Fuel operation accident radioactive consequence analysis method based on staged release mechanism

Abstract

The invention provides a fuel operation accident radioactive consequence analysis method based on a staged release mechanism. The fuel operation accident radioactive consequence analysis method comprises the following steps: S1, setting an initial source item composition of damaged fuel release, wherein the initial source item composition comprises iodine in an initial form and inert gas; s2, radioactive release is divided into two stages, and release models of the two stages are constructed: the first stage is bubble instantaneous release, and the second stage is element iodine re-precipitation release; s3, calculating a dynamic decontamination factor of the element iodine in the water layer according to the first stage and the second stage; s4, establishing a release path and dose calculation model of the radioactive substance; and S5, setting an acceptance criterion and judging a result. According to the invention, refined simulation of accident release behaviors and accurate prediction of key position doses are realized. The method can support the optimization design, reduce the construction cost, improve the application passing rate of the license, and provide a scientific basis for accident alleviation and emergency response.

Description

Technical Field

[0001] This invention relates to the field of nuclear power plant radiation safety and accident analysis technology, and in particular to a method for analyzing the radioactive consequences of fuel operation accidents based on a phased release mechanism. Background Technology

[0002] Fuel handling accidents are typical design basis accidents (DBAs) during nuclear power plant operation, primarily occurring during refueling overhauls. Causes typically include mechanical failure, lifting equipment malfunction, or human error, leading to fuel assemblies falling from the transfer channel, impacting the fuel storage racks in the pool, causing cladding rupture, and releasing fission products (such as 131I, 133Xe, and 135Xe) into the pool water. These products then migrate through the plant and its ventilation system to critical areas within the plant or to the external environment.

[0003] Currently, the industry generally conducts relevant assessments based on the standard "NB / T 20444-2017 Design Basis Accident Source Term Analysis Criteria for Pressurized Water Reactor Nuclear Power Plants". However, traditional methods often have the following significant shortcomings:

[0004] 1. Oversimplification of source term morphology: It is assumed that cesium iodide (CsI) released from the fuel cladding gaps is completely converted into elemental iodine (I2) in the pool water, ignoring its complex chemical evolution process of dissolution-hydrolysis-oxidation-re-precipitation, which leads to an underestimation of long-term release behavior.

[0005] 2. Constant value of the decontamination factor: The washing efficiency of the water layer for elemental iodine was estimated by using a fixed decontamination factor (DFe=500), without considering the influence of factors such as bubble size, rising speed, water depth, and pH value of the pool water. This resulted in highly conservative analysis results and affected the space for design optimization.

[0006] 3. Instantaneous treatment of the release process: Only focusing on the short-term (within a few hours) bubble release stage, ignoring the process of CsI slowly hydrolyzing in acidic pool water to generate I⁻, then being oxidized to form volatile I2 and continuously released (which can last from several hours to several days), leading to deviations in the cumulative dose prediction in the main control room.

[0007] As experimental and mechanistic studies have deepened, the physical inconsistencies of the aforementioned simplified assumptions have become increasingly apparent. In recent years, international nuclear safety review agencies have also gradually emphasized the need to use more realistic dynamic source term models for safety analysis.

[0008] Therefore, existing technologies cannot accurately reflect radioactive release behavior under real accidents, resulting in overly conservative safety analysis results. This may lead to unnecessary design margins, increased construction costs, and even affect the efficiency of permit application.

[0009] Given the urgent need in the field for a scientifically sound, physically clear, and parameter-adjustable method for analyzing the radioactive consequences of fuel operation accidents in order to improve the accuracy and economy of nuclear power plant safety assessments, the inventors of this application have designed a method for analyzing the radioactive consequences of fuel operation accidents based on a phased release mechanism. Summary of the Invention

[0010] The technical problem to be solved by the present invention is to overcome the shortcomings of the traditional radioactive release analysis methods in the prior art, such as overly simplified source term setting, single value of decontamination factor, and neglect of long-term release process, and to provide a method for analyzing the radioactive consequences of fuel operation accidents based on a staged release mechanism.

[0011] The present invention solves the above-mentioned technical problems through the following technical solution:

[0012] A method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism, characterized in that the method includes the following steps:

[0013] S1. The initial source term composition of the damaged fuel release is set to include iodine and inert gas in their initial form;

[0014] S2. The radioactive release is divided into two stages, and a two-stage release model is constructed: the first stage is the instantaneous release of bubbles, and the second stage is the re-precipitation and release of elemental iodine.

[0015] S3. Calculate the dynamic decontamination factor of elemental iodine in the water layer for the first stage and the second stage.

[0016] S4. Establish a model for calculating the release pathway and dose of radioactive materials;

[0017] S5. Set acceptance criteria and result judgment.

[0018] According to one embodiment of the present invention, the iodine in the initial form in step S1 includes cesium iodide (CsI), elemental iodine (I2), and organic iodine (CH3I), and the inert gas is released in a free gaseous state and does not participate in the aqueous phase reaction.

[0019] According to one embodiment of the present invention, the cesium iodide (CsI) is 95%, the elemental iodine (I2) is 4.85%, and the organic iodine (CH3I) is 0.15%.

[0020] According to one embodiment of the present invention, the instantaneous bubble release time in the first stage of step S2 is 0 hours to 2 hours, and the elemental iodine re-precipitation and release time in the second stage is 2 hours to 30 days.

