JPH0812270B2 - Method and device for measuring effective multiplication factor of fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention relates to a method and an apparatus for measuring an effective multiplication factor of a fuel assembly including a spontaneous neutron source such as a spent fuel assembly. .

(Prior Art) Generally, nondestructive measurement of the effective multiplication factor of a fuel assembly including a spontaneous neutron source, particularly a spent fuel assembly including Pu-240, Cm-244, etc. generated by neutron irradiation. Confirmation is extremely important for ensuring the criticality safety of various treatments such as cutting and dissolution and storage, and for the measurement control of fissile nuclide concentration and safeguards.

Conventionally, the effective multiplication factor of a spent fuel assembly is measured by an active neutron method or an external neutron source in which an external neutron source is placed on one side of the spent fuel assembly and a neutron detector is placed on the opposite side. It was carried out by a method called multiplication method.

Further, the present inventors, without using an external neutron source, arranges a neutron effector such as a neutron absorber or a neutron reflector on the outer periphery of a spent fuel assembly placed in water, and there is no neutron effector Case and a method that can be called the so-called passive neutron method or spontaneous neutron multiplication method for obtaining the effective multiplication factor from the ratio of the neutron count rates measured by the neutron detector when present Collection (Part I, B33, page 105, March 1986) ”and so on.

(Problems to be solved by the invention) However, in the conventional method of measuring the effective multiplication factor of the spent fuel assembly described above, for example, in the active neutron method, Cf-252 having a relatively short half-life is used as an external neutron source. Therefore, it is necessary to regularly replace the external neutron source. In addition, there is a problem that maintenance is complicated, for example, it is necessary to keep the neutron measurement conditions of the neutron detector constant with high accuracy over a long period of time.

Also, in the passive neutron method to measure the effective multiplication factor from the ratio of the presence and absence of neutron effector at one measurement point of the spent fuel assembly, the neutron effector is removable or There is a problem that it is necessary to measure the spent fuel assembly by moving it in the axial direction and the like, which requires time for the measurement and the work efficiency is poor. further,
Even with the same passive neutron method, if the axial composition of the spent fuel assemblies is uniform, neutron detectors are placed on the sides of the spent fuel assemblies at regular intervals and one detector is used. There is also a method of arranging a neutron acting body on the outer periphery of a spent fuel assembly only for, and obtaining an effective multiplication factor from the ratio of neutron count rates of both neutron detectors, but in this method,
Instead of requiring means such as attaching and detaching the neutron effector or moving the spent fuel assembly in the axial direction, in the actual spent fuel assembly, the burnup between two points at regular intervals,
There are differences in fission nuclide concentration, effective multiplication factor, etc., which are relatively small, and the neutron detection efficiencies of neutron detectors cannot be completely the same, and the efficiencies may change independently over time. However, there is a problem that the measurement result is likely to have an error in the lower measurement.

The present invention has been made in response to such a point, there is no need to replace the external neutron source, etc., maintenance of the measuring device and the like is easier than in the past, work efficiency is high and measurement error may occur. It is also an object of the present invention to provide an effective multiplication factor measuring method for a small number of fuel assemblies and a measuring device.

[Structure of Invention]

(Means for Solving the Problems) That is, the method for measuring the effective multiplication factor of a fuel assembly of the present invention is to cover the side of the fuel assembly containing the spontaneous neutron source placed in water with a thermal neutron absorber. At least one neutron detector is placed in the neutron absorber between the fuel assembly and the neutron detector, when the neutron absorber is close to the fuel assembly From the ratio of neutron count rates measured by the neutron detector when
An effective multiplication factor of the fuel assembly is calculated.

Further, the effective multiplication factor measuring device for a fuel assembly of the present invention is
At least one neutron detector, which is arranged on the side of a fuel assembly containing a spontaneous neutron source arranged in water and whose outer periphery is covered with a thermal neutron absorber, and the fuel assembly and the neutron detector And a neutron absorbing plate which is disposed between the neutron absorbing plate and the fuel assembly so as to be able to approach and separate from the fuel assembly.

