Method for coded marking and tracking of fuel pellets in a lead bismuth pool destruction core melt migration accident

Abstract

This invention discloses a coding and tracking method for fuel pellets in lead-bismuth reactor core molten material migration accidents, aiming to solve the problem of tracing the migration path of molten material under conditions of no visibility. The method uses tungsten carbide rods to simulate damaged fuel pellets, and uses eight characteristic heavy metal oxides—zirconium oxide, scandium oxide, hafnium oxide, yttrium oxide, antimony oxide, molybdenum oxide, tantalum oxide, and barium oxide—as coding elements. Different combinations of these oxides are incorporated into tungsten carbide powder according to a preset binary coding rule. After mixing, ball milling, and sintering, rods with specific elemental markings are formed. This coding system assigns unique identification information to each rod, including its location within the rod bundle and its axial position. After the experiment, the elemental composition of the recovered fragments is analyzed using a handheld X-ray fluorescence spectrometer, allowing for decoding and inversion of their initial positions, thus enabling precise tracking of their migration trajectory within the molten lead-bismuth.

Description

Technical Field

[0001] This invention belongs to the field of reactor damage research technology under severe nuclear reactor accident conditions, specifically relating to a method for coding, marking and tracking fuel pellets in a lead-bismuth reactor core molten material migration accident. Background Technology

[0002] Lead-bismuth fast reactors have become a necessity for power supply in remote areas and on islands and reefs due to their inherent safety.

[0003] From a design perspective, the high boiling point (1670℃) of the lead-bismuth alloy, the reactor's physical characteristics, and its integrated layout essentially eliminate the possibility of severe accidents. There is no possibility of main loop pressurization, boiling, or thermal explosion caused by core overheating under conditions exceeding design reference. Therefore, lead-bismuth reactors possess several inherent safety features: 1) Lead has poor moderation capabilities, resulting in minimal impact on the core energy spectrum, which is beneficial for reactor safety and fuel breeding; 2) Throughout the core burnup period, the cavitation reactivity coefficient of the lead-cooled fast reactor is negative (when the coolant-to-fuel volume ratio is greater than 1); 3) The temperature reactivity coefficient of the lead-cooled fast reactor remains negative regardless of whether cavitation is present or absent. 4) The process is extremely simple and has great self-stability. It does not require the protection system required by traditional nuclear power plants, which greatly reduces the probability of human error. 5) The boiling point of lead-bismuth alloy is high, about three times that of normal operation, so the possibility of boiling is extremely small. 6) The freezing point of lead-bismuth alloy is low. It can self-seal under normal temperature and pressure conditions, which eliminates coolant vaporization and prevents a large loss of coolant caused by leakage in the main circuit. It can also retain fission products such as radioactive elements such as iodine and cesium, and retain steel-based elements, so as to minimize the long-term consequences. 7) Lead-bismuth alloys have low chemical reactivity and are unlikely to react with water and air, eliminating the possibility of fire or explosion in the event of coolant leakage into the reactor building or due to SG heat transfer tube rupture; 8) The reactor adopts an integrated design with strong primary loop natural circulation capability, allowing passive removal of residual heat. Even if all other heat removal systems are lost, the reactor vessel can still be cooled by natural circulation of air or water surrounding the vessel, preventing core overheating and damage; 9) Lead-bismuth alloys have a small solidification volume and large plasticity, allowing for planned multiple "solidification-melting" operations without deformation or damage to reactor equipment components. In summary, even if events such as containment failure, reactor room floor failure, large-scale primary loop seal failure, and direct contact between lead-bismuth coolant in the unit container and air are superimposed, reactor overspeed, explosion, and fire will not occur. Radioactive emissions will be below the level required for the evacuation of nearby residents. There is no possibility of core meltdown. The reactor can be quickly and passively shut down under any abnormal conditions and can be naturally cooled through circulation without time constraints. In the event of emergency overheating of the coolant, the pressure will not increase, and there is no risk of overpressure or reactor thermal explosion.

[0004] Given the multiple inherent safety features of lead-bismuth stacks, they can be widely used for power supply in remote areas and on islands and reefs, and their application scope can be expanded as needed.

