JP7707141B2 - Method for clearing steel scrap and method for recycling steel scrap - Google Patents

Description

The present invention relates to a processing method for steel scrap stored as radioactive waste in a nuclear facility so that it can be subject to the clearance system (hereinafter also referred to as the "clearance processing method"), and a method for reusing this steel scrap.

Radioactive waste is generated during the decommissioning of nuclear facilities, including the dismantling of equipment and buildings. Radioactive waste must be stored in a controlled area within the nuclear facility.

As decommissioning of nuclear facilities progresses in earnest, it is expected that the amount of radioactive waste will increase. For this reason, it is becoming increasingly important to reduce the amount of stored steel scrap, which is particularly large and bulky among radioactive waste, and is hard and difficult to reduce in volume.

Currently, in light of resource depletion, recycling of various items is progressing, and recycling of metals, which are consumed in large quantities, has been carried out for some time. If the steel scrap generated at nuclear facilities could also be recycled in this way, the amount of material stored at the facilities could be reduced.

However, there is a clearance system in place where waste generated at a nuclear facility has an extremely low level of radioactivity, with negligible impact on human health, and if certain conditions are met, it can be removed from the facility as clearance material, which does not need to be treated as radioactive waste, and can be reused or disposed of as industrial waste. The standard value for radioactivity concentration in this clearance system, which is set by the government, is called the clearance level, and is usually expressed in units of Bq/g.

For nuclear operators to apply for the clearance system, they must first receive approval from the government for the methods of measuring and evaluating radioactivity concentrations. They must then measure and evaluate the radioactivity concentrations of the objects for which they wish to apply the system (hereinafter referred to as "clearance objects") in accordance with the approved methods, submit documents such as an application that lists the results to the government, and receive government confirmation that the objects do not exceed the clearance level.

Under this clearance system, the process of measurement and evaluation by nuclear operators and confirmation by the government is called "inspection." If this inspection can be carried out efficiently and steel scrap can be smoothly removed from nuclear facilities, it will help resolve issues related to the storage of radioactive waste.

Such a clearance system and a clearance processing system related to verification are disclosed in Patent Document 1. Specifically, Patent Document 1 discloses a highly efficient and reliable radioactive waste separation and clearance processing system, which is characterized by having a decontamination device that removes highly contaminated parts from clearance objects separated from dismantled waste resulting from the dismantling and removal of nuclear facilities, a pre-clearance measurement device that stores the decontaminated clearance objects in a first measurement container and measures the surface contamination density to confirm the presence or absence of highly contaminated parts in the clearance objects, and a clearance measurement device that stores the clearance objects confirmed to have no highly contaminated parts as a result of the pre-clearance measurement in a second measurement container larger than the first storage container, measures the average radioactive concentration of the clearance objects using the second measurement container as an evaluation unit, and confirms that the clearance objects are below the clearance level.

In the clearance processing system described in Patent Document 1, all gamma rays emitted from a clearance object that has been confirmed to have no highly contaminated areas are measured from the outside. Specifically, a second measurement container containing a clearance object that has been confirmed to have no highly contaminated areas is placed on a transport conveyor, and the radioactivity concentration is measured with radiation detectors arranged above, below, and to the sides of the transport conveyor.

Patent No. 4268970

In the method described in Patent Document 1, a clearance object that has been confirmed to have no highly contaminated areas is placed in a large measurement container with a volume of 1.6 ^m3 , and the total gamma rays emitted are measured.

The measurement method described in Patent Document 1 measures extremely low levels of gamma rays, and therefore involves uncertainties in the measurement of gamma rays, measurement uncertainties due to the distance between the radiation detector and the object being measured, and measurement uncertainties such as the weight of the object being measured. The standard value for whether a measurement value of radioactivity concentration is eligible for the clearance system is set taking into account these measurement uncertainties.

In other words, the more uncertain factors there are in the measurement, the greater the possibility that the measured value of radioactivity concentration will be lower than the true value, or conversely, higher than the true value. In the latter case, even if the true value of the radioactivity concentration is below the standard value and should be considered to have passed, if the measured value of the radioactivity concentration is higher than the true value and exceeds the standard value, it may be considered to have failed. In other words, if the radioactivity concentration is measured as a value higher than the actual value (true value), it is possible that even clearance objects that actually meet the clearance level will fail, and there will be fewer clearance objects that can be transported out of the nuclear facility as clearance objects.