(Operation) In the method for measuring the effective multiplication factor of the fuel assembly of the present invention, at least one neutron detector covered with a thermal neutron absorber is provided on the side of the fuel assembly including the spontaneous neutron source arranged in water. One is arranged, a neutron absorbing plate is arranged between the fuel assembly and the neutron detector, and when the neutron absorbing plate is brought close to the fuel assembly, for example, separated by a thermal neutron migration distance in water or more. The effective multiplication factor of the fuel assembly is calculated from the ratio of the neutron count rates measured by the neutron detector when

k ° = {A (φ / φ °) -1} / {AB (φ / φ °) -1}
(I) Further, the apparatus for measuring the effective multiplication factor of the fuel assembly of the present invention is
Between the fuel assembly and the neutron detector, and at least one neutron detector, which is arranged laterally of the fuel assembly containing the spontaneous neutron source and is surrounded by thermal neutron absorber, And a neutron absorbing plate that is arranged so as to be able to approach and separate from the fuel assembly.

Therefore, in the method for measuring the effective multiplication factor of the fuel assembly of the present invention and the measuring apparatus, there is no need to replace the external neutron source, etc., maintenance of the measuring apparatus and the like is easier than in the past, the work efficiency is high, and the neutron counting is high. Since the ratio ratio φ / φ ° is obtained by the same detector, the measurement error can be reduced.

(Example) Hereinafter, the present invention will be described in detail with reference to one example shown in the drawings.

FIG. 1 and FIG. 2 show an effective multiplication factor measuring apparatus according to an embodiment of the present invention, which is formed in a rectangular cross section in conformity with the shape of the fuel assembly 1 arranged in the substantially central portion. Inside the frame 2, four neutron absorbing plates 3 are supported so as to surround the periphery of the fuel assembly 1 by an arm 4 and a drive rod 5 which are rotatably attached to the frame 2.

A cylinder 6 driven by, for example, air, liquid, etc. is connected to the drive rod 5, and the neutron absorbing plate 3 and the fuel assembly are rotated by rotation of the drive rod 5 with the mounting portion 7 of the drive rod 5 as a fulcrum. The distance from the body 1 is variable. The neutron absorbing plate 3 and the frame 2 are each provided with a spacer 8 and a stopper 9 in order to set the distance between the neutron absorbing plate 3 and the fuel assembly 1.

A neutron detector 10 is fixed inside the frame 2 outside the neutron absorbing plate 3 at a position corresponding to substantially the center of the neutron absorbing plate 3, and at the upper and lower portions of the frame 2, the fuel assembly 1 is attached. The guide roller 11 is attached so that the insertion of the can be performed smoothly.

As shown in FIG. 3, the neutron detector 10 has a detector main body 100 surrounded by a neutron moderator 101 such as polyethylene, and the periphery thereof is a thermal neutron absorber such as cadmium or cadolinium.
The container 102 is covered with a sheet having a thickness of, for example, about 0.5 to 1.0 mm and enclosed in a container 103. In such a neutron detector 10, fast neutrons and epithermal neutrons that have passed through the neutron absorbing plate 3 are decelerated by the neutron moderator 101,
Thermal neutrons with high detection efficiency are used for detection. Further, since the thermal neutron absorber 102 prevents the inflow of thermal neutrons,
The neutron absorbing plate 3 is not affected by the distributed thermal neutron flux. The neutron detector 10 is the same as the fuel assembly 1-4.
It is preferable that the fuel assemblies 1 are arranged on all the surfaces, but when they are arranged on two surfaces, for example, the positions where the fuel assembly 1 is sandwiched are preferable.
This is due to the effect of offsetting the positioning error of the fuel assembly 1 in the apparatus and the difference in the fuel design and the neutron irradiation characteristics in the reactor in that the two facing surfaces of the fuel assembly 1 have the most different properties.

