JPS5826775B2 - Neutron detector assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a neutron detector assembly for nuclear instrumentation in a nuclear reactor, and more particularly to improving the sensitivity of a neutron detector assembly.

In neutron detector assemblies used for external nuclear instrumentation of nuclear reactors, those in the source area and those in the intermediate area are surrounded by moderators such as polyethylene to increase neutron sensitivity. .

The shape of the neutron detector body is generally cylindrical, and therefore a cylindrical hole is provided in the center of the moderator and the neutron detector body is housed in the hole.

It is housed in a housing made of this moderator A/.

The situation is shown in Figure 1. The neutron detector assembly described above is placed in a nuclear reactor as shown in FIG.

Recently, there has been a strong need for improved sensitivity and longer life in these assemblies.

First, to improve the sensitivity of the former, we have developed a method of inserting two or more neutron detector bodies into the same assembly and simultaneously instrumenting the signals from them (as shown in Figure 2), and a method of increasing the length of the latter assembly. For the purpose of life extension, two or more neutron detectors are inserted into the same assembly, for example, one neutron detector is used at a time, and the other neutron detector is used at the end of the life of one (as shown in Figure 3). ) etc. are being considered.

However, in these cases, the external dimensions of the assembly, that is, the moderator, are the same, so there is less moderator per neutron detector, which reduces the number of thermal neutrons incident on each detector, as well as the number of other neutrons. Due to the absorption of neutrons by the detector, the number of thermal neutrons per neutron detector will decrease compared to a system with one neutron detector inserted, and the counting rate per neutron detector will decrease. As shown in FIG. 4, the neutron detector assembly is placed at a predetermined distance from the core part 6 of the reactor core vessel 10, and connected to the MI cable (high voltage application and signal extraction) 9.
It is connected to an external detection signal processing circuit through.

The present invention eliminates these drawbacks and provides a neutron detector assembly in which sensitivity decreases less per neutron detector even when two or more neutron detectors are inserted.

Furthermore, according to the present invention, in the case of an assembly in which one neutron detector is inserted, it is possible to obtain a higher sensitivity than the conventional one.

An embodiment of the present invention shown in FIG. 5 will be described below.

In this embodiment, the center 8 of the neutron detector insertion hole 3 is located at a position away from the center 7 of the moderator 2, and the neutron detector 4 is installed.

In other words, the thickness of the moderator 2 is set to the neutron incident side (core side).
compared to the opposite side).

In this case, when the neutron detector assembly 5 is installed in a nuclear reactor, the reactor center 6, the center 8 of the neutron detector insertion hole (ie, the neutron detector 4), and the center 7 of the moderator are arranged in a straight line in this order.

At that time, the sensitivity was about 1.5 compared to the conventional arrangement shown in Figure 1.
Double the sensitivity.

The effects of the present invention will be explained below in comparison with conventional products.

The arrangement of a conventional neutron detector assembly is as shown in Figure 1, where neutron beams entering from any direction perpendicular to the central axis of the neutron detector assembly 5 are decelerated and heated at the same rate. The idea was to measure the neutrons converted into neutrons through a 1°B(nα)'Li nuclear fission reaction in the neutron detector 4.

However, the neutron flux near the ex-core nuclear instrumentation of a nuclear reactor is composed of fast neutrons and thermal neutrons at a ratio of approximately 1=1, and the neutral beam at the location where the detector assembly is installed is It is a parallel beam, and a very high percentage of it comes from the direction of the core.

Based on the above, the neutron detector assembly of the present invention reduces the moderator on the neutron beam incident side, rather than surrounding the neutron detector with a moderator of uniform thickness, and reduces the reflection of the incident thermal neutron flux. While reducing attenuation, by thickening the moderator on the rear side of the beam incidence, the fast neutron flux is decelerated and reflected, increasing the number of thermal neutrons that enter the detector. We were able to obtain 1.5 times the sensitivity compared to the conventional method.

Although the above description has been made regarding the case of a set of one neutron detector, the present invention is not limited to this, and may be used in the case of a set of two neutron detectors arranged as shown in FIG. 6.

If it is desired to use the device with even lower sensitivity, the present invention can be applied to an arrangement as shown in FIG.

In this case, the sensitivity can be controlled to be low by increasing the thickness of the moderator on the neutron incident side to promote absorption of thermal neutrons.

As described above, in the neutron detector according to the present invention, by changing the arrangement of the neutron detector in the moderator, it is possible to obtain a high-sensitivity neutron detector, and if necessary, a low-sensitivity neutron detector can be obtained. You can also get it.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a conventional neutron detector assembly;
Figure 2 is a cross-sectional view of a conventional neutron detector in which two or more neutron detectors are placed in the same housing to increase sensitivity, and Figure 3 is a conventional neutron detector that detects two or more neutrons. FIG. 4 is a configuration diagram showing the vicinity of the core of a nuclear reactor; FIG. 5 is a configuration diagram showing an embodiment of the present invention; FIG. 6 , FIG. 7 is a block diagram showing another embodiment of the present invention. In the figure, 1 is a housing, 2 is a moderator, 3 is a neutron detector insertion hole, 4 is a neutron detector, and 5 is a neutron detector assembly. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] 1. A neutron detector assembly comprising a neutron detector and a moderator surrounding it, the thickness of the moderator being different between the neutron incident side and the reflection side.