Any items subject to clearance that cannot be transported outside the nuclear facility must be treated as radioactive waste. Radioactive waste is costly to store, transport and dispose of, and there are problems with this, such as the need to secure space for storage within the nuclear facility site.

The present invention was made in consideration of these problems, and aims to provide a method for clearance processing of steel scrap and a method for recycling steel scrap that will enable the clearance system to be applied to a larger amount of steel scrap from among the waste generated at nuclear facilities.

After extensive investigation, the inventors have discovered that the above object can be achieved by the following invention.

A method for clearance of steel scrap according to one aspect of the present invention is a method for clearance of steel scrap stored as radioactive waste in a nuclear facility, comprising:
melting the steel scrap;
a step of melting the steel scrap and separating the resulting molten material into molten metal and slag;
casting a portion of the molten metal into a sample for analysis;
casting the remainder of the molten metal into an ingot;
and measuring the radioactivity concentration of the analysis sample with a germanium semiconductor detector.

A method for recycling steel scrap according to another aspect of the present invention is a method for recycling steel scrap whose radioactivity concentration has been measured by the above-mentioned measuring method, comprising the steps of:
removing the ingot from the nuclear facility if the measured value of the radioactivity concentration of the analysis sample is equal to or lower than a predetermined clearance level;
When the measured value of the radioactivity concentration of the analysis sample is greater than the clearance level, the ingot is stored at the nuclear facility at least until the radioactivity concentration calculated based on the measured value decays to below the clearance level, and then the ingot is transported from the nuclear facility.

The present invention provides a method for clearance processing of steel scrap and a method for recycling steel scrap that enable the clearance system to be applied to a larger amount of steel scrap from among waste generated at nuclear facilities.

The following describes a method for clearance processing of steel scrap and a method for recycling steel scrap according to one embodiment of the present invention.

The steel scrap clearance processing method according to this embodiment is a steel scrap clearance processing method stored as radioactive waste in a nuclear facility, and includes the steps of melting the steel scrap, separating the molten material obtained by melting the steel scrap into molten metal and slag, casting a portion of the molten metal into an analysis sample, casting the remaining portion of the molten metal into an ingot, and measuring the radioactivity concentration of the analysis sample with a germanium semiconductor detector.

This measurement method allows the radioactivity concentration of steel scrap to be measured with high accuracy, so the standard value for whether the measured value of radioactivity concentration is eligible for the clearance system can be set to a value closer to the clearance level set by the government than the value set for the conventional measurement method described in the above-mentioned Patent Document 1. This makes it possible for the clearance system to be applied to a larger amount of steel scrap.

In addition, in the conventional measurement method described in Patent Document 1, radiation detectors must be placed above, below, and to the side of a conveyor that transports a large measurement container containing the object to be measured, and a large-scale measurement device must be installed on a large site. However, in the measurement method according to the present embodiment, the object to be measured is a small analysis sample, and the germanium semiconductor detector is also small, so a large site is not required and measurements can be performed even in small facilities.

<Scrap Steel>
The nuclear facility according to the present embodiment is, for example, a refining facility and processing facility for nuclear fuel materials such as uranium and thorium, a nuclear reactor facility for testing and research or power generation, a spent fuel reprocessing facility, etc. The steel scrap according to the present embodiment is scrap containing iron generated by decommissioning or refurbishment of nuclear equipment within the nuclear facility, and is stored within the site of the nuclear facility after the decommissioning or refurbishment of the nuclear equipment. The steel type of the steel scrap may be either carbon steel or special steel.

In this embodiment, carbon steel refers to an iron alloy with a carbon (C) content of 2.0 mass% or less, a small content of alloying elements, and does not fall under the category of special steel. Special steel refers to an iron alloy that contains a specified amount or more of one or more alloying elements other than carbon, with the balance being iron (Fe) and unavoidable impurities. Examples of alloying elements that can be used include nickel (Ni), chromium (Cr), silicon (Si), manganese (Mn), molybdenum (Mo), tungsten (W), vanadium (V), cobalt (Co), titanium (Ti), boron (B), and niobium (Nb).

