JPH0613510Y2 - Neutron detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a neutron detection device for neutrons emitted during nuclear fission of transuranic elements contained in wastes produced by related facilities of a nuclear fuel cycle. .

[Prior Art] Conventionally, a so-called active neutron detection device for transuranic elements irradiates a waste drum 13 as an object to be inspected with neutrons from a neutron source 16 as shown in FIG. Then, the nuclear fission of the transuranium element trapped inside the waste tram can 13 is induced, and the neutrons generated at this time are measured by the neutron detector 14, and the waste drum 13 and the neutron detector 14 described above are measured. And neutron source
16 is surrounded by a neutron moderator 12 such as graphite, and the outside thereof is surrounded by a neutron shield 15 such as a water jacket.

If it is the detection device 11 described above, the fission neutrons generated inside the drum 13 need to be detected separately from the neutrons generated from the neutron source 16, so the measurement of neutrons is based on the principle of the neutron annihilation time difference method. Has been done. Fourth
(A), explaining the principle of the neutron annihilation time difference method with reference to (b), the fast neutron 16a is irradiated from the neutron source 16, the fast neutron 16a, the neutron moderator 12
Is decelerated / heated by and becomes thermal neutron 16b in drum 1
It enters into 3 and induces fission of transuranium element 17, and newly generates high-speed fission neutron 16c.

At this time, since this kind of neutron detector 14 is designed to hardly react to thermal neutrons 16b, the neutron time measured by the neutron detector 14 is shown in FIG. 4 (b). Like That is, fast neutrons from the neutron source 16
When 16a is pulse-irradiated, the irradiated fast neutron 16a is measured by the neutron detector 14 in a very short time after irradiation,
After that, fission neutrons 16c generated by fission of transuranium element 17 are measured. As described above, the thermal neutron 16b that is a factor inducing fission is hardly detected by the neutron detector 14, so all neutrons measured after decay of the fast neutron 16a generated from the neutron source 16 are all fission neutrons. Is evaluated.

[Problems to be Solved by the Invention] When fission neutrons 16c are measured using the detection device 11 described above, the amount of fission neutrons generated is attenuated along with the elapsed time after pulse irradiation. For neutron detection, it is desirable that the neutron detector 14 has high sensitivity. However, in the first half of the elapsed time after pulse irradiation, the amount of fission neutrons generated is large, so if the sensitivity of the neutron detector is too high, on the contrary, a dead state called a choking phenomenon occurs. Since the correct measurement becomes impossible, there is a limit to increase the sensitivity of the neutron detector 14, and in the end, a detection device that is not always satisfactory in terms of sensitivity is used.

Therefore, it is an object of the present invention to provide a neutron detector which can detect transuranium elements with high accuracy without causing the insensitive state as described above.

[Means for Solving the Problems] To this end, the present invention provides a deceleration container that defines a closed space for accommodating an inspection object and a fast neutron that irradiates the inspection object. In a neutron detector comprising a neutron source and a neutron detector for detecting neutrons generated from the inspection object by receiving irradiation of thermal neutrons by thermalization of the fast neutrons from the neutron source, The neutron detector is characterized by including a first neutron detector having a neutron sensitivity of 0.01 cps or less at an energy of 1 Mev and a second neutron detector having a neutron sensitivity of 0.1 cps or more at an energy of 1 Mev.

[Operation] The fast neutrons emitted from the neutron source are decelerated by the neutron moderator in the moderator to be thermalized, and the thermal neutrons irradiate the inspection object. If the inspection object contains transuranic elements, the transuranic elements undergo fission and generate high-speed fission neutrons.

The fission neutrons are measured by the first and second neutron detectors by the neutron annihilation time difference method.

In the case of the low sensitivity first neutron detector, since the detector sensitivity is insufficient and the accuracy of the data is poor after the latter half of the time elapsed from the pulse irradiation until the fission neutron disappears,
The first neutron detector is used to detect fission neutrons in the first half of the elapsed time. On the other hand, in the case of the highly sensitive second neutron detector, in the first half of the time elapsed from the pulse irradiation until the disappearance of the fission neutrons, the suffocation state or dead state occurs in the measurement system because the detector sensitivity is too high. Therefore, it is used for the detection of fission neutrons in the latter half of the passage of time.

In a preferred embodiment, the data thus detected by the first and second neutron detectors is sent to a processing device, wherein the time-shifted data of the measuring range of the first neutron detector, The data in the measurement range of the second neutron detector is combined, and thus the measurement is suitably performed.

[Embodiment] Next, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows a partially broken view of a neutron detection device 1 according to the present invention. The detection device 1 is similar to a conventional device, and defines an internal space for accommodating an inspection object such as a drum can 3. And a moderator 2 made of a neutron moderator such as a graphite block, and neutron shields 5a to 5f such as water jackets surrounding the moderator 2 to reduce the neutron dose rate outside the apparatus (bottom part in the embodiment). Is also covered by the shield 5f, but this may be omitted) and a neutron source or generator 6 housed in the graphite block and generating neutrons.
It has and. In the drum 3 is stored nuclear fuel waste (not shown) containing transuranium elements with atomic numbers of 92 or more, such as plutonium. Further, in FIG. 1, in order to take in and out the drum can 3, the neutron shield 5d is shown in a state of being pulled out obliquely downward in the drawing by the drive device 8, that is, in a direction away from the neutron shield 5e. ing.

