JPH05232238A - Calibration of neutron monitor device - Google Patents

Abstract

PURPOSE:To set the alarm value of the neutron count value corresponding to the concn. of controlled plutonium by instantaneously and certainly detecting the abnormality of the concn. of plutonium. CONSTITUTION:The neutron generated from a small amount of Pu contained in a waste extract is detected by a neutron detection part 1 surrounded by a neutron decelerating material. The signal from the neutron detection part 1 is outputted to an alarm generator 3 as a count ratio through a count circuit 2. A calibrating neutron source 3 is received in a neutron source cask 5 during operation. The calibrating neutron source 4 is inserted in the predetermined place of the neutron detection part 1 prior to operation and it is confirmed that said neutron source 3 is set to a predetermined count ratio. The alarm value 6 of a neutron count ratio is set to the alarm generator 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a neutron monitor method used for an apparatus for performing criticality control by plutonium (Pu) concentration attached to co-decontamination step and refining step in reprocessing and extraction operation of spent fuel The present invention relates to a calibration method of a neutron monitor device for setting an alarm value of a Pu leak detection device.

[0002]

2. Description of the Related Art Among the co-decontamination / distribution process and the refining process of a reprocessing plant, the neutron effective multiplication factor of less than 1.0 has been applied to equipment handling nuclear fuel materials such as plutonium (Pu) and uranium (U). In other words, the criticality safety design is done so as not to become critical.

That is, in each of these devices, the acidity of the liquid to be supplied, the concentration of nuclear fuel substances such as Pu and U, and the concentration of Pu and U components in the nitric acid aqueous solution / organic solvent are also taken into consideration. It is designed so that the size and volume are limited so that it is not critical to changes that can fluctuate.

On the other hand, a device for performing criticality control according to the Pu concentration (Pu content in nitric acid aqueous solution or organic solvent liquid) attached to each of the above-mentioned devices has a small amount of Pu or hardly flows into it during normal operation. In consideration of processing efficiency, the design is relaxed in the shape and volume of the equipment.

By the way, if something happens, Pu
There is a possibility that it will leak and become critical, and by installing detectors for neutron rays, gamma rays, etc. at various places, an increase in radiation level is detected and an alarm is issued to notify the operator and each personnel of the danger. Alternatively, measures such as stopping the process at the discretion of the person are taken.

[0006]

However, its radiation level does not merely increase in proportion to the nuclear fuel material concentration. For example, in criticality safety management, criticality control of Pu is particularly important, but it is known that the neutron generation rate from Pu changes greatly depending on the Pu isotope, and this isotope composition ratio is the initial concentration of reprocessed fuel. The level of the neutron beam is not necessarily proportional only to the amount of Pu because of a great difference depending on the irradiation degree and the irradiation history.

Therefore, the Pu concentration abnormality cannot be detected merely by detecting the radiation level, and an excessive margin is taken in performing criticality safety control, which may reduce the facility utilization rate. .. The present invention has been made to solve the above problems, and in a device for performing criticality control based on plutonium concentration, which belongs to the co-decontamination / distribution process and the purification process of the main reprocessing step, the neutron counting from Pu in it In the neutron monitor device that measures the rate and immediately and surely detects an abnormality in Pu concentration and issues an alarm, an alarm value of the neutron count value corresponding to the control Pu concentration (leakage Pu maximum allowable concentration in process control) A method for calibrating a neutron monitor device for setting the above is provided.

[0008]

SUMMARY OF THE INVENTION The present invention is a signal generator for counting neutrons from plutonium that leaks into a device that performs criticality control based on plutonium concentration, and compares it with a preset alarm value to issue an alarm. In the method for calibrating a neutron monitor device, the alarm value is set based on a count value by a neutron source for calibration.

