Lagrangian and Eulerian approach to predict the movement of radionuclides in selected potential sites in Malaysia during the monsoon period

In computing the movement of particles through the fluid, three types of dispersion models are used which are Gaussian plume models, Lagrangian puff models, and small-scale numerical models. Lagrangian particle dispersion models are increasingly used for nuclear applications. In this study, the usage of the Lagrangian model is implemented in a Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) which combines with the Eulerian model to predict the movement of radionuclides from the simulated nuclear power plant accident in selected locations in Malaysia. These include nuclear risk studies in these areas, emergency response systems, and source term analyses. This study aims to compare the risk from the dispersion of 137Cs from a simulated Nuclear Power Plant (NPP) at Mukah and Tasik Kenyir during the Northeast monsoon (8th October 2022) and Southeast monsoon (16th May 2022). The HYSPLIT model was set up based on the meteorological data during both monsoon periods and was simulated for 5 days. The outcomes revealed that the concentration of 137Cs is higher at Tasik Kenyir compared to Mukah for both during the Northeast monsoon period (3.5 x 107 Bq/m2) while for Southwest monsoon, the result is vice versa where the concentration of 137Cs at Mukah is (2.2 x 107Bq/m2). The activity concentration is then converted into rate effective dose measurement and compared with the limit suggested by ICRP which is 1 Sv per year. It is concluded that Tasik Kenyir is a more suitable location to build NPP compared to Mukah as the movement of radionuclides was dispersed to the location that contained less population.


Introduction
The world's population has been growing at a pace rarely seen before, and this demographic shift has a big impact on how much energy the world needs.According to the United Nations [1], the world's population has increased to over 8 billion in 2023.This increase, together with industrialization and urbanization, has resulted in a rising demand for dependable and sustainable energy sources to power our contemporary society.In Southeast Asia, the absence of nuclear power plants is notable; however, as the population continues to grow and energy demand escalates, it becomes imperative to proactively identify potential sites for such facilities.With energy demand in Malaysia steadily rising at an annual rate of approximately 3-4%, it is evident that the country urgently requires a nuclear power plant to address its growing power needs effectively.However, due to the harmful effects that 1308 (2024) 012006 IOP Publishing doi:10.1088/1757-899X/1308/1/012006 2 have resulted from previous nuclear accidents (e.g., Fukushima and Chernobyl), many people are apprehensive about the use of nuclear energy.In nuclear safety, it is important to predict the movement of radionuclides from the source to protect humans from being exposed to them.Many types of atmospheric models can be used to predict the movement of particles such as AERMOD, HOTSPOT, CALPUFF, and HYSPLIT.
HYSPLIT is an atmospheric model developed by the National Oceanic and Atmospheric Administration (NOAA) in 1980.The model calculation in HYSPLIT is a hybrid model between the Lagrangian-Eulerian System.The HYSPLIT model is made to support a variety of simulations including the atmospheric transport and dispersion of pollutants and hazardous materials as well as the deposition of these materials to the Earth's surface.Tracking and predicting the release of radioactive material, volcanic ash, wildfire smoke, dust, and pollutants from different fixed and mobile emission sources are some of the usages [2].HYSPLIT Model of the Air Resources Laboratory of NOAA database was also used along with isotopic compositions to study atmospheric moisture by measuring the concentration of Oxygen-18 and Hydrogen-2 in the Bojnourd area, Iran [3].Other than that, in the nuclear safety sector, a study has been made to analyze the radiological component on a potential site at Baiji.The results of the effective dose were compared with the dose limits that are suggested by the International Commission on Radiological Protection (ICRP), and it exceeded by 1 Sv for the areas that are near the potential site [4].Thus, in this study, nuclear incidents were simulated for some potential NPP sites in Malaysia during critical periods in the Northeast Monsoon and Southwest monsoon.

