Modification of logic module in the reactor protection system of JOYO

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Since the initial criticality of Joyo was achieved in 1977, the reactor has been operated thoroughly and the reactor protection system which consists of control rods, acontrol rod drive system, monitoring instrumentations and logic modules has been functioned as designed. As the time elapsed, the maintenance work was gradually expanded due to degradation of the system. A modification of the logic module was conducted to increase its reliability. This paper describes the modification and its results of the logic module which was conducted during 5th periodical inspection. The logic module is essential for the reactor protection function, and provides scram signals on determing conditions. Reliability of the module is strongly required and function check of the module is also required on the reactor operation, for assurance of plant safety. The results of the modification are summerized as follows, (1)The solid-state integrated circuits used in the logic module as the basic logic switching scheme was modified from type "HTL" into type "C-MOS" which is excellent noise proof device. And further faile-safe design was completely applied in both logic circuits and out-put mechanical relay circuits. (2)Reflecting the experience obtained through the operation, the operational board and circuits were modified in order to improve operability of the system. New functions were introduced as indication of first actuated signal and self-testing circuits which enable to check regardless the operation conditions. Further step-wise checking was also introduced to diagnose integrity of individual signal train. (3)In order to increase maintenability, defect indication was newly provided, which displays circuit defect if any, and circuit boards were modified to access easily for checking.