calculation result of the process control computer 6 is inputted to a higher mode calculator 7, which is started in synchronization with the process control computer 6 and solves the above equation (2). Thus, the higher-order mode of the neutron flux calculated by the process control computer 6 is calculated. This calculation method is described in, for example, the document "Detailed Numerical Calculation Exercise" written by Hayato Togawa and published by Kyoritsu Shuppan.

The higher mode calculator 7 also takes in LPRM count values from the data sampler 5 until requested by an operator and performs the following calculations.

[0025]

(Equation 3) Here S ^M: LPRM count value S _n ^C: LPRM, which is calculated from the neutron flux of the n-order mode
The calculated value of the equation (8) is different from the equation (5) in that the LPRM count value and the LPRM are used instead of the neutron flux itself.
In the sense that the calculated value is used, it is a substitute value of the intensity of the mode.

The calculation result of the higher mode calculator 7 is shown in FIG.
As shown in FIG. 7, the output from the process control computer 6 and the input / output device 8 are input to the input / output device 8. In displaying and executing the prediction function of the process control computer 6,
Used to specify the core state.

FIGS. 3 (a) and 3 (b) show an example in which the present invention is applied to an output vibration of a 1.1 million kwe class reactor. FIG. 3 (a) shows a case where the output of the entire core vibrates. FIG. 3B shows an example in which the output causes a regional vibration.

As is clear from FIGS. 3A and 3B, when the power of the entire core vibrates, the intensity of the primary mode hardly changes, while the intensity of the fundamental mode greatly increases. When the output is causing regional oscillation, the intensity of the primary mode vibrates greatly, but the intensity of the fundamental mode hardly changes, contrary to the above. I understand.

As described above, according to the present embodiment, the regional vibration is grasped from the viewpoint of the higher-order mode of the neutron flux, and the intensity of each higher-order mode is shown with respect to time. This makes it possible to operate the reactor safely and efficiently.

[0030]

As described above, according to the present invention,
When a regional vibration occurs, it can be quickly detected and notified to the operator, and the reliability of the reactor power distribution monitoring can be improved.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram showing a reactor power distribution monitoring device according to one embodiment of the present invention.

FIG. 2 is an explanatory diagram showing an arrangement state of LPRMs in a core.

3A is a graph showing an example in which the present invention is applied to whole core vibration, and FIG. 3B is a graph showing an example in which the present invention is applied to regional vibration.

4A is an explanatory diagram showing a fundamental mode of a neutron flux distribution in a reactor core, FIG. 4B is an explanatory diagram showing a similar primary mode, FIG.
(C) is an explanatory view showing a similar secondary mode.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Nuclear reactor 2 Core 3 LPRM 4 Core current data measuring device 5 Data sampler 6 Process control computer 7 Higher mode calculator 8 Input / output device

Claims (1)

(57) [Claims] And 1. A nuclear reactor plurality of neutron flux measuring apparatus disposed in the core of the neutron flux distribution calculating means for calculating the neutron flux distribution based on the LPRM counts S ^M from the respective neutron flux measuring device, Higher mode calculating means for calculating a higher mode of the neutron flux based on the calculation result of the neutron flux distribution calculating means, and LPRM calculated from the neutron flux of the higher mode
Based the calculated value _S ^{n C} to said LPRM count value ^{S M,} Equation 1] A reactor power distribution monitoring device comprising: an intensity change calculating unit that calculates a change in intensity of each mode by calculating the following equation; and an output unit that outputs a calculation result of the intensity change calculating unit.