Radioactivity assessment method based on smoke plume diffusion mode in limited space

Abstract

The invention provides a radioactivity assessment method based on a smoke plume diffusion mode in a limited space, which is mainly used for assessing radioactivity concentration distribution in the limited space, and comprises the following steps: equivalent treatment of a radioactivity diffusion process: considering the radioactivity diffusion process as a smoke plume beam composed of smoke puff, obtaining radioactive concentration distribution through a Gaussian puff diffusion model; a traditional Gaussian puff model is improved by considering radioactive source intensity change, gravity settlement and environmental wind speed, change of puff radioactive intensity is considered according to radioactive decay, and offset of gravity settlement to a smoke plume center line is considered according to a Stokes formula. According to the parallelogram rule, the environment wind speed and the radioactive self-diffusion speed are synthesized into a superposition diffusion speed; and according to the improved puff model, superposing the radioactivity concentrations of all puff, and evaluating the radioactivity concentration in the limited space.

Description

Technical Field

[0001] The present invention belongs to the field of nuclear radiation protection, and in particular relates to a radioactivity assessment method based on a plume diffusion pattern in a confined space. Background Art

[0002] During the construction and operation of nuclear facilities, and even after decommissioning, especially when nuclear facilities seriously deviate from normal operating conditions or accidents occur, their radioactivity will have a radiation impact on the environment and the public. Therefore, nuclear radiation assessment is of great significance. Among them, the calculation of the radiation amount of radioactive airborne substances diffused through the atmosphere to the environment is an important indicator for assessing nuclear radiation hazards.

[0003] Radioactive hazards differ from common environmental pollution in that they are often difficult to detect but can be very powerful. They release radioactive substances instantly or in a short period of time, causing severe pollution and damage to the environment and the public. The Chernobyl and Fukushima nuclear power plant accidents are terrifying. Even nuclear facilities operating under normal operating conditions are subject to some degree of radioactive contamination. To protect the health and safety of the public around nuclear facilities, appropriate protective measures must be implemented, and this requires an assessment of the scope and intensity of radioactive contamination. At present, the diffusion model widely used in nuclear assessment is the Gaussian plume model. However, the traditional Gaussian plume model is suitable for relatively uniform and stable radioactive airborne flows, and does not comprehensively consider the influence of relevant factors. The reliability and accuracy of the assessment results in actual applications are often low: 1. The influence of radioactive decay is not considered, and the concentration of the radioactive source is often assumed to be stable, which does not conform to the actual situation that the concentration changes with time; 2. The influence of gravity sedimentation is not considered. When the particle size of the radioactive material is larger than a certain scale, the gravity sedimentation effect will cause the center line of the plume to deviate significantly, thereby affecting the plume diffusion path, resulting in the radioactive concentration distribution derived by the traditional plume model no longer being applicable; 3. The influence of ambient wind speed is not considered. The superposition of ambient wind speed and the diffusion speed of the radioactive material itself will change the diffusion trajectory and diffusion range of the radioactive material, further affecting the distribution of radioactive concentration.

[0004] The radioactivity assessment method based on the plume diffusion pattern in a confined space adopts an improved Gaussian plume model, regards the radioactive plume as a composition of air masses, considers the change in the intensity of the radioactive source through the radioactive decay characteristics, and takes into account the offset of the plume centerline caused by gravity sedimentation. At the same time, by superimposing the diffusion rate of the radioactive material itself with the ambient wind speed in a confined space, the superimposed diffusion rate, which plays a key role in gas diffusion, is obtained, thereby better evaluating the concentration distribution of radioactive material diffusion in space. Summary of the Invention

[0005] To address the shortcomings of the traditional Gaussian plume model for gaseous diffusion assessment, the present invention aims to provide an improved plume model for estimating radioactivity concentration within a confined space. This method, based on the traditional Gaussian plume model, improves upon it in three key aspects: By treating the plume as individual puffs, the variation in radioactive source concentration during diffusion is accounted for; by considering the offset of the plume centerline, the gravitational settling of radioactive materials is accounted for; and by superimposing the diffusion velocity of the radioactive material itself with the ambient wind speed to obtain a superimposed diffusion velocity. This method better reflects actual diffusion conditions and allows for a more accurate assessment of radioactivity intensity within a confined space.