[0005] Although the lead-bismuth fast reactor has multiple inherent safety features, eliminating the possibility of main loop pressurization, boiling, and thermal explosion caused by core overheating in the event of an over-design-baseline accident, the special physicochemical properties of lead-bismuth coolant may lead to some flow heat transfer and corrosion phenomena within the lead-bismuth reactor. These phenomena may also bring about thorny safety problems to the reactor.

[0006] From the perspective of severe accidents, the aforementioned factors, including flow heat transfer, corrosion, and typical initiation events, can lead to fuel blockage accidents in the reactor core. For example, lead and lead-nitrogen alloys exhibit strong corrosive effects on structural materials under high temperatures and high-speed flow, and corrosion products deposited in the core can easily cause blockage accidents. Similarly, fatigue and detachment of equipment components caused by thermal oscillations can also block the coolant channels in the core. Furthermore, wire or grid breakage, thermal expansion, and radiation swelling of fuel elements can also lead to blockage accidents. When these blockage accidents occur, the coolant flow area near the blockage decreases, and a backflow zone appears behind the blockage, leading to deteriorated flow heat transfer, a significant increase in local temperature, and a threat to the integrity of the cladding. More seriously, under extreme conditions such as typical initiation events combined with safety system failures, blockage accidents can also lead to localized core meltdown. In this case, the safety assessment of the core and containment will be of paramount importance for evaluating radioactive release.

[0007] Due to the opacity and lack of visibility of lead and bismuth, studying the core melting and molten material migration in lead-bismuth reactors has become an urgent problem to be solved. A large number of experimental studies both domestically and internationally have focused on sodium-cooled fast reactors or have been conducted using alternative materials. The phenomena and processes of severe accidents in lead-bismuth reactors differ significantly from those in pressurized water reactors and sodium-cooled fast reactors; therefore, experimental research on severe accidents in lead-bismuth reactors is still insufficient.

[0008] REF01: The invention patent "Experimental System and Method for Migration of Molten Material in Damaged Lead-Bismuth Reactor Core" (CN120293778A) discloses an experimental system and method for the migration of molten material in damaged lead-bismuth reactor cores. The experimental system includes an invisible experimental section, simulated fuel rods, a DC power supply heating device, a camera, and instruments. This invention can be used to conduct experiments on the migration of molten material in damaged lead-bismuth reactor cores. However, it can only photograph the upper surface area of ​​the lead-bismuth pool and cannot trace the source of fuel pellet fragments. Therefore, it cannot analyze the migration path of fuel pellets and other complex behaviors. Summary of the Invention

[0009] To overcome the problems existing in the prior art, the purpose of this invention is to provide a method for encoding, marking and tracking fuel pellets in lead-bismuth reactor core molten material migration accidents. This method provides the possibility of conducting core damage, cladding melting, pellet fragmentation and interaction with coolant in the extreme environment of invisible molten lead-bismuth, as well as a series of complex migration movements carried by coolant, and determining the initial and final positions of pellet fragments. It also provides comparative experimental data for the development and verification of numerical simulation programs, thereby elucidating the relevant mechanisms of lead-bismuth reactor core molten material migration.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: A method for coding, marking and tracking fuel pellets in a lead-bismuth reactor core molten material migration accident involves constructing a binary binary coding system to convert the spatial location information of simulated fuel elements into unique elemental fingerprint information and solidify it in the sample, ultimately reconstructing the migration trajectory through the elements. The specific implementation steps are as follows: Step 1: Using tungsten carbide, which has a high melting point, good high-temperature stability, and controllable density, as the matrix material, uranium oxide ceramic pellets are made to simulate real fuel pellets; high-purity tungsten carbide powder is further refined into ball mill powder, which is mixed with high-purity heavy metal element oxide ball mill powder, and after mixing, it is subjected to high-temperature sintering process to form a green body, which is then made into regular cylindrical rods as fuel pellet fragments after wire cutting and concentric grinding; Step 2: Use the first set of coded heavy metal element oxides, namely scandium oxide, zirconium oxide, and hafnium oxide, to distinguish the positions of the seven fuel rods; the "presence" of each heavy metal oxide is defined as binary "1", and the "absence" is defined as binary "0"; by combining the "presence" or "absence" of the three heavy metal oxides, eight unique element combinations can be generated, which are sufficient to uniquely identify the seven fuel rods in the experiment. Step 3: Use the second group of coded heavy metal element oxides, namely yttrium oxide, antimony oxide, molybdenum oxide, Barium oxide and tantalum oxide are used to achieve precise axial positioning within a single fuel rod; these five heavy metal oxides are used for binary coding, with a coding capacity of up to 32 types, to meet the unique identification requirements of 32 axial stacked layers. Step 4: Use water jet cutting to cut the experimental section after the experiment is completed; Step 5: Use a handheld X-ray fluorescence spectrometer to perform non-destructive elemental analysis on the water-cut samples to ensure that each sample contains only one fuel pellet fragment. Identify the characteristic heavy metal elements contained in the sample and compare and decode them with a pre-established "code-location" mapping database to deduce the rod position and layer position of the original source of the fragment.

