JP3278537B2 - Criticality warning device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is to notify a worker by detecting a criticality in a nuclear fuel facility such as a nuclear fuel reprocessing facility or a nuclear fuel processing factory, which handles nuclear fuel material, by detecting the criticality and issuing an alarm. The present invention relates to a criticality warning device that prompts evacuation and stops the operation of the facility so that the criticality event does not continue.

[0002]

2. Description of the Related Art Criticality warning devices are installed in facilities handling nuclear fuel materials such as nuclear fuel reprocessing facilities and nuclear fuel processing factories. If a criticality event occurs, the criticality event is immediately detected and an alarm is issued. In this way, a function is provided to notify the worker, to prevent the worker from being exposed to excessive radiation, and to immediately stop the operation of the facility so that the criticality event does not continue.

[0003] Therefore, the criticality warning device is an extremely important device for ensuring the safety of workers and also an important device for determining stable operation of facilities. This means that the operation of the criticality warning device immediately leads to the shutdown of the facility. If it malfunctions, there is no problem for the workers to be exposed to radiation, but the facility will operate every time. Is stopped, and the operation rate decreases.

[0004] In particular, in a facility involving continuous process operation such as a nuclear fuel reprocessing facility, once the operation is stopped, it takes a very long time to return to the normal operation state after the operation is returned. The impact on facility availability is extremely significant.

Conventionally, various improvements have been made in the criticality warning device so as not to cause such a malfunction. In early criticality warning systems, a single gamma ray detector was used as a radiation detector (hereinafter abbreviated as a detector), a threshold was set at a higher dose rate than normal, and the gamma ray dose rate detected by the gamma ray detector was set. However, there has been a method in which when the threshold value is exceeded, it is determined to be critical.

However, according to this method, even if the detector picks up a noise signal or the like having a value higher than the threshold value, it malfunctions immediately. Therefore, in order to reduce such malfunction, for example, FIG. As shown in the block diagram of
When the gamma ray dose rate detected by at least two detectors in the logic circuit 2 exceeds the threshold value N, the three detectors 1a to 1c are regarded as one set, and are determined to be critical.

FIG. 3 shows the relationship between the time change of the dose rate detected by each of the detectors 1a to 1c and the threshold value in the three cases (a) to (c). Show.
In the case of (a) and (b) in FIG. 3, the dose rate fa in at least two detectors 1a and 1b.
Since (t) and fb (t) exceed the threshold value N, the alarm device 3 operates.

However, in the case of FIG. 3 (c), the alarm device 3 does not operate because only one detector 1a exceeds the threshold value N (fa (t)> N: fb (t), fc
(T) ≦ N). Then, in order to further improve the reliability, in addition to the logic of judging that the dose rate is critical by confirming that the dose rate exceeds the threshold value N in at least two detectors described above, Time difference to reach Δ
In some cases, a simultaneous detection condition that t must be within a specific time width (gate time T) is added.

More specifically, using the case of FIG. 3B, the dose rates fa (t) and fb (t) in the two detectors 1a and 1b exceed the threshold value N. Therefore, in at least the two detectors, the alarm device 3 is activated in a manner that only considers that the dose rate exceeds the threshold value N.

However, considering the simultaneous detection condition, for example, when the gate time T is set to 0.5 sec, the alarm device 3 does not operate unless the time difference Δt ≦ 0.5 sec. That is, the criticality judgment logic of the logic circuit 2 in this criticality alarm device can be summarized as shown in the logic flow diagram of FIG. 7, and the conventional criticality alarm device has such a two-stage criticality judgment logic in the logic circuit 2. In order to prevent malfunction, the use of a device has been devised.

[0011]

Although these measures reduce the malfunction rate of the criticality warning devices, some criticality events cannot be detected by these criticality warning devices. Normally, as shown in the schematic diagram of FIG. 4, each detector 1 a to 1 c stores the nuclear fuel material 4 due to restrictions on the layout of the building, as shown in the schematic diagram of FIG. 4. Are not arranged at the same distance from the monitoring target device 5 and differ from each other as distances La to Lc.

Further, since the detectors 1a to 1c are generally installed outside the shielding body 6, radiation is emitted from each of the detectors 1a to 1c.
The thicknesses da to dc of the shielding body 6 passing therethrough also differ before reaching the positions a to 1c, and the effect of this becomes more remarkable as the installation location of the detectors 1a to 1c is further away from the monitoring target device 5. .

