## 1 Introduction  
Recently in nuclear power plants (NPPs), there have been an increasing number of attempts to replace wired systems with wireless communication systems, because wired monitoring systems that use complex and heavy wire cables have installation and maintenance drawbacks [1, 2]. However, wireless systems emit electromagnetic fields for communication purposes, which can be a potential hazard to instrumentation and control (I&C) equipment in NPP. This is because electromagnetic fields exceeding the tolerable threshold may cause malfunctions of the I&C equipment. To protect the I&C equipment from these unpredictable electromagnetic fields, the I&C equipment must be safely placed outside the exclusion zone (EZ). A guideline for the EZ is introduced in _Regulatory Guide 1.180_[3], but as this assumes a free space environment, it may not be suitable for an actual NPP that includes many interior structures. Previous studies have investigated the indoor electromagnetic fields inside the NPP when arbitrary wireless devices are in operation [4, 5, 6], but there has been no detailed discussion on the EZ. In a recent study, the EZ for the actual indoor environment was examined [7, 8], but it is very difficult to determine the boundary between the EZ and the safe zone because the indoor EZ boundary is very complicated and irregularly distributed, which makes it difficult to apply in practice. To obtain an EZ that can be used in practice, the boundary of the EZ should not be too complicated, so that the I&C equipment can be easily placed in the safe zone. In addition, the EZ needs to be extended into a 3D volume space, since the EZ in previous studies is limited to a 2D plane with a fixed height.  
In this paper, we propose a statistical indoor EZ that can be efficiently applied to an NPP by investigating electromagnetic fields inside the actual NPP. The proposed indoor EZ has much simpler boundaries than those reported in [7; 8], since the EZ is achieved using a statistical regression analysis for practical use in the NPP. This means that the proposed statistical EZ is more suitable for real NPPs than conventional methods that have a complicated boundary. To obtain the statistical indoor EZ, the indoor environment of the simulator room of the Korea Institute of Nuclear Safety (KINS) is modeled using the Wireless InSite commercial electromagnetic simulation software [9]. In this simulation model, the transmitting and receiving antennas use the measured radiation patterns of the dipole antenna in order to make the simulation results more consistent with the actual values. The received power at the receiving antenna, caused by the transmitting antenna of the wireless system, is analyzed in all classified indoor observation regions with a 30\({}^{\circ}\) angle range. The statistical regression model [10; 11] is applied to each piece of data included in the observation region. The boundaries of the EZ for each observation region can be achieved using a calculated regression model. Then, the EZ is extended into a 3D volume space by investigating at various heights, which helps us to understand the EZ in a 3D environment. The statistical EZs in the NPP are then compared with those in a free space case. To verify the proposed method, the electromagnetic field strengths according to the azimuth angle and distance from each transmitting antenna are measured, and are confirmed to be in good agreement with the simulation results. The results demonstrate that the proposed statistical indoor EZ can be properly obtained not only from the simulation data but also from the measurement data.