JPS5857720B2 - Insulation resistance abnormality test device for neutron measurement equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an abnormality detection device for a measurement device consisting of a large number of neutron detectors three-dimensionally arranged in a reactor core for measuring the neutron flux distribution of a nuclear reactor, and particularly relates to an abnormality detection device for a measurement device consisting of a large number of neutron detectors arranged three-dimensionally within a reactor core. Furnace (hereinafter referred to as B
The present invention relates to a device suitable for abnormality detection of a local neutron detector (hereinafter abbreviated as LPRM) of a WR (abbreviated as WR).

Conventionally, in order to detect abnormalities in the neutron detectors of nuclear reactors, operators have to observe the detected values and select based on their intuition.
A second method of automatically monitoring the relative relationship of detected values, a third method of comparing theoretical calculation results and detected values, and a fourth method of using absolute correction means.
The first and second methods are not effective unless the neutron flux distribution is smooth and the detector array density is high, and the third method requires solving a three-dimensional diffusion equation, so it takes a long time. There are problems both in terms of accuracy and accuracy.

Although the fourth method is accurate, the number of absolute correction means is generally small and it takes a lot of time to calibrate all of the large number of detectors.

For example, in the core of a BWR with an output of 460 MWe, as shown in Figure 1A, there are axial directions A, B, C, I)
4 LPRMs are arranged to form one string 4, and 22 LPRMs are arranged horizontally at regular intervals (
Figure 1 mouth).

The indicated value of LPRA/I is constantly monitored and used for safe core operation and performance evaluation.

But BWR. In the case of odor, the neutron distribution within the reactor changes considerably locally, so it is difficult to detect initial weak abnormalities using the first and second conventional methods.

The BWR is equipped with a traveling neutron detector (
Several TIPs (hereinafter referred to as TIPs) are provided, and the TIPs are inserted from outside the furnace through a guide tube to the position of the LPRM that requires verification, allowing for strict verification.

However, it takes a long time of 2 to 6 hours to inspect all LPRMs, and there is a limit to the number of times the TIP can be run during the life of the plant due to wear and tear on the TIP structure, so it is unnecessary to run the LPRM unnecessarily frequently. It is not possible to test it.

In addition, the shape of the neutron flux distribution inside the reactor changes slowly with a time constant of about 5 to 6 hours after the power change due to the Zenon poisoning effect caused by nuclear fission, and due to changes in nuclear properties due to fuel combustion. , even if the core flow rate is held constant, the shape of the neutron distribution changes.

As described above, the conventional techniques have problems in terms of effectiveness and operability from the viewpoint of easily detecting abnormality in a detector in a short time.

SUMMARY OF THE INVENTION An object of the present invention is to provide an abnormality testing device that can quickly and easily test a large number of neutron detectors for abnormalities, and also determine the cause of the abnormality.

Generally, the causes of abnormal readings from neutron detectors in nuclear reactors can be divided into two types: abnormal sensitivity of the detector and deterioration of insulation resistance including signal transmission wiring from the detector to the indicator. In this method, the ratio of change in the indicated value of LPRM is used to estimate leakage current due to insulation resistance deterioration.

In particular, in BWR, the ratio of change in true neutron flux is calculated by taking advantage of the fact that the neutron distribution shape does not change significantly in response to short-term core flow rate changes in which the effect of Zenon poisoning does not appear. The leakage current value is estimated and calculated by accurately estimating the leakage current value, and an abnormality in the insulation resistance of the LPR, M is tested based on this.

A specific example of the insulation resistance abnormality test according to the present invention will be described in detail below.

LPRM (/i) As shown in FIG. 2, the LPRM consists of an ionization chamber 10, a signal transmission coaxial cable 20, a constant voltage source 30, and a current-vehicle pressure conversion amplifier 40.

17 is a bypass terminal.

The ionization chamber 10 includes an anode 11, a cathode 12, a support material 13, and a gas sealing material 14, and a camp 15 is filled with argon gas.

Further, a fissile material 16 such as U235 is coated on the inner surface of the cathode 12, and nuclear fission occurs on the inner surface of the cathode by thermal neutrons inside the reactor.

The electrically charged fission fragments generated at this time fly through the argon gas, ionizing the argon gas.

The generated charged ions have a voltage of V. It is taken out as an ionizing current at the anode cathode.

The ionizing current is led out of the furnace by a coaxial cable 20,
The paper pressure signal is amplified by the current-voltage conversion amplifier 40, and is given to the operator.