According to the present invention, the detection device 1 comprises two types of neutron detectors which are separated from the neutron source 6 and embedded in a graphite block.
4a, 4b (first and second neutron detectors) are provided as shown. In the preferred embodiment, the high sensitivity neutron detector 4b has a sensitivity of 0.2 cps for neutrons having an energy of 1 MeV as in the conventional case.
And the low-sensitivity neutron detector 4a has an energy of 1 MeV
Has a neutron sensitivity of 0.01 cps. However, the neutron detector 4b may have a sensitivity of 0.1 cps or higher, and the neutron detector 4a may have a sensitivity of 0.01 cps or lower.

Furthermore, the detection device 1, as shown schematically in FIG.
It may be provided with a processing device 9 connected to the neutron detectors 4a and 4b and processing the measurement results thereof, and a display device 10 connected to the processing device 9 and displaying the processing results.

Next, the operation of the above-described detection device 1 will be described. First, the drive device 8 is operated to bring the drum 3 into the inside and place it in the closed space surrounded by the graphite block.
After that, the neutron source 6 is driven to inspect the transuranium element. Fast neutron 16a generated from neutron source 6
(See Fig. 4) is decelerated and heat-treated by the graphite block,
It is supplied into the closed space surrounded by the graphite block.
The thermal neutrons 16b are transferred to the drum 3 and the uranium element 17
When (see Fig. 4) exists, it induces fission and newly generates fast neutrons 16c (see Fig. 4).

This high-speed fission neutron 16c is measured by neutron detectors 4a and 4b. 2 (a) and 2 (b) show that 1 mg of
^{When 239} Pu is contained, the time spectrum of the fission neutron 16c from ²³⁹ Pu is independently measured by the neutron detectors 4a and 4b using the principle of neutron ablation time difference method. As can be seen from Fig. 2 (a), in the case of the low sensitivity neutron detector 4a, fission neutrons are emitted after pulse irradiation.
After time t _{2 in} the time elapsed until 16c disappears,
Since the detector sensitivity is insufficient and the accuracy of the data is poor, the neutron detector 4a is used for detecting the fission neutrons 16c in the first half of the time passage. On the other hand, the sensitivity is as good as the conventional one.
In the case of 4b, in the first half of the time lapse from the pulse irradiation until the fission neutron 16c disappears, as described at the beginning in relation to the conventional technique, the detector sensitivity is too high and the measurement system is suffocated. Alternatively, it is used to detect fission neutrons 16c in the latter half of the passage of time because a dead state occurs.

Therefore, the data thus detected by the neutron detectors 4a and 4b are sent to the processing device 9, where the measurement range of the neutron detector 4a, that is, FIG. 2 is shown, as shown in FIG. 2 (c).
Data from time t ₁ (time until the irradiation fast neutrons 16a disappear) to almost the first half t ₂ of the elapsed time in (a) and the measurement range of the neutron detector 4b, that is, the elapsed time in FIG. 2 (b) During the latter half of the period, the data of t ₃ −t ₂ is combined and displayed on the display device 10.

Thus Effect of invention], the detection device of the present invention, since the sensitivity is provided with two different neutron detectors, since disappeared irradiating fast neutrons 16a is at time t _1, at time t ₃ Fission neutron 16c
Can be suitably measured over the entire range until disappearance, and has a sensitivity of about 1.
Can be expanded five times.

[Brief description of drawings]

FIG. 1 is a partially cutaway perspective view of the neutron detector according to the present invention, and FIGS. 2 (a) and 2 (b) show two types of neutron detectors in the neutron detector of FIG. 2 (c) is a graph obtained by synthesizing the graphs of FIGS. 2 (a) and 2 (b) based on the present invention, and FIG. 3 is a graph showing separately measured data. FIG. 4 is a schematic sectional view of a conventional detection device.
(a) And (b) is explanatory drawing of the measurement principle by a neutron annihilation time difference method. 1 ... Neutron detector, 2 ... Deceleration container 3 ... Inspection object (drum can) 4a ... 1st neutron detector with energy 1 Mev neutron sensitivity of 0.01 cps or less 4b ... Energy 1 Mev neutron sensitivity 0.1 cps Second neutron detector 6 ...... Neutron source 16a …… Fast neutron, 16b …… Thermal neutron 16c …… Fission neutron

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Claims (1)

[Scope of utility model registration request] 1. A deceleration container defining a closed space for accommodating an inspection object, a neutron generation source for generating fast neutrons for irradiating the inspection object, and the neutron generation source. In a neutron detector comprising a neutron detector for detecting fission neutrons generated from the inspection object by receiving thermal neutron irradiation due to thermalization of fast neutrons, the neutron detector has a neutron sensitivity of energy 1 Mev First neutron detector with 0.01cps or less and energy 1M
A neutron detector comprising a second neutron detector having a neutron sensitivity of ev of 0.1 cps or more.