JP6919468B2 - Interface level measurement system and method - Google Patents

Description

The present invention relates to an interface measurement system and method capable of measuring the level of an interface between a molten metal layer and a slag layer containing a radionuclide.

Radioactive waste is generated at nuclear facilities, nuclear fuel facilities, radiation utilization facilities, etc. Radioactive waste includes, for example, metals with radionuclides attached, fly ash, concrete debris, and the like. In order to reduce the volume of these radioactive wastes, the radioactive wastes are melted in a furnace such as a plasma furnace.

When radioactive waste is melted, a slag layer is formed at the top and a molten metal layer is formed at the bottom due to the difference in specific gravity. It has been confirmed that some radionuclides adhering to radioactive waste are transferred to the slag layer by melting treatment. This is due to the chemical properties of radionuclides. ^{For example, 137} Cs, which is one of the radionuclides, has a low boiling point of 670.8 ° C., cannot be stably present in the molten metal layer, and is chemically supplemented by the silicate present in the slag layer.

By the way, a gamma ray level meter is known as a level meter for measuring the level of a liquid level in a closed container such as a furnace (see Patent Document 1). In this gamma ray level meter, a gamma ray source and a gamma ray detector are arranged so as to face each other across a container, and gamma rays are passed horizontally across the container. If there is a substance that blocks gamma rays, the number of measurements at the detector will decrease. Therefore, the level of the liquid level can be measured. Using multiple gamma ray sources or multiple gamma ray detectors can measure several levels. The gamma ray level meter has a feature that the level of the liquid level can be measured from the outside of the container, and the level of the liquid level can be measured even when the liquid is in a molten state at a high temperature.

Japanese Unexamined Patent Publication No. 8-5441

When the above radioactive waste is melted in a furnace, if the level of the interface between the molten metal layer and the slag layer can be measured, clean molten metal and slag containing radionuclides can be efficiently separated. However, even if the level of the liquid level of the slag layer can be measured by using a conventional gamma ray level meter, it is difficult to measure the level of the interface between the molten metal layer and the slag layer.

Therefore, an object of the present invention is to provide an interface level measurement system and method capable of measuring the level of the interface between the molten metal layer and the slag layer.

In order to solve the above problems, one aspect of the present invention is a level measurement system used in a furnace for melting radioactive waste containing ^{137 Cs of radionuclide to produce molten metal and slag containing 137} Cs. At least one detector that is located outside the furnace and contains slag containing molten metal and ¹³⁷ Cs to detect the intensity of gamma rays emitted from ¹³⁷ Cs inside the furnace, and the output of the detector. based on a level measurement system interface and a measuring device for estimating the level of the interface between the position where the gamma ray intensity is peak emitted molten metal layer and the slag layer from ^{137 Cs.}

Another aspect of the present invention is a level measuring method used in a furnace for melting radioactive waste containing ^{137 Cs of a radionuclide to produce molten metal and slag containing 137} Cs ^{, wherein the molten metal and 137} Cs. The gamma ray intensity emitted from ¹³⁷ Cs inside the furnace containing the slag containing the above is detected by a detector located outside the furnace, and is emitted from ¹³⁷ Cs based on the output of the detector. gamma ray intensity is level measurement method of the interface of estimating the level of the interface between the position where the peak melting metal layer and the slag layer.

Slag containing radionuclides itself emits radiation. A three-dimensional model of the slag layer, molten metal layer, container, and detector, and simulating the radiation incident on the detector outside the container from the radionuclides of the slag layer, the radiation near the interface between the slag layer and the molten metal layer. It was found that the intensity peaked and the radiation intensity dropped sharply below the interface. Below the interface, the molten metal attenuates and absorbs radiation. As in the present invention, the interface between the molten metal layer and the slag layer can be measured by detecting the radiation emitted from the radionuclide of the slag layer with a detector.

It is a schematic diagram of the interface measurement system of the 1st Embodiment of this invention. It is a graph which shows the simulation result when the height of a slag layer is changed to 3 patterns (FIG. 2 (a) shows an example of the height of a slag layer of 50 to 60 cm, and FIG. 2 (b) is an example of a slag layer. An example in which the height is 30 to 40 cm is shown, and FIG. 2C shows an example in which the height of the slag layer is 20 to 30 cm). It is a schematic diagram of the interface measurement system of the 2nd Embodiment of this invention. It is a schematic diagram of the interface measurement system of the 3rd Embodiment of this invention. It is a graph which shows the simulation result.