In such a device, the characteristics of LPII (,M) at location (i) at output level (n) are expressed by equation (1).

L (n, i) −KA(i) (Sφ(i)−
φ (n, i) + S r (+) ・ γ (i) - t
-4(i)) ... (However, L(n, i) is LPR, and the indicated value of M φ(n, i) is the true neutron flux level r(1)
is the gamma ray intensity Sφ(i) is the neutron flux sensitivity Sγ(1) is the gamma ray sensitivity t (i) is the leakage current KA (i) is the amplifier gain where the gamma rays generated by nuclear fission in the reactor penetrate the ionization chamber 10 When the argon gas is directly ionized, and the time constant associated with changing the output is extremely long, (1
) The term 5r(t)·r(i) in the equation becomes a kind of measurement error.

However, this is usually not a problem since the neutron flux sensitivity Sφ is used in a sufficiently large region compared to the gamma ray sensitivity S.

The output of BWR can be controlled by the core flow rate.

Now, when the core flow rate is increased, for example, ramp-like (linearly) so that the output level becomes (n layer) to (n+1), the indicated value of LPR,M at location (i) becomes L(n,
Suppose that there is a change from i ) to L(n+i,i).

At this time, the leakage current t(i) at location (i) is (
By eliminating Sφ(1) in equation (1), it is given by equation (2).

However, it is not possible to directly measure the value of β(i) without using absolute correction means.

Therefore, in the present invention, this value is estimated as follows.

Generally, in a BWR, as shown in FIG. 3, it is known that the shape of the neutron distribution does not change significantly with respect to changes in the core flow rate, and this is called the distribution similarity law due to flow rate control.

However, this law of distribution similarity due to flow rate control does not hold completely, and as can be seen in Figure 4, the slope of the change in the LPRM indicated value due to the change in the core flow rate is gi (i = A, B, C
.

D) is not the same, but differs for each string, and its inclination also differs depending on the axial position (A, B, C, D) of the same string.

The reason for this is that even if the core flow rate is changed at a constant rate, the proportion of the flow rate flowing through each channel is different, and even the same string is affected by the void distribution.

Therefore, the ratio of the true neutron flux level φ at location (1) expressed by equation (3) is estimated using the distribution similarity law based on this flow rate control.

An approximate method for this is to locate locations within the reactor core that are likely to be in the same fluidic situation, such as at the same distance from the center of the reactor, at the same horizontal location, and at the flow distribution orifice at the bottom of the channel. LPRM with the same opening degree
The method is to collect and average the ratios of the indicated values before and after the output level of the output level.

That is, β(i) at the location (i) to be detected is estimated using equation (4).

Here, M is the number of LPR,M determined to be under fluidly equivalent conditions.

Among the M LPRs, M taken into equation (4), there may already be some abnormalities, but by taking a large value of M, the influence can be kept small. I can do it.

The second term on the right side of equation (2) indicates the constant current due to the r line.

Gamma ray sensitivity ST has a proportional relationship with neutron flux sensitivity Sφ, considering the ionization process of argon gas.

If the proportionality constant CT determined for each individual LPR, M is determined by known means, the gamma ray sensitivity S is given by equation (5).

S T(t) = Cr・Sφ(1) ・・・・・・
(5) And the sensitivity Sφ is expressed by equation (6).

t is time.

Now, if the output level of BWR is increased ramp-like (linearly) from (,) to (n+1) for JT (-1°+1 to) time, the output of each part in the reactor will also be proportional to the core flow rate. Therefore, the numerator of the second term on the right side of equation (6) can be found from the indicated value as.

However, since the denominator a'/at cannot be determined accurately unless an absolute correction method is used, in the present invention, using the above-mentioned distribution similarity law on the flow rate side @Jl, it is calculated as follows. Estimate this value.

In other words, there are places within the reactor core that are considered to be in the same fluid situation, for example, at the same distance from the center of the reactor.
The slopes of the indicated values of LPR and M at the same horizontal plane position are collected and averaged.

That is, aφ/a1 of the detector at the location (O) to be detected is estimated using equation (8).

Here, M is the number of LPR and M determined to be under fluidly equivalent conditions Co /dKA(j) ・Sφ(j) y C6 (constant) A constant (7) By substituting the values obtained by Equations (8) and (8) into Equation (6), the neutron flux sensitivity Sφ(1) of the LPRM at location (1) at that time will be estimated.

In this way, the gamma ray sensitivity S is determined.