As described above, a difference Δt in the dose rate detected by each of the detectors 1a to 1c reaches the threshold value N due to the difference in the arrangement condition of the plurality of detectors. Critical events with a slowly increasing dose rate (eg late criticality)
In this case, the time difference Δt tends to increase.

As described above, in the conventional criticality alarm device, instead of taking measures to reduce the malfunction rate by adding the simultaneous detection condition, due to the condition of the detector arrangement and the slowness of the dose rate increase rate, Since the time difference Δt does not fall within the gate time T and the criticality cannot be detected in some cases, there is a problem that the demand for reducing the exposure of the worker to any criticality event cannot always be met.

It is an object of the present invention that the radiation dose rate detected by at least two detectors exceeds a threshold value, and coincidence of the doubling times is detected, so that the probability of malfunction is low and the critical An object of the present invention is to provide a criticality alarm device with high detection accuracy.

[0016]

In order to achieve the above object, a criticality warning device according to the first aspect of the present invention comprises a plurality of line detectors, a logic circuit and a warning device, and is installed in a nuclear fuel facility to detect a criticality event. In the criticality alarm device that performs detection,
The logic circuit is determined to be critical when the dose rates of at least two detectors simultaneously exceed a threshold value and the doubling times of the dose rates detected by at least two of the detectors match. It is characterized by doing.

The criticality warning device according to the second aspect of the present invention is characterized in that the logic circuit determines that the criticality is reached when the dose rate doubling time is equal to or longer than a certain value. The criticality warning device according to the invention according to claim 3, wherein in the criticality warning device, the logic circuit determines the doubling time of the dose rate detected by each detector from the time-dependent information of the dose rate detected by each detector. It is characterized by obtaining.

A criticality warning device according to a fourth aspect of the present invention is characterized in that the logic circuit calculates the doubling time of the dose rate detected by each detector using a logarithmic differentiation method. A criticality warning device according to a fifth aspect of the present invention is characterized in that in the criticality warning device, three detectors are provided. The criticality warning device according to the invention of claim 6 is
In the criticality warning device, a gamma ray detector is used as a detector.

[0019]

According to the first aspect of the present invention, radiation detection is performed by a plurality of detectors. In a logic circuit, at least two detectors simultaneously have a dose rate higher than a threshold value and each of the radiation rates is higher than a threshold value. When the doubling times are the same, it is determined to be critical, an alarm is issued from the alarm device, and the facility is stopped. However, if the doubling times do not match, even if the detected dose rate exceeds the threshold,
The dose rate is not determined to be critical due to noise signals or the like.

According to a second aspect of the present invention, in the logic circuit, when the doubling time of the dose rate detected by each detector is larger than the minimum value of the doubling time obtained from past critical accident cases and analysis, the critical circuit is determined to be critical. to decide. According to the third aspect of the present invention, in the logic circuit, the doubling time of the dose rate in each detector can be easily obtained from the time-dependent information of the dose rate detected by each detector.

According to a fourth aspect of the present invention, in the logic circuit, the doubling time is calculated from the logarithm of the dose rate detected by each of the detectors by a logarithmic differentiation method for time-differentiating the logarithm to determine the coincidence. . According to the fifth aspect of the present invention, by providing three detectors as a set, even if one of the detectors becomes abnormal, the criticality can be detected if the two detectors are sound. Yes, high reliability can be obtained.

According to a sixth aspect of the present invention, by employing a gamma ray detector, even if the detector is installed in a place away from a nuclear fuel material which is easy to maintain, the gamma ray penetrates the monitored equipment and so on. Reaching relatively easily allows criticality detection.

[0023]

An embodiment of the present invention will be described with reference to the drawings. The detailed description of the same reference numerals as those of the above-described conventional technology is omitted. As shown in the block diagram of FIG. 2, the criticality alarm device includes a plurality of detectors 1 a to 1 c connected to a logic circuit 2, and is configured by an alarm device 3 that operates according to a command from the logic circuit 2. Here, three detectors are shown as an example.

Next, the operation of the above configuration will be described with reference to the logic flow diagram of FIG. 1 using an example in which three detectors 1a to 1c are installed as shown in the schematic diagram of FIG. When the three detectors 1a to 1c are arranged outside the shielding body 6 with respect to the monitoring target device 5 as shown in FIG. 4, the dose of radiation detected at a certain time t by each of the detectors 1a to 1c. Let the rates be fa (t), fb (t) and fc (t).