[0006] The technical solution of the present invention includes the following processes:

[0007] Step 1: Based on the puff model, the smoke plume formed during the radioactive diffusion process is considered to be a stream composed of continuous puffs. The concentration obtained according to the Gaussian puff model is:

[0008]

[0009] Where c(x,y,z,t,H) represents the concentration at the spatial point (x,y,z); H is the effective height, for example, when H=0, it is the diffusion mode of the ground radioactive source; Q is the intensity of the radioactive source; u is the radioactive diffusion rate; σ x , σ y , σ z are the standard deviations of the puff in the x, y, and z directions respectively.

[0010] Step 2: Consider the change of the intensity Q of the radioactive source. In reality, the intensity of the radioactive source changes with time and generally obeys the exponential distribution of the decay law:

[0011] Q=Q0*e -λt

[0012] Where Q0 is the initial estimate of the intensity of the radioactive source, λ is the decay constant, and t is the time from the initial estimate.

[0013] Step 3: Consider gravity settling and its impact on the plume model. Generally, particles larger than 10 μm have significant gravity settling. Gravity settling affects the plume diffusion trajectory, thus affecting the radioactive diffusion effect. This can be considered in two steps:

[0014] Step 31: Calculate the settling velocity. The settling velocity depends on the balance between air resistance and gravity. Use Stokes' formula to calculate the settling velocity:

[0015]

[0016] Where V sis the sedimentation velocity; ρ is the particle density; g is the acceleration due to gravity; D is the particle diameter; μ is the dynamic viscosity coefficient of air.

[0017] Step 32: Calculate the offset of the plume centerline. Figure 1 The gravity sedimentation effect is superimposed on the center line of the plume, causing the center line to shift downward. The effect is equivalent to the radioactive particles in the process of plume diffusion. s The speed of the vertical downward movement is:

[0018] ΔH=V s t

[0019] Where ΔH is the height of the plume centerline offset, t is the settling time, and is the time it takes to move to coordinate x at speed u:

[0020]

[0021] Therefore, after taking into account gravity settlement, the effective height is:

[0022] H-ΔH

[0023] Step 4: Obtain the superimposed diffusion velocity. The gas diffusion velocity at a spatial point is determined by the wind speed and the diffusion velocity of the gas itself, as shown in the figure. Figure 2 The relationship between wind direction, observation point, and the location and angle of the radiation source is given in

[15] , which is used to calculate the superposition diffusion velocity. Within a confined space, the ambient wind speed is essentially stable, with a stable wind direction and speed. Calculating the superposition diffusion velocity involves three steps.

[0024] Step 41: Determine the angle ψ between the wind direction and the diffusion direction. The angle θ between the line connecting the spatial point and the radiation source and the x-axis of the coordinate system with the radiation source as the origin, and the angle δ between the wind direction and the x-axis, is obtained as:

[0025] ψ=θ-δ

[0026] Step 42: Determine the wind speed conversion coefficient. Convert the wind speed to the direction of the spatial point and the radiation source according to the principle of orthogonal decomposition. The wind speed conversion coefficient λ in this direction is:

[0027] λ=cosψ

[0028] Where λ = 1 indicates downwind diffusion, 0 < λ < 1 indicates partial downwind diffusion, λ = 0 indicates no-wind diffusion, -1 < λ < 0 indicates partial upwind diffusion, and λ = -1 indicates upwind diffusion.

[0029] Step 43: Determine the superimposed diffusion velocity. The superimposed diffusion velocity ζ is obtained by combining the wind speed conversion coefficient and the diffusion velocity of the radioactive material itself.

[0030] ζ=λv+u

[0031] Where u is the diffusion speed of radioactive material and v is the wind speed.

[0032] Step 5: The puff model concentration formula obtained after improvements in steps 2, 3, and 4 is:

[0033]

[0034] Step 6: Calculate the sum of the radioactivity concentrations of all puffs in the plume. Within a limited space, according to the plume model, the radioactivity concentration is equivalent to the cumulative effect of all radioactivity in the plume. Within the time range of interest [0, T], the radioactivity concentration at a spatial point is obtained by integration:

[0035]

[0036] 2. The radioactivity assessment method based on the plume diffusion model in a confined space according to claim 1 is characterized in that the radioactivity diffusion process is quantitatively calculated through two equivalent calculations: once the radioactivity diffusion process is equivalent to a plume model, and once the plume model is equivalent to a plume bundle composed of multiple smoke puffs, so that the decay characteristics of radioactive materials can be fully considered in the radioactivity assessment model.

