KR100740779B1 - Method for reducing radioactivity of spent pwr cladding hull by using pulse laser - Google Patents

Abstract

The present invention relates to a method for reducing radioactivity of spent nuclear fuel sheathed spent light reactors, and more particularly, transuranic (TRU) elements and nuclear fission products that are adhered to the surface of spent fuel hulls generated in a dry process. And a method for reducing spent nuclear waste-covered tube radioactivity, wherein the radioactive product is removed using a pulse laser to form a low to medium level waste having a radioactivity of less than 100 nCi / g and a heat generation rate of less than 2 mW / m 3. According to the present invention, the high radioactive layer of the inner surface of the closed coating tube is removed by the thickness of the contaminated pulse laser, thereby lowering the total radioactivity of the closed coating tube, thereby reducing the disposal cost, and removing the high level radioactive material. It allows the production of good ingots.

Nuclear fuel, encapsulated tube, radioactivity, cutting

Description

METHODS FOR REDUCING RADIOACTIVITY OF SPENT PWR CLADDING HULL BY USING PULSE LASER}

1 is a graph showing the oxygen potential of the inner surface of the closed coating tube,

Figure 2 is a graph showing the results of EPMA analysis of the inner surface of the lung coating tube,

3 is a photograph showing the surface treatment by a pulse laser according to the present invention,

4 is a photograph showing surface treatment by a conventional continuous wave laser.

The present invention relates to a method for reducing the radioactivity of spent fuel spent pipes for use in hard water, and more particularly, transuranic (TRU) elements fixed to the inner surface of spent fuel spent pipes generated in a dry process, and nuclear fission. The present invention relates to a method for reducing the radioactivity of spent fuel lung coat by removing a product or the like using a pulse laser.

Nuclear power is carried out by loading nuclear fuel into fuel cladding and reacting it with neutrons. The nuclear fuel is loaded into a nuclear fuel cladding tube in the form of pellets made of uranium dioxide, and the nuclear fuel cladding tube is in direct contact with uranium dioxide and should exhibit excellent physical properties during nuclear power generation. For this reason, the fuel cladding tube is made of a zirconium alloy, and specifically, zircaloy-2 and zircaloy-4 are used. Among them, Zircaloy-4 is the most commonly used nuclear fuel cladding, which is 1.20 to 1.70 wt% Sn, 0.18 to 0.24 wt% Fe, 0.07 to 1.13 wt% Cr, Ni <0.007 wt%, O 900 to 1500 ppm and Zr glass It is a cladding tube consisting of parts.

The spent fuel bundles burned in nuclear power plants are stored and stored in the tanks without being processed at present, but as the power generation continues, the amount accumulates gradually, requiring vast space. That is, as described above, a nuclear fuel made mainly of uranium dioxide is loaded in a Zircalloy cladding tube and loaded into a nuclear reactor in the form of a bundle of nuclear fuel. The cladding pipe occupies more than double. Spent fuels and fission products are thermally, physically and chemically fixed in the spent cladding that has been burned and discharged for a certain period of time, so that they are classified and managed as high-level wastes. Therefore, treatment methods for these fuel claddings are required.

In response to these demands, several proposals have been made regarding the method of storing the spent fuel. For example, Korean Patent Application No. 194-006237 discloses that the sides of the storage vessel are completely sealed and are soluble at the top or the top and bottom of the container. And a filter cartridge containing a built-in synthetic zeolite capable of effectively adsorbing radioactive materials, thereby effectively preventing contamination of the cooling water due to defective nuclear fuel.

