Nuclear reactor kept running for 12 hours after it should have been shut down

Posted on : 2019-05-21 17:09 KST Modified on : 2019-05-21 17:09 KST
S. Korean nuclear safety commission assigns special judicial police officers to investigate
The Hanbit 1 reactor in Yeonggwang County
The Hanbit 1 reactor in Yeonggwang County

According to South Korea’s nuclear power regulator, a nuclear reactor whose thermal output exceeded safety limits was kept running for nearly 12 hours when it should have been shut down manually at once. Furthermore, the regulator said, an individual who wasn’t licensed to operate the reactor was holding the control rods, which regulate the reactor’s output, at the time. A continuing increase in output could have led to a thermal runaway, potentially causing the reactor to explode.

The nuclear regulator, called the Nuclear Safety and Security Commission (NSSC), has assigned special judicial police officers to the case. Korea Hydro & Nuclear Power (KHNP), the public company that operates the country’s nuclear reactors, is likely to be slapped with administrative measures and may be held criminally liable as well.

“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.

The accident occurred on May 10 in the middle of a test to measure the control capacity of the control rods at Hanbit-1. The control rods are devices that are inserted into or withdrawn from the reactor in order to control or halt output. The control rod measurement test began at 3 am, and an abnormal situation occurred at 10:30 am. The thermal output soared from 0% to 18% in a single minute, causing the temperature of the reactor coolant to rise rapidly, along with the steam generator level. According to the technical operations manual, the upper limit for thermal output is 5%. The rising level of the steam generator caused the main water pump to halt. This activated the auxiliary water pump, which adds cooling water in the event of an emergency.

KHNP didn’t proceed with a manual shutdown until 10:02 pm that day, in violation of the operations manual, which calls for an “immediate” manual shutdown of the reactor if the thermal output exceeds the upper limit. In short, the reactor remained in operation for 12 hours after it was supposed to be shut down. 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.”

Person handling control rods wasn’t licensed operator

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 10 million won (US$8,383).

Experts in nuclear power equipment describe this as an extremely dangerous situation that could easily have led to a thermal runaway, an event that can cause violent blowouts of reactor contents and catalysts. “This incident was not a technical disagreement between experts or a mistake by the operator—it was a crime,” said an industry insider who spoke on condition of anonymity.

“This was an incident that almost caused a thermal runaway under low output conditions. The rapid increase of output from 0 is the exact principle used in a nuclear weapon,” said Han Byeong-seop, director of the Research Institute for Safe Nuclear Power.

“KHNP took a terrifying risk in not shutting down the reactor at once. Since this could well have led to a serious accident, it needs to be thoroughly investigated,” said Jang Jeong-uk, a professor at Matsuyama University in Japan.

The 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, through July 20, the NSSC plans to inspect not only the reactor’s equipment but also the company’s safety culture, a term in the international nuclear power industry that refers to the ability and attitude of the individuals and organizations that handle nuclear power.

“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.

By Choi Ha-yan, staff reporter

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