[0021] According to an embodiment of the present invention, the first stage of calculating the decontamination factor of elemental iodine in step S3 includes:

[0022] S 31 Determine the bubble rise time. Where t represents the bubble rise time; p represents the pressure inside the fuel rod;

[0023] S 32 Determine the bubble diameter. Where d represents the bubble diameter and p represents the pressure inside the fuel rod;

[0024] S 33 Determine the detergency factor of elemental iodine.

[0025] According to an embodiment of the present invention, step S 33 Includes:

[0026] When the water depth is in the range of 5.8-7m, calculate the decontamination factor of elemental iodine penetrating the water layer. ;

[0027] When the water depth is less than 5.8m but greater than 7m, the decontamination factor is determined by combining fluid dynamics simulation. ;

[0028] Among them, K eff Let be the mass transfer coefficient during the bubble's ascent, t represent the bubble's ascent time, and d represent the bubble's diameter.

[0029] According to an embodiment of the present invention, the cleaning agent of elemental iodine in the second stage of step S3 includes:

[0030] S 34 Identify representative nuclides, including radioactive and non-radioactive iodine in the pool;

[0031] S 35 The amount of iodine in the molded fuel gap, including the calculation of the molar amount of radioactive iodine and the molar amount of non-radioactive iodine;

[0032] S 36 Calculate the volatile iodine content in the pool;

[0033] S 37 Calculate the evaporation rate of elemental iodine.

[0034] According to one embodiment of the present invention, the radioactive iodine comprises 130 I, 131 I, 132 I, 133 I, 134 I, 135 I; the non-radioactive iodine includes 127 I.

[0035] According to an embodiment of the present invention, step S 35 The calculation of the molar amount of radioactive iodine described in the text is as follows:

[0036] ;

[0037] Wherein, N1 represents the molar amount of radioactive iodine released into the pool during a fuel handling accident; F gap Indicates the fraction of iodine within the fuel cladding gap; F peak Indicates the radial peak power factor of the reactor core fuel; N represents the number of fuel assemblies in the reactor core; A i λ represents the amount of radioactive iodine isotope i accumulated in all fuel assemblies of the reactor core; i represents the decay constant of iodine isotope i; t represents the time interval from reactor shutdown to fuel operation accident.

[0038] According to an embodiment of the present invention, step S 35 The calculation of the non-radioactive iodine molar quantity as described in the document is as follows:

[0039] ;

[0040] Wherein, N2 represents the molar amount of non-radioactive iodine released into the pool during a fuel handling accident; m 127 Indicates all fuel assemblies in the reactor core 127 The quality of I; M 127 express 127 Atomic weight of I; F gap Indicates the fraction of iodine within the fuel cladding gap; F peak This represents the radial power peak factor of the reactor core fuel; N represents the number of fuel assemblies in the reactor core.

[0041] According to an embodiment of the present invention, step S 36 include:

[0042] S 361 Taking into account both radioactive and non-radioactive iodine, the total iodine concentration in the pool is calculated as follows: C = (N1 + N2) / V; where C represents the total iodine concentration released into the pool; V represents the volume of water; N1 represents the molar amount of radioactive iodine released into the pool during the fuel operation accident; and N2 represents the molar amount of non-radioactive iodine released into the pool during the fuel operation accident.

[0043] S 362 Calculate the H⁺ concentration based on the pH value of the pool water: C H =[H + ]=10 -pH ;

[0044] Among them, C H pH indicates the concentration of hydrogen ions; pH indicates the acidity of water.

[0045] S 363 Calculate [I2] / [I] based on chemical equilibrium. - ] 2 ratio:

[0046] R i =[I2] / [I - ] 2 = C H 2 / (6.05×10 -14 +1.47×10 -9 × C H );

[0047] In the formula, R i Represents [I2] / [I - ] 2 The ratio of; C H Indicates hydrogen ion concentration;

[0048] S 364 Determine the proportion of volatile iodine (I2):

[0049]

[0050] Where Frc represents the proportion of iodine atoms present as I2; C represents the concentration of total iodine released into the pool; R i Represents [I2] / [I - ] 2 The ratio of .

[0051] According to an embodiment of the present invention, step S 37 include:

[0052] S 371 Based on the mass transfer coefficient, the ratio of water tank surface area to volume, and the proportion of I2 to I, calculate the reprecipitation removal coefficient λ. e : ;

[0053] S 372 The release rate of elemental iodine from the pool was modeled using a removal coefficient to determine the evaporation rate F of elemental iodine released from the pool. e : ;

[0054] Where S represents the total surface area of ​​the pool; K L Frc represents the mass transfer coefficient; Frc represents the proportion of iodine atoms present as I2; V represents the volume of water.

[0055] According to an embodiment of the present invention, step S4 includes:

[0056] S41 Establish release pathways: If the accident occurs inside the containment, consider the direct release pathway after the containment cleanup system is shut down; if it occurs in an auxiliary building, assume that the radioactive material is discharged directly into the environment without filtration.

[0057] S 42 Establish a public dose calculation model;

[0058] S 43 Establish a dose calculation model for personnel in the main control room.