In addition, since the response of γ-rays is sufficiently slow, if the average neutron flux level before leakage current detection is φ(i), then
The gamma ray intensity γ(i) is calculated by equation (9) using a proportionality constant determined by known means for each nuclear reactor.

γ(i)=to-a(i) ・・・・・・・・・(
9) γ Therefore, the influence of γ rays in equation (2) can be calculated in advance.

Therefore, (4), (5). By substituting β(1) t S , (t), and γ(i) obtained from equation (9) into equation (2), the leakage current value at location (i) can be estimated.

On the other hand, the output level is changed from (n) to (n+1) at a time interval such that the distribution similarity law due to flow rate control is not affected by Zenon poisoning and the gamma rays can change sufficiently to follow the neutron flux level. In this case, β(i) can be estimated using equation (4), and the second term in equation (2) can be set to zero.

The estimated leakage current obtained in this way, ffex(i)
The actual configuration of LPRM (dimensions, shape, materials used, etc.)
The deviation Ez(i) from this is compared with a reference value such as the theoretical leakage current 7th(i) calculated from 7th(i), and is determined by an appropriate means, for example, equation (10), and the deviation is larger than the specified value εt. At this time, the abnormality of the LPR,M and its location (i) can be displayed.

As the reference value tth, it is also possible to use the leakage current that flows when a specified voltage is applied before using the detector, that is, before installing a normal detector in the furnace.

FIG. 5 is a flowchart showing the operation of the abnormality testing device for the neutron measuring device of the present invention explained above.

There is no need to explain this further; it will be fully understood from the following description.

As is clear from the above description, according to the present invention, many LPRMs can be verified in a short period of time, which is extremely useful for safe operation of a nuclear reactor.

It should be noted that there is no need to go to the trouble of changing the reactor output level for verification purposes, and timing such as load following operation can be used to change the reactor output level.In this way, there will be no operational problems, and the output level will be changed. When applying this method under the condition that B is held constant, for example, B
In WR, the total flow rate of the two recirculation pumps (for changing the core flow rate) is kept constant, and the flow rate of one is changed to be larger than the other, resulting in a change in the flow distribution within the core. It is also possible to use

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the arrangement of the LPRM inside the core of a boiling water reactor, Figure 2 is a schematic diagram of the LPRM structure and measurement system, and Figure 3 is the similarity law of the neutron distribution shape when the core flow rate changes. Figure 4 is a diagram showing the indication characteristics of four L PR, M on the same string when the core flow rate changes, Figure 5 is a flow diagram showing an example of the procedure for L PR, M abnormality verification. It is a line diagram. 2...LPR, M (local neutron detector), 3.
...Control rod, 4...String, 11.
... Anode, 12 ... Cathode, 20
...Coaxial cable, 40...Current-voltage conversion amplifier.

Claims (1)

[Claims] 1. Isolines for transmitting and instructing measurement signals from a plurality of neutron detectors arranged three-dimensionally in the center of each nuclear reactor;
In a terminal device consisting of an amplifier and an indicator, different output levels are reached by means of increasing and decreasing the output of each spatial part of the core in a substantially similar manner, and the ratio of the indicated value of the indicator before and after the output change is determined. The ratio of the true neutron flux levels before and after the output change at the location where the neutron detector to be tested is present is determined by the ratio of the true neutron flux levels before and after the output change for a group of neutron detectors that are under conditions substantially fluidly equivalent to the neutron detector to be tested. An abnormality is determined when the estimated leakage current, which is estimated from the ratio of the values and obtained using the estimated value of the gain of the amplifier, the ratio of the indicated value, and the ratio of the true neutron flux level, exceeds a specified value. Insulation resistance abnormality test device for neutron measurement equipment. 2. Select a group of neutron detectors under substantially equivalent fluid conditions based on the radial position, horizontal position, symmetry, and opening degree of the flow distribution orifice installed at the bottom of the core. An insulation resistance abnormality testing device for a neutron measuring device as set forth in claim 1. 3. Claim No. 3, characterized in that when changing the reactor output, the change is made in such a short time that the influence of Zenon dynamic characteristics does not appear significantly, and slowly enough that gamma rays can follow changes in neutron flux. An insulation resistance abnormality testing device for a neutron measurement device according to item 1. 4. A patent characterized in that the influence of gamma rays is corrected for the estimated leakage current using the estimated detector sensitivity calculated from the neutron detector indicated value and the average value of the neutron flux level over a long period of time before the time of detection. An insulation resistance abnormality testing device for a neutron measuring device according to claim 1.