(570.5417) Green Korea United - The 14th "Steam Generator Tube Rupture (SGTR)" accident in the world occurred in Ulchin nuclear power plant (unit 4) located in the north-eastern part of South Korea. [See for a full list www.wise-paris.org]. Due to this rupture accident 45 ton of the primary coolant was poured into the second coolant system. The accident happened around 18 hours after the shut down for the regular inspection and fuel exchange, and the emergency core cooling system (ECCS) didn't work, so operators had to manually input the emergency coolant to the reactor.
Even though the accident happened on 5 April, the Korean Hydro & Nuclear Power (KHNP) and the regulator (Ministry of Science & Technology) hid the severity of the accident for the past two months by deliberately describing it as a "coolant leakage incident" in their press release. Korean nuclear activists didn't recognize this press release seriously at first due to their distorted information. The Korean Nuclear Safety Committee announced last 25 May that the "incident" is Level 1 on the 7-level International Nuclear Event Scale (INES), insisting that there was no radiation release to the outside and no one affected by radiation from this event.
Ulchin 4, which started operating in Dec. of 1999, was inspected by the licensee, Korea Hydro and Nuclear Power (KHNP), three times before this accident. The inspection methods called Bobbin and MRPC (Motorized Rotating Pancake Coil) revealed slight crack indications, but KHNP ignored the indications, claiming that they were not significant. According to the final investigation report of Korean Nuclear Safety Committee on this accident (which they describe as an "incident"), the main causes of the accident were several cracks around the Expansion Transition - just above the tube sheet. However, the report deliberately avoids commenting on the problematic material of the tube - the so-called Alloy 600 (HTMA), and even slightly denies the probability that the material may have caused the developing of the cracks. Nevertheless, according to the Korean Atomic Energy Research Institute, the exact cause of the accident has not been confirmed yet.
There have been 11 SGTR accidents in the world so far, once in Belgium and Japan respectively and 9 times in the US. All the reactors that have experienced SGTR accidents had steam generators made of Alloy 600 (or Inconel 600), and the material has been considered as a main cause of rapid deterioration of the tubes. Unfortunately, the so-called Korean Standard Nuclear Plants from Yonggwang 3,4,5,6 to Ulchin 3,4 have installed the same type of steam generators so far. The KHNP affirms that it can probe or predict this kind of crack on steam generators' tubes since it now has experience, and will prevent same kind of "incident" from other nuclear reactors. However, it didn't explain clearly why it had failed to investigate the cracks appropriately and how it plans to avoid repeating the same problem.
Source and contact: Kwanghoon Seok, Energy and Nuclear Safety issues, Green Korea United, 113-34, Seongbuk-dong, Seongbuk-gu, Seoul, South Korea, 136-821
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An unnamed official of the Korea Institute of Nuclear Safety (KINS) said it would take several months for the KHNP to find out the exact cause and process of the steam generator tube rupture. However, he added that the nuclear industry will hesitate to reveal the exact cause since "this kind of investigation may harm the reliability of whole nuclear industries as well as the steam generator supplier".
Such potentially embarrassing revelations about nuclear accidents are often hidden in "commercially confidential" reports. Occasionally parts of these reports are revealed. For example, in a report into the Davis-Besse incident, one such report was quoted as saying that cracks in reactor vessel head penetrations were discovered over twice as often in French reactors as in reactors in other countries. Matthieu Schuler from the French nuclear safety authority suggested this was because the French use eddy-current testing to detect cracks rather than the ultrasonic methods often used elsewhere, including in the US (see WISE/NIRS Nuclear Monitor 568.5402, "Large numbers of undetected cracks in the world's PWRs").
However, the recent Korean incident throws doubt even on the eddy-current testing method, since the two types of testing used - Bobbin and Motorized Rotating Pancake Coil (MRPC) - are both forms of eddy-current testing. True, they revealed crack indications, but the utility was unable to predict that these would lead to catastrophic failure of the tube. Indeed, despite decades of research in universities all over the world and data from hundreds of reactors, there is still no reliable method of predicting crack growth.
This, of course, has not stopped the US Nuclear Regulatory Commission from extending the licenses of some reactors to as much as 60 years. Since no power reactor has been in operation for this long, this means that the old reactors will enter unknown territory, with increasing numbers of cracks, and more chance that the cracks may lead to catastrophic failure as the metal becomes more and more embrittled from decades of irradiation.
Green Korea United report, 19 June 2002; WISE Amsterdam