Second quarter of 2002
Steam Generator Tube Rupture Accident in South Korea
The fourteenth Steam Generator Tube Rupture (SGTR) accident of the nuclear history occurred on 5 April 2002 at the Uljin nuclear power plant (unit 4) in the North-Eastern part of South Korea. This rare event is potentially disastrous and can lead to significant releases of radioactivity to the environment. In this case, 45 tons of primary coolant water leaked into the secondary coolant circuit.
WISE-Paris, 27June 2002
[Posted 28/06/2002]
Uljin-4, a 900 MW pressurized water reactor (PWR), is one of the 16 operating nuclear reactors in South Korea, of which 12 are of this PWR type and 4 more use pressurized heavy water (PHWR).
The steam generator tube rupture, or SGTR accident occurred 18 hours after shut down for regular inspection and refueling. It seems that the Emergency Core Cooling System (ECCS) was deficient and operators had to manually activate the emergency injection of coolant to the reactor core. Approximately 45 tons of water from the primary coolant leaked into the secondary coolant circuit.
Despite the high risks inherent to such an accident, the Korean Hydro & Nuclear Power (KHNP) and the Ministry of Science & Technology (MOST), the regulator, qualified the accident of «coolant leakage incident». (1) It was classified at level 1 of the INES scale (International Nuclear Event Scale, graduated from 0 to 7) by the Korean Nuclear Safety Committee (KNSC). According to the available information, the KNSC declared there was no radiation release outside of the reactor containment and no one has been affected by radiations. The final investigation report by the KNSC concluded that the main causes were several cracks above the tube sheet. The tubes are made of a particular nickel, cobalt and steel alloy, called Inconel-600, that has proven sensible to high temperatures, high pressure and the acidic conditions in the pressure vessel. The particular operating conditions of a nuclear reactor lead to material stress in key components like the steam generators tubes but also reactor vessel heads made of Inconel-600. While the exact cause of the accident has not been confirmed yet, the nature of the guillotine rupture about 7.5 cm above the tube sheet has been clearly identified as triggered by significant circumferential and longitudinal cracking in the tube.
Thirteen single tube ruptures are listed before this accident. Cracks have shown up in PWRs and especially in those with steam generator tubes made from Inconel-600. Many operators have decided to replace aging and leaking steam generators while others have preferred to shut down the reactors for good once the condition of the steam generators had deteriorated beyond a certain number of plugged leaking tubes.
Contrary to other primary leaks, most of the water transferred from the primary to the secondary circuit might be lost outside of the reactor containment. The risk of a core melt accident is particularly high if a single or multiple tube rupture is combined with other degradations (station blackout, steam line break, etc.).
Many different degradation mechanisms have been identified. The speed and shape of crack development is very diversified and difficult to forecast.
Known Steam Generator Tube Rupture Accidents in the World 1975-2002
* military installation. Other SGTR on military installations could have occured but do not appear in this list because of the lack of informaton related to military installations incidents.
Source : WISE-Paris, «The Nuclear Steam Generator : The One Millimeter Containment», May 1997 ; IPSN 2000
- Personal communication with Kwanghoon Seok, Green Korea United, 8 June 2002. However the information was not communicated neither from the MOST nor from the KHNP
Back
to contents