[0011] The dimensions, types of doping elements, and mass fractions of doping elements in step 1 include: (1) The fuel pellet fragments are cylinders with a diameter of 1 mm and a length of 25 mm to simulate the shape of real uranium dioxide ceramic pellets after fragmentation; (2) The doped heavy metal oxides include: scandium oxide, zirconium oxide, hafnium oxide, yttrium oxide, antimony oxide, molybdenum oxide, barium oxide and tantalum oxide; (3) The total mass fraction of the heavy metal element oxides should be controlled between 1% and 3% to facilitate instrument detection while reducing the impact on the density of fuel pellet fragments.

[0012] The "encoding-location" mapping database established in step 5 contains the following details: (1) The coding order of heavy metal element oxides used to locate the position of the rod is: scandium oxide, zirconium oxide and hafnium oxide; the element is "present" and is "0"; for example, the characteristic heavy metal elements of rod No. 1 are scandium "not present", zirconium "not present" and hafnium "present", and its rod position number is "001"; (2) The coding order of heavy metal element oxides used to locate the layer position is: yttrium oxide, antimony oxide, molybdenum oxide, barium oxide and tantalum oxide; the element is "present" and is "0"; for example, the intermediate characteristic heavy metal elements of the 9th layer of a certain rod are yttrium "not present", antimony "present", molybdenum "not present", barium "not present" and tantalum "present", and its layer position is numbered "01001"; (3) The numbering format used to locate the fuel pellet fragments is “rod position - layer position”; for example, the fuel pellet fragments in the 9th layer of rod 1 are numbered “001-01001”; when performing elemental analysis, if the characteristic heavy metal elements are found to be scandium “not present”, zirconium “not present”, hafnium “present”, yttrium “not present”, antimony “present”, molybdenum “not present”, barium “not present”, and tantalum “present”, then it is determined that the fuel pellet fragments originated from the 9th layer of rod 1.

[0013] Compared with the prior art, the present invention has the following advantages: (1) It pioneered the ability of precise spatial positioning and tracking in invisible environments. Compared with the prior art, the present invention encodes the "rod position" and "layer position" information into a unique element binary code table, and realizes the precise positioning of the three-dimensional initial position of the fuel pellet migration fragments, namely the initial rod position and layer position, in the invisible lead-bismuth molten pool, thereby drawing a detailed migration path map; (2) The labeling system has extremely high environmental robustness and thermal stability. The eight selected characteristic heavy metal element oxides all have melting points far exceeding the experimental temperature (>2000℃) and excellent chemical inertness, ensuring that they do not volatilize, decompose, or react violently with the matrix in the extreme lead-bismuth environment of 1430℃. The labeling signal is stable and reliable throughout the experiment, overcoming the risk of labeling failure of low-melting-point metals or volatile oxides; (3) The marking system has a large coding capacity, strong scalability, and low cost. Using the binary coding principle, only 8 characteristic heavy metal element oxides are needed to provide up to 256 unique codes, covering the complex requirements of 224 positional information points across 7 bars and 32 layers. It has strong scalability; adding one element can double the coding capacity. Furthermore, compared to the stringent and expensive neutron radiography method, the method of this invention uses only 1%-3% heavy metal oxides by mass, making it low-cost and requiring no safety permits or special protection. (4) The labeling system decoding process is fast, non-destructive, and easy to implement on-site. Using a handheld X-ray fluorescence spectrometer for decoding, the recovered fragments can be analyzed on-site quickly and non-destructively, and the elemental composition results can be obtained within seconds. Since it is only necessary to distinguish the presence or absence of elements without distinguishing the content distribution of each element, the requirements for instrument precision are low, and there is no need to send the samples to a professional laboratory for complex pretreatment and a long analysis cycle, which greatly improves the analysis efficiency and test speed. Attached Figure Description

[0014] Figure 1 This is a flowchart of the method of the present invention.

