Design Improvement on Ulchin Nuclear Power Plant Units 5 and 6

Since the completion of the construction of Yonggwang Nuclear Power Plant Units 3 and 4 (YGN 3 and 4), all project team developed its design improvement program and tried to adopt the advanced technology to the Nuclear Steam Supply System (NSSS) design. As previous project team did, the project team of the Ulchin Nuclear Power Plant Units 5 and 6 (UCN 5 and 6) made an effort to incorporate several improvement items into the system design. The reference plant of the UCN 5 and 6 is Ulchin Nuclear Power Plant units 3 and 4 (UCN 3 and 4), which is the first Korean Standard Nuclear Power Plant (KSNP). Even though the main system configuration is the same as the reference plant, UCN 5 and 6 is designed to provide improvements in safety, reliability and costs by applying both advanced proven technology and experiences gained from the construction and operation of previous plants. The result of the preliminary probabilistic safety assessment study for UCN 5 and 6 shows that the core damage frequency is lowered significantly. Several design improvement items incorporated into the system design contributed for lowering the core damage frequency value. Among the design improvements, digital PPS and digital ESFAS are the key to the UCN 5 and 6 design. Korean regulatory body evaluated the design concept of the digital PPS including digital ESFAS and concluded to be acceptable for the plant safety. Also, in-depth review on the detail design of the digital PPS/ESFAS and the special evaluation/audit for the software design process are underway to secure the quality of the software. The safety and the licensability of the UCN 5 and 6 system design have been evaluated through a two-year review on the preliminary safety analysis report. As a result, the construction permit was issued on May 17, 1999 by the government. In this paper, the design characteristics of UCN 5 and 6 focused on the design improvements are discussed. And some safety analysis results as well as the licensing status are presented. (authors)