South Korea's Nuclear Safety and Security Commission (NSSC) said on 20 May that it will expand its special investigation into the manual shutdown of the Hanbit 1 reactor earlier this month, in view of concerns that it may have been triggered by a violation of safety protocol. On 10 May, Korea Hydro & Nuclear Power (KHNP) carried out a diagnostic test on the effectiveness of control rods, during which the reactor's power output surged 18% exceeding the 5% upper limit range. Instead of shutting down the reactor immediately, it was almost 12 hours before the unit was taken off line, NSSC said. Initial investigations showed that a member of staff who was not licensed to handle the control rods had managed the critical component that regulates the fission reaction in reactors.

NSSC has assigned special judicial police officers to the case and KHNP could face administrative measures and may also be held criminally liable, local press reports said. “In the process of the special investigation that we initiated on May 16 in regard to the manual shutdown of the reactor at Hanbit 1 on May 10, we found evidence that the KHNP had taken inadequate safety measures and violated the Nuclear Safety Act. We will proceed with a special investigation to suspend use of the reactor and assign the case to special judicial police officers,” the NSSC announced in a press release issued on May 20. This is the first time that special judicial police officers have investigated nuclear reactors in Korea since commercial operation of nuclear power began in South Korea at the Gori-1 reactor in 1978.

KHNP only shut down the reactor on orders from experts from the Korea Institute of Nuclear Safety (KINS), who arrived that afternoon and stated that the reactor was not in compliance with the operations manual. Failure to comply with the manual is a violation of Article 26 of the Nuclear Safety Act. KHNP stated on Monday that staff “hadn’t known that it was necessary to immediately shut down the reactor if the thermal output exceeds the upper limit.”

The NSSC secured testimony that an individual who was not licensed as a reactor operator had operated the control rods during the control rod measurement test. The NSSC also found evidence that the head of power generation who was responsible for supervising the test (being licensed for reactor operation and supervision) had not provided adequate instructions or oversight. This is a violation of Article 84 of the Nuclear Power Act, which states that the reactor must be operated by a license holder or at least supervised by a supervisory license holder. Such a violation carries a penalty of up to one year in prison or a fine of up to KRW10 million ($8,383).

NSSC said that Hanbit 1 is currently being kept safe, and that there was no leak of radioactivity either inside or outside the facility. For two months, until 20th July, NSSC plans to monitor the reactor’s equipment and the company’s safety culture. “Since the thermal output rose so suddenly, we’ll also have to check the integrity of the nuclear fuel. After thoroughly ensuring that the nuclear rods and nuclear fuel are both safe, we’ll take action related to nuclear power legislation,” said an NSSC official.

The 950MWe pressurised water reactor in Yeonggwang had recently been approved to restart operations after undergoing maintenance checks that started last year. South Korea currently operates 24 reactors, generating about 30% of its electricity but has adopted a phase-out plan that aims to replace nuclear power with renewable energy sources.