Hereinafter, the interface level measurement system and method according to the embodiment of the present invention will be described in detail with reference to the accompanying drawings. However, the interface level measurement system and method of the present invention can be embodied in various forms, and are not limited to the embodiments described in the specification. The present embodiment is provided with the intention of allowing those skilled in the art to fully understand the invention by adequately disclosing the specification.
(First Embodiment)

FIG. 1 shows a schematic diagram of an interface measurement system according to the first embodiment of the present invention. A furnace 1 such as a plasma furnace is charged with radioactive waste such as metal, fly ash, and concrete shavings to which radionuclides are attached. When these radioactive wastes are melted by plasma heating or the like, a slag layer 2 is formed at the upper part and a molten metal layer 3 (hereinafter, the molten metal layer is referred to as a metal layer 3) is formed at the lower part due to the difference in specific densities. ^{Most of the radionuclides such as 137} Cs adhering to the radioactive waste are transferred to the slag layer 2 by the melting treatment. Most of the concentrations of radionuclides in the metal layer 3 are below the detection limit, and the metal layer 3 substantially does not contain radionuclides.

A plurality of detectors 4 are arranged in the vertical direction on the outside of the furnace 1 in a non-contact state with the furnace 1. FIG. 1 is an example in which m detectors 4 are arranged. The pitch at which the plurality of detectors 4 ₁ to 4 _m are arranged may be constant or different. Detectors 4 ₁ to 4 _m are GM counters, ionization chambers, or scintillation detectors. The detectors 4 ₁ to 4 _m detect the gamma ray intensity emitted from the radionuclide inside the furnace 1 to the outside of the furnace 1 without detecting the gamma ray intensity emitted from the radioactive source outside the furnace 1.

A measuring device 5 composed of a computer is electrically connected to the detectors 4 ₁ to 4 _m. The measuring device 5 estimates the level of the interface 6 between the metal layer 3 and the slag layer 2 based on the output (gamma ray intensity) of the _{detectors 4 1} to 4 _m. Specifically, the level at the interface 6 is determined to be the position where the gamma ray intensity measured by the _{detectors 4 1} to 4 _{m peaks or the vicinity of the peak.} In the example of FIG. 1, it is determined that there is a level of the interface 6 between the detector 4 ₃ and the detector 4 _4.

FIG. 2 shows the results of simulating the gamma rays incident on the detector 4 from the radionuclides of the slag layer 2. FIG. 2 shows the simulation results when the height of the slag layer 2 is changed to three patterns. In FIG. 2 (a), the slag layer 2 is located at a position of 50 to 60 cm of the furnace 1, and in FIG. 2 (b), the slag layer 2 is located at a position of 50 to 60 cm. It is located at a position of 30 to 40 cm of the furnace 1, and in FIG. 2C, it is located at a position of 20 to 30 cm of the furnace 1. The graph in the figure shows the intensity distribution of gamma rays emitted to the outside of the furnace 1 obtained by simulation, that is, the count number of simulated gamma rays detected by each of _{the simulated detectors 4 1} to 4 _{n arranged in the vertical direction.} show.

The simulation begins with a three-dimensional model of the slag layer 2, the metal layer 3, the furnace 1, and the detectors 4 ₁ to 4 _m. That is, the shape, material, size, position, etc. of the slag layer 2, the metal layer 3, the furnace 1, and the detectors 4 ₁ to 4 _m are used as parameters, and a three-dimensional model simulating these is created. ^{Next, simulated radionuclides such as 137} Cs are uniformly dispersed in the simulated slag layer 2'at a predetermined concentration, and simulated gamma rays are randomly generated from the simulated furnace 1'. Then, the simulated gamma rays incident on the simulated _{detectors 4 1} to 4 _{n are counted.} Note that n indicating the number of simulated detectors is the number of simulations performed to obtain the intensity distribution of gamma rays. The larger n is, the finer the gamma ray intensity distribution is obtained. Therefore, it is possible to accurately estimate the level of the interface 6 which is the peak position of the gamma ray intensity or the vicinity of the peak position.

As shown in FIGS. 2 (a) to 2 (c), _{the count number of simulated gamma rays detected by the simulated detectors 4 1} to 4 _n peaks near the simulated interface 6'and is lower than the simulated interface 6'. It drops sharply at. This is because the simulated metal layer 3 attenuates and absorbs simulated gamma rays below the simulated interface 6'. Since the graph shows the same tendency under any of the conditions of FIGS. 2 (a) to 2 (c), it can be used as an interface level meter by utilizing this phenomenon.