At this time, the value of the dose rate fa (t) in the detector 1a closest to the monitoring target device 5 is the largest, and the order of the dose rates fb (t) and fc (t) follows. In addition, the linear distances La to Lc from the detectors 1a, 1b, and 1c to the monitoring target device 5 increase in the order, and the distances da to dc across the shield 6 of the concrete wall also increase.

The time dependency n (t) of the number of neutrons generated by nuclear fission in the nuclear fuel material 4 is generally expressed by the following equation (1) using the neutron lifetime 1 and the neutron doubling rate k. _{n (t) = n 0 ·} exp ((k-1) t / l) ... (1)

When the radiation to be detected is gamma rays generated by nuclear fission (primary gamma rays), the dose rate (t) at time t, which is the time dependency f of the gamma ray dose rate, is also the same as in equation (1). In the form, it is expressed as equation (2). _{f (t) = f 0 ·} exp ((k-1) t / l) ... (2)

When critical, k ≧ 1 and (k−1) / l ≧
Since it is 0, the following equation (3) is obtained by replacing (k-1) / l with λ. f (t) = f ₀ · exp (λt) (3)

Here, if the doubling time τ, which is the time when the dose rate is doubled, is used, λ becomes as shown in equation (4). λ = 1n2 / τ (4)

From equations (3) and (4), the following equation (5) is obtained. _{f (t) = f 0 ·} exp ((1n2 / τ) t) ... (5)

From this equation (5), it can be seen that the time-dependent change of the dose rate in the critical state is like the relationship between the doubling time τ and the dose rate shown in the characteristic diagram of FIG. Further, the rate of increase of the dose rate is characterized by the magnitude of the doubling time τ, and is all expressed by Expression (5).

In the criticality warning device according to the present invention, the value of the doubling time τ is obtained from the time-dependent change information of the dose rate detected by each of the detectors 1a to 1c, instead of the simultaneous detection condition using the gate time T conventionally employed. Is obtained, and when it is determined that the values of τ detected by at least two detectors are the same, it is determined to be critical.

Thus, a critical event exceeding the gate time T which cannot be detected conventionally can be detected. As a specific method of obtaining the value of the doubling time τ, there is a logarithmic differentiation method of taking the logarithm of the dose rate and further obtaining the doubling time τ from λ obtained by differentiating it with time.
The method will be described below.

From the above equation (3), the dose rate is f (t) = f _0.
exp (λt). Taking the logarithm of the dose rate and then time-differentiating it yields the following equation (6).

D / dt (1n (f ₀ · exp (λt))) = d / dt (1n (f ₀ · exp (λt)) / (f ₀ ·
exp (λt)) = λ (6)

The doubling time τ is obtained from λ of the equation (6) using the equation (4) as follows. τ = 1n2 / λ. The coincidence of the doubling time τ obtained for each detector is determined in the logic circuit 2.

[0037] Here, as in the time variation of the dose rate shown in dose rate characteristic diagram of FIG. 6, a time t ₁ at two detectors 1a, the dose rate at 1b, exceeds the threshold value N (Fa (t), fb (t)> N: fc (t) <N). At this time, when it is determined that the doubling times τ of the dose rates detected by the two detectors 1a and 1b are the same, it is determined to be critical.

That is, the dose rate fa (t ₂ ) obtained at time t ₂ is twice that of the dose rate fa (t ₁ ) obtained by _one detector 1a at time t ₁ (fa).
(T ₂ ) = 2fa (t ₁ )). If the same is confirmed for the dose rate detected by the other detector 1b (fb (t ₂ ) = 2fb (t ₁ )), the two detectors 1
The doubling time τ of the radiation detected by a and 1b is equal, and the doubling time at that time is τ− (t ₂ −t ₁ ).

When the doubling times τ are equal, the logarithm of the dose rate is plotted on the ordinate and the time t is plotted on the abscissa, as shown in FIG.
The slopes are equal. In the case where the dose rate takes a value higher than the threshold value N at the same time in all the detectors 1a to 1c, when there are at least two detectors for which the doubling times τ of the radiation are determined to be the same. The logic circuit 2 activates the alarm device 3 when judging that it is critical.