[0009]

The alarm level of the count rate corresponding to the control Pu concentration is set accurately based on the calibration neutron source used for checking the measurement system and the theoretical calculation result. Neutron count rate at Pu solution control concentration: C _fuel Detector position neutron flux at Pu solution control concentration: φ _fuel Neutron count rate by calibration neutron source: C _calb Detector position neutron flux by calibration neutron source: φ _calb C _fuel = β (φ _fuel / φ _calb ) · C _calb β is a value close to 1.0, and the neutron flux is a theoretical calculation value. By taking the ratio, the systematic error is offset.

By setting an alarm value with high accuracy, the criticality safety can be continuously and reliably monitored, and by detecting an abnormal Pu concentration and issuing an alarm, the criticality can be prevented.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a neutron monitor calibration method according to the present invention will be described with reference to the drawings. FIG. 1 shows the configuration of a neutron monitor device for carrying out the calibration method of the present invention, in which the neutron monitor device carries out criticality control according to the Pu concentration attached to the co-decontamination step and the purification step of the reprocessing extraction step. This is for continuously monitoring the criticality safety generated from Pu leaking into the equipment.

The device illustrated in FIG. 1 has a small amount of Pu in the waste liquid.
FIG. 3 is a vertical cross-sectional view showing the step direction of a mixer-settler as a solvent extraction device for washing and removing the, showing the positional relationship between the Pu solution and the neutron detector 1. This mixer-settler M is used in the fuel extraction step, and the Pu solution generated in the reprocessing step flows into the fuel solution section F and comes into contact with the organic solvent for liquid extraction. A neutron absorber N such as cadmium is arranged below the fuel solution portion F, and a neutron detector 1 surrounded by a neutron moderator is provided below the neutron absorber N.

That is, a He-3 counter in which neutrons generated from a small amount of Pu contained in the extraction waste liquid (nitric acid aqueous solution or organic solvent liquid) supplied to the mixer settler M is surrounded by a neutron moderator (polyethylene or the like), B- It is detected by the neutron detector 1 composed of 10 counters.

The signal from the neutron detector 1 is output to the alarm generator 3 as a count rate via the counting circuit 2. The counting circuit 2 is composed of a preamplifier, an amplifier, a wave height discriminator, a counter and the like. The alarm generating device 3 has a function of comparing the input count rate with an alarm value and generating an alarm when the value exceeds the alarm value.

Further, the calibration neutron source 4 is housed in the neutron source cask 5 during operation, and is inserted in a predetermined location of the neutron detector 1 prior to operation in order to confirm normal operation of the measurement circuit. It is confirmed that it is a predetermined count rate.

Next, the warning value 6 of the neutron count rate obtained by the method according to the present invention is set in the warning generator 3. This neutron count rate warning value 6 is the uncertainty of the detection efficiency ratio β and the neutron flux ratio (φ _fuel / φ _calb ) in the above-mentioned equation (4), and the correction of the neutron amplification factor evaluated by calculation. Management Pu taking into consideration the uncertainty of
It is practical to set a value smaller than the neutron count rate C _fuel at the concentration.

Pu emits neutrons by spontaneous fission reaction, and also emits neutrons by (α, n) reaction between α particles and surrounding light elements such as oxygen accompanying alpha decay.
It is known that the magnitude s of the neutron production rate S per unit weight varies depending on the Pu isotope, as shown in Table 1.

[0018]

[Table 1]

On the other hand, the Pu isotope composition ratio (Pu vector) in the reprocessed fuel Vary greatly depending on the initial enrichment of the reprocessed fuel, the irradiation history, and the like. Figure 2 shows P-240 with Pu-240 as the representative composition.
It is the figure which illustrated the range of u isotope composition ratio. From the figure,
It can be seen that the neutron production rate S per unit volume in the Pu solution of the device to be measured fluctuates within a certain range even if the Pu concentration is constant. Further, the neutron count rate C counted from a device including Pu is proportional to S and depends on the effective neutron multiplication factor k _eff of the system, and is as follows.