[0059] According to an embodiment of the present invention, step S 41 Includes:

[0060] Regarding the first stage, considering inert gases and iodine, the activity of iodine radionuclides released into the environment 2 hours after the accident is as follows: The calculation formula is:

[0061] For inert gases ;

[0062] For iodine ;

[0063] Regarding the second stage, considering the release of iodine after the conversion of particulate iodine to elemental iodine, the cumulative release of iodine nuclides into the environment at time T after the accident would be... The calculation formula is:

[0064]

[0065] Among them, Fr e Indicates the initial fraction of elemental iodine; Fr o Indicates the initial share of organic iodine; F gap Indicates the fraction of iodine within the fuel cladding gap; F peak Indicates the radial peak power factor of the reactor core fuel; A i This indicates the amount of radioactive iodine isotope i accumulated in all fuel assemblies of the reactor core. Represents the reduction term due to nuclide decay; N represents the number of fuel assemblies in the reactor core; DF e DF indicates the detergency factor of elemental iodine as it penetrates the water layer. o Fr represents the detergency factor of organic iodine penetrating the water layer, taken as 1; p Indicates the initial fraction of cesium iodide; F e λ represents the evaporation rate of elemental iodine released from the pool; V represents the volume of water; λ i represents the decay constant of iodine isotope i; t represents the time interval from reactor shutdown to fuel operation accident.

[0066] According to an embodiment of the present invention, step S 42 Includes:

[0067] Taking into account both external irradiation and inhalation internal irradiation, the total effective dose calculation model for the public outside the factory is as follows:

[0068] ;

[0069] The calculation model for thyroid dose caused by inhalation of radioactive materials by the public outside the factory is as follows:

[0070]

[0071] Wherein, D1 represents the effective dose to the general public outside the factory; D2 represents the thyroid dose to the general public outside the factory; R i,j This represents the activity of nuclide i released into the environment during time period j; (χ / Q) j DCF represents the atmospheric dispersion factor within time period j. 1,i DCF represents the effective dose conversion factor for external irradiation of radionuclide i. 2,i DCF represents the internal radiation effective dose conversion factor for radionuclide i. 3,i BR represents the thyroid dose-converting factor for radionuclide i; j This represents the respiratory rate of individuals within time period j.

[0072] According to an embodiment of the present invention, step S 43 Includes:

[0073] Taking into account both external irradiation and internal irradiation from inhalation, the effective dose calculation model for personnel in the main control room is as follows:

[0074] The calculation model for the thyroid dose caused by the inhalation of radioactive material by personnel in the main control room is as follows:

[0075]

[0076] Where D3 represents the effective dose to personnel in the main control room; D4 represents the thyroid dose to personnel in the main control room; and GF represents the geometric correction factor for the main control room, GF = 351.6 / V. 0.338 V represents the volume of the main control room; DCF 1,i IAR represents the external radiation effective dose conversion factor for radionuclide i. i,j This represents the cumulative concentration of radionuclide i in the main control room during time period j; (χ / Q) j O represents the atmospheric dispersion factor within time period j; j This represents the percentage of time personnel spend in the main control room during time period j; DCF 2,i DCF represents the internal radiation effective dose conversion factor for radionuclide i. 3,i BR represents the thyroid dose-converting factor for radionuclide i; jThis represents the respiratory rate of individuals within time period j.

[0077] According to one embodiment of the present invention, the acceptance criteria in step S5 include: the effective dose received by the public at the boundary of the non-residential area within any 2 hours after the incident, and the effective dose received by the public at the boundary outside the planned restriction zone during the incident is ≤100mSv, and the thyroid dose is ≤1000mSv;

[0078] For main control room staff, the effective dose ≤50mSv and the thyroid dose ≤500mSv within 30 days after the accident.

[0079] The positive and progressive effects of this invention are as follows:

[0080] This invention provides a method for analyzing the radioactive consequences of fuel handling accidents based on a phased release mechanism. By introducing three core technologies—a two-stage release model, dynamic decontamination factor calculation, and quantification of re-precipitation mechanisms—it achieves refined simulation of accident release behavior and accurate prediction of doses at key locations.

[0081] The proposed method for analyzing the radioactive consequences of fuel handling accidents enhances the physical authenticity and engineering applicability of nuclear power plant safety analysis. It can support optimized design, reduce construction costs, increase the approval rate of license applications, and provide a scientific basis for accident mitigation and emergency response. Attached Figure Description

[0082] The above and other features, properties and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings and embodiments, in which the same reference numerals always denote the same features, wherein:

[0083] Figure 1 This is a flowchart of the method for analyzing the radioactive consequences of fuel handling accidents based on a phased release mechanism, as described in this invention.

[0084] Figure 2 This is a schematic diagram of the VES mode ventilation in the main control room of the present invention, which is based on the staged release mechanism for analyzing the radioactive consequences of fuel handling accidents.

[0085] Figure 3 This invention relates to a method for analyzing the radioactive consequences of fuel handling accidents based on a phased release mechanism. The diagram shows the ventilation mode of the main control room's VBS fresh air filtration system. Detailed Implementation

[0086] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0087] Embodiments of the invention will now be described in detail with reference to the accompanying drawings. Preferred embodiments of the invention will now be described in detail, examples of which are shown in the drawings. Wherever possible, the same reference numerals will be used in all the drawings to denote the same or similar parts.

[0088] Furthermore, although the terminology used in this invention is selected from commonly known and used terms, some terms mentioned in this specification may have been selected by the applicant in his or her judgment, and their detailed meanings are explained in the relevant sections of the description herein.

[0089] Furthermore, the invention should be understood not only through the actual terminology used, but also through the meaning implied by each term.

[0090] like Figures 1 to 3 As shown, this invention discloses a method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism, applicable to the safety analysis of fuel handling areas in pressurized water reactor nuclear power plants. The method includes the following steps:

[0091] Step S1: Set the initial source term composition for the release of damaged fuel, including iodine and inert gas in their initial form.

[0092] Suppose that the fuel assembly with the highest power consumption in the reactor falls during the transfer process, causing damage to the cladding, and the fission gas and iodide in the gap between the damaged fuel rods are released into the pool.

