The Nuclear Safety And Security Commission said on the 9th that it allowed the restart of Hanbit Unit 5 in Yeonggwang, South Jeolla, which had been undergoing a regular inspection since Apr. 4.
Through the regular inspection that began on Apr. 7, the commission said it carried out repairs for a reactor head penetration nozzle leak and addressed an automatic start event of an emergency diesel generator, confirming the overall nuclear plant safety.
First, regarding the reactor head penetration nozzle leak, the commission explained that in Jul., a 0.7 mm hole was found in a newly replaced penetration nozzle (the passage through which the device that moves the control rods entering the reactor passes). The investigation found that bubbles remained inside the metal when the penetration nozzle was manufactured, and pressure applied in the reactor caused the hole.
The cause of the defect was identified as pores (air) remaining inside the metal during penetration nozzle fabrication. Micro-pores formed during fabrication remained in the thin section of the nozzle, and continuous pressure from hydrostatic tests and other processes was the problem. In connection with this, a detailed ultrasonic test on similar areas and expanded tests, including hydrostatic tests, on all penetration nozzles (83) found no other defects.
Also, in Apr., when the reactor was shut down, one emergency generator started by itself, and the commission determined that the cause of the incident was a control device failure. It replaced the faulty part and inspected other devices, confirming safety.
Of the 97 total inspection items, all 86 items that must be verified before restarting the reactor were completed.
The commission said, "Based on the results of this regular inspection, we allowed the criticality of Hanbit Unit 5," and added, "We plan to finally confirm safety through follow-up tests (11), including a power ascension test." Criticality refers to a state in which the neutrons produced and the neutrons lost are equal, forming an equilibrium as a sustained nuclear fission chain reaction occurs in a reactor.