[0015] Figure 2 This is a coding diagram for the position of the rod bundle.

[0016] Figure 3 This is a schematic diagram of layer location encoding.

[0017] Figure 4 It is a handheld X-ray fluorescence spectrometer. Detailed Implementation

[0018] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments: like Figure 1 As shown, a coding, marking, and tracking method for fuel pellets in a lead-bismuth reactor core molten material migration accident is proposed. By constructing a binary binary coding system, the spatial location information of the simulated fuel elements is converted into unique elemental fingerprint information and solidified in the sample. Finally, the migration trajectory is reconstructed through the elements.

[0019] The specific implementation steps are as follows: Step 1: Using tungsten carbide, which has a high melting point, good high-temperature stability, and controllable density, as the matrix material, uranium oxide ceramic pellets are produced to simulate real fuel pellets. High-purity tungsten carbide powder is further refined into ball mill powder, which is then mixed with high-purity heavy metal oxide ball mill powder. After mixing, a high-temperature sintering process is carried out to form a green body. After wire cutting and concentric grinding, regular cylindrical rods with a diameter of 1 mm and a length of 25 mm are produced as fuel pellet fragments. Step 2: Use the first set of coded heavy metal oxides, namely scandium oxide, zirconium oxide, and hafnium oxide, to distinguish the positions of the seven fuel rods. The "presence" of each heavy metal oxide is defined as binary "1", and the "absence" is defined as binary "0". By combining the "presence" or "absence" of the three heavy metal oxides, eight unique element combinations can be generated, which are sufficient to uniquely identify the seven fuel rods in the experiment; Step 3: Use the second set of coded heavy metal oxides, namely yttrium oxide, antimony oxide, molybdenum oxide, barium oxide, and tantalum oxide, to achieve precise axial positioning within a single fuel rod. Using these five heavy metal oxides for binary coding provides a coding capacity of up to 32 types, satisfying the unique identification requirements for 32 axially stacked layers. Step 4: Use water jet cutting to cut the experimental section after the experiment is completed; Step 5: Use a handheld X-ray fluorescence spectrometer to perform non-destructive elemental analysis on the water-cut samples to ensure that each sample contains only one fuel pellet fragment. Identify the characteristic heavy metal elements contained in the sample and compare and decode them with a pre-established "code-location" mapping database to deduce the rod position and layer position of the original source of the fragment.

[0020] like Figure 2 As shown, in this invention, the central bar of the 7-bar bundle is set as bar number 1, corresponding to binary code 001; the remaining outer bars are set as bars number 2, 3, 4, 5, 6, and 7 in clockwise order, corresponding to binary codes 010, 011, 100, 101, 110, and 111, respectively.

[0021] like Figure 3 As shown, in this invention, the first layer of fuel pellets in a certain rod is assigned a layer number of 1 from top to bottom, corresponding to a binary code of 00001; the last layer of fuel pellets is assigned a layer number of 32, corresponding to a binary code of 00000; the remaining rods are assigned layer numbers sequentially, and their binary codes are the binary numbers corresponding to their layer numbers. like Figure 4As shown, the handheld X-ray fluorescence spectrometer used in this invention is a Thermo Scientific Niton XL5 Plus. Its analytical element range covers all the characteristic heavy metal elements involved in this invention. It is portable, easy to operate, and can analyze all the characteristic heavy metal elements in a sample within seconds.

[0022] This invention creatively introduces various characteristic heavy metal oxides into fuel pellets. By encoding the "rod position" and "layer position" information into a unique binary element encoding table, it addresses the issue of invisible lead oxides. This invention enables precise positioning of the initial three-dimensional location of migrating fragments within the molten bismuth pool, specifically the initial rod position and layer position, thereby generating a detailed migration path diagram. This invention can serve as an auxiliary method for experiments on the migration of molten material from damaged cores in lead-bismuth reactors, providing experimental data for the analysis of severe accidents in lead-bismuth reactors and offering a reference for the safety design of lead-bismuth reactors.