Study Area
Tasik Kenyir (2.8921° N, 112.0922°E) and Mukah (5.0040° N, 102.6388°E) are the selected NPP potential sites in Malaysia.Both locations are located near the sea and the population density is less than the limit set by AELB which is 200 people/m2 [5].The area surface of Tasik Kenyir and Mukah is about 270 km 2 and 2536 km 2 respectively.Previously, there was a study about the site selection in Malaysia in Johor State, where Tenggaroh and Jemaluang are the most suitable locations among the other sites potential.Based on the AELB procedure, they outline a three-step process to determine the location of the NPP [6]: i.
It cannot be affected by phenomena against which protection through the design is impracticable.ii.
The probability of occurrence and the severity of destructive phenomena are not too high.iii.The site characteristics are such that the consequences of a potential accident would be at acceptable limits.

Methodology
In predicting the movement of particles in the atmosphere, the Lagrangian model is one of the atmospheric models that is usually used in many fields and it is also considered as one of the emergency preparedness tools.The Lagrangian technique predicts the movement and dispersion of pollutants by following the trajectories of a large number of "particles" that originate from a source point.Equation 1shows the calculation of x-y components for a 2-dimensional grid combined with equation 2 which calculates the altitude, and z of the particle by considering the average turbulence velocity of the wind [7].
There are two ways to calculate particles: vertically or simultaneously in both vertical and horizontal directions.There are two possible horizontal distributions for calculations with a vertical particle distribution: top-hat and Gaussian puff.On the other hand, instead of being calculated at a grid point, particle computations are added to a grid cell.The area of a cell, which corresponds to the distance between adjacent nodes in half, is specified at the node's center.One particle of mass m is defined as contributing an incremental amount of concentration to a cell as a 3D particle.The concentration of pollutant's mass at a specific grid inside the atmosphere is calculated as shown in equation 3 [7].
On the other hand, the Eulerian model is an approach that specifies a specific grid to calculate the velocity and concentration of particles concerning space and time, The mass balance in the Eulerian model is centered on a differential volume.The air motion is computed using the velocity field [8]. Figure 1 shows the fixed three-dimensional grid which acts as a frame of reference to compute the pollutant air concentrations.via (https://www.ready.noaa.gov/data/archives/gdas1). Meteorological data that is inserted in this model is called a Global Data Assimilation System (GDAS) which is considered global meteorological data that contains variables such as temperature, pressure, relative humidity, and wind direction.This parameter is simulated by HYSPLIT to produce a trajectory that might travel by radionuclides in the simulation.
By using meteorological data from 2022 in May and October, the trajectory simulation was done for a daily emission throughout 5 days.The emission input is designed to be equal to the predicted release of cesium during the Fukushima accident in 2011 to obtain a clear observation of air movement in an intense nuclear emission scenario [9].As the emission origin, the emission height was chosen at 0 meters above sea level as it is a significant height for humans in that area.After computing the simulation, the activity concentration for both locations in both periods, the activity concentration is converted into rate effective dose rate by using the dose rate coefficient for 137 Cs and Equation 3.
where e is the dose rate coefficient, A is the activity concentration and t is the duration of a person being exposed to the pollutant. 137Cs are chosen for this simulation because it has a long half-life of 30 years and can emit gamma radiation.Since 137 Cs stay longer in the environment, they can travel a long distance across the globe.Long-term exposure to Caesium can cause stochastic health effects such as cancer or tumors [10].Table 1 shows the radionuclide input data and the selected period in this simulation.

Analyze meteorological data
Based on the GDAS meteorological data, parameters such as temperature, wind speed, and wind direction were extracted and then compared within both monsoons.The interval time for these parameters' measurement is 3 hours.Based on figure 2(a), shows that the temperature at Tasik Kenyir has a higher range compared to the Mukah area where the differences in the temperature during the day and night are large for Tasik Kenyir.The Southwest monsoon (Figure 2(b)) follows a similar pattern for both sites, but Tasik Kenyir's maximum temperature is greater than the Northeast monsoon's, which is roughly 34.3°C.To visualize the wind data before converting it into trajectory form, the wind speed and wind direction are then plotted in a wind rose diagram (figure 3(a) and 3(b)).According to the data, the wind direction for Mukah during the Southwest monsoon moved in a wide range of directions and did have an on the dispersion of radionuclides, which dispersed into plenty of locations.The same goes for the movement of radionuclides at Tasik Kenyir during the Northeast monsoon where it dispersed in many directions.Similarly, radionuclides moved over from Tasik Kenyir during the northeast monsoon and scattered in all directions.

