# Digitalization of the Ex-Core Neutron Flux Monitoring System for APR1400 Nuclear Power Plant
## 4 Conclusions  
The Ex-core Neutron Flux Monitoring System (ENFMS), which is one of the core systems in a nuclear power plant (NPP), controls the reactor power level and protects the reactor from excessive thermal power. These functions are required to meet relevant nuclear regulations for every type of nuclear reactor: from research reactors to commercial power reactors or fusion reactors. All the ENFMSs operating in Korea, as well as Korean-built ones abroad, are analog-based. Thus, there have been repeated demands for improvements using new digital technologies. As an effort to respond to these demands, this study was performed to review the feasibility of the digitalization of the analog ENFMS currently being used for Advanced Power Reactor 1400 (APR1400) NPPs. As such, the feasibility study aims to answer whether a digitalization of the analog ENFMS is possible; and if so, which strategies and steps must be taken for a successful and effective digitalization. Based on the literature review, a simplified model of a fission chamber neutron flux detection and instrumentation using the MATLAB software (R2010b) was designed. The accuracy of the model was evaluated by comparing the simulation results of the model with data gathered from tests and plant operations of currently operating ENFMSs. Simulation results showed that the model data was in good agreement with actual data and hence was used to evaluate the performance of the proposed digital ENFMS. A conceptual design for the digitalization of the ENFMS for APR1400 nuclear power plants is proposed through a combined structure that digitalizes only part of the system. In this design, the detector signal pre-amplification, which needs to process very low signals with high speed and short response time, remains in analog form. However, the reactor power calculation function and other connected signal conditioning and processing functions from pulse shaping to output data and signal generation will be digitalized. Grounded on the technical analysis of currently used digital components and equipment, it is judged that the proposed scheme of the digital ENFMS can satisfy all the functional and performance requirements of the ENFMS. Furthermore, it can provide design flexibility to implement advanced instrumentation algorithms and functions in the ENFMS. Simulation results showed that the true MSV calculated by the digital ENFMS maintained a linear relationship between the real and estimated reactor powers in the wide-range compared to the averaged magnitude squared value calculation of analog ENFMS. It proved that the proposed digital ENFMS has a better performance than the analog ENFMS for the wide-range monitoring instrumentation. This work also utilized extended Kalman filter (EKF) algorithm to estimate the reactor power and the reactor period from the measurement signals that are contaminated with gamma ray interactions and electric noise. EKF was chosen since it performed better compared to the discrete low-pass filter in reducing the noise. In conclusion, this study proved that a successful digitalization can be realized which will satisfy all functional and performance requirements of the ENFMS. Furthermore, it will improve the system functions and performances due to application of sophisticated digital signal processing algorithms such as the EKF and true MSV calculation. The designed model can also be used further for various \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reactor Power State** & \
\begin{tabular}{c} **Measurement** \\ **w/Noise** \\ \end{tabular} &
\begin{tabular}{c} **Low Pass Filter** \\ **(Cut-Off freq 1/10/30)** \\ \end{tabular} & **EKF** \\ \hline Steady at \(2\times 10^{-8}\) [\%] & 0.2497 & 0.0134/0.0448/0.0802 & 0.0457 \\ Increasing, continuous step of 2 [sec] \& 50\% & 0.2497 & 0.1755/0.0509/0.0807 & 0.0457 \\ duty cycle bet 0.2 to 200.2 [sec] & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Reactor power estimation RMSE.