[0023] The above description provides a further detailed explanation of the present invention in conjunction with specific operational procedures, but it is not intended to limit the implementation of the present invention. For those skilled in the art, any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for coding, marking, and tracking fuel pellets in a lead-bismuth reactor core molten material migration accident, characterized in that: By constructing a binary binary coding system, the spatial location information of fuel elements is transformed into unique elemental fingerprint information and solidified in the sample, ultimately enabling the reconstruction of migration trajectories through elemental analysis. The specific implementation steps are as follows: Step 1: Using tungsten carbide, which has a high melting point, good high-temperature stability, and controllable density, as the matrix material, uranium oxide ceramic pellets are made to simulate real fuel pellets; high-purity tungsten carbide powder is further refined into ball mill powder, which is mixed with high-purity heavy metal element oxide ball mill powder, and after mixing, it is subjected to high-temperature sintering process to form a green body, which is then made into regular cylindrical rods as fuel pellet fragments after wire cutting and concentric grinding; Step 2: Use the first set of coded heavy metal element oxides, namely scandium oxide, zirconium oxide, and hafnium oxide, to distinguish the positions of the seven fuel rods; the "presence" of each heavy metal oxide is defined as binary "1", and the "absence" is defined as binary "0"; by combining the "presence" or "absence" of the three heavy metal oxides, eight unique element combinations can be generated, which are sufficient to uniquely identify the seven fuel rods in the experiment; Step 3: Use the second set of coded heavy metal element oxides, namely yttrium oxide, antimony oxide, molybdenum oxide, barium oxide and tantalum oxide, to achieve precise axial positioning inside a single fuel rod; use these five heavy metal element oxides for binary coding, with a coding capacity of up to 32 types, to meet the unique identification requirements of 32 axial stacked layers; Step 4: Use water jet cutting to cut the experimental section after the experiment is completed; Step 5: Use a handheld X-ray fluorescence spectrometer to perform non-destructive elemental analysis on the water-cut samples to ensure that each sample contains only one fuel pellet fragment. Identify the characteristic heavy metal elements contained in the sample and compare and decode them with a pre-established "encoding-location" mapping database to deduce the rod position and layer position of the original source of the fragment.

2. The method for coding, marking, and tracking fuel pellets in a lead-bismuth reactor core molten material migration accident according to claim 1, characterized in that: The dimensions, types of doping elements, and mass fractions of doping elements in step 1 include: (1) The fuel pellet fragments are cylinders with a diameter of 1 mm and a length of 25 mm to simulate the shape of real uranium dioxide ceramic pellets after fragmentation; (2) The doped heavy metal oxides include: scandium oxide, zirconium oxide, hafnium oxide, yttrium oxide, antimony oxide, molybdenum oxide, barium oxide and tantalum oxide; (3) The total mass fraction of the heavy metal element oxides should be controlled between 1% and 3% to facilitate instrument detection while reducing the impact on the density of fuel pellet fragments.

3. The method for coding, marking, and tracking fuel pellets in a lead-bismuth reactor core molten material migration accident according to claim 1, characterized in that: The "encoding-location" mapping database established in step 5 contains the following details: (1) The coding order of heavy metal element oxides used to locate the position of the rod is: scandium oxide, zirconium oxide and hafnium oxide; the element is "present" and is "0" and the element is "absent". If the characteristic heavy metal element of rod No. 1 is scandium "absent", zirconium "absent", and hafnium "present", the rod position number is "001". (2) The coding order of heavy metal element oxides used for locating layer positions is: yttrium oxide, antimony oxide, molybdenum oxide, barium oxide and tantalum oxide; the element is "present" and is "0"; if the intermediate characteristic heavy metal element of the 9th layer of a certain rod is "not present", "present", "not present", "not present", "not present", and "present", the layer position number is "01001"; (3) The numbering format used to locate the fuel pellet fragments is "rod position-layer position"; if the fuel pellet fragments in the 9th layer of rod 1 are numbered "001-01001"; if the characteristic heavy metal elements are found to be scandium "not present", zirconium "not present", hafnium "present", yttrium "not present", antimony "present", molybdenum "not present", barium "not present", and tantalum "present" during elemental analysis, it is determined that the fuel pellet fragments originated from the 9th layer of rod 1.

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