JPH04256897A - Neutron monitor device - Google Patents

Abstract

PURPOSE:To monitor critical safety continuously by detecting abnormality in Pu density in a mixer settler immediately and certainly and generating an alarm. CONSTITUTION:Neutrons from Pu leaked into mixer settler 1 are detected with a neutron detector 4. The signal from the neutron detector 4 is input to a counter circuit 5 to obtain neutron counting rate. The neutron counting rate and the alarm limit value of the neutron counting rate are compared in an alarm generation circuit 6. When the neutron counting rate exceeds the alarm limit value of the neutron counting rate, the alarm generation circuit 6 generates an alarm.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a neutron monitor device used in equipment that performs criticality control using Pu (plutonium) concentration attached to the co-decontamination step and purification step of the reprocessing extraction step, and particularly relates to a neutron monitor device used in equipment that performs criticality control using Pu (plutonium) concentration attached to the co-decontamination step and purification step of the reprocessing extraction step. The present invention relates to an apparatus for detecting Pu leakage using the method.

0002

[Prior Art] Conventionally, plutonium (Pu) and uranium (
For equipment that handles nuclear fuel materials such as U), criticality safety design is performed so that the effective neutron multiplication factor is less than 1.0, that is, it does not become critical. In other words, in each of these devices, the acidity of the supplied liquid, the concentration of nuclear fuel materials such as Pu and U in it, and abnormalities in the concentration of Pu and U components in the nitric acid aqueous solution and organic solvent can be changed. The design limits the shape and volume so that it is subcritical to changes.

On the other hand, in the equipment that performs criticality control using the Pu concentration (Pullium content in the nitric acid aqueous solution or organic solvent solution) attached to each of the above equipment, the amount of Pu is small or hardly flows during normal operation. In consideration of processing efficiency, the design has relaxed restrictions on the shape and volume of the equipment. However, there is a possibility that Pu may leak for some reason and become critical, so by installing detectors for neutron beams, gamma rays, etc. in various places, it is possible to detect an increase in the radiation level and generate an alarm. Measures are taken, such as notifying employees and other personnel of the danger, and halting the process based on human judgment.

0004

However, the level does not simply increase in proportion to the nuclear fuel material concentration. For example, Pu is particularly important for criticality safety management, but Pu
It is known that the neutron generation rate from the Pu It's not just proportional to quantity. For this reason, it is not possible to detect an abnormal Pu concentration simply by detecting the radiation level, and the current situation is that an excessive margin is taken in criticality safety management.

[0005] The present invention has been made in consideration of the above points, and is designed to prevent abnormal Pu concentration in equipment that performs criticality control based on Pu concentration attached to the co-decontamination/distribution process and the purification process of the main reprocessing process. An object of the present invention is to provide a neutron monitor device that can immediately and reliably detect and issue an alarm.

[0006]

[Means for Solving the Problems] As a means for achieving the above object, the present invention provides a device for criticality control based on Pu concentration that is attached to the co-decontamination/distribution step and the purification step of the main reprocessing step. A neutron detection unit detects neutrons from Pu leaking into the interior, a counting circuit calculates a neutron count rate by inputting a signal from this neutron detection unit, and compares this neutron count rate with a neutron count rate alarm value to detect neutrons. The present invention is characterized in that an alarm generating device that issues an alarm when the counting rate is equal to or higher than a neutron counting rate alarm value is provided.

[0007]

[Operation] In the neutron monitor device according to the present invention, P
In equipment that performs criticality control based on u concentration, neutrons from Pu leaking into the equipment are detected by a neutron detection unit, and the signal from this neutron detection unit is input to a counting circuit to determine the neutron counting rate. . The obtained neutron count rate is compared with a neutron count rate alarm value in the alarm generating device, and an alarm is issued when the neutron count rate is equal to or higher than the neutron count rate alarm value. Therefore, it becomes possible to quickly and reliably detect an abnormal Pu concentration and issue an alarm.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows an example of a neutron monitoring device according to the present invention, and in the figure, reference numeral 1 is a mixer settler that washes and removes a trace amount of Pu in waste liquid.
A neutron detection section 4 is arranged on the back side of the fuel solution section 2 with a neutron absorber (cadmium or the like) 3 interposed therebetween.