[0093] Preferably, the initial source term composition is set as follows in step S1:

[0094] The initial forms of iodine include cesium iodide (CsI), elemental iodine (I2), and organic iodine (CH3I). For example, the cesium iodide (CsI) is 95%, the elemental iodine (I2) is 4.85%, and the organic iodine (CH3I) is 0.15%.

[0095] The inert gas is released in a free gaseous state and does not participate in the aqueous phase reaction.

[0096] Step S2: Divide the radioactive release into two stages and construct a two-stage release model: the first stage is the instantaneous release of bubbles, and the second stage is the re-precipitation and release of elemental iodine.

[0097] Step S3: Calculate the dynamic decontamination factor of elemental iodine in the water layer for the first stage and the second stage.

[0098] In step S2, the instantaneous bubble release time in the first stage is 0 to 2 hours.

[0099] CsI ​​is highly soluble in water, while I2 and CH3I can rise through the water layer in airborne form along with bubbles. Among them, CH3I is a water-washing detergency agent (DF). o) Take 1 (almost impossible to remove); the cleaning effect of I2 is affected by factors such as water depth, bubble size, and rising speed, and needs to be calculated dynamically.

[0100] When the water depth is in the range of 5.8-7m, the first stage of step S3 calculates the decontamination factor (DF) of elemental iodine. e )include:

[0101] Step S 31 1. Determine the bubble rise time. The rise time of the gas cannon varies exponentially with the pressure inside the fuel rod.

[0102] Referring to international empirical formulas, the bubble rise time is determined by the internal pressure of the fuel rod: .

[0103] Where t represents the bubble rise time; p represents the pressure inside the fuel rod.

[0104] Step S 32 Determine the bubble diameter.

[0105] The bubble diameter is closely related to the internal pressure of the fuel rod: .

[0106] Where d represents the bubble diameter and p represents the pressure inside the fuel rod.

[0107] Step S 33 The decontamination factor of elemental iodine was determined, and the dynamic decontamination factor of elemental iodine in the water layer was calculated based on water depth, bubble diameter, rise time, and internal pressure of fuel rods.

[0108] Preferably, step S 33 Includes:

[0109] When the water depth is in the range of 5.8-7m, calculate the decontamination factor of elemental iodine penetrating the water layer. .

[0110] When the water depth is less than 5.8m but greater than 7m, the decontamination factor is determined by combining fluid dynamics simulation. .

[0111] Among them, K eff Let be the mass transfer coefficient during the bubble's ascent, t represent the bubble's ascent time, and d represent the bubble's diameter.

[0112] The advantage of this method is that it breaks through the limitations of traditional methods for elemental iodine (DF). e By using the fixed assumption of "=500", dynamic modeling of decontamination efficiency is achieved.

[0113] Alternatively, the embodiments of this application can simplify the first-stage model, omitting detailed calculations of bubble diameter and rise time, and directly obtaining the elemental iodine decontamination factor from a table based on water depth (e.g., establishing a DF-h lookup table), which is suitable for rapid evaluation in the preliminary design stage.

[0114] The second stage of elemental iodine re-precipitation and release takes 2 hours to 30 days.

[0115] In the first stage, the non-volatile CsI is gradually hydrolyzed into free I⁻ in the low pH pool water, and then oxidized into volatile I₂ under acidic conditions and slowly released into the atmosphere, forming a continuous release source.

[0116] Preferably, the second-stage elemental iodine cleaning agent in step S3 includes:

[0117] Step S 34 Identify representative nuclides, including radioactive and non-radioactive iodine in the pool.

[0118] Radioactive and non-radioactive iodine in the pool jointly affect the I2 precipitation equilibrium. The radioactive iodine includes... 130 I, 131 I, 132 I, 133 I, 134 I, 135 I; the non-radioactive iodine includes 127 I.

[0119] Step S 35 The amount of iodine in the molded fuel gap, including the calculation of the molar amount of radioactive iodine and the molar amount of non-radioactive iodine.

[0120] Preferably, step S 35 The calculation of the molar amount of radioactive iodine described in the text is as follows:

[0121] .

[0122] Or, conservatively assuming that all radioactive iodine is... 131 I, replaced with:

[0123] .

[0124] Wherein, N1 represents the molar amount of radioactive iodine released into the pool during a fuel handling accident; F gap Indicates the fraction of iodine within the fuel cladding gap; F peak Indicates the radial peak power factor of the reactor core fuel; N represents the number of fuel assemblies in the reactor core; A i λ represents the amount of radioactive iodine isotope i accumulated in all fuel assemblies of the reactor core; iλ represents the decay constant of iodine isotope i; t represents the time interval from reactor shutdown to a fuel operation accident; λ 131 denoted as the decay constant of ¹³¹I.

[0125] The step S 35 The calculation of the non-radioactive iodine molar quantity as described in the document is as follows:

[0126]

[0127] Wherein, N2 represents non-radioactive iodine (e.g., iodine) released into the pool during a fuel handling accident. 127 I) molar quantity; m 127 Indicates all fuel assemblies in the reactor core 127 The quality of I; M 127 express 127 Atomic weight of I; F gap Indicates the fraction of iodine within the fuel cladding gap; F peak This represents the radial power peak factor of the reactor core fuel; N represents the number of fuel assemblies in the reactor core.

[0128] Step S 36 Calculate the volatile iodine content in the pool. Calculate the re-precipitation rate of elemental iodine in the second stage using a chemical equilibrium and mass transfer model.

