Experts warn of possibility of significant damage to generator
Korea Hydro and Nuclear Power (KHNP), which is in charge of the construction and management of domestic nuclear power plants, was revealed to have neglected for over a year the significant corrosion of a cooling system at the No. 6 nuclear generator in Uljin.
In a recent interview with the Hankyoreh, two executives of Korea Power Engineering Co., an affiliate of Korea Electric Power Corp. (KEPCO) that designs nuclear power plants, said that KHNP hid significant corrosion in tube joints of the cooling system at the No. 6 generator after finding it during a routine inspection between March 2 and 26 last year. The executives asked to remain anonymous.
According to an image of the cooling system, taken March 21 last year during the period of inspection and obtained by the Hankyoreh, significant corrosion is seen on the back side of the cooling system. An official of Korea Power Engineering Co., also asking to remain anonymous, said that during the inspection period, “the No. 6 generator was in trial operation. However, I heard KHNP hid the fact of corrosion on fears of delaying completion of the construction scheduled for August the same year.”
Responding to the revelation, Lim Hoon-taek, manager of KHNP’s department of technology, said, “We found light rust on the cooling system of Uljin nuclear power generator No. 6 during the inspection in March last year, but no corrosion was seen. During this year’s inspection, between March 25 and May 11, we found corrosion for the first time and removed it,” Lim said.
KHNP had previously belatedly admitted the existence of corrosion in the Uljin nuclear power generator No. 5, after the fact was reported by media.
Corrosion in a cooling system may prove critical for a nuclear power plant. KHNP’s web site said that “a power plant may be prompted to cease operations due to the possibility of corrosion in the tubes of its steam generator.” In an official letter to KEPCO in December 18, 1996, Korea Heavy Machinery (currently Doosan Heavy Machinery) warned that significant damage in plant operations would be done if water were released into a damaged cooling system.
Safety concerns are emerging as KHNP is building cooling systems for New Gori and New Wolseong nuclear power generators Nos. 1 and 2, with the same experimental methods used for Uljin Nos. 5 and 6 generators. KHNP said, "We plan to make tubes for the cooling system by welding super-stainless steel to titanium and a coating will be placed on the cover." However, experts say the method has not yet been proven safe.
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Regading the article above, Korea Hydro & Nuclear Power Co. said in reply that:
This is to rebut an article published in the Hankyoreh on July 24, 2006, under the title called “Corrosion cover-up revealed at nuclear power plant”.
Before we start, there is a need for a correction to be made to the names of component described in the article above. “No. 6 (nuclear) generator” and “generator No. 5” were mentioned in the article incorrectly. The correct description shall be “condenser” instead of “generator”. The presented article is NOT related to either steam generators or turbine-generators in any aspects.
In October last year the SBS (Seoul Broadcasting System) had reported the same issue, which incident had triggered not only a national assembly audit but also a Ministry of Commerce, Industry and Energy’s expert panel investigation with the experts representing the industry and academia. The expert panel investigation focused on the UCN unit 5 condenser operations and produced a final report in March 2006.
KHNP did not try to conceal the corrosion of the condenser.
During the semi overhaul of UCN unit 6 in March 2005, the corrosion was not detected because investigative effort was concentrated on plant structural integrity. The final report produced from the Ministry of Commerce, Industry and Energy had explained it in detail.
In the case of UCN unit 5’s condenser, it had already been repaired by removing the rust and re-coated during the outage of June 2005 before SBS made the report in October 2005.
If the rust at the unit 6 condenser had been detected during the semi overhaul of UCN unit 6 in March 2005, it was obvious that the same remedy would have been taken to repair the unit 6 condenser as the unit 5 was repaired. Therefore, it does not make sense that a cover up of corrosion was attempted to proceed with commercial operation.
The new materials have been used at the overseas power plants.
The article in the Hankyoreh asserts that KHNP had pushed through the use of the super stainless steel in place of the titanium alloy against the warning of using the super stainless steel. However, the decision was made based on a careful study of material wear resistance characteristics between the super stainless steel and the titanium alloy against steam and foreign particle impingement of the turbine exhaust. As a result, the use of the super stainless steel started in 1997.
Above all, it is not that KHNP has started to use the super stainless steel in an experimental basis, but that the overseas nuclear power plants have already been using it since early 1970’s.
The decision of using the new material was made after a sufficient discussion between KHNP and the manufacturer.
The Hankyoreh insists that KHNP has pushed through the use of the super stainless steel unilaterally.
Although there were differences in opinion when use of the super stainless steel was considered initially, it was merely a result of an open technical discussion addressing every possible option on the subject to bring out the most optimum conclusion. During the discussion, which took four (4) years and four (4) months from July 1994 to November 1998, various case studies on the overseas application have been made.
The initial reservation about using the super stainless steel was based on the first generation of super stainless steels. However, the final decision to use the super stainless steel was made after considering the improved material characteristics of the second generation material and the US industry experience of using AL-6XN, the second generation material, at more than twenty (20) power plants using seawater as a cooling medium.
Application of the new material to nuclear power plants was approved by the academia.
The results of investigation have ascertained that the selection of materials was not directly related to the corrosion, but improper sealing between condenser tubes and tube sheets caused base metal corrosion which bled out on the surface of condenser tube sheet.
For Shin-Kori 1&2 and Shin-Wolsong 1&2, a sealing modification between super stainless tubes and tube sheets has been implemented to prevent base metal corrosion by seawater intrusion.
Furthermore, ARC (Advanced Reinforced Composite) coating method applied to prevent the crevice corrosion was proved by the Ministry of Commerce, Industry and Energy’s expert panel to be effective.
Based on these judgments, super stainless steel tubes, which are superior in mechanical strength, wear resistance and ductility to titanium alloy tubes, were applied to Shin-Kori units 1&2 and Shin-Wolsong units 1&2. The tube materials for next units will be decided evaluating its merits of technical and economical considerations at the time of the procurement contract.
The corrosion in condenser is not related to a leakage of any radioactive materials.
Condenser is a large heat exchanger designed to cool the steam exhausted from a turbine. The steam in shell side is condensed by transferring its heat to seawater circulating in tube side. Since the condenser is a component isolated from the reactor coolant path, which contains radioactive materials, corrosion in condenser will not have adverse influence on a leakage of radioactive materials.