The number of simulated gamma ray counts of the simulated detectors 4 ₁ to 4 _n correlates with the gamma ray intensity measured by the _{actual detectors 4 1} to 4 _m. Therefore, it can be estimated that the interface 6 is the position where the gamma ray intensity detected by the actual detectors 4 ₁ to 4 _{m peaks.}

When estimating the level of the interface 6 more accurately, the measuring device 5 estimates the interface 6 as follows by using the above simulation result. The measuring device 5 previously calculates the number of simulated gamma ray counts associated with the positions of the _{plurality of simulation result libraries (simulated detectors 4 1} to 4 _n ) as shown in the graphs of FIGS. 2 (a) to 2 (c). The library to represent) is memorized. In addition, in FIGS. 2A to 2C, the slag thickness is constant, but the library in which the slag thickness is changed is also stored. Then, the measuring device 5 compares the actual detector 4 ₁ to 4 _m gamma ray intensity was measured and the library selects the actual detector 4 ₁ to 4 _m closest library to gamma intensities measured. The level of interface 6 may be estimated from this selected library.
(Second embodiment)

FIG. 3 shows a schematic diagram of an interface measurement system according to a second embodiment of the present invention. Since the configurations of the furnace 1, the slag layer 2, the metal layer 3, and the measuring device 5 are substantially the same as those of the first embodiment, the same reference numerals are given and the description thereof will be omitted. In the first embodiment, a plurality of detectors 4 ₁ to 4 _m are arranged in the vertical direction, whereas in the second embodiment, one detector 7 is configured to be movable in the vertical direction. As a device for moving the detector 7 in the vertical direction, for example, a feed screw mechanism, a linear motor, or the like can be used. By making the detector 7 movable in the vertical direction, the gamma ray intensity can be measured at a plurality of measurement points in the vertical direction even though it is one detector 7. Therefore, the level of the interface 6 between the metal layer 3 and the slag layer 2 can be estimated in the same manner as when a plurality of detectors 4 ₁ to 4 _{m are arranged in the vertical direction.}
(Third Embodiment)

FIG. 4 shows a schematic diagram of an interface measurement system according to a third embodiment of the present invention. Since the configurations of the furnace 1, the slag layer 2, the metal layer 3, and the measuring device 5 are substantially the same as those of the first embodiment, the same reference numerals are given and the description thereof will be omitted. In the first embodiment, a plurality of detectors 4 ₁ to 4 _m are arranged in the vertical direction, whereas in the third embodiment, one detector 8 faces the lower part of the furnace 1. Deploy.

When the metal is discharged from the lower part of the furnace 1, the level of the interface 6 between the metal layer 3 and the slag layer 2 is lowered. As shown in the simulation result of FIG. 5, the position where the gamma ray count number peaks also moves downward due to the hot water discharge. The gamma ray intensity measured by the detector 8 is almost zero when the detector 8 is sufficiently lower than the level of the interface 6, but increases sharply as the detector 8 approaches the level of the interface 6. The height of the detector 8 when the gamma ray intensity suddenly increases can be estimated as the level of the interface 6 between the metal layer 3 and the slag layer 2.

The present invention is not limited to the above embodiment, and can be embodied in other embodiments without changing the gist of the present invention.

For example, in the above embodiment, a furnace is used as a container, but a ladle, a tundish, or the like can be used as a container as long as it can contain molten radioactive waste.

Radionuclides not limited to ^{137 Cs,} it is also possible to use other radionuclides instead of addition or ¹³⁷ Cs to ¹³⁷ Cs.

1 ... Furnace (container)
2 ... Slag layer 3 ... Metal layer (molten metal layer)
4 ... Detector 5 ... Measuring device 6 ... Interface 7 ... Detector 8 ... Detector

Claims (4)

A level measurement system used in a reactor that melts radioactive waste containing ¹³⁷ Cs of radionuclides ^{to produce molten metal and slag containing 137} Cs.
Disposed outside the furnace slag containing molten metal and ¹³⁷ Cs are accommodated, and at least one detector for detecting the gamma ray intensity emitted from the ¹³⁷ Cs of the interior of the furnace,
On the basis of the output of the detector, the level measurement system interface and a measuring device for estimating the level of the interface between the position where the gamma ray intensity is peak melting metal layer and the slag layer to be emitted from the ^{137 Cs.} The level measurement of the interface according to claim 1, wherein a plurality of the detectors are arranged in the vertical direction, or at least one of the detectors is configured to move in the vertical direction with respect to the furnace. system. The level measurement system for an interface according to claim 1, wherein the detector is arranged so as to face the lower part of the furnace. A level measurement method used in a reactor that melts radioactive waste containing ¹³⁷ Cs of radionuclides ^{to produce molten metal and slag containing 137} Cs.
The gamma ray intensity emitted from ¹³⁷ Cs inside the furnace containing the molten metal and ^{slag containing 137} Cs was detected by a detector located outside the furnace.
A method for measuring the level of an interface in which the position where the intensity of gamma rays emitted from ¹³⁷ Cs peaks is estimated to be the level of the interface between the molten metal layer and the slag layer based on the output of the detector.

Images