[0020]

[Equation 1]

FIG. 3 schematically shows the relationship between the Pu concentration n and the neutron count rate C. The shaded portion surrounded by the curves A and B in the figure indicates Pu in the received reprocessed fuel. It corresponds to the composition range. In the figure, it is shown that if the alarm value Cd of the count rate corresponding to the control Pu concentration n _d is set on the curve B on the side where the count rate is the minimum, the count rate becomes the minimum and the safe side is set. ing.

Now, Cd is, for example, Pu on the curve B in the figure.
Neutron transport calculation (or diffusion calculation) of a subcritical multiplication system containing a neutron source of the system under the condition of composition and Pu concentration n _d
You can go and ask. However, it is impossible to confirm the validity of the calculation by actually filling the device with the Pu solution and counting the neutrons. Therefore, in the present invention, the level of the alarm value is set by the following method based on the theoretical calculation and the count value of the neutron source for calibration whose source intensity is known.

The counting rate C is considered to be approximately proportional to the neutron flux at the detector position. If the count rate in the case of the Pu solution and the neutron flux at the detector position are C _fuel and φ _fuel, and those in the calibration neutron source are C _calb and φ _calb , then C _fuel = α · φ _fuel ... ( 2) C _calb = α '· φ _calb (3) Here, α and _α'are constants (detection efficiency), and if the neutron flux spectrum at the detector position is thermalized, they are almost equal. I think. Therefore, φ _fuel , φ on the right side
_calb is calculated by theoretical calculation. From equations (2) and (3),
Alarm value setting C _{fu el} (Cd) is determined as follows. C _fuel = _β (φ _fuel / φ _calb ) _{C calb} (4) where β is the ratio of detection efficiencies and is close to 1.0. In general, the calculated neutron flux may have a systematic error (bias) due to the accuracy of the nuclear reaction cross section used, etc., and the neutron flux ratio (φ _fuel / φ _calb ) should be taken as shown in the above equation. The error tends to be canceled by the above, and the present invention utilizes the property. As the calculation code, the calculation accuracy used in general nuclear calculation has been verified, and a Monte Carlo transport calculation code, an S _N transport calculation code, a diffusion calculation code, etc. are used.

As described above, according to the method of the present invention,
The neutron count rate corresponding to the controlled Pu concentration can be accurately obtained, and the alarm value with a more appropriate safety margin can be set, which can contribute to the improvement of the facility utilization rate.

The alarm value is determined by taking into consideration the detection efficiency and the uncertainty of correction of the neutron multiplication effect which is automatically evaluated when calculating the neutron flux, and the influence of the background count.
_It is practical to set a value smaller than _fuel . As the neutron source for calibration, ²⁵² Cf or the like can be used.

The neutron monitor device targeted by the present invention is
The present invention is not limited to the one installed in the above-mentioned mixer-settler, and is also applied to other devices containing Pu such as those installed in the lower part of the pulse column or the bottom part of the storage tank.

[0027]

According to the present invention, by setting the alarm value with an appropriate safety margin, P is compared with the alarm value.
It is possible to immediately detect an abnormality in the u concentration and generate an alarm to prevent the criticality and improve the facility utilization rate.

[Brief description of drawings]

FIG. 1 is a system diagram showing an apparatus for carrying out an embodiment of a neutron monitoring apparatus calibration method according to the present invention.

FIG. 2 is a curve diagram showing a range of Pu isotope composition ratios.

FIG. 3 is a schematic diagram showing setting of alarm values in relation to Pu concentration and neutron count rate.

[Explanation of symbols]

1 ... Neutron detector, 2 ... Counting circuit, 3 ... Alarm generator,
4 ... Calibration neutron source, 5 ... Neutron source cask, 6 ... Neutron count rate alarm value.

Claims (1)

[Claims] 1. The number of neutrons from the plutonium leaked into the device for criticality control based on the plutonium concentration is counted,
In a calibration method of a neutron monitor device comprising a signal generator that emits an alarm in comparison with a preset alarm value, the alarm value is set based on a neutron count value by a neutron source for calibration. Calibration method for neutron monitor.