Trajectory analysis
Wind trajectory analysis is a valuable tool for understanding the movement of air masses and their potential impacts on our environment and industries.By tracking wind patterns and predicting their behavior, it can be used as an initial risk assessment right after the onset of a nuclear accident.Immediate emergency evacuation or sheltering orders can be carried out through this analysis, thus minimizing the affected population's exposure to radioactive deposition.Based on figure 4(a), shows the wind direction moving to the north-east direction for both locations.However, during the southwest monsoon period in Mukah (Figure 4(c)), the wind direction for Mukah was predicted to move and accumulate in the region that has a high population in Sarawak state. (

Activity concentration analysis
The dispersion of 137 Cs has been evaluated for the first 5 days after the hypothetical accident starting at UTC 00:00 at the ground level.The main purpose of the 0 m altitude is to evaluate the value that is most exposed to human beings.To give a route for escape in the event of an unexpected catastrophe, this prediction should be helpful in any emergency preparedness plans.Based on Table 2 and Table 4, the dispersion of 137 Cs was estimated to reach Tanjung Beringin, West Kalimantan, Indonesia after 5 days of a nuclear accident.The movement of radionuclides in Mukah has affected the region that contains a large population.For Tasik Kenyir during the northeast monsoon, the movement of radionuclides reached some regions in Sabah state on the 5th day but the concentration was not as high as in Mukah.During the southwest monsoon, the movement of 137 Cs has reached Peninsular Malaysia from Mukah.It is because of the wind direction that spread to various directions as observed in Figure 3 (c).The dose rate coefficient for 137 Cs at 0 m for newborns, teenagers, and adult persons are 9.23 x 10-18 Sv Bq-1 s m2, 7. x 10-18 Sv Bq-1 s -1 m2 and 7.85 x 10-18 Sv Bq-1 s -1 m2 respectively [11].This dose rate coefficient is used to convert activity concentration into effective dose rate measurement.This study focuses on the dose rate coefficient for adult people and the highest amount of effective dose are tabulated in Tables 4 and 5.

Conclusion
Based on this study, it is found that Tasik Kenyir is the more suitable location to build NPP as the dispersion of radionuclides does not affect a large number of the population compared to Mukah.However, it is important to provide an emergency plan for the population to evacuate if a nuclear accident occurs.Thus, several atmospheric models such as the Lagrangian and Eulerian models might be useful for making a prediction movement of radionuclides in a certain period.

Figure 1 .
Figure 1.Movement of particles through the specific grid in the Eulerian approach by respect to time and space, In this study, HYSPLIT software is used to simulate the movement of radioactive particles in the environment utilizing atmospheric dispersion modeling.The National Oceanic and Atmospheric Administration (NOAA) of the USA created the computer model HYSPLIT to measure the paths of air parcels and the concentration of air contaminants.As mentioned earlier, this software applies both Lagrangian and Eulerian approaches to simulate the movement of particles in an interval period.HYSPLIT requires the input of meteorological data and usually the input is obtained from The National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) of the USA supply open access [2]via (https://www.ready.noaa.gov/data/archives/gdas1). Meteorological data that is inserted in this model is called a Global Data Assimilation System (GDAS) which is considered global meteorological data that contains variables such as temperature, pressure, relative humidity, and wind direction.This parameter is simulated by HYSPLIT to produce a trajectory that might travel by radionuclides in the simulation.By using meteorological data from 2022 in May and October, the trajectory simulation was done for a daily emission throughout 5 days.The emission input is designed to be equal to the predicted release of cesium during the Fukushima accident in 2011 to obtain a clear observation of air movement in an intense nuclear emission scenario[9].As the emission origin, the emission height was chosen at 0 meters above sea level as it is a significant height for humans in that area.After computing the simulation, the activity concentration for both locations in both periods, the activity concentration is converted into rate effective dose rate by using the dose rate coefficient for 137 Cs and Equation 3.

Figure 2 .
Figure 2. (a) Temperature for both locations during the Northeast monsoon and (b).during the Southwest monsoon.

Figure 4 .
Figure 4. Trajectory analysis in the Tasik Kenyir and Mukah region during (a) Northeast and (b) Southwest monsoon.