Modeling of the Radiological Atmospheric Dispersion and Consequences of Hypothetical Nuclear Accident at an Coastal Site using MACCS2 Code

INTRODUCTION

For calculating radioactive atmospheric dispersion.^[1,2-12] and its effects, MACCS2 is a Gaussian plume model. Since the issuance of DOE-STD-3009-94, MACCS2 has primarily been used for deterministic consequence analysis in Department of Energy (DOE) applications. In nuclear facilities, safety control selection has been based on code results.^[1,13-15] By utilizing either the instantaneous concentration

(Bq m^-3 or g m^-3) or time-integrated concentration (Bq s m^-3 or g-s m^-3). The dispersion of the atmosphere is typically represented in terms of χ/Q, where χ is the pollutant concentration in the air at a given downwind point (x, y, z).^[16,17] Q can be either the pollutant’s overall source strength (in becquerels or grams) or its steady rate of discharge (in Bq s^-1 or g s^-1). The plume’s relative concentration since it moves windward from a discharge spot is denoted by χ/Q.

When determining the dose to an individual and the emergency response process,^[18,19] the χ/Q is an important element that ultimately defines the type of safety controls required^{[13,14,20-22]}. In many countries worldwide, numerical models are employed to calculate exposure for co-located workers when the facility’s aerodynamic impacts could affect the release. In building, the χ/Q value offers a conservative dispersion estimate; otherwise, the source term is an important tool in determining the dose to an individual.^[13–27] As indicated in Table 1,^[28] the source term used in the input data was determined by Maciej Lipka. For example,^[28-31] assuming that the radionuclide source term for I-131 was 3.0E+6 Bq, the study was conducted. When a nuclear power plant accident occurs, a large amount of radioactive fission products is released from the core due to meltdown, polluting the environment.

MATERIAL & METHODS

a. The Gaussian distribution equation can be stated as:

Where:

X(x,y,z): Concentration of air (Bq m^-3) at a point with coordinates x,y,z,

H: height of effective release (m)

Q: rate of release (Bq s^-1)

x: distance downwind (m),

y: distance from the crosswind (m),

z: height in relation to the ground (m),

U: average speed of wind (m s^-1),

σ_y, σ_z : factors for crosswind and vertical dispersion (m).

Dispersion variables of the Gaussian model σ_y, σ_z could be expressed as a function of downwind distance x and atmospheric stability (Apismon and Goddard, 1987) as in Equations (2) and Table 2:^[32]

Where:- a,b,c, and d depend on the atmospheric stability

Equation 1 calculates the atmospheric dispersion by taking into account atmospheric parameters, including hypothetical data for (temperature, wind direction, and wind speed). The plume’s size is determined by its Gaussian distribution in the horizontal and vertical planes, which is reliant on the atmosphere’s stability and the plume’s dispersion downwind from the release point in both horizontal and vertical directions [Figure 1]. Keep in mind that this calculation does not specifically include the downwind plume distance, x. Values for σ_y and σ_z are estimated as a function of downwind distance, x, and D.B. Turner’s implementation. The atmosphere’s stability. Alternatively, Equation (3) can be obtained by using the Gaussian plume equation (Equation 1), a plume centerline receptor, and a ground-level release (y = z = H = 0), assuming there is no building.^[5] Using the coefficients of dispersion that were acquired by means of the Eimutis-Konicek parameterization once more,^[32]

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Figure 1:

Plume of Gaussian distribution and coordinate system source.

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Where:

U: speed of wind, which reduces the plume’s intensity (m s^-1);

σ_y: factor for crosswind dispersion (m).

σ_z: factor for vertical dispersion (m).

RESULTS AND DISCUSSION

For the E stability class, to make the calculation of χ/Q values, as wind speed increases, χ/Q decreases, as shown in Tables 3 and 4 and Figures 2 and 3, Where the value as shown in Table 3, Figure 2 at U=0.5 m s^-1 the χ/Q = 3.67E-03 and at U= 6 m s^-1 the χ/Q = 3.06E-04 and Table 4 and Figure 3 gives at U=0.5 the χ/Q = 1.42E-and at U= 6 m s^-1 the χ/Q = 1.18E-04, this attributed to wind transports air more quickly as its speed increases, airborne particles or gases are more likely to mix and disperse, spreading the material over a bigger volume of air and decreasing its specific concentration., else shown as the distances increases, χ/Q decreases as in Table 3 and Figure 2,which show that at D=100 m, U=0.5 and 6 m s^-1 the χ/Q = 3.67E-03 and 3.06E-04 respectively, while at D= 200 m, U= 0.5 and 6 m s^-1 the χ/Q = 1.42E-03 and 1.18E-04 respectively, as shown in Table 4 and Figure 3, and it is noticeable that the value of χ/Q at 200 m is less than at 100 m, as shown in Figure 4, because when a gas or aerosol is released into the air, it starts to spread out; as it moves away from the source, the air volume increases, which means the concentration of the substance in any given volume of air decreases, because dilution and dispersion causes the relative air concentration of a substance to decrease as the distance from the source increases, as shown in Tables 5 and 6, Figure 5. For stability class E, the value of χ/Q at 100m at U=1.5 m s^-1 is 3.67E-3, which is in agreement with 3.3E-3 s m^-3 in NUREG-1140 and 3.5E-3 s m^-3 in DOE-STD-1189-2008. ^[31]

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Figure 2:

χ/Q against wind speed for stability class E at 100 m.