As the neutron absorbing plate 3, a cadmium plate is optimal and inexpensive, but boron, cadolinium, or the like can also be used. It is preferable to coat these with a stainless steel sheet or the like because they may be dissolved in water even if a trace amount thereof and adversely affect the inside of the pool. Further, the length of the neutron absorbing plate 3 is preferably 30 cm or more above and below the effective length of the neutron detector 10. In order to improve the measurement accuracy,
It is preferable to dispose the neutron absorbing plate 3 on all side surfaces of the fuel assembly 1, but it is not always necessary to dispose on all side surfaces.

In the effective multiplication factor measuring device having the above-described configuration, the supporting portion 12 projecting outwardly on the upper portion of the frame 2 is locked to the supporting member 13 arranged at a predetermined position in water, and is movable in the horizontal direction. Is installed in.

Then, with the neutron absorbing plate 3 positioned outside,
The fuel assembly 1 is hung by a hanging tool and inserted. At this time, when the central axis of the inserted fuel assembly 1 and the central axis of the device are deviated from each other, the device is moved in the horizontal direction to match the central axes of the devices in order to improve the measurement accuracy.

Next, as shown in FIG. 3, the neutron count rate φ ° when the neutron absorbing plate 3 and the fuel assembly 1 are separated by a distance x _2
2 and the neutron absorption plate 3 and the fuel assembly 1 are brought close to the distance x ₁ by the operation of the cylinder 6, the neutron count rate φ is measured by the neutron detector 10, and the above-mentioned neutron count rate ratio The effective multiplication factor of the fuel assembly 1 is calculated by the equation (I).

Here, the constant A in the formula (I) is a value obtained by calculation in advance, but since the neutron absorbing plate 3 simply moves between the fuel assembly 1 and the neutron detector 10, it is almost from 1.00. It does not change. The constant B is effective k ° ₂ when spaced fuel assembly 1 to effective k and the distance x ₂ when brought close neutron absorbing plate 2 at a distance x ₁
The ratio is calculated in advance.

In the graph of FIG. 4, the ratio x of the effective multiplication factor k when the distance between the neutron absorbing plate 3 and the fuel assembly 1 is x and the effective multiplication factor k when it is infinite is k / k ° for the distance x. Show the relationship. As is clear from this graph, the value of k / k ° is almost 1.0 when the distance x is about 5 cm or more. Therefore, if the distance x ₂ separating the neutron absorbing plate 3 from the fuel assembly 1 is about 5 cm or more, k ° ₂ becomes almost equal to k °. This distance is about twice the thermal neutron migration distance in water.

The k ° ₂ obtained by the measurement in this way is within an error range of 2 to 3% from the actual k °, and even if the error of the effective multiplication factor k ° of the deep subcritical system is about 10%, it is sufficient. Since it has been performed, it is sufficient to make no correction, but it can be corrected by the following calculation.

The effective multiplication factor k based on the critical equation of the one-point furnace theory is k = _k∽ / (1 + M ² B ² ). Here, k _∽ is an infinite multiplication factor, M ² is a neutron moving area, and B ² is a geometric buckling. Therefore, k ° / k ° ₂ = (1 + M ² B ₂ ² ) / (1 + M ² B ² ), and this ratio does not depend on k ° and k ° ₂ , and is 1.
The value is very close to 00, but can be corrected in advance by calculation.

The graph of FIG. 5 shows the relationship of the neutron count rate ratio φ / φ ° with respect to the distance d between the center of the neutron detector 10 and the surface of the fuel assembly 1, and the solid line a shows the neutron absorbing plate 3 at a distance x ₁ , The neutron count rate ratio when the neutron absorbing plate 3 is moved to the x ₂ position, and the broken line b indicates the neutron count rate when the neutron absorbing plate 3 is placed at the x ₁ position and separated to infinity. Represents the ratio,
It can be seen that neither changes regardless of the distance d.
Therefore, as long as the neutron detector 10 is fixed, the measurement error of the neutron count rate ratio due to the displacement of the neutron detector 10 does not occur. If x ₂ ≧ 5 cm, the difference between the solid line a and the broken line b is so small that it can be ignored.

