 INES 1 |
Contamination of Demineralized Water System & Uncontrolled Minor Radioactivity Release |
Event Description
1. Circumstances at the initial stage of the Event
On December 22, 2003, at 23:32, just prior to the plant start-up, an "Alert" alarm of the radiation monitor (RM, RE-054) monitoring the radiation level of intake air of Technical Support Center HVAC actuated. Plant start-up was proceeding in parallel with checking of the cause of RM alarm actuation. Full power was achieved on December 24, 2003.
In the following days, from December 23 to December 27, the cause for RE-054 alert was investigated. The aerosol monitor, RE-054, which detected the increased radiation was checked for a possible failure of the installed equipment. In addition entrance air samples were analyzed. No airborne radiation was detected and it was verified that the function of RE-054 was normal.
The investigation was expanded and on Dec. 27, 2003, a high radiation level was detected near the humidifier overflow piping located at the Technical Support Centre (TSC) room. Because the humidifier is provided with demineralized water, it was suggested that the high radiation level was caused by contamination of the Demineralized Water System (DWS), the DWS is a non-radioactive system that has to be kept free of contamination.
To verify the cause of the radiation, the pipings of the DWS was subsequently surveyed. Finally, on December 27, 2003, a very high radiation level was detected at the valve connected to the Train A of the Post Accident Sampling System (PASS) interface.
After recognition of the contamination of DWS, the Yonggwang Unit 5 was decreased the power and shutdown at 19:00 on December 29, 2003 to scrutinize the cause of the contamination of DWS and to conduct the further actions including the prevention of contamination spread and decontamination.
2. Follow-up Actions and Investigation Details
On December 28, 2003, the event was notified to the regulatory body of Korea, Ministry of Science & Technology (MOST).
The Korea Institute of Nuclear Safety (KINS) conducted the investigation following the notification under the delegation of MOST, for 1) the cause and the range of the contamination, 2) the quantity of released radioactive material to the environment, 3) the effect to the plant workers and vicinity residents by the contamination and release, and 4) the adequacy of plant counter activities for the event.
As a result of the investigation for the cause and leakage path, it was verified that the leakage occurred through the interfaces between the PASS connected with the SCS and DWS.
Further investigation confirmed that, due to the leakage, the six (6) systems including Condensor A hotwell and several humidifiers supplied demi-water from the DWS were contaminated and water drained from those contaminated equipments and systems flowed into the corresponding sumps, collected general water treatment sump and sewage water sump and discharged to the sea via the drainage water pathway.
Upon recognition of contamination of the central water treatment sump and sewage water sump, the discharge path from those facilities to the sea were stopped on December 27, 2003 and on January 2, 2004, respectively.
3. Safety Assessment
As a consequence of the event, the quantity of g-radioactivities released to the sea was estimated as 64MBq (1.73mCi) and the assessed radiological effects by this event to the plant workers and vicinity residents were negligible.
Also, it was evaluated that the dose for the inhabitants at the site exclusion area boundary was about 4.93E-07mSv which is approximately equivalent to 1/60,000 of the regulatory limit, 0.03mSv, based on the calculated released inventory composed of Co-58(60.1MBq), Mn-54(4.06MBq), and H-3(1,790MBq). In addition, it was confirmed that there was no specific increment in space radioactivity based on trend check for the records of radiation monitors installed around the site.
For the effect to the plant workers, summoned for the investigation and follow-up actions including system flushing and decontamination activities, it was evaluated that the maximum dose for a worker was 0.18mSv which is equivalent to the 1/100 of annual average dose regulatory limit, 20mSv, and collective dose was estimated as 1.06mSv. In addition, the internal exposure doses for the plant workers exhibited no indication of specific radioactive nuclide.
Event date: Tue, 23-12-2003