[0129] Taking into account both the total iodine concentration and the pH value of the pool water, a mass transfer model was used to calculate the re-precipitation rate, specifically including:

[0130] The step S 36 include:

[0131] Step S 361 Taking into account both radioactive and non-radioactive iodine, calculate the total iodine concentration in the pool: C=(N1+N2) / V.

[0132] Where C represents the total concentration of iodine released into the pool; V represents the volume of water; N1 represents the molar amount of radioactive iodine released into the pool during the fuel handling accident; and N2 represents the molar amount of non-radioactive iodine released into the pool during the fuel handling accident.

[0133] Step S 362 Calculate the H⁺ concentration based on the pH value of the pool water: C H =[H + ]= 10 -pH .

[0134] Among them, C H This indicates the concentration of hydrogen ions; pH indicates the acidity of water.

[0135] Step S 363 Calculate [I2] / [I] based on chemical equilibrium. - ]2 ratio:

[0136] R i =[I2] / [I - ] 2 = C H 2 / (6.05×10 -14 +1.47×10 -9 × C H ).

[0137] In the formula, R i Represents [I2] / [I - ] 2 The ratio of; C H This indicates the concentration of hydrogen ions.

[0138] Step S 364 Determine the proportion of volatile iodine (I2):

[0139] .

[0140] Where Frc represents the proportion of iodine atoms present as I2; C represents the concentration of total iodine released into the pool; R i Represents [I2] / [I - ] 2 The ratio of .

[0141] Step S 37 Calculate the evaporation rate of elemental iodine.

[0142] The step S 37 include:

[0143] Step S 371 Based on the mass transfer coefficient, the ratio of water tank surface area to volume, and the proportion of I2 to I, calculate the reprecipitation removal coefficient λ. e : .

[0144] Step S 372 The release rate of elemental iodine from the pool was modeled using a removal coefficient to determine the evaporation rate F of elemental iodine released from the pool. e : .

[0145] Where S represents the total surface area of ​​the pool; K L Frc represents the mass transfer coefficient; Frc represents the proportion of iodine atoms present as I2; V represents the volume of water.

[0146] Step S4: Establish a model for calculating the release pathway and dose of radioactive materials.

[0147] Step S4 includes:

[0148] Step S 41 Establish release pathways: If the accident occurs inside the containment, consider the direct release pathway after the containment cleanup system is shut down; if it occurs in an auxiliary building, assume that the radioactive material is discharged directly into the environment without filtration.

[0149] Preferably, step S 41 Includes:

[0150] For the first stage (i.e., the instantaneous bubble release stage (0-2 hours)), considering the release of inert gases and iodine (elemental iodine and organic iodine), the activity of iodine nuclides released into the environment 2 hours after the accident is as follows: The calculation formula is:

[0151] For inert gases .

[0152] For iodine .

[0153] Regarding the second stage (elemental iodine re-precipitation and release (2 hours to 30 days)), the main consideration is the release after the conversion of particulate iodine to elemental iodine. Therefore, the cumulative release of iodine nuclides to the environment at time T after the accident will be significant. The calculation formula is:

[0154]

[0155] Among them, Fr e Indicates the initial fraction of elemental iodine; Fr o Indicates the initial share of organic iodine; F gap Indicates the fraction of iodine within the fuel cladding gap; F peak Indicates the radial peak power factor of the reactor core fuel; A i This indicates the amount of radioactive iodine isotope i accumulated in all fuel assemblies of the reactor core. Represents the reduction term due to nuclide decay; N represents the number of fuel assemblies in the reactor core; DF e DF indicates the detergency factor of elemental iodine as it penetrates the water layer. o Fr represents the detergency factor of organic iodine penetrating the water layer, taken as 1; p Indicates the initial fraction of cesium iodide; F e λ represents the evaporation rate of elemental iodine released from the pool; V represents the volume of water; λ i represents the decay constant of iodine isotope i; t represents the time interval from reactor shutdown to fuel operation accident.

[0156] Step S 42 Establish a public dose calculation model.

[0157] By using a Gaussian plume diffusion model combined with meteorological data from the plant site (such as wind speed, wind direction, atmospheric stability, frequency distribution, etc.), the concentration distribution of radioactive materials after release in key areas outside the plant (such as the boundaries of non-residential areas and the boundaries of planned restricted areas) is calculated, thereby assessing the dose consequences to the public.

[0158] In assessing the public dose consequences outside the plant, three radiation pathways are typically considered: external radiation from plume annihilation, internal radiation from inhalation, and external radiation from ground deposition. In this invention, to simplify the analysis process and focus on the characteristics of the early accident phase, the dry / wet deposition, radioactive decay, and chemical transformation processes of radionuclides during atmospheric transport are conservatively ignored. That is, it is assumed that all released radioactive materials are completely transported to the receiving point in gaseous form without any mass loss.

[0159] Therefore, only the effects of the first two irradiation pathways—external irradiation from plume immersion and internal irradiation from inhalation—are considered, while the contribution of external irradiation from ground deposition is ignored. This assumption is applicable to emergency response assessments in the initial stages of an accident and is both engineeringly sound and acceptable for review.

[0160] Preferably, step S 42 Includes:

[0161] Taking into account both external irradiation and inhalation internal irradiation, the total effective dose calculation model for the public outside the factory is as follows:

[0162] .

[0163] The calculation model for thyroid dose caused by inhalation of radioactive materials by the public outside the factory is as follows:

[0164] .

[0165] Wherein, D1 represents the effective dose to the general public outside the factory; D2 represents the thyroid dose to the general public outside the factory; R i,j This represents the activity of nuclide i released into the environment during time period j; (χ / Q) j DCF represents the atmospheric dispersion factor within time period j. 1,i DCF represents the effective dose conversion factor for external irradiation of radionuclide i. 2,i DCF represents the internal radiation effective dose conversion factor for radionuclide i. 3,i BR represents the thyroid dose-converting factor for radionuclide i; j This represents the respiratory rate of individuals within time period j.

