The Nuclear Safety and Security Commission said it allowed Hanbit Unit 4 to reach criticality on the 26th after beginning a periodic inspection on Oct. 25 last year.
Criticality refers to a state in which nuclear fission of nuclear fuel continues so that a nuclear power plant can reach normal output. Allowing criticality means permitting normal operation of the plant.
The Nuclear Safety and Security Commission said, "During this periodic inspection, corrosion caused by a boric acid water leak was confirmed at the upper part of the reactor coolant pump casing, and we conducted a root-cause analysis of the leak and expanded inspections," adding, "We confirmed that the causes of the leak were a defect in one small-diameter pipe of the reactor coolant pump and damage to two gaskets (ring-shaped components inserted between the pipe and the body)." The defect in the small-diameter pipe was seen as having occurred when impurities remaining inside the metal during the pipe's manufacturing process gradually detached.
In this regard, the Nuclear Safety and Security Commission said that an expanded inspection of all 39 similar small-diameter pipes in Unit 4 found no other defects, and the pipe with the confirmed defect was replaced with a new one. It added that all gaskets were replaced, and a technical review and safety analysis by Westinghouse confirmed there were no integrity issues.
Going forward, the Nuclear Safety and Security Commission plans to strengthen inspection and testing methods when manufacturing the new pipes to prevent a recurrence.
The Nuclear Safety and Security Commission said, "Of the 90 total items in the periodic inspection, we conducted inspections of the 79 items that must be completed before criticality and confirmed that the reactor can safely achieve criticality," adding, "Based on the periodic inspection results to date, we are allowing Hanbit Unit 4 to reach criticality, and we will finally verify safety through follow-up tests (11 items), including power ascension tests."