Republic of Korea Patent Application No. 195-000102, "In order to reduce the storage and disposal space of spent fuel in light water, the spent fuel rods are remotely withdrawn from the spent fuel assembly in a dry state and rearranged densely and stored in a storage or disposal container. It is a core device of the rod consolidation system that is inserted and repacked.An impact protection device that mitigates the shock that may be applied to the fuel rod when the fuel rod is taken out and the fuel rod dense material which is the next process after the fuel rod withdrawal Horizontal feeder for horizontally feeding the fuel rods in the alignment process,

Republic of Korea Patent Application No. 2001-0046539, "In the volume reduction device of the spent fuel assembly structure waste; The volume reduction device is a compression member waste is seated, the transport member for fixing the end of the compressed structure waste and intermittent / transfer; A compression / cutting member for compressing and cutting the compressed structural waste connected and installed on one side of the conveying member by a plurality of rams, and a hydraulic member connected / installed with the compression / cutting member to supply a power source; Spent fuel assembly structured waste, comprising a discharge member connected to and installed with the compression / cutting member to discharge the compressed structured waste, and the compressed structure waste is transported horizontally and compressed / cut in the same vertical direction. Compression / cutting reduction apparatus.

However, although the above methods are useful as a method of storing spent fuel in their own way, they are insufficient as a recent method of treating a fuel cladding tube.

On the other hand, in order to recycle the spent fuel, the fuel rods stored and stored for a certain period of time go through a powdering treatment process, which is performed by a wet treatment process and a dry treatment process.

First, the wet process is to cut the fuel cladding tube into short lengths and then dissolve the fuel in nitric acid solution (Chop and leach). In this process, uranium dioxide pellets are dissolved in nitric acid and lead to the next process for recycling. The cladding tube containing the pellets remains solid waste. At this time, the remaining nuclear fuel cladding tube is called "hull."

In addition, in the dry process, the rod cut obtained by shortly cutting the nuclear fuel cladding tube into a predetermined length is thinly divided in the longitudinal direction, and the oxidation and reduction process is repeated while heating to separate the uranium powder and the cladding tube. Pulmonary sheath, a high-level waste, develops.

As described above, the waste coating tube generated in the powdering treatment process (Hull) is buried with undissolved fuel residue in the process of dissolving the fuel, which is generally known to be about 0.1% of the initial fuel amount. That is, the waste coating tube generated as described above has a nuclear fuel dissolution solution in addition to the nuclear fuel residues such as uranium or plutonium, nuclear fission products, and radioactive products resulting from the radiation of metal elements contained in the coating material and wet treatment. Due to its large amount of coating, it is classified as a high-level radioactive waste.

When classified as high-level radioactive waste, it is more expensive than the low and medium-level radioactive waste and has a difficult handling procedure.

Waste coating tube, which is a high-level radioactive waste, is usually compressed, melted, and then reduced, and then solidified to a suitable matrix and disposed of in a packaged state.

However, if the closed tube is compressed or melted as it is, having high radioactivity, it is classified as a high-level radioactive waste, which is expensive and difficult to handle, and in addition to nuclear fission residues remaining on the surface of the closed tube. Due to the product, properties such as ductility, hardness, corrosion resistance, etc. of the closed coating pipe (zirconium alloy) are deteriorated, and thus, it is difficult to manufacture ingots having excellent metallic properties and to ensure long-term disposal stability.

As a method of pretreatment of the lung coat tube, a method of disposing it in the form of high quality metal ingot by melting it after chemical pretreatment using radioactive decontamination technology has been studied. There is no proposal for lowering the level of radioactivity by directly removing radioactive material from the surface of the lung coat. However, in Korean Patent Application No. 2003-0089056, filed by the present inventors, etc., entitled "Reduction of Radiation in LWR spent fuel pipes", "The amount of radioactivity produced by cutting nuclear fuel waste pipes after nuclear power generation is 100 nCi. A method for reducing the radioactivity of a nuclear fuel cladding tube, which is formed by a medium-low-level nuclear fuel cladding tube of less than / g and a heat generation rate of less than 2 mW / m 3. Although a method using a laser is proposed as the method, the laser may use a so-called CW (Continuous Wave) laser, and as shown in FIG. 4, the melt may remain in the coating tube.

Accordingly, the present inventors completed the present invention while intensively studying a method for eliminating the disadvantages of the method using the so-called C.W. (Continuous Wave) laser of the prior art.

The purpose of the present invention is to remove the radioactive materials such as TRU stuck on the inner surface of the nuclear fuel waste pipe, which is a high-level radioactive waste generated after nuclear power generation, to determine the total radioactivity 100 nCi / g (0.1) By lowering mCi / kg) or less, it is intended to provide a method for reducing the radioactivity of spent fuel waste canals that can convert them into non-TRU wastes.