[0166] Step S 43 Establish a dose calculation model for personnel in the main control room.

[0167] Following an accident, radioactive materials can enter the main control room through the ventilation system and personnel movement, potentially exposing those residing there to radiation. For passive nuclear power plants, the dose assessment of the main control room must consider radiation scenarios under both VES (Voltage Emergency Habitability System) and VBS (Voltage Baseline Non-Radioactive Ventilation Filtering) ventilation modes. A ventilation diagram is shown below. Figure 2 and Figure 3 As shown. For active nuclear power plants, the primary assessment focuses on the radiation impact on personnel under VBS ventilation mode.

[0168] Preferably, step S 43 Includes:

[0169] Taking into account both external irradiation and internal irradiation from inhalation, the effective dose calculation model for personnel in the main control room is as follows:

[0170]

[0171] The calculation model for the thyroid dose caused by the inhalation of radioactive material by personnel in the main control room is as follows:

[0172]

[0173] Where D3 represents the effective dose to personnel in the main control room; D4 represents the thyroid dose to personnel in the main control room; and GF represents the geometric correction factor for the main control room, GF = 351.6 / V. 0.338 V represents the volume of the main control room; DCF 1,i IAR represents the external radiation effective dose conversion factor for radionuclide i. i,j This represents the cumulative concentration of radionuclide i in the main control room during time period j; (χ / Q) j O represents the atmospheric dispersion factor within time period j; j This represents the percentage of time personnel spend in the main control room during time period j; DCF 2,i DCF represents the internal radiation effective dose conversion factor for radionuclide i. 3,i BR represents the thyroid dose-converting factor for radionuclide i; j This represents the respiratory rate of individuals within time period j.

[0174] Step S5: Set acceptance criteria and result judgment. Using atmospheric diffusion and dose models, assess the effective dose and thyroid dose at key locations, and determine compliance.

[0175] Preferably, according to GB6249 Environmental Radiation Protection Regulations for Nuclear Power Plants, the acceptance criteria in step S5 include: the effective dose received by the public at the boundary of the non-residential area within any 2 hours after the accident, and the effective dose received by the public at the boundary outside the planned restricted area during the accident are ≤100mSv, and the thyroid dose is ≤1000mSv.

[0176] According to the "Emergency Preparedness and Emergency Response of the Operating Unit of HAD002 / 01 Nuclear Power Plant", the main control room staff are required to have an effective dose ≤50mSv and a thyroid dose ≤500mSv within 30 days after the accident.

[0177] If all receiving points meet the limits, the design scheme is deemed to meet the safety requirements.

[0178] As described above, compared with the prior art, the method for analyzing the radioactive consequences of fuel handling accidents based on a phased release mechanism in this invention has significant advantages in terms of modeling system, calculation accuracy, and engineering applicability, specifically reflected in the following aspects:

[0179] I. More realistic physical mechanism: The "two-stage release" mechanism (instantaneous bubble release + long-term re-precipitation) is introduced to fully reflect the kinetic process of iodine release in fuel operation accidents; corresponding to the two-stage model design in step S3.

[0180] II. Dynamic Decontamination Factor (DF): A dynamic DF calculation model based on water depth, bubble parameters, and pool water pH is proposed, avoiding the limitations of traditional methods. e =500 leads to excessive conservatism. Corresponding step S 33 DF e Dynamic calculation formula.

[0181] III. Quantitative Re-precipitation Effect: A chemical equilibrium and mass transfer model was established to quantitatively predict I2 re-precipitation after CsI hydrolysis, significantly improving the accuracy of long-term dose assessment in the main control room. (Corresponding step S) 36 Step S 37 Chemistry and mass transfer model.

[0182] IV. Strong compatibility: Applicable to safety analysis of fuel operation areas for different types of reactors (such as AP1000, CAP1400, and Hualong One).

[0183] V. Significant Engineering Benefits: It has been applied and verified in multiple engineering projects. Compared with the old model, the dose in some areas has been reduced by more than 50%, which helps to reduce unnecessary design margins and save construction costs.

[0184] In summary, the present invention provides a method for analyzing the radioactive consequences of fuel handling accidents based on a phased release mechanism. By introducing three core technologies—a two-stage release model, dynamic decontamination factor calculation, and quantification of re-precipitation mechanisms—it achieves refined simulation of accident release behavior and accurate prediction of doses at key locations.

[0185] The proposed method for analyzing the radioactive consequences of fuel handling accidents enhances the physical authenticity and engineering applicability of nuclear power plant safety analysis. It can support optimized design, reduce construction costs, increase the approval rate of license applications, and provide a scientific basis for accident mitigation and emergency response.

[0186] This invention presents a method for analyzing the radioactive consequences of fuel handling accidents based on a phased release mechanism. It is particularly applicable to the detailed modeling and assessment of the release behavior of radioactive materials (especially iodine isotopes) and their effective dose impact on the environment and personnel during refueling and overhauls of pressurized water reactor nuclear power plants, specifically in the event of an accident where irradiated fuel assemblies fall and cause cladding damage in the spent fuel pool area of ​​the reactor building or auxiliary buildings. This analytical method can be widely applied to nuclear power plant design review, operational safety analysis, emergency preparedness and response, safety margin assessment, and license application support, possessing good engineering applicability and software integration potential.