<Clearance processing method for steel scrap>
In the clearance processing method for steel scrap according to the present embodiment, first, the steel scrap is melted. If steels with different chemical compositions are mixed and melted, it becomes complicated to adjust the compositions when recycling, so it is preferable to separate the steel scrap by steel type before melting it.

A typical electric furnace can be used as a melting device for melting the steel scrap, and an induction melting furnace can also be used. However, it is preferable to use an induction melting furnace, which has a stronger stirring power than a typical electric furnace, because it actively stirs the molten steel scrap, making the chemical composition of the molten metal (molten steel) less biased and closer to uniform, and therefore ensuring the representativeness of the samples described below. The size and capacity of the melting device can be determined according to the amount of steel scrap to be melted.

The melting equipment may be installed at the nuclear facility where the scrap steel is generated, or if the nuclear facility where the scrap steel is generated does not have a melting equipment, one installed at another nuclear facility may be used. However, to simplify the transportation process and reduce the human burden on workers during transportation, it is preferable to use the melting equipment installed at the nuclear facility where the scrap steel is generated.

When melting the steel scrap, only the steel scrap may be fed into the melting device, or the slag product may be fed together with the steel scrap. The slag product is made of calcium oxide and the like, and melts with the heat of the molten metal to become liquid, covering the top surface of the molten metal and suppressing metal loss due to oxidation of the molten metal. The steel scrap may be fed into the melting device all at once, or the entire amount may be divided into several portions and fed into the melting device while being melted sequentially. The slag product may be fed after the steel scrap is completely melted, or may be fed simultaneously with the steel scrap. Even if the slag product is not fed, a small amount of slag will be generated from impurities and contaminants contained in the steel scrap.

For example, lime, fluorite, etc. can be used as the slag product. The amount (mass) of the slag product added is preferably 2 to 4% of the total amount (mass) of the steel scrap.

Next, the molten material obtained by melting the steel scrap is separated into molten metal and slag. This separation can be done in any way, for example by discharging only the molten metal from a discharge hole provided on the side of the melting device, leaving the slag in the melting device. The slag can also be removed by scooping it up with a scraping tool from above the melting device.

A portion of the molten metal separated from the slag is taken and poured into a mold to cast a sample for analysis of a given shape. The size and shape of the sample for analysis may be any size that can be stored in a germanium semiconductor detector. Furthermore, in the case of a typical germanium semiconductor detector in which a cylindrical detector is inserted from below into a cylindrical lead shield, a cylindrical or disk-shaped sample can be stored inside the cylindrical lead shield, but samples smaller than the diameter of the detector are easier to measure, and a thinner sample compared to its diameter is less likely to block the emitted radiation by the sample itself, making it easier to measure. Therefore, it is appropriate to make the shape of the sample for analysis a disk with a diameter of 30 to 60 mm and a thickness of 10 to 40 mm. One or more analytical samples may be cast.

The remainder of the molten metal, from which a portion has been taken for an analytical sample, is cast into an ingot. There are no particular limitations on the size or shape of the ingot, and it should be of a size and shape that is easy to store and transport. A single ingot may be cast from the remaining molten metal, or multiple ingots may be cast.

The cast analytical sample is subjected to measurement of the content of radioactive nuclides contained in the analytical sample, and thus the radioactivity concentration, using a germanium semiconductor detector.

Germanium semiconductor detectors are gamma ray detectors, but because they have higher energy resolution than other detectors and can perform accurate gamma ray spectral analysis, they are used to identify radioactive nuclides contained in samples and evaluate radioactivity. However, the target object must be stored in an internal space that uses a shield to reduce background noise, and the sample must be small. For this reason, they cannot be used for waste itself (large targets) as in the invention described in Patent Document 1. However, small analytical samples taken from the molten material obtained by melting steel scrap, as in this embodiment, can be used as samples in germanium semiconductor detectors.