Alternatively, when the doubling time τ of the radiation obtained from each detector takes a value larger than the past critical accident example or the minimum value of the doubling time τ obtained from the analysis, the logic is determined to be critical. May be added.

On the other hand, two or three detectors 1a to 1
If the doubling time τ does not match even if 1c simultaneously detects a dose rate equal to or greater than the threshold N, it is determined that the signal determined to be equal to or greater than the threshold N is due to a noise signal or the like. Thus, the logic circuit 2 does not activate the alarm device 3.

As a result, the conventional critical event in which the dose rate rises slowly, and the arrangement of the plurality of detectors 1a to 1c,
For example, depending on the distance from the monitored device 5 and the thickness of the shielding body 6 through which the radiation passes, for example, the monitored device 5
, Each detector 1a-1c
Will cause a difference in the time to reach the threshold,
It could not be detected because it did not fit within the gate time T. However, the criticality warning device of the present invention can also detect these, so that the detection accuracy can be improved and the radiation exposure of the worker can be reduced.

Although the above embodiment has been described in connection with the case where three detectors are installed, by setting the three detectors as a set, even if one of the detectors fails, the other two detectors can be used. If the detector is sound, criticality can be detected and reliability is high. Further, by employing a gamma ray detector as a detector, even if the detector is separated from the monitored device 5 and installed outside the shielding body 6, sufficient detection of criticality can be performed. It is easy to avoid radiation exposure.

[0044]

As described above, according to the present invention, the dose rate of radiation detected simultaneously by at least two detectors by a logic circuit for a plurality of installed detectors, and the doubling time of the dose rate by the detectors The criticality is determined from. As a result, a critical event in which the dose rate rises slowly or a critical event can be detected regardless of the arrangement of the detectors, so that the detection accuracy is improved and the radiation exposure of the worker is reduced.

[Brief description of the drawings]

FIG. 1 is a logic flow diagram of criticality determination in a criticality warning device according to one embodiment of the present invention.

FIG. 2 is a block diagram of a criticality warning device.

3A and 3B are dose rate characteristic diagrams, wherein FIG. 3A shows a case where three detectors exceed a threshold value, FIG. 3B shows a case where two detectors exceed a threshold value,
(C) indicates that one detector has exceeded the threshold.

FIG. 4 is a schematic diagram of a criticality warning device showing an arrangement of detectors.

FIG. 5 is a characteristic diagram of doubling time and dose rate.

FIG. 6 is a dose rate characteristic diagram when the doubling times of two detectors are equal.

FIG. 7 is a logic flow diagram of a criticality judgment in a conventional criticality alarm device.

[Explanation of symbols]

1 to 1c detector, 2 logic circuit, 3 alarm device, 4 ...
Radioactive material, 5: monitored equipment, 6: shielding body.

Continuation of the front page (56) References JP-A-6-3485 (JP, A) JP-A-5-288859 (JP, A) JP-A-5-264737 (JP, A) JP-A-62-198793 (JP) , A) JP-A-58-47278 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G21C 17/06 G01T 1/17 G01T 7/12 G21C 19/40

Claims (6)

(57) [Claims] 1. A criticality warning device comprising a plurality of radiation detectors and a logic circuit and a warning device and installed in a nuclear fuel facility for detecting a criticality event, wherein a radiation dose rate is simultaneously thresholded in at least two radiation detectors. Exceeds the value,
A criticality warning device characterized in that the logic circuit determines that the criticality occurs when the doubling times of the radiation dose rates detected by at least two of the radiation detectors match. 2. The criticality warning device according to claim 1, wherein the logic circuit further judges that the radiation dose rate is double when the doubling time of the radiation dose rate is equal to or longer than a certain value. 3. The criticality warning device, wherein the logic circuit obtains a doubling time of the radiation dose rate detected by each radiation detector from time-dependent information of the radiation dose rate detected by each radiation detector. The criticality warning device according to claim 1 or 2, wherein 4. The criticality warning device according to claim 3, wherein the logic circuit calculates a doubling time of the radiation dose rate detected by each radiation detector using a logarithmic differentiation method. 5. The criticality warning device according to claim 1, wherein three radiation detectors are provided.
The criticality warning device as described in the above. 6. The criticality warning device according to claim 1, wherein a gamma ray detector is used as the radiation detector in the criticality warning device.