[0187] For those skilled in the art, the above disclosure is merely illustrative and does not constitute a limitation of this application. Although not explicitly stated herein, those skilled in the art may make various modifications, improvements, and corrections to this application. Such modifications, improvements, and corrections are suggested in this application and therefore remain within the spirit and scope of the exemplary embodiments of this application.

[0188] Furthermore, this application uses specific terms to describe embodiments of the application. For example, "an embodiment," "one embodiment," and / or "some embodiments" refer to a particular feature, structure, or characteristic related to at least one embodiment of the application. Therefore, it should be emphasized and noted that "an embodiment," "one embodiment," or "an alternative embodiment" mentioned twice or more in different locations in this specification do not necessarily refer to the same embodiment. In addition, certain features, structures, or characteristics in one or more embodiments of the application can be appropriately combined.

[0189] Similarly, it should be noted that, in order to simplify the description of the embodiments disclosed in this application and thus aid in the understanding of one or more embodiments of the invention, the foregoing description of the embodiments of this application sometimes combines multiple features into a single embodiment, drawing, or description thereof. However, this disclosure method does not imply that the subject matter of this application requires more features than those mentioned in the claims. In fact, the embodiments have fewer features than all the features of a single embodiment disclosed above. Some embodiments use numbers describing the number of components or attributes; it should be understood that such numbers used in the description of embodiments are modified in some examples by the terms "approximately," "about," or "generally."

[0190] While specific embodiments of the present invention have been described above, those skilled in the art should understand that these are merely illustrative examples, and the scope of protection of the present invention is defined by the appended claims. Those skilled in the art can make various changes or modifications to these embodiments without departing from the principles and essence of the present invention, but all such changes and modifications fall within the scope of protection of the present invention.

Claims

1. A method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism, characterized in that, The method for analyzing the radioactive consequences of fuel handling accidents includes the following steps: S1. The initial source term composition of the damaged fuel release is set to include iodine and inert gas in their initial form; S2. The radioactive release is divided into two stages, and a two-stage release model is constructed: the first stage is the instantaneous release of bubbles, and the second stage is the re-precipitation and release of elemental iodine. S3. Calculate the dynamic decontamination factor of elemental iodine in the water layer for the first stage and the second stage. S4. Establish a model for calculating the release pathway and dose of radioactive materials; S5. Set acceptance criteria and result judgment.

2. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 1, characterized in that, The initial form of iodine in step S1 includes cesium iodide (CsI), elemental iodine (I2), and organic iodine (CH3I). The inert gas is released in a free gaseous state and does not participate in the aqueous phase reaction.

3. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 2, characterized in that, The cesium iodide (CsI) is 95%, the elemental iodine (I2) is 4.85%, and the organic iodine (CH3I) is 0.15%.

4. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 1, characterized in that, In step S2, the instantaneous bubble release time in the first stage is 0 to 2 hours, and the elemental iodine re-precipitation and release time in the second stage is 2 hours to 30 days.

5. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 1, characterized in that, The first stage of calculating the decontamination factor of elemental iodine in step S3 includes: S 31 Determine the bubble rise time. Where t represents the bubble rise time; p represents the pressure inside the fuel rod; S 32 Determine the bubble diameter. Where d represents the bubble diameter and p represents the pressure inside the fuel rod; S 33 Determine the detergency factor of elemental iodine.

6. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 5, characterized in that, The step S 33 Includes: When the water depth is in the range of 5.8-7m, calculate the decontamination factor of elemental iodine penetrating the water layer. ; When the water depth is less than 5.8m but greater than 7m, the decontamination factor is determined by combining fluid dynamics simulation. ; Among them, K eff Let be the mass transfer coefficient during the bubble's ascent, t represent the bubble's ascent time, and d represent the bubble's diameter.

7. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 1, characterized in that, The second-stage elemental iodine cleaning agent in step S3 includes: S 34 Identify representative nuclides, including radioactive and non-radioactive iodine in the pool; S 35 The amount of iodine in the molded fuel gap, including the calculation of the molar amount of radioactive iodine and the molar amount of non-radioactive iodine; S 36 Calculate the volatile iodine content in the pool; S 37 Calculate the evaporation rate of elemental iodine.

8. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 7, characterized in that, The radioactive iodine includes 130 I, 131 I, 132 I, 133 I, 134 I, 135 I; the non-radioactive iodine includes 127 I.

9. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 7, characterized in that, The step S 35 The calculation of the molar amount of radioactive iodine described in the text is as follows: ； Wherein, N1 represents the molar amount of radioactive iodine released into the pool during a fuel handling accident; F gap Indicates the fraction of iodine within the fuel cladding gap; F peak Indicates the radial peak power factor of the reactor core fuel; N represents the number of fuel assemblies in the reactor core; A i λ represents the amount of radioactive iodine isotope i accumulated in all fuel assemblies of the reactor core; i represents the decay constant of iodine isotope i; t represents the time interval from reactor shutdown to fuel operation accident.

10. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 7, characterized in that, The step S 35 The calculation of the non-radioactive iodine molar quantity as described in the document is as follows: Wherein, N2 represents the molar amount of non-radioactive iodine released into the pool during a fuel handling accident; m 127 Indicates all fuel assemblies in the reactor core 127 The quality of I; M 127 express 127 Atomic weight of I; F gap Indicates the fraction of iodine within the fuel cladding gap; F peak This represents the radial power peak factor of the reactor core fuel; N represents the number of fuel assemblies in the reactor core.

11. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 7, characterized in that, The step S 36 include: S 361 Taking into account both radioactive and non-radioactive iodine, the total iodine concentration in the pool is calculated as follows: C = (N1 + N2) / V; where C represents the total iodine concentration released into the pool; V represents the volume of water; N1 represents the molar amount of radioactive iodine released into the pool during the fuel operation accident; and N2 represents the molar amount of non-radioactive iodine released into the pool during the fuel operation accident. S 362 Calculate the H⁺ concentration based on the pH value of the pool water: C H =[H + ]=10 -pH ; Among them, C H pH indicates the concentration of hydrogen ions; pH indicates the acidity of water. S 363 Calculate [I2] / [I] based on chemical equilibrium. - ] 2 ratio: R i =[ I2] / [I - ] 2 = C H 2 / (6.05×10 -14 +1.47×10 -9 × C H ); In the formula, R i Represents [I2] / [I - ] 2 The ratio of; C H Indicates hydrogen ion concentration; S 364 Determine the proportion of volatile iodine (I2): Where Frc represents the proportion of iodine atoms present as I2; C represents the concentration of total iodine released into the pool; R i Represents [I2] / [I - ] 2 The ratio of .

12. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 7, characterized in that, The step S 37 include: S 371 Based on the mass transfer coefficient, the ratio of water tank surface area to volume, and the proportion of I2 to I, calculate the reprecipitation removal coefficient λ. e : ; S 372 The release rate of elemental iodine from the pool was modeled using a removal coefficient to determine the evaporation rate F of elemental iodine released from the pool. e : ; Where S represents the total surface area of ​​the pool; K L Frc represents the mass transfer coefficient; Frc represents the proportion of iodine atoms present as I2; V represents the volume of water.

13. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 1, characterized in that, Step S4 includes: S 41 Establish release pathways: If the accident occurs inside the containment, consider the direct release pathway after the containment cleanup system is shut down; if it occurs in an auxiliary building, assume that the radioactive material is discharged directly into the environment without filtration. S 42 Establish a public dose calculation model; S 43 Establish a dose calculation model for personnel in the main control room.

14. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 13, characterized in that, The step S 41 Includes: Regarding the first stage, considering inert gases and iodine, the activity of iodine radionuclides released into the environment 2 hours after the accident is as follows: The calculation formula is: For inert gases For iodine ; Regarding the second stage, considering the release of iodine after the conversion of particulate iodine to elemental iodine, the cumulative release of iodine nuclides into the environment at time T after the accident would be... The calculation formula is: Among them, Fr e Indicates the initial fraction of elemental iodine; Fr o Indicates the initial share of organic iodine; F gap Indicates the fraction of iodine within the fuel cladding gap; F peak Indicates the radial peak power factor of the reactor core fuel; A i This indicates the amount of radioactive iodine isotope i accumulated in all fuel assemblies of the reactor core. Represents the reduction term due to nuclide decay; N represents the number of fuel assemblies in the reactor core; DF e DF indicates the detergency factor of elemental iodine as it penetrates the water layer. o Fr represents the detergency factor of organic iodine penetrating the water layer, taken as 1; p Indicates the initial fraction of cesium iodide; F e λ represents the evaporation rate of elemental iodine released from the pool; V represents the volume of water; λ i represents the decay constant of iodine isotope i; t represents the time interval from reactor shutdown to fuel operation accident.

15. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 13, characterized in that, The step S 42 Includes: Taking into account both external irradiation and inhalation internal irradiation, the total effective dose calculation model for the public outside the factory is as follows: ； The calculation model for thyroid dose caused by inhalation of radioactive materials by the public outside the factory is as follows: Wherein, D1 represents the effective dose to the general public outside the factory; D2 represents the thyroid dose to the general public outside the factory; R i,j This represents the activity of nuclide i released into the environment during time period j; (χ / Q) j DCF represents the atmospheric dispersion factor within time period j. 1,i DCF represents the effective dose conversion factor for external irradiation of radionuclide i. 2,i DCF represents the internal radiation effective dose conversion factor for radionuclide i. 3,i BR represents the thyroid dose-converting factor for radionuclide i; j This represents the respiratory rate of individuals within time period j.

16. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 13, characterized in that, The step S 43 Includes: Taking into account both external irradiation and internal irradiation from inhalation, the effective dose calculation model for personnel in the main control room is as follows: The calculation model for the thyroid dose caused by the inhalation of radioactive material by personnel in the main control room is as follows: Where D3 represents the effective dose to personnel in the main control room; D4 represents the thyroid dose to personnel in the main control room; and GF represents the geometric correction factor for the main control room, GF = 351.6 / V. 0.338 V represents the volume of the main control room; DCF 1,i IAR represents the external radiation effective dose conversion factor for radionuclide i. i,j This represents the cumulative concentration of radionuclide i in the main control room during time period j; (χ / Q) j O represents the atmospheric dispersion factor within time period j; j This represents the percentage of time personnel spend in the main control room during time period j; DCF 2,i DCF represents the internal radiation effective dose conversion factor for radionuclide i. 3,i BR represents the thyroid dose-converting factor for radionuclide i; j This represents the respiratory rate of individuals within time period j.

17. The method for analyzing the radioactive consequences of fuel handling accidents based on a staged release mechanism as described in claim 1, characterized in that, The acceptance criteria in step S5 include: the effective dose received by the public at the boundary of the non-residential area within any 2 hours after the incident, and the effective dose received by the public at the boundary outside the planning restriction zone during the incident is ≤100mSv, and the thyroid dose is ≤1000mSv. For main control room staff, the effective dose ≤50mSv and the thyroid dose ≤500mSv within 30 days after the accident.

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