[0037] 3. The radioactivity assessment method based on the plume diffusion model in a confined space according to claim 1 is characterized by taking into account the sedimentation effect of radioactive particles and obtaining the sedimentation velocity by balancing air resistance and gravity according to the Stokes equation. The sedimentation velocity acts on the centerline of the plume, causing the centerline of the plume to be offset in the vertical direction, thereby changing the effective height of radioactivity diffusion and thus affecting the distribution of radioactivity concentration.

[0038] 4. The radioactivity assessment method based on the plume diffusion model in a confined space according to claim 1 is characterized by taking into account the influence of ambient wind speed on radioactivity diffusion. The angle between wind direction and diffusion direction can not only determine whether a spatial point is in the downwind direction or upwind direction of radioactivity diffusion, but also combine the ambient wind speed with the radioactivity diffusion velocity to form a superimposed diffusion velocity, which further affects the radioactivity concentration distribution in the plume model, and fully consider wind speed, an important factor affecting radioactivity concentration, in the plume model.

[0039] 5. The radioactivity assessment method based on the smoke plume diffusion model in a confined space according to claim 1 is characterized in that the radioactivity diffusion effect of each smoke puff is obtained according to the improved smoke puff model, and the radioactivity concentration in the confined space is the accumulation of the diffusion effects of each smoke puff. According to the time period of radioactivity assessment, the diffusion effects of all smoke puffs in the time period of interest are accumulated to obtain the overall radioactivity concentration of the smoke plume model as an important result of the radioactivity assessment in the confined space. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 The relationship between the ambient wind speed, radioactive source, and spatial point is shown as an example;

[0041] Figure 2 The offset of the plume centerline caused by gravity settling is shown as an example. DETAILED DESCRIPTION

[0042] The present invention is further explained below with reference to the accompanying drawings and specific embodiments. The method comprises the following main steps:

[0043] Step 1: Based on the puff model, the smoke plume formed during the radioactive diffusion process is considered to be a stream composed of continuous puffs. The concentration obtained according to the Gaussian puff model is:

[0044]

[0045] Where c(x,y,z,t,H) represents the concentration at the spatial point (x,y,z); H is the effective height, for example, when H=0, it is the diffusion mode of the ground radioactive source; Q is the intensity of the radioactive source; u is the radioactive diffusion velocity; σ x , σ y , σ z are the standard deviations of the puff in the x, y, and z directions respectively.

[0046] Step 2: Consider the change of the intensity Q of the radioactive source. In reality, the intensity of the radioactive source changes with time and generally obeys the exponential distribution of the decay law:

[0047] Q=Q0*e -λt

[0048] Where Q0 is the initial estimate of the intensity of the radioactive source, λ is the decay constant, and t is the time from the initial estimate.

[0049] Step 3: Consider gravity settling and its impact on the plume model. Generally, particles larger than 10 μm have significant gravity settling. Gravity settling affects the plume diffusion trajectory, thus affecting the radioactive diffusion effect. This can be considered in two steps:

[0050] Step 31: Calculate the settling velocity. The settling velocity depends on the balance between air resistance and gravity. Use Stokes' formula to calculate the settling velocity:

[0051]

[0052] Where V s is the sedimentation velocity; ρ is the particle density; g is the acceleration due to gravity; D is the particle diameter; μ is the dynamic viscosity coefficient of air.

[0053] Step 32: Calculate the offset of the plume centerline. Figure 1 The gravity sedimentation effect is superimposed on the center line of the plume, causing the center line to shift downward. The effect is equivalent to the radioactive particles in the process of plume diffusion. s The speed of the vertical downward movement is:

[0054] ΔH=V s t

[0055] Where ΔH is the height of the plume centerline offset, t is the settling time, and is the time it takes to move to coordinate x at speed u:

[0056]

[0057] Therefore, after taking into account gravity settlement, the effective height is:

[0058] H-ΔH

[0059] Step 4: Obtain the superimposed diffusion velocity. The gas diffusion velocity at a spatial point is determined by the wind speed and the diffusion velocity of the gas itself, as shown in the figure. Figure 2 The relationship between wind direction, observation point, and the location and angle of the radiation source is given in

[15] , which is used to calculate the superposition diffusion velocity. Within a confined space, the ambient wind speed is essentially stable, with a stable wind direction and speed. Calculating the superposition diffusion velocity involves three steps.

