Numerical Simulation of radon migration influenced by heat-moisture coupled transfer in aerated concrete wall under different outdoor temperature and humidity conditions

. It is of great significance for indoor radon radiation protection to study the mechanism of radon migration systematically caused by heat-moisture coupled transfer in building walls. The radon migration model of porous building walls under the influence of the heat-moisture coupled model is built. On the base of it, four types of different temperature and humidity conditions, including high temperature and high humidity, high temperature and low humidity, low temperature and high humidity and low temperature and low humidity, and the real ambient temperature, relative humidity conditions in a southern city of China in July, are set for the numerical simulation. The migration mechanism of the radon through the wall influenced by outdoor temperature and humidity conditions, including its emanation, diffusion and condensation, is studied by numerical simulation, and the main factors leading the results are analyzed, which provides some reference for indoor radon radiation protection.


1.Introduction
Radon can easily accumulate in indoor environments where most people spend the majority of their time, therefore the harm to human health from the indoor radon cannot be ignored.The indoor radon comes principally from 4 sources: soil, underground water, building materials and outdoor air, while the building walls are one of the major way for radon to the indoor space. Therefor it is of great significance for indoor radon radiation protection to study the mechanism of radon migration in building walls systematically.
Much research has been done on radon exhalation, radon migration and indoor radon concentration distribution.Ref. [1] has carried out a comprehensive investigation and related simulation study on indoor radon in ten cities in China. The main sources of indoor radon, the current indoor radon concentration level in China, and the corresponding control measures have been generally summarized. Ref. [2] measured the radon emission concentration of several common building materials with a radon detector, and calculated the contribution ratio of each building material to indoor radon. Ref. [3] used a radon detector to measure the radon exhalation rate of several building materials, and carried out numerical simulation on the distribution of radon concentration in double-layer porous air emitting medium and indoor. Ref. [4] conducted numerical simulation research on radon migration in adits under different indoor and outdoor pressure differences. Internationally, some scholars have also measured indoor radon concentration in various buildings [5][6][7][8]. The coupled heat and humidity transfer is an important research topic in the research field of building walls, and the migration rule of radon in building walls is also closely related to the coupled heat and humidity transfer rule, but this correlation is rarely considered in existing studies [9]. Therefore, this article regards a kind of aerated concrete wall as the research object, four different outdoor climate conditions including low temperature and low humidity, high temperature and low humidity, low temperature and high humidity, high temperature and high humidity, and the real ambient temperature, relative humidity and indoor radon condensation conditions in a southern city of China in July, are set for the numerical simulations of the radon diffusion coefficient, source term and activity concentration distributions in the wall, and the migration mechanism of radon under the influence of heat-moisture coupled model through the wall in different outdoor climate conditions is analyzed.

Heat and humidity transfer model
According to the Ref. [10], basing on the Fick's Law, Darcy's Law, Kelvin relation and Fourier's law, the wall heat and moisture transfer model is established:  The outer boundary conditions are described by Eq. to the relative humidity of the outer surface of the wall.

Radon migration model
According to Ref.
[11], radon migration in uniform porous media is described by Eq. (7) : In the Eq. The radon source term is described by Formula (9) : In the Eq.  (13) Assuming that the ambient radon concentration in contact with the wall surface is C air and the porosity of the wall material is ε, the radon concentration at the wall surface C 0 can be described by Formula (14) :      Table 2.

Simulation results and analysis
In the first case, the numerical simulation results of heat-moisture-radon transfer of wall in four climates are shown in Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7,   Fig. 8, Fig. 9, which correspond to temperature distribution, water saturation distribution, source term of radon, radon diffusion coefficient distribution and radon concentration distribution of the four outdoor temperature and humidity conditions respectively. The wall radon source term distributions are shown in Fig. 4. According to Eq. (9) and Eq. (10), radon source term is unrelated to temperature and positively correlated with water saturation. Fig. 3 and Fig. 4 indicates that the overall level and variation trend of radon source term and water saturation of the four climates are similar, which means that the distribution of water saturation can reflect the distribution of radon source in wall well.
The distribution of radon diffusion coefficient on the wall is shown in Fig.5. From the perspective overall level, the value of the high temperature and low humidity climate is always highest, because it has always the highest temperature and the lowest water saturation. The value of low temperature and high humidity climate is always the lowest, From around 75mm to 200mm away to the outer surface, since it has always the lowest temperature and the highest water saturation, From 0mm to 75mm to the outer surface, it has low water saturation, but its temperature is much lower than that of the high temperature and high humidity condition. At this period, temperature plays a leading role in radon diffusion coefficient, resulting in the low diffusion coefficient. In most areas of the wall, i.e. 0mm to 120mm from the outer surface, the distribution of radon diffusion coefficient in low temperature and low humidity climate is higher than that in high humidity climate. It can be seen that humidity has a more significant effect on radon diffusion coefficient than temperature. Heating, Refrigerating and Air-Conditioning Engineers).
The numerical simulation results are shown in Fig.10,   Fig.11, Fig.12, Fig.13, Fig.14 and Fig.15. The transient distribution of temperature and relative humidity from the outer surface to the inner surface in wall is shown in Fig.10 and Fig.11. As the city is located in southern China in summer, the outdoor temperature and relative humidity are always at a high level.
The water saturation of the wall material depends largely on its hygroscopic capacity. The isothermal hygroscopic curve ω in Table 1 shows that the hygroscopic capacity of the material is weak, therefore, the water saturation in wall is always at a low level (less than 10%) as shown in Fig.12.
According to Eq.(9) and Eq.(10), radon source is positively correlated with water saturation, therefore Fig. 12 and 13 are similar. The outdoor relative humidity is high at night and low in the daytime, and the radon source in the wall is also higher at night than in the daytime. Indoor relative humidity is always lower than outdoor relative humidity, so the radon source of wall gradually decreases from outdoor side to indoor side.
The water saturation distribution in wall shown in Fig. 12 and the radon diffusion coefficient distribution in wall shown in Fig. 14  materials plays a crucial role in radon diffusion coefficient.
The radon concentration distribution of radon is shown in Figure 15. Wall is a continuous source of radon, so the radon concentration in wall keeps rising. When it reaches nearly 360h, radon concentration tends to be stable, and its value is about 33Bq/m 3 .

Conclusion
Combined with the wall heat and humidity coupled transfer model and radon transfer model, the following four different outdoor climate conditions, including low temperature and low humidity, high temperature and low humidity, low temperature and high humidity, high temperature and high humidity, and the real ambient temperature, relative humidity and indoor radon condensation conditions in a southern city of China in July are set, on this basis, the numerical simulation of radon migration in one kind of aerated concrete wall is conducted. The main conclusions are as follows: 1. The main factor affecting the migration of radon on the wall is the transfer of water, which can effectively inhibit the diffusion of radon; Heat transfer promotes radon diffusion to a certain extent. Temperature plays a leading role in radon diffusion under similar humidity.
2.For the wall containing radon emanation media, the radon concentration in the wall mainly depends on the radon source term, which is positively correlated with the relative humidity in the wall and the hygroscopic capacity of the wall material. Under different outdoor temperature and humidity conditions, the general distribution of radon concentration in the wall from high to low is: low temperature and high humidity, high temperature and high humidity, low temperature and low humidity, high temperature and low humidity.