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Figure 3:

χ/Q against wind speed for stability class E at 200 m.

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Figure 4:

χ/Q versus wind speed for stability class E at 100 m and 200 m.

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Figure 5:

χ/Q versus distance at U= 1.5 m s^-1 for stability class E.

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Figures 6-9 and Table 7 show The results for total effective dose (TEDE) of I- 131 in stability class E at 100,200 and 300 m at U= 1.5 m s^-1, which reached about 3.13E-07, 1.2E-07 and 6.37E-08 for Adult (outdoor worker) & 2.7E-07, 1.06E-07 and 6.11E-8 for Adult (indoor worker (pensioner)) & 1.8E-07, 7.3E-08 & 4.2E-8 for (10 -year –old) and 6.4E-08, 2.5E-08, 1.43E-08 for (1 -year -old) respectively. The total effective dose (TEDE) decreased with increasing distances, which attributed to result of atmospheric dispersion factor, where dilution and dispersion cause the concentration of substance to decrease with

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Figure 6:

Total effective dose as a function of distance for an adult (outdoor worker)of I-131at stability class E.

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Figure 7:

Total effective dose as a function of distance for an adult (indoor worker (pensioner) of I-131 at stability class E.

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Figure 8:

Total dose of effectiveness as a distance function for a ten-year-old of I-131 at stability class E.

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Figure 9:

Total dose of effectiveness as a distance function for a one-year-old.

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increasing distance from a source, and the length or intensity of exposure also reduces, resulting in a reduced total effective dose, else notice that the total effective dose decreases as wind speed increases as shown in Tables 7 and 8, Figures 10 and 11, This is because higher wind speeds enhance the dispersion and dilution of airborne radioactive materials, When stronger winds disperse radioactive particles over a greater area, the radioactive material is mixed with clean air more quickly, lowering its concentration at any one time and lowering the dose that people in the affected area get.

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Figure 10:

Total dose of effectiveness as a distance function for Adult (outdoor worker), Adult (indoor worker or elderly), (ten years old), and (one-year-old) of I-131at stability class E and wind speed 0.5 m.

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Figure 11:

Total dose of effectiveness as a distance function for Adult (outdoor worker), Adult (indoor worker or elderly), (ten years old), and (one-year-old) of I-131at stability class E and wind speed 1.5 m.

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Figure 10, Table 8 give the total dose of effectiveness as a distance function for Adult (outdoor worker), Adult (indoor worker or pensioner), (10-year-old), and (1-year-old) of I-131 at stability class E and wind speed 0.5 m s^-1. The maximum TEDE of 9.38 E-07, 8.26 E-07, 5.68 E-07 and 1.9E-07 for Adult (worker outside), Adult (worker indoors), (Ten years old) and (One-year-old)at stability class E respectively at 100m from the release source, this value is less than IAEA for indoor worker ((occupational exposure limit of 2E-02 Sv per year).^[31] It is evident from Figures 10 and 11 that the total effectiveness dose for an adult outdoor worker is greater than that for an adult indoor worker, an older adult, a 10-year-old, or a 1-year-old, which could be because the total effective dose of a substance can vary greatly depending on several factors, such as exposure time, physical activity, metabolic rate, and susceptibility. The adult outdoor worker usually has the largest total effective dose due to longer exposure time and higher activity levels (which result in greater inhalation), and perhaps increased pollution concentrations, The elderly and indoor workers have lower overall exposure and spend less time inexposed locations, which results in a lower total dose and younger children in particular have a faster respiratory rate in relation to their body size. However, because of their shorter exposure times and the environmental controls, they are still likely to receive a lower overall dose. Still, because of their susceptibility to poisons, even tiny amounts can have a big effect.

CONCLUSION

The χ/Q lowers as the wind speed rises and distances increase. For stability class E, χ/Q against wind speed is greater at 100 m than at 200 m. For stability class E, the value of χ/Q at 100m at U=0.5 m s^-1 equals 3.67E-03,which is consistent with 3.3x10-3 s m^-3 in NUREG-1140 and 3.5x10-3 s m^-3 in DOE-STD-1189-2008.

At 100 meters from the release source, the maximum effective dose (TEDE) of I-131 for adults (outdoor workers), adults [indoor workers (pensioners)], and children aged 10 and 1, respectively, at stability class E is higher than at

At 200 meters, however, this value is lower than the IAEA occupational exposure limit for indoor workers (2E-02 Sv per year). Due to age and hours spent working, the total effective dose as a function of distance is higher for adults working outside and those working indoors (pensioners) at stability class E and wind speed of 0.5 m s^-1.