In order to achieve the above object, the present invention is a non-TRU fuel with a total radiation generation less than 100 nCi / g and a heat generation rate of less than 2 kW / ㎥ by cutting the surface of the spent fuel closed tube with a pulse laser (pulse laser) Provided is a method for reducing radioactivity of a nuclear fuel closed tube formed by the closed tube.

Hereinafter, the present invention will be described in detail.

The present invention relates to a method for reducing radioactivity of a high-level closed capillary tube generated during the powdering process for recycling spent nuclear fuel after nuclear power generation. It is to provide a method for reducing radiation.

The method for reducing radioactive tube waste radiation according to the present invention utilizes the surface radiochemical characteristics of the nuclear fuel waste tube, which is a high-level waste generated after nuclear power generation, and a difference occurs in the surface radiochemical characteristics of the waste tube according to the fuel cladding and powdering process.

The closed coating tube of the present invention is a closed coating tube formed from a zircaloy-4 cladding tube generally used for nuclear power generation, and the zircaloy-4 cladding tube is 1.20 to 1.70 wt% of Sn, 0.18 to 0.24 wt% of Fe, and 0.07 to 1.13 wt. Of Cr. %, Ni <0.007 wt%, O 900-1500 ppm and Zr balance.

After the cladding tube is used for nuclear power generation, it is formed as a closed cladding tube through a treatment process, in which the treatment process may be performed by a wet treatment process or a dry treatment process, but preferably, a dry treatment process is performed.

In the dry treatment process, the rod cut obtained by cutting the nuclear fuel rod is thinly divided in the longitudinal direction, and the uranium powder and the waste coating tube are separated by repeating the oxidation and reduction process while heating. Inside, an oxide layer and a radioactive layer of constant thickness are formed.

The present invention utilizes the surface radiochemical characteristics of the above-described closed coating tube, and can form a fuel closed tube of non-TRU waste by cutting the closed coating tube formed through the dry treatment process by the contaminated thickness using a pulse laser. have.

At this time, the cutting is to remove the required thickness from the surface of the closed coating tube, in the present invention is carried out to a depth of 20 ㎛ or more. It contains high-level TRU wastes that generate radiation within about 10 μm, and it is desirable to cut to a depth of at least 20 μm to sufficiently cut it. This cutting can reduce the amount of radiation and the amount of emitted heat to medium and low levels.

The present invention belongs to the pretreatment process of the waste coating tube treatment method for converting high-level waste into medium- and low-level waste by cutting the inner surface of the waste coating tube using a pulse laser, and the following treatment methods are conventional in the field to which the present invention belongs. It can process by the phosphorus method. At this time, by converting to low and medium level waste through the pretreatment process of the present invention, it is possible to easily and easily reduce the treatment method and treatment cost.

Hereinafter, the present invention will be described in detail by way of examples.

However, the following examples are merely to illustrate the present invention is not limited to the contents of the present invention.

<Experimental Example 1> Radiochemical Characteristics of Waste Coated Tubes in Dry Reprocessing Process

SEM and EPMA analyzes were performed on the waste coating tubes generated during the dry treatment process of PWR spent fuel with a combustion degree of 32,000 MWd / tU and a cooling period of 15 years. The results are as follows.

(1) oxide layer analysis

The inner surface of the closed coating tube forms an oxide layer, a TRU such as Pu, and a fission product penetration layer as the coating tube is irradiated. Α-nuclides, such as Pu, can collide with the heavy fission fragments and implant into the oxide layer, but they decrease exponentially with depth from the surface of the lung coating tube and can penetrate about 1 μm. Known. In order to determine the thickness of the oxide layer present in the closed coating tube, the oxygen potential was measured and shown in FIG. 1.

In Fig. 1, since the oxygen potential is increased from about the point where α-nuclides such as plutonium is maximum to about 3 μm, it is understood that fission products exist in the form of oxide within this distance. _It is also thought to form an oxide layer of ₂ .