The germanium semiconductor detector is a small detector, and the analytical sample is also small, so that measurements can be made even in small-scale facilities. The germanium semiconductor detector can also accurately measure the content of radioactive nuclides contained in the above-mentioned analytical sample, and thus the radioactivity concentration, and can measure the amount of gamma rays emitted from the analytical sample for each energy when measuring the radioactivity concentration. Radioactive substances, i.e., radioactive nuclides, decay by emitting gamma rays, and the amount of radiation gradually decreases. Since the energy of gamma rays differs for each radioactive nuclide, the germanium semiconductor detector can identify the radioactive nuclides contained in the analytical sample and detect the content of each radioactive nuclide. The value obtained by dividing the content of each radioactive nuclide contained in the analytical sample by the mass of the analytical sample is the radioactivity concentration for each nuclide in the analytical sample.

Here, we will explain radionuclides, which are the source of radioactive contamination of steel scrap. In nuclear power plants, power is generated by circulating cooling water within the reactor system. Steel products with high corrosion resistance, such as stainless steel, are selected for the reactor steel materials, but they corrode very slightly in high-temperature environments. When corrosion products circulate within the system and adhere to the core, they are irradiated with neutrons and activated, peel off, circulate within the system again, and adhere to parts such as piping. Thus, corrosion products are the main source of radioactive contamination of steel scrap in nuclear power plants. The main components of reactor constituent materials are transition elements such as iron, and the main source of radioactive contamination are radioactive isotopes of these transition elements. Specific radioactive isotopes, or radioactive nuclides, include cobalt-60 and manganese-54, for example.

Generally, when metals or alloys such as scrap iron and steel are melted, molten metal and slag are produced, and the constituent elements of the metal or alloy migrate to both the molten metal and the slag. The ease of migration between the molten metal and the slag varies depending on the constituent elements, but the inventors have found that when scrap iron and steel is melted, elements with redox potentials close to that of iron, the main component of the molten metal, i.e., transition elements that are less likely to undergo oxidation reactions with the atmosphere when melted, migrate into the molten metal and remain there.

On the other hand, elements other than transition elements that do not easily undergo oxidation reactions with the atmosphere during melting, such as the typical elements cesium and strontium, are oxidized by the atmosphere and are either collected as oxides in the slag or collected as dust in the ventilation system. For this reason, radioactive isotopes of transition elements such as cobalt-60 and manganese-54, which are sources of radioactive contamination, mainly migrate to the molten metal, with only a small amount migrating to the slag.

In addition, these nuclides emit characteristic gamma rays of 1.173 Mev/1.323 Mev (cobalt-60) and 0.835 Mev (manganese-54), respectively, so the radioactivity concentration (Bq/g) of each nuclide can be evaluated by analyzing the gamma ray spectrum. Therefore, by melting steel scrap, the main sources of radioactive contamination, such as cobalt-60 and manganese-54, can be transferred to the molten metal, and the content of radioactive nuclides can be evaluated by gamma ray spectrum analysis.

In addition, because the chemical composition of the molten metal becomes uniform and unbiased due to natural convection during melting, the chemical composition of the analysis sample can be considered to be the average chemical composition of the molten metal of the steel scrap and the ingot cast from that molten metal. In other words, it can be said that the chemical composition of the analysis sample is representative of the chemical composition of the ingot cast from the molten metal. Therefore, by measuring the content (Bq) of radionuclides in the analysis sample and evaluating the radioactivity concentration (Bq/g) calculated from this measurement result and the mass of the analysis sample, it is possible to evaluate the content (Bq) of radionuclides and the average radioactivity concentration (Bq/g) in the ingot produced by casting the molten metal.

In the case of nuclear facilities other than nuclear power plants, the main source of contamination may not be radioactive isotopes of transition elements. However, even in such cases, by melting the steel scrap, radioactive isotopes of transition elements can be incorporated into the molten metal and other radioactive nuclides can be discharged into the slag, so the effectiveness of the process of measuring isotopes of transition elements in steel scrap is the same as in the case of nuclear power plants.

