Press reports in South Korea said the report concluded that workers miscalculated the power output of the 995-MW pressurised water rector unit and mishandled the control rods, and violated safety protocols, including failing to convene a meeting before beginning testing.
Last month the regulator said it was expanding its special investigation into the manual shutdown of Hanbit-1 because of concerns it may have been the result of a violation of safety protocol.
The commission ordered the suspension of operations at the plant and sent a team of special judiciary police officers to carry out a special inspection.
The commission said 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 Kori-1 reactor in 1978.
On 10 May station owner and operator Korea Hydro and Nuclear Power Company 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 offline. Initial investigations showed that a member of staff who was not licensed to handle the control rods had been directly involved in the process.
At the time the regulator said it had found “some proof” that KHNP “did not apply an adequate level of safety measures and violated the Nuclear Safety Act”.