Design of the Fully Digitalized SOE System in Nuclear Power Plants

With the spread of the digital technologies, the Instrumentation and Control (I and C) systems including the Sequence of Events (SOE) system in the nuclear power plants gradually follow these general trends. This paper discusses the methods for calculating SOE events' occurrence time in each of the non-safety systems and the safety systems. This paper presents the structure of a fully digitalized SOE system in the I and C systems and takes the important design elements of the SOE systems into consideration, including the time resolution, time protocol, and the properties of the data communication networks in the non-safety systems and the safety systems. The feature of the SOE system is that the processing of the SOE events' occurrence time is distributed in their individual systems except for the safety systems processed in a gateway. The commercial data communication networks adopting TCP/IP are used in the non-safety systems and the safety systems use the deterministic data communication networks in order to produce their output within restricted time. Under the two different data communications networks, the methods for establishing the SOE events' occurrence time which are classified into a safety grade and a non-safety grade are applied here. The Network Time Protocol (NTP) is used to synchronize the time keeping among the time servers and the clients in the non-safety systems. When the SOE events occur, the clients record the time information from their own local clocks. The safety systems are designed to precisely calculate the SOE events' occurrence time. The equation defined as a function of a transmission time, a transmission waiting time and an arrival time to a gateway is presented here. This paper analyzed the time errors of the SOE events in non-safety systems by using NTP through an experimental environment. In the case of the safety systems, the principle for the calculation of the SOE events' occurrence time is explained by an example. (authors)