Non-compliant Welding Procedures at Two Units of South Korea's Hanbit Nuclear Power Plant

The Hanbit Nuclear Power Plant headquarters of Korea Hydro & Nuclear Power Co., Ltd. (KHNP) recently confirmed that Units 1 and 2 of the plant had used welding procedure specifications (WPS) that had not undergone impact testing during maintenance of the component cooling water heat exchangers. A retrospective inspection found that among 14 repair activities conducted since the relevant heat exchangers were replaced in 2007, 6 cases involved similar non-compliance.

The issue was identified during the 28th planned preventive maintenance outage of Hanbit Unit 1. Inspections revealed that some welding procedure specifications used for repairs to the water chambers of the component cooling water heat exchangers did not meet requirements, involving repair work on worn areas inside the heat exchanger water chambers in the cooling systems of Units 1 and 2.

The Hanbit Nuclear Power Plant headquarters stated that welding work should have been performed in accordance with the American Society of Mechanical Engineers (ASME) Code. The cause of this misapplication was that personnel, when interpreting the ASME Code, mistakenly determined that the welding procedure specification could be exempted from impact testing. The headquarters also stated that a review of past work showed no differences in the variables involved in actual maintenance between welding procedure specifications that had undergone impact testing and those that had not.

The Hanbit Nuclear Power Plant headquarters plans to perform rework on previously repaired areas whose specific locations can be confirmed; for areas where the problem location is difficult to determine, components will be replaced. These measures are scheduled to be completed by April next year. The headquarters also stated that procedures have been improved by incorporating maintenance and replacement activity checklists and component inspection checklists into the planning phase, and requiring that the basis be clearly documented when technical standards are not applicable.

The Hanbit Nuclear Power Plant explained that the function of the heat exchangers in question is to lower the temperature of cooling water supplied to major equipment, and they are not directly related to reactor safety or radioactive material release. Nevertheless, since improper implementation of maintenance procedures at the plant has been confirmed, the relevant rework or component replacement must still proceed.

As originally planned, Hanbit Units 1 and 2 are scheduled to undergo continued operation reviews in the first half of next year. Unit 1 is planned to cease power generation and undergo equipment improvement work starting in December 2025; Unit 2 was originally scheduled to shut down on the 11th of this month for similar equipment maintenance. Previously, South Korea's nuclear safety regulatory authority had proposed shortening the review period for continued operation of nuclear power units approaching the end of their design life by streamlining the regulatory review process.