The neutron detection section 4 is composed of a He-3 counter, a B-10 counter, etc. surrounded by a neutron moderator (polyethylene, etc.). The system measures neutrons generated from a small amount of Pu contained in a waste solution (nitric acid aqueous solution or organic solvent solution). The signal from this neutron detector 4 is input to a counting circuit 5 composed of a preamplifier, an amplifier, a pulse height discriminator, a counter, etc., and the neutron counting rate is determined in the counting circuit 5. ing.

[0010] This neutron counting rate is input to the alarm generating device 6 and compared with a preset neutron counting rate alarm value. An alarm will be issued when the value exceeds this value.

Next, the operation of this embodiment will be explained. In addition to emitting neutrons through spontaneous fission reactions, Pu emits neutrons (
α, n) The reaction releases neutrons. It is known that the magnitude of these neutron generation rates differs depending on the Pu isotope, as shown in Table 1.

0012

[Table 1] On the other hand, the Pu isotope composition ratio in reprocessed fuel varies greatly depending on the initial enrichment level of the reprocessed fuel, irradiation history, etc., but Figure 2 shows the Pu isotope composition using Pu-240 as a representative composition. It is a figure which illustrated the range of a ratio. Therefore, the neutron generation rate per unit volume in the Pu solution of the device to be measured varies within a certain range even if the Pu concentration is constant.

Based on this, in the present invention, we consider the range of Pu isotope composition that encompasses all reprocessed fuel specified in the acceptance specifications, and calculate the Pu isotope composition ratio (Pu vector) as shown in FIG. The relationship between Pu and the neutron generation rate s(Pu) per unit weight of Pu is determined in advance. In addition, Figure 4 shows the keff of the auxiliary extractor that washes and removes trace amounts of Pu in the waste liquid.
This is an example of the effective neutron multiplication factor ke of the system.
ff is a function of Pu concentration and Pu, and is determined based on theoretical calculations and the like. In the present invention, an abnormality in the Pu concentration is detected by setting an alarm value for the number of neutrons in advance based on these relationships and comparing the number of neutrons measured by the neutron detector 4 with this value. That is, if the counting rate corresponding to the alarm value of the number of neutrons is corrected by the detection efficiency and neutron multiplication effect, and the neutron generation rate per unit volume in the Pu solution is calculated, and this is set as the alarm value SA, then the following is obtained. The Pu concentration abnormality can be detected by the principle described.

In order to calculate the neutron generation rate S value per unit volume in the Pu solution, the Pu concentration n is evaluated as follows. The neutron counting rate C is proportional to the neutron generation rate S per unit volume of Pu solution, and the effective neutron multiplication rate keff of the system
It also depends on. It is also the product of the Pu concentration n and the neutron generation rate s per unit weight of received Pu.

That is, [Equation 1] C=α・s/(1−keff) [Equation 2] S=n・s(Pu) Here, α is a constant (detection efficiency), and the specifications of the source are as follows. It is a constant that is determined by measurement using a known system. Further, keff of the system is a function of n and Pu, and is expressed by [Formula 3] keff = keff (n, Pu).

From formulas 1 to 3 above, the relationship between C and n is as follows:
It is determined as follows.

[Formula 4] C=α・n・s(Pu)/(1−keff (n, Pu
)) In other words, the count rate C measured by the neutron detector is
If corrected by the detection efficiency and neutron effective multiplication factor using equation 4,
n is found, and from Equation 2, the relationship between S and n is found.

FIG. 5 is a schematic diagram showing the relationship between the neutron generation rate S per unit volume of Pu solution and the Pu concentration n. S
and n are in a proportional relationship, and S can be determined within the range determined by the maximum neutron generation rate per unit weight of Pu, Smax and Smin (within the shaded area in the figure, which corresponds to the range of change in the Pu isotope composition). It shows. Therefore, the minimum value S of the neutron generation rate at the management Pu concentration nd
If dmin exceeds the maximum value Snmax of the neutron generation rate in the case of the normal Pu concentration nn, the management Pu concentration can be distinguished from the normal Pu concentration. That is, the managed Pu concentration nd can be managed by the counting rate Cdmin corresponding to the neutron generation rate Sdmin.

This identification condition is expressed by the following equation.