(2) EPMA analysis of the inner surface of the lung coat tube

The lung coat tube was cut to about 1 cm in length for EPMA analysis. EMPA analysis results are shown in FIG. 2.

As shown in Figure 2, it can be seen that the nuclear fission product is distributed to the point of about 10 ㎛ from the inner surface of the closed coat tube. Among the elements, Zr showed almost 98.9% and the fission product showed about 1.1%.

<Example> Reduction of radioactivity of the lung coat tube using pulse laser

After the nuclear power generation, the inner surface of the simulated closed tube which was subjected to the dry treatment process was cut to a depth of about 20 μm using a pulse laser (model name: Nd-YAG Pulse Laser).

The external surface state after such cutting is as shown in FIG. Then, the changes in the amount of radiation and the amount of emission for the cut closed tube is as follows.

(1) radioactivity

The total radioactivity emitted from the closed cut tube by the above embodiment is the sum of the radioactivity of the fission product and the radioactivity of the α-nuclide, which is theoretically TR = 0.00000 + 6.65 × 0.005 = 0.03325 mCi / kg It becomes Zry. In fact, the measured results were obtained almost the same as those calculated numerically, and these values were below 100 nCi / g (0.1 mCi / kg), which is the TRU waste standard.

(2) the amount of heat released

The zircaloy-4 cladding contains 3.5 ppm of uranium uniformly, which can generate TRU during the fuel investigation. Radioactivity by TRU produced by such uranium was calculated using ORIGEN II, a nuclide decay calculation code, and the results are shown in Table 1.

Applying 2kW / m3 of heat generation, TRU classification standard, to Zircaloy closed-coated tube, it corresponds to about 0.31 W / kg (100% of ingot density).

first 5 years 10 years 15 years 20 years 25 years 30 years 35 years 40 years 8.163W / kg 0.0192 0.0084 0.0039 0.0019 0.0009 0.0004 0.0003 0.0001

Table 1 is a calorific value generated when the uranium present in the closed coating itself changes to TRU during the irradiation process. On the other hand, the TRU waste attached to the inner surface of spent fuel Zircaloy-4 waste coating tube is 199 times (99.5% /0.5%=199) of TRU waste due to waste tube radiation. In the case of W / kg (0.0039 + 199X0.0039 = 0.78) and 20 years cooler, it is 0.38W / kg (0.0019 + 0.0019X199 = 0.38) and it will fall below the TRU standard only after 20 years or more.

However, in case of removing the TRU attached to the inner surface of the closed coating pipe by cutting the inner surface of the closed coating pipe about 20㎛, the total discharged heat amount is 0.38W / kg (0.0192 + 0.0192X199X0.005 = 0.0383) at 5 years of cooling period. Will fall below.

As such, when the inner surface of the closed coating pipe is removed to remove the TRU attached to the inner surface of the closed coating pipe, the heat release rate is rapidly reduced compared to the case where the closed coating pipe is not cut, and the heat below the TRU classification standard is maintained even if it is left for 5 years which is much shorter than 20 years. The discharge amount can be satisfied.

The method for reducing the radioactivity of the nuclear fuel lung coating tube of the present invention configured as described above reduces the total radioactivity of the lung coating tube by removing the high radioactive layer of the lung coating tube with a uniform thickness, thereby reducing the volume of the lung coating tube to be specially managed. In addition to reducing disposal costs, the pretreatment process removes impurities, such as fission products, from the waste coating tube, enabling the production of ingots with better metallic performance, resulting in more environmentally friendly disposal of the waste coating tube. .

Claims (3)

Sn 1.20-1.70 wt%, Fe 0.18-0.24 wt%, Cr 0.07-1.13 wt%, Cr <0.007 wt%, O 900-1500 ppm and residues of Zr generated after nuclear power generation. The surface of the nuclear fuel cladding tube is cut to a depth of 20 µm or more with a pulse laser to form a medium and low level nuclear fuel cladding tube having a radioactivity of less than 100 nCi / g and a heat generation amount of less than 2 mW / m 3. A method for reducing radioactivity of a nuclear fuel cladding tube, characterized in that delete delete

Images