Second quarter of 2002
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
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
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