[원자력안전위원회][January 5, 2023] NSSC Approved Resumption of Kori Unit 3

NSSC Approved Resumption of Kori Unit 3

The Nuclear Safety and Security Commission (Chairperson Yoo Guk-hee, hereinafter referred to as the ” NSSC”) finalized checking the safety of the Kori Unit 3 which had been under investigation since automatic trip on December 22, and approved the resumption of the reactor.

At that time, the turbine/generator and the reactor of the Kori Unit 3 automatically tripped as a ground fault* had occurred at the connection of the generator excitation transformer** cable.

** Ground fault is a phenomenon in which current flows to ground due to insulation damage, etc.

* Excitation transformers supply power to the exciter that turns the generator rotor into an electromagnet

As a result of the investigation of the event, it was confirmed that the ground fault occurred in a gap (less than 1 mm) caused by the heat shrinkable tube* at the cable connection part of the excitation transformer not being completely adhered to the insulator.

The gap was found to have occurred because the heat shrinkage of the tube was not uniform during the installation of the cable connection part (in 2011).

* To insulate the cable connection part, the tube is heated and adhered to the insulator (heat shrink)

The Korea Hydro and Nuclear Power replaced the problematic connection with an assembly type that does not require heat shrinkage. The NSSC additionally confirmed the integrity of other facilities using the heat-shrinkable tubes as well as the suitability of replaced parts.

In addition, the NSSC confirmed that the actions of the reactor operators were carried out in accordance with the relevant procedures during the process of automatic trip of the nuclear reactor, that safety facilities such as reactor cooling* were operating normally, and that there were no unusual issues in the radiation level inside and outside the power plant.

* When the reactor automatically tripped, the auxiliary feed water pump for cooling was automatically started.

The NSSC plans to thoroughly check the safety of the relevant facilities during power uprating process after approving restart of the Kori Unit 3.