Therefore, the radioactivity concentration of each radionuclide in the analysis sample measured by the steel scrap clearance processing method according to this embodiment is equivalent to the radioactivity concentration of each radionuclide in the ingot. Therefore, the future change in the radioactivity concentration of each radionuclide contained in the ingot can be calculated from the measured value of the radioactivity concentration of each radionuclide and the half-life of each radionuclide, and the transition of the radioactivity concentration of each radionuclide in the ingot can be predicted. The half-life of transition element radionuclides is, for example, 5.27 years for the above-mentioned cobalt-60 and 312 days for manganese-54. In contrast, the half-life of cesium-137, a typical element radionuclide that easily migrates to slag, is 30.1 years, and the half-life of strontium-90 is 28.79 years. Thus, because the half-lives of transition element radionuclides are relatively short, in the recycling method for steel scrap described below, if the clearance level is not met, it is effective to continue storing the steel scrap in the hope of recycling it after its radioactivity has decayed.

In the steel scrap clearance processing method according to the present embodiment, it is preferable to have a step of removing slag mixed in the molten metal from the surface of the analysis sample before the step of measuring the radioactivity concentration of the analysis sample. Slag may be caught and mixed in the molten metal in the melting device. In this case, during the solidification process of the analysis sample, a part of this slag, which has a smaller specific gravity than the molten metal, may precipitate on the surface of the analysis sample. As described above, radioactive nuclides different from those in the molten metal are transferred to the slag, and if the slag is mixed into the sample, the composition of the nuclides will be different from that of the molten metal, which will hinder the representativeness of the measurement. For this reason, by scraping off the surface of the analysis sample and removing the slag from the surface of the analysis sample, the radioactivity concentration of the radioactive nuclides in the analysis sample can be made closer to the radioactivity concentration of the radioactive nuclides in the ingot. As a method for removing slag from the surface of the analysis sample, for example, a method of scraping off the surface of the analysis sample with a grinder or the like is available.

In the steel scrap clearance processing method according to this embodiment, it is preferable to store the analytical samples after measurement at the nuclear facility. By storing the analytical samples at the nuclear facility and recording their quality, it is possible to verify that the ingots removed from the nuclear facility are eligible for the clearance system even after they have been reused. This makes the legitimacy of the ingots clear, and can gain the trust of consumers.

<How to reuse scrap steel>
The method for recycling steel scrap according to this embodiment is a method for recycling steel scrap whose radioactivity concentration has been measured by the above-mentioned clearance treatment method for steel scrap, and if the measured value of the radioactivity concentration of an analytical sample is below a predetermined clearance level, the ingot is transported from a nuclear facility, and if the measured value of the radioactivity concentration of the analytical sample is greater than the clearance level, the ingot is stored at the nuclear facility at least until the radioactivity concentration calculated based on the measured value decays to below the clearance level, and then the ingot is transported from the nuclear facility.

If the measured value of the radioactivity concentration of the analytical sample is below the clearance level, the ingot cast from the steel scrap from which the analytical sample was taken can be subject to the clearance system if it meets certain conditions. This means that the ingot can be removed from the nuclear facility as a clearance item that does not need to be treated as radioactive waste, and can be reused or disposed of. In this embodiment, the "clearance level" is 0.1 Bq/g for the above-mentioned cobalt-60.

On the other hand, if the measured radioactivity concentration of the analytical sample is greater than the clearance level, specifically if the sum of the ratios (D/C) of the measured radioactivity concentration of each radioactive substance (D) to the national standard value (C) is greater than 1, the ingots cast from the steel scrap from which the analytical sample was taken will not be subject to the clearance system.

Here, the radioactivity concentration of the analytical samples is measured for each radionuclide, so it is possible to predict the decay progress of the radioactivity concentration of each radionuclide as described above, and calculate the period until the radioactivity concentration of the radionuclide in the ingot decays to the clearance level.

In the method for recycling steel scrap according to this embodiment, the period until the radioactivity concentration of the ingot decays to the clearance level can be clarified. If the ingot is stored at the nuclear facility until at least this period has passed, the ingot can then be transported from the nuclear facility as a clearance item and reused or disposed of. If the ingot is stored and the radioactivity concentration can be allowed to decay to the clearance level, costs can be reduced compared to disposing of it as radioactive waste without being subject to the clearance system. In addition, if an analytical sample is stored at the nuclear facility along with the ingot, the radioactivity concentration of the analytical sample can be measured again using a germanium semiconductor detector after the above period has passed, and it can be confirmed that the radioactivity concentration of the ingot transported from the nuclear facility satisfies the clearance level.