[0060] Step 41: Determine the angle ψ between the wind direction and the diffusion direction. The angle θ between the line connecting the spatial point and the radiation source and the x-axis of the coordinate system with the radiation source as the origin, and the angle δ between the wind direction and the x-axis, is obtained as:

[0061] ψ=θ-δ

[0062] Step 42: Determine the wind speed conversion coefficient. Convert the wind speed to the direction of the spatial point and the radiation source according to the principle of orthogonal decomposition. The wind speed conversion coefficient λ in this direction is:

[0063] λ=cosψ

[0064] Where λ = 1 indicates downwind diffusion, 0 < λ < 1 indicates partial downwind diffusion, λ = 0 indicates no-wind diffusion, -1 < λ < 0 indicates partial upwind diffusion, and λ = -1 indicates upwind diffusion.

[0065] Step 43: Determine the superimposed diffusion velocity. The superimposed diffusion velocity ζ is obtained by combining the wind speed conversion coefficient and the diffusion velocity of the radioactive material itself.

[0066] ζ=λv+u

[0067] Where u is the diffusion speed of radioactive material and v is the wind speed.

[0068] Step 5: The puff model concentration formula obtained after improvements in steps 2, 3, and 4 is:

[0069]

[0070] Step 6: Calculate the sum of the radioactivity concentrations of all puffs in the plume. Within a limited space, according to the plume model, the radioactivity concentration is equivalent to the cumulative radioactivity of all puffs in the plume. Generally, one puff is generated per unit time. From the initial time to time T, a total of N puffs are generated. The sum of these puffs yields the radioactivity concentration at the spatial point:

[0071]

[0072] 2. The radioactivity assessment method based on the plume diffusion model in a confined space according to claim 1 is characterized in that the radioactivity diffusion process is quantitatively calculated through two equivalent calculations: once the radioactivity diffusion process is equivalent to a plume model, and once the plume model is equivalent to a plume bundle composed of multiple smoke puffs, so that the decay characteristics of radioactive materials can be fully considered in the radioactivity assessment model.

[0073] 3. The radioactivity assessment method based on the plume diffusion model in a confined space according to claim 1 is characterized by taking into account the sedimentation effect of radioactive particles and obtaining the sedimentation velocity by balancing air resistance and gravity according to the Stokes equation. The sedimentation velocity acts on the centerline of the plume, causing the centerline of the plume to be offset in the vertical direction, thereby changing the effective height of radioactivity diffusion and thus affecting the distribution of radioactivity concentration.

[0074] 4. The radioactivity assessment method based on the plume diffusion model in a confined space according to claim 1 is characterized by taking into account the influence of ambient wind speed on radioactivity diffusion. The angle between wind direction and diffusion direction can not only determine whether a spatial point is in the downwind direction or upwind direction of radioactivity diffusion, but also combine the ambient wind speed with the radioactivity diffusion velocity to form a superimposed diffusion velocity, which further affects the radioactivity concentration distribution in the plume model, and fully consider wind speed, an important factor affecting radioactivity concentration, in the plume model.

[0075] 5. The radioactivity assessment method based on the smoke plume diffusion model in a confined space according to claim 1 is characterized in that the radioactivity diffusion effect of each smoke puff is obtained according to the improved smoke puff model, and the radioactivity concentration in the confined space is the accumulation of the diffusion effects of each smoke puff. According to the time period of radioactivity assessment, the diffusion effects of all smoke puffs in the time period of interest are accumulated to obtain the overall radioactivity concentration of the smoke plume model as an important result of the radioactivity assessment in the confined space.

