Determination of representative elementary surface for accurately measuring radon exhalation rate in masonry wall.

The average radon exhalation rate of the building wall surface is a key factor affecting indoor radon concentration, and its accurate measurement is of great significance for the evaluation and design of radon protection for walls. The main layer of the masonry walls is composed of bricks and bonded cement mortar. Due to the possible differences in radon exhalation rates from brick surfaces and cement mortar joints. General methods for measuring radon exhalation rate of the wall (such as the closed-loop method, the opened-loop method and the local static collection method) all involve the use of radon collection hoods. However, the measurement results of radon exhalation rate are significantly affected by the position and size of the radon collection hood covering the wall surface. Therefore, the Computational Fluid Dynamics (CFD) method was used to study the radon exhalation rules on the surface of masonry wall under diffusion and seepage-diffusion conditions, respectively. The simulation results show that the radon exhalation rates on the brick surfaces and cement mortar joints of masonry walls are different, and a representative elementary surface (RES) should be selected when accurately measuring the radon exhalation rate of the wall surface. It is worth noting that the shape of the RES is not restricted, and the representative surface for measuring radon exhalation rate is not unique; it can be an integer multiple surface of RES area. Furthermore, experiments have confirmed the accuracy and effectiveness of the determined RES. The proposed RES achieves accurate measurement of the average radon exhalation rate of walls by integrating the radon exhalation rates at the joints between bricks and cement mortar.