Summary of the disclosed technology
As described above, this specification discloses various aspects of the technology, the main technologies of which are summarized below.

As described above, a clearance treatment method for iron and steel scrap according to one aspect of the present invention is a clearance treatment method for iron and steel scrap stored as radioactive waste in a nuclear facility, the method including the steps of: melting the iron and steel scrap; separating a molten material obtained by melting the iron and steel scrap into a molten metal and a slag; casting a part of the molten metal into a sample for analysis; and casting the remaining part of the molten metal into an ingot.
and measuring the radioactivity concentration of the analysis sample with a germanium semiconductor detector.

This method allows the transition elements, which are the main source of contamination, to be transferred to the molten metal obtained by melting the scrap steel, and the analytical sample taken from the molten metal is representative of the radioactivity concentration of the scrap steel. Therefore, by measuring the content of radioactive nuclides contained in the analytical sample with a germanium semiconductor detector, the radioactivity concentration of the scrap steel can be measured with high accuracy.

As a result, the standard value for whether or not a radioactive concentration measurement is eligible for the clearance system can be set closer to the clearance level set by the government than with conventional measurement methods, making it possible for more steel scrap to be subject to the clearance system.

In addition, in the measurement method according to this embodiment, the object to be measured is a small analytical sample, and the germanium semiconductor detector is also small, so large-scale facilities are not required and measurements can be performed in a small laboratory.

The above-described steel scrap clearance processing method may include a step of removing the slag mixed in the molten metal from the surface of the analysis sample prior to the step of measuring the radioactivity concentration of the analysis sample.

With this configuration, the radioactivity concentration of the entire steel scrap calculated from the measurement results using the analytical sample can be made closer to the actual radioactivity concentration of the entire steel scrap. This makes it possible to apply the clearance system to even more steel scrap.

In the above-described method for clearance processing of scrap iron and steel, the scrap iron and steel may be separated by steel type, and the scrap iron and steel may be melted for each separated steel type.

If the steel is not separated by type, there is a risk that the resulting chemical composition after melting will only allow it to be used for specific purposes. However, separating the steel by type reduces the possibility of such limitations on use.

In the above-described steel scrap clearance processing method, the analysis samples may be stored at the nuclear facility.

With this configuration, even after the ingots removed from a nuclear facility are reused, it is possible to verify that the ingots are eligible for the clearance system, thereby gaining the trust of consumers.

A method for recycling steel scrap according to another aspect of the present invention is a method for recycling steel scrap whose radioactivity concentration has been measured by the above-mentioned measurement method, in which if the measured value of the radioactivity concentration of the analysis sample is equal to or lower than a predetermined clearance level, the ingot is transported from the nuclear facility, and if the measured value of the radioactivity concentration of the analysis sample is greater than the clearance level, the ingot is stored at the nuclear facility at least until the radioactivity concentration calculated based on the measured value decays to or below the clearance level, and then the ingot is transported from the nuclear facility.

This method allows more steel scrap to be subject to the clearance system. In addition, because steel scrap is cast as ingots and has a small volume, more steel scrap can be stored in a small area during the storage period. Furthermore, the period until the radioactivity concentration of the ingot decays to the clearance level can be calculated from the measured value of the radioactivity concentration of the analytical sample and the half-life of each radioactive nuclide. According to this method, ingots that have passed this period and are eligible for the clearance system can be removed from the nuclear facility, allowing for efficient management of the ingots.

The steel scrap clearance processing method according to the above-mentioned embodiment uses an analytical sample, and applies the so-called sampling method. The following simulation test was conducted to confirm whether the chemical composition of this analytical sample is unbiased and can be considered to be the average chemical composition of the entire steel scrap, i.e., to confirm the representativeness of the analytical sample.

<sample>
As a simulated material for steel scrap, a steel material having the composition shown in Table 1 was used. This steel material corresponds to SS400 specified in JIS G 3101:2020, which is a general structural rolled steel widely used in nuclear facilities.