That is, in the method for measuring the effective multiplication factor of the fuel assembly and the measuring apparatus of this embodiment, since the simple operation of bringing the neutron absorbing plate 3 into close contact with or away from the fuel assembly 1, there is little erroneous operation, The mechanism can be simplified.

Further, since the neutron count rate ratio is measured by the same neutron detector 10, the error due to each detector and the error due to the drift of the detector sensitivity are small, and the measurement accuracy can be improved.

Furthermore, since the neutron detector 10 is covered with the thermal neutron absorbing material 102 such as cadmium, the error due to the positional deviation is small.

〔The invention's effect〕

As is apparent from the above description, in the method for measuring the effective multiplication factor of the fuel assembly of the present invention and the measuring device, there is no need to replace the external neutron source, etc. High efficiency and less possibility of measurement error.

[Brief description of drawings]

FIG. 1 is a longitudinal sectional view showing an effective multiplication factor measuring apparatus for a fuel assembly according to an embodiment of the present invention, FIG. 2 is a transverse sectional view of FIG. 1, and FIG. 3 is an enlarged view of a main part of FIG. Fig. 4 shows the effective multiplication ratio k / k ° when the distance between the neutron absorbing plate and the fuel assembly is x and infinity, and the neutron absorbing plate and the fuel assembly FIG. 5 is a graph showing the relationship with the distance x, and FIG. 5 is a graph showing the relationship of the neutron count rate ratio φ / φ ° to the distance d between the center of the neutron detector and the surface of the fuel assembly. 1 ... Fuel assembly 2 ... Frame 3 ... Neutron absorbing plate 6 ... Cylinder 10 ... Neutron detector

Claims (6)

[Claims] 1. At least one neutron detector covered with a thermal neutron absorber is arranged laterally to a fuel assembly containing a spontaneous neutron source arranged in water, and the fuel assembly and the neutron detector are arranged. A neutron absorber is placed between the neutron absorber and the fuel assembly from the ratio of the neutron count rates measured by the neutron detector when the neutron absorber is placed close to and separated from the fuel assembly. A method for measuring an effective multiplication factor of a fuel assembly, which comprises calculating an effective multiplication factor of a body. 2. At least one neutron detector disposed laterally of a fuel assembly containing a spontaneous neutron source disposed in water and having an outer periphery covered with a thermal neutron absorber, the fuel assembly and the neutron detector. An effective multiplication factor measuring device for a fuel assembly, comprising: a neutron absorbing plate disposed between the neutron detector and capable of approaching and separating from the fuel assembly. 3. The effective multiplication factor measuring device for a fuel assembly according to claim 2, wherein the neutron detector and the neutron absorbing plate are arranged in four directions so as to surround the fuel assembly. 4. The neutron absorbing plate is arranged in four directions so as to surround the fuel assembly, and is configured to move coaxially around the axis of the fuel assembly by a driving means. An effective multiplication factor measuring device for the fuel assembly described. 5. The neutron detector comprises a neutron detector body, a neutron moderator arranged so as to surround the neutron detector body, and a thermal neutron absorbing material arranged so as to further surround the neutron moderator. The effective multiplication factor measurement of the fuel assembly according to claim 2, wherein the neutrons that have passed through the thermal neutron absorber are converted into thermal neutrons by the neutron moderator and the thermal neutrons are measured by the neutron detector body. apparatus. 6. The neutron detector and the neutron absorbing plate are held by a frame and configured to be movable in the horizontal direction together with the frame, and the relative position with respect to the fuel assembly can be adjusted. The effective multiplication factor measuring device for a fuel assembly according to claim 2.