Root Cause Analysis of the Leak in the Safety Injection System Pipeline Completed

The Root Cause Analysis (RCA) of the leak in one of the two pipelines of the Safety Injection System (SI-53), which led to the unexpected shutdown of the power plant in October 2023, has been completed. The analysis has been submitted to the Slovenian Nuclear Safety Administration (SNSA).

Destructive testing of materials was conducted in a hot cell at a specialized laboratory in the USA to determine the direct cause of the leak. Experts from the Institute of Metals and Technology (IMT) in Ljubljana were present during the tests.

The RCA of the failure involved experts from the supplier, Westinghouse, and organizations authorized by the SNSA from Slovenia and Croatia, including the Faculty of Mechanical Engineering (Ljubljana), the Jožef Stefan Institute (Ljubljana), and the Faculty of Electrical Engineering and Computing (Zagreb), along with experts from NEK.

The analysis revealed that the root causes of the failure were linked to the inadequate design of the pipeline for safety injection into the reactor vessel (SI-53). The design did not adequately consider the thermal stratification of the pipeline during certain operating phenomena in the power plant, nor the temperature changes caused by the constant circulation of water in the pipeline during plant operation (thermal fatigue).

As previously reported, during the emergency outage in 2023, we completely replaced the leaking pipeline. Simultaneously, as a preventive measure, we also replaced the other pipeline, which serves the same function but has a slightly different configuration. After further examinations of the second pipeline, no deviations were found.

Thermomechanical and strength analyses are currently underway, which will serve as the basis for any further measures and enhancements that NEK plans to implement to increase the pipelines' resistance to the conditions they face.