Claims

1. A radioactivity assessment method based on plume diffusion pattern in a confined space, used to assess radioactivity concentration in a confined space, characterized in that: The following procedures are included: Step 1: Based on the puff model, the smoke plume formed during the radioactive diffusion process is considered to be a stream composed of continuous puffs. The concentration obtained according to the Gaussian puff model is: Where c(x,y,z,t,H) represents the concentration at the spatial point (x,y,z); H is the effective height, for example, when H=0, it is the diffusion mode of the ground radioactive source; Q is the intensity of the radioactive source; u is the radioactive diffusion rate; σ x , σ y , σ z are the standard deviations of the puff in the x, y, and z directions respectively. Step 2: Consider the change of the intensity Q of the radioactive source. In reality, the intensity of the radioactive source changes with time and generally obeys the exponential distribution of the decay law: Q=Q0*e -λt Where Q0 is the initial estimate of the intensity of the radioactive source, λ is the decay constant, and t is the time from the initial estimate. Step 3: Consider gravity settling and its impact on the plume model. Generally, particles larger than 10 μm have significant gravity settling. Gravity settling affects the plume diffusion trajectory, thus affecting the radioactive diffusion effect. This can be considered in two steps: Step 31: Calculate the settling velocity. The settling velocity depends on the balance between air resistance and gravity. Use Stokes' formula to calculate the settling velocity: Where V s is the sedimentation velocity; ρ is the particle density; g is the acceleration due to gravity; D is the particle diameter; μ is the dynamic viscosity coefficient of air. Step 32: Calculate the offset of the plume centerline. In Figure 1, the gravity sedimentation effect is superimposed on the plume centerline, causing the centerline to shift downward. The effect is equivalent to the radioactive particles moving at V during the plume diffusion process. s The speed of the vertical downward movement is: ΔH=V s t Where ΔH is the height of the plume centerline offset, t is the settling time, and is the time it takes to move to coordinate x at speed u: Therefore, after taking into account gravity settlement, the effective height is: H-ΔH Step 4: Obtain the superimposed diffusion velocity. The gas diffusion velocity at a spatial point is determined by both the wind speed and the gas's own diffusion velocity. Diagram 2 shows the relationship between wind direction, observation point, and the location and angle of the radiation source, from which the superimposed diffusion velocity is calculated. Within a confined space, the ambient wind speed is essentially stable, with consistent wind direction and speed. Calculating the superimposed diffusion velocity involves three steps. Step 41: Determine the angle ψ between the wind direction and the diffusion direction. According to the angle θ between the spatial point, the line connecting the radiation source and the x-axis of the coordinate system with the radiation source as the origin, and the angle δ between the wind direction and the x-axis, the angle ψ between the wind direction and the diffusion direction is obtained: ψ=θ-δ Step 42: Determine the wind speed conversion coefficient. Convert the wind speed to the direction of the spatial point and the radiation source according to the principle of orthogonal decomposition. The wind speed conversion coefficient λ in this direction is: λ=cosψ Where λ = 1 indicates downwind diffusion, 0 < λ < 1 indicates partial downwind diffusion, λ = 0 indicates no-wind diffusion, -1 < λ < 0 indicates partial upwind diffusion, and λ = -1 indicates upwind diffusion. Step 43: Determine the superimposed diffusion velocity. The superimposed diffusion velocity ζ is obtained by combining the wind speed conversion coefficient and the diffusion velocity of the radioactive material itself. ζ=λv+u Where u is the diffusion speed of radioactive material and v is the wind speed. Step 5: The puff model concentration formula obtained after improvements in steps 2, 3, and 4 is: Step 6: Calculate the sum of the radioactivity concentrations of all puffs in the plume. Within a limited space, according to the plume model, the radioactivity concentration is equivalent to the cumulative effect of all radioactivity in the plume. Within the time range of interest [0, T], the radioactivity concentration at a spatial point is obtained by integration:

2. The radioactivity assessment method based on the plume diffusion model in a confined space according to claim 1 is characterized in that ,The radioactive diffusion process is quantitatively calculated twice by equivalent calculation. ,Once the radioactive diffusion process is equivalent to a plume model, and ,the plume model is equivalent to a plume bundle composed of numerous smoke ,clusters, so that the decay characteristics of radioactive materials ,can be fully considered in the radioactive assessment model.

3. The radioactivity assessment method based on the plume diffusion model in a confined space according to claim 1 is characterized in that Considering the sedimentation effect of radioactive particles, according to the Stokes formula, the sedimentation velocity is obtained by the balance between air resistance and gravity. The sedimentation velocity acts on the center line of the plume, causing the center line of the plume to be offset in the vertical direction, changing the effective height of radioactive diffusion, and thus affecting the distribution of radioactive concentration.

4. The radioactivity assessment method based on the plume diffusion model in a confined space according to claim 1 is characterized in that , the influence of ambient wind speed on radioactive diffusion is taken into account. Through the angle between wind direction and diffusion direction, not only can we know whether the spatial point is in the downwind direction or upwind direction of radioactive diffusion, but also the ambient wind speed and the diffusion speed of radioactivity itself are combined to form a superimposed diffusion speed, which further affects the radioactive concentration distribution of the plume model, and fully consider the wind speed, an important factor affecting the radioactive concentration, into the plume model.

5. The radioactivity assessment method based on plume diffusion model in a confined space according to claim 1 is characterized in that ,According to the improved puff model, the radioactive diffusion effect of each puff is obtained, and the radioactive concentration in the confined space is the accumulation of the diffusion effect of each puff.,According to the time period of radioactive assessment, the diffusion effects of all puffs in the time period of interest are accumulated to obtain the overall radioactive concentration of the plume model, which is used as an important result of radioactive assessment in the confined space.

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