In addition, to simulate the behavior of radioactive nuclides contained in steel scrap when it is melted, a chemical tracer was used instead of a radioactive nuclide. A metal reagent of natural cobalt was used as the chemical tracer. Natural cobalt is not radioactive, but it has the same chemical properties as cobalt-60, which is the element with the highest concentration (and most abundant amount) in nuclear facilities, and it is thought that when steel scrap is melted, it will behave in the same way as cobalt-60 in the molten material. Note that the actual amount of radioactive nuclides is extremely small and difficult to detect using chemical analysis methods, so in this simulation test, a chemical tracer was used in an amount that can be detected using chemical analysis methods.

Test conditions:
Tests were conducted using the amounts of steel and chemical tracer shown in Test Nos. 1 and 2 in Table 2 under the following melting conditions:

Melting furnace: High-frequency induction melting furnace (rated output 50 kW, frequency 3000 Hz)
Atmosphere: air Melting temperature: 1550°C
The test procedure was as follows:

A chemical tracer and a portion of the steel were sequentially placed into the melting furnace before operation, and the melting furnace was started. After the molten metal was formed in the melting furnace, the remaining part of the steel was placed and it was confirmed that it had completely melted. The slag was then removed from the melting furnace, the temperature of the molten metal was measured, and samples were taken from the molten metal.

Test Results
The test results are also shown in Table 2. In both Test No. 1 and Test No. 2, the chemical tracer concentration relative to the steel material before melting was the same, and the chemical tracer content of the obtained samples was also the same. From this result, it was possible to confirm the representativeness of the analytical samples in the clearance processing method for steel scrap according to the above embodiment.

In addition, in both Test No. 1 and Test No. 2, the concentration of the chemical tracer in steel before melting was 0.77 mass%, while the content of the chemical tracer in the obtained samples was 0.73 mass%, and 95% of the chemical tracer remained in the samples. From this result, it was confirmed that in the steel scrap clearance processing method according to the above embodiment, the content of radioactive nuclides contained in the analysis sample was equivalent to the content of radioactive nuclides in the entire steel scrap, that is, the radioactivity concentration of the analysis sample was equivalent to the radioactivity concentration in the entire steel scrap. Note that the content of the chemical tracer in the sample was 5% lower than the concentration of the chemical tracer in steel before melting, which is thought to be due to the transfer of the chemical tracer to slag and exhaust gas during melting.

Claims (8)

A method for clearance of steel scrap stored as radioactive waste in a nuclear facility, comprising the steps of:
melting the steel scrap in an induction melting furnace ;
a step of melting the steel scrap and separating the resulting molten material into molten metal and slag;
casting a portion of the molten metal into a sample for analysis;
casting the remainder of the molten metal into an ingot;
and measuring the radioactivity concentration of the analysis sample with a germanium semiconductor detector. 2. The method for clearance processing of scrap iron and steel according to claim 1, wherein a slag product is charged together with the scrap iron and steel in the step of melting the scrap iron and steel in an induction melting furnace. The steel scrap clearance processing method according to claim 1, further comprising a step of removing the slag mixed in the molten metal from the surface of the analysis sample before the step of measuring the radioactivity concentration of the analysis sample. The method for clearance processing of steel scrap according to claim 1, wherein the steel scrap is separated into steel types, and the steel scrap is melted for each separated steel type. The steel scrap clearance processing method according to claim 1, wherein the analytical samples are stored at the nuclear facility. 2. The clearance processing method for steel scrap according to claim 1, wherein in the step of casting into the analysis sample, a part of the molten metal is taken and poured into a mold to cast into the analysis sample having a predetermined shape and a size that can be accommodated in a germanium semiconductor detector. 7. The clearance processing method for steel scrap according to claim 6, wherein the shape of the analysis sample is a disk shape having a diameter of 30 to 60 mm and a thickness of 10 to 40 mm. A method for recycling steel scrap whose radioactivity concentration has been measured by the measurement method according to claim 5 , comprising the steps of:
removing the ingot from the nuclear facility if the measured value of the radioactivity concentration of the analysis sample is equal to or lower than a predetermined clearance level;
When a measured value of the radioactivity concentration of the analysis sample is greater than the clearance level, the ingot and the analysis sample are stored at the nuclear facility at least until the radioactivity concentration calculated based on the measured value decays to or below the clearance level, and then the ingot is transported from the nuclear facility.