Shin Kori 3 - construction of which began in October 2008 - is the first Korean-designed Advanced Pressurised Reactor-1400 (APR-1400) to start up.
Having been issued with an operating licence for the unit by the Nuclear Safety and Security Commission on 30 October, KHNP began loading 241 fuel assemblies into the reactor on 4 November. The unit achieved first criticality on 29 December.
KHNP has since been conducting commissioning tests at the unit. These tests involve checking the unit's performance as its output is gradually increased to full capacity.
Shin Kori 3 is expected to enter commercial operation in May following the completion of these tests, KHNP said. It becomes South Korea's 25th operable power reactor.
Unit 4 at Shin Kori - also an APR-1400 - is expected to start operating in early 2017. Unit 3 had originally been due to begin operating at the end of 2013, with unit 4 following in September 2014. However, their operation has been delayed by the need to test safety-related control cabling and its subsequent replacement.
Two more of the 1350 MWe pressurized water reactors are under construction as units 1 and 2 of the Shin Hanul site in South Korea. Those units are expected to enter service in April 2017 and February 2018, respectively.
Two further APR-1400 units are planned for both the Shin Kori and Shin Hanul sites.
Four more APR-1400s are under construction at Barakah in the United Arab Emirates. All four are scheduled to be in operation by 2020.