JPH056678B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a nondestructive measuring device for irradiated fuel that measures the power distribution or burnup distribution of irradiated fuel in a nuclear reactor in a fuel storage pool.

[Technical Background of the Invention and Problems Therewith] Gamma ray measurement, Thierenkov photometry, and the like are known as nondestructive measurement methods for the power distribution or burnup distribution of nuclear reactor irradiated fuel.

The conventional Thierenkov light measurement method is a method in which the intensity of Thierenkov light generated by gamma rays emitted from highly radioactive irradiated fuel placed in pool water is remotely measured using a photometer from above the pool water surface. Depending on the length of the irradiation, the approximate burnup or output at the end of irradiation can be determined.

However, with this method, the fuel to be measured is easily affected by ambient light, so the fuel to be measured is placed in water isolated from other fuels, completely blocking ambient light, and Thierenkov light is transmitted from the outside of the fuel. There is a restriction that it must be observed. Also, the distance between the photometer and the fuel to be measured is long (approximately 4 to
10 m), the drawback was that it was quite difficult to accurately measure the axial Thierenkov luminosity distribution of a fuel installed vertically underwater.

[Object of the invention] The present invention has been made to address the above problems, and
By accurately measuring the axial Thierenkov luminous intensity distribution of fuel installed vertically underwater without being affected by ambient light using an optical fiber, we can measure the burnup distribution of the fuel or the power distribution at the end of irradiation. It is an object of the present invention to provide a non-destructive measuring device for irradiated fuel that can easily and accurately determine the irradiated fuel.

[Summary of the Invention] That is, the present invention detects Thierenkov light or scintillation light emitted by radiation emitted from each part of an irradiated fuel assembly or fuel rod underwater, along the axial direction of the fuel, and transmits it to a subsequent measuring device. A large number of radiation-resistant optical fibers are used for transmission, a light intensity measuring device is connected to these optical fibers and measures the amount of transmitted light, and the axial distribution of the light intensity measured by the light intensity measuring device is called burnup distribution. or a display device that converts and displays the output distribution.

[Embodiment of the Invention] The present invention will be described in detail below with reference to an embodiment shown in the drawings.

FIG. 1 schematically shows an embodiment of the present invention. In the figure, reference numeral 1 indicates a fuel rod constituting a fuel assembly 2. As shown in FIG. The nondestructive measurement device for irradiated fuel of this embodiment includes an optical fiber detection section 3 consisting of a large number of optical fibers whose tips reach each measurement position in the axial direction of a fuel rod 1, and an optical fiber detection section 3 connected to this detection section 3. Light amount measuring device 4 that quantifies the amount of light from each optical fiber
and a display device 5 that converts the axial distribution of light amount measured by the light amount measuring device 4 into a burnup distribution or output distribution at the end of irradiation and displays the result.

The tips of the optical fibers constituting the optical fiber detection unit 3 are held perpendicular to the axial direction of the fuel rod 1, and arranged so that the distance between the tip of the core of each optical fiber and the fuel rod 1 is constant. Thus, only Thierenkov light or scintillation light generated near a local portion of the fuel rod 1 is guided to each optical fiber. For example, as shown in FIG. 2, an optical fiber cladding tube 31 is attached to a fuel rod.
By contacting the surface, it blocks light from the surroundings, and by installing an extremely small light-shielding box 33 filled with water or a scintillator between the tip of the core 32 and the fuel rod 1, radiation from the fuel rod 1 is blocked. The configuration is such that only the Thierenkov light or scintillation light generated by the optical fiber is introduced into the tip of the optical fiber.

In the middle part of the optical fiber detection unit 3, each optical fiber is sequentially bundled and held in a straight line parallel to the fuel rod 1, but the part above the fuel rod 1 up to the light intensity measuring device 4 has a flexible structure. has been done. The optical fiber used here is
For example, a pure silica optical fiber, which is radiation resistant and has no change in light transmittance against gamma rays of about 10 ⁵ R, is suitable.

In fact, when measuring a fuel assembly using such an optical fiber detection section 3, the optical fiber detection section 3 is inserted from above into the gap between the fuel rods inside the channel box with the channel box attached. Thierenkov light intensity distribution that is not affected by light from outside the body can be measured.

Furthermore, when the optical fiber detection section 3 is disposed on the outer periphery of the fuel assembly, either the tip end has a structure as shown in FIG. 2, or the optical fiber detection section 3
By covering the entire structure with an opaque cladding tube, measurements can be performed without being affected by external light.

Furthermore, the intensity of gamma rays around a fuel assembly decreases to about 1/10 when the distance is about 25 to 35 cm underwater, so if the fuel assembly to be measured is installed further away from adjacent fuel, It can be measured with almost no influence. Therefore, it is also possible to carry out measurements in a fuel storage rack, for example.

The light intensity measuring device connected to the optical fiber detector 3 can be a measuring device using multiple photoelectric conversion elements that simultaneously measure the amount of light passing through each optical fiber, or a micro photoelectric amplification device if the amount of incident light is very weak. A measuring device that uses an element to amplify and quantify the amount of light can be used.

The intensity of Thierenkov light or scintillation light based on the radiation emitted from the irradiated fuel is approximately proportional to the output at the end of irradiation when the cooling period after irradiation is short, about 3 months or less; In the case of a long period of one year or more, the burnup is approximately proportional to the burnup during the entire irradiation period. Therefore, the axial distribution of Thielenkov light intensity or scintillation light intensity measured immediately after fuel removal represents the power distribution, and the light intensity distribution obtained by measurement after a sufficient cooling period, for example about two years, indicates burnup. distribution is obtained.

Furthermore, by calibrating the correlation between the amount of light and the burnup in some way with the cooling period known, it is possible to evaluate not only the distribution of the burnup but also its absolute value by measuring the amount of Thierenkov light.
Burnup calibration methods include, for example, a method using calculated values, a method using non-destructive measurement values such as gamma ray spectroscopy or passive neutron measurement, or a method using destructive measurement values if possible.

[Effects of the Invention] As is clear from the above description, according to the present invention, the optical fiber detection unit is compact and does not take up much space for installation, and is almost unaffected by light from the surroundings and can detect the irradiated fuel in the axial direction. Since the amount of Thierenkov light can be accurately measured, the burnup distribution of the fuel or the output distribution at the final stage of irradiation can be easily and accurately measured depending on the length of the cooling period after irradiation.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing an embodiment of the irradiated fuel non-destructive measuring device of the present invention, and FIG. 2 is a sectional view showing an embodiment of the tip portion of the optical fiber detection section of the present invention. 1...Fuel rod, 3...Optical fiber detection unit, 4
...Light measuring device, 5...Display device.

Claims (1)

[Scope of Claims] 1. A large number of optical fibers that detect and transmit Thierenkov light emitted by radiation emitted from each part of an irradiated fuel assembly or fuel rod in water along the axial direction of the fuel. , a large number of optical fibers, each tip of which is arranged at each measurement position in the axial direction of the fuel, are sequentially bundled from bottom to top and held parallel to the fuel; A display device for converting the axial distribution of the light amount measured by the light amount measuring device into a burnup distribution or a power distribution and displaying the result. Non-destructive measurement equipment. 2. The non-destructive measurement device for irradiated fuel according to claim 1, wherein the tip of the core of each optical fiber is held perpendicular to the fuel to be measured and at a constant interval. 3. The non-destructive measuring device for irradiated fuel according to claim 1 or 2, characterized in that a small scintillator is provided at the tip of the optical fiber.