[Equation 5] Maximum value of neutron generation rate per unit volume of Pu solution at normal Pu concentration Snmax (=nn ·Smax)<
Minimum value of neutron generation rate per unit volume of Pu solution at controlled Pu concentration Sdmin (=nd ・Smin) or [Formula 6]

[0021]

[Equation 1] In other words, the normal time and management P determined by the process design
By limiting the range of the accepted Pu isotope composition, if necessary, so that Equation 6 is satisfied for the u concentration, the neutron generation rate SA per unit volume of Pu solution corresponding to the alarm value in the equipment subject to Pu concentration management can be calculated. Using this, it is possible to distinguish between normal and controlled Pu concentrations. In actual operation, even if the range of the accepted Pu isotopic composition ratio is managed, the specification of the isotopic composition ratio Pu of the accepted Pu in the process is not necessarily given, and therefore, in general, the Pu concentration is calculated using Equation 4. However, according to the present invention, the Pu concentration can be managed by setting the alarm value as follows.

First, regarding the alarm value of the neutron generation rate,
If the value is set between the maximum value of the neutron generation rate per unit volume of Pu solution at the normal Pu concentration and the minimum value of the neutron generation rate at the control Pu concentration, abnormalities in the Pu concentration can be reliably detected. The alarm value level SA should theoretically be set to the minimum value Sdmin of the neutron generation rate at the controlled Pu concentration, but due to S considering the influence of counting etc.
It is practical to set it to a value smaller than dmin.

Next, once SA is determined, the corresponding alarm value CA of the neutron count rate is uniquely determined from Equation 2. The alarm value CA of the neutron counting rate is determined by formula 4 when the Pu concentration is nd and the neutron generation rate per unit weight of Pu is Smin.
The neutron counting rate is Cdmin when the Pu concentration is nn.
If Cnmax is the neutron counting rate when the neutron generation rate per unit weight of Pu is Smax, then if it is set between the two, an abnormality in the Pu concentration can be reliably detected.

[0024] CA is preferably set to the minimum value Cdmin of the neutron counting rate at the controlled Pu concentration; however, there are uncertainties in the correction of the detection efficiency and effective neutron multiplication factor according to Equation 4 above, the influence of background counting, etc. Taking this into consideration, it is practical to set it to a value smaller than Cdmin. In addition, although the said Example took the mixer settler 1 as an example and demonstrated, the target apparatus is not limited to the mixer settler 1, For example, it is applicable to other apparatuses which incorporate Pu, such as a pulse column and a storage tank. Furthermore, even when specifications are given for the isotopic composition ratio Pu of the Pu accepted in the process, the method based on the present invention can be used as a system that can set an alarm value and detect abnormalities in the Pu concentration with high accuracy. can.

[0025]

Effects of the Invention As explained above, according to the present invention, P
In equipment that performs criticality control based on u concentration, the number of neutrons from leaked Pu in the equipment is continuously measured to determine the counting rate, and this counting rate is compared with the alarm value set in advance on the safe side. Therefore, an abnormal Pu concentration can be detected immediately and reliably and an alarm can be issued, and its criticality safety can be continuously monitored.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing a neutron monitor device according to an embodiment of the present invention.

FIG. 2 is a graph showing the range of isotopic composition ratios of Pu.

FIG. 3 is a graph showing the relationship between the neutron generation rate per unit weight of Pu and the Pu isotope composition ratio (represented by Pu-240).

FIG. 4 is a graph showing the relationship between effective neutron multiplication factor and Pu concentration.

[Figure 5] Neutron count rate per unit volume of Pu solution and Pu
A graph showing the setting of alarm values in relation to concentration.

[Explanation of symbols]

1　Mixasetra 4 Neutron detection section 5 Counting circuit 6 Alarm generating device

Claims (1)

[Claims] 1. A neutron detection unit that detects neutrons from Pu leaking into the equipment in equipment that is attached to the co-decontamination/distribution process and the purification process of the main reprocessing process and performs criticality control based on Pu concentration; A counting circuit that calculates a neutron count rate by inputting a signal from the neutron detection unit, and an alarm that compares this neutron count rate with a neutron count rate alarm value and issues an alarm when the neutron count rate is greater than or equal to the neutron count rate alarm value. A neutron monitor device comprising: a generator.