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2006.06.28 Hee Geun Kim Radiation and Chemistry Group Nuclear Power Laboratory

Operational Experiences and Occupational Dose Assessment of the Radioactivity Release Event at YG NPP Unit 5. 2006.06.28 Hee Geun Kim Radiation and Chemistry Group Nuclear Power Laboratory Korea Electric Power Research Institute. Topics for Presentation. Introduction

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2006.06.28 Hee Geun Kim Radiation and Chemistry Group Nuclear Power Laboratory

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  1. Operational Experiences and Occupational Dose Assessment of the Radioactivity Release Event at YG NPP Unit 5 2006.06.28 Hee Geun Kim Radiation and Chemistry Group Nuclear Power Laboratory Korea Electric Power Research Institute

  2. Topics for Presentation • Introduction • Description of the Release Event at YG Unit 5 • Details on the Event and Countermeasures • Occupational Dose Assessment • Summary

  3. Installed Condition Power Plants Generating Comment Capacity(MW) Kori #1 587 Westinghouse Kori #2 650 ” Kori #3 950 ” Kori #4 950 ” Wolsong #1 679 CANDU Wolsong #2 700 ” Wolsong #3 700 ” Wolsong #4 700 ” In Yonggwang #1 950 Westinghouse Operation Yonggwang #2 950 ” Yonggwang #3 1,000 CE Yonggwang #4 1,000 ” Yonggwang #5 1,000 ” 1,000 CE Yonggwang #6 Ulchin #1 950 Framatome Ulchin #2 950 ” Ulchin #3 1,000 CE Ulchin #4 1,000 ” Ulchin #5 1,000 ” Ulchin #6 1,000 ” 20 Total 16,716 1. Introduction-General • Activity of Korean Nuclear Power Plants

  4. operation(units) man-mSv 18000 20 man-mSv 18 16000 In operation(units) 16 14000 14 12000 12 10000 10 8000 8 6000 6 4000 4 Year 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 1. Introduction-General • No. of Korean NPPs and Occupational Radiation Exposure

  5. 1. Introduction-General • Annual Average Dose (man-Sv/unit and mSv/man)

  6. 1. Introduction-General • Annual Soild Radioactive Waste (Drums/unit)

  7. 1. Introduction-YG NPPs The View of Yonggwang Nuclear Power Plants

  8. 1. Introduction-YG NPPs • Yonggwang Nuclear Power Plant Unit 5 • Located in the western sea side of Korean peninsular (The Republic of Korea) • Design Criteria: 10CFR50 Appendix A(GDC for NPPs) and Korean Radiation Protection Criteria (ex; Reduced dose limits(ICRP-60, etc)) • CE type and Pressurized Water Reactor (PWR) • 2,825 MWth(1,000 MWe) Power • 2 Loop Steam Generator/2,250 psia • 4 Reactor Coolant Pumps, etc

  9. 2. Description of the Release Event • Description of the Release Event at YG Unit 5 [1] • December 21, 2003: Disconnection from the grid to perform a normal shutdown (S/D) process. The S/D system is connected to the primary Rector Coolant System (RCS) at a pressure of approximately 395 psig(29kg/cm2 absolute). • December 22: Increased radiation level of aerosol monitor(RE-054). The alert was indicated in the Technical Support Center (TSC) and reported to the MCR. The alarm status, that would have required immediate actions to be taken, was not reached. • December 23 to 27 : Investigation of the cause for this alert by HP staff. Check of the possible failure of the RMS (electrical and radiogical, etc).air samples and No airborne radiation. The cause for the alert was not identified at that time.

  10. 2. Description of the Release Event • Description of the Release Event at YG Unit 5 [2] • December 23: Restart and 100% power on December 24. • December 27: A high radiation level in the TSC. The point of measurement was near the humidifier overflow piping. The DWS is a non-radioactive system that has to be kept free of contamination. To verify the cause of the radiation the piping of the DWS was subsequently measured. Finally a very high dose level was detected at the valve in the PASS interface. The Post Accident Sample System (PASS) is connected to the SCS, that can be contaminated by the primary reactor coolant, and to the clean (non-radioactive) DWS. • December 29: shutdown for additional checks and investigations to detect the cause of the high radiation level.

  11. 2. Description of the Release Event • Description of the Release Event at YG Unit 5 [3] • December 29 to 31: The PASS interface was checked to verify the possibility of contaminated fluid leaking from the S/D system into DWS. Leak rate tests were performed on site. A raised internal leak rate of one isolation valve (V 041) in the PASS interface was found. The allowed leak maximum of 1.5 cm3/h (under specified test conditions) was very clearly exceeded by a measured leak rate of approximately 600 cm3/h. The leaking valve was dismantled and later disassembled by service staff. A belt of hardened suspended foreign material was found on the valve disc inside the valve. The valve was cleaned and foreign material was removed. After cleaning, reassembling and fitting in the valve, leak rate tests were performed again. The measurements showed no valve leakage. Due to the contamination a higher level of radiation in the piping remained. The contamination had to be eliminated by cleaning the piping, and in part by exchanging pipes, after the radioactivity release event.

  12. 2. Description of the Release Event The leakage pathways for contamination of SCS, PASS and DWS

  13. 2. Description of the Release Event The release pathway of contaminated water to the offsite • Release activity: 64 MBq (1.73 mCi) • Public dose: 4.93x10-7 mSv (Dose limit: 0.03 mSv) • Occupational dose: 0.18 mSv (Dose limit: 100 mSv/5yr)

  14. 3. Details on the Event and Countermeasures • Flow Path • No radiation monitoring that could directly indicate the contamination and the spread of radioactivity in the DWS • The contamination of the DWS was detected later by radiation measurement taken to investigate the cause of alert of RMS • In further investigation it was verified by valve leakage tests performed at PASS interface that a small flow contaminated coolant caused by an internal valve leakage could pass this interface and could contaminate the clean secondary system • Flow path: SCS (Train A) → SCS Isolation Valve V041 → Check Valve V1018 → Check valve V1016 → Isolation Valve V098 → Check Valve V1020 → DWS Isolation Valve V097 → DWS • The related systems: Shutdown Cooling system (SCS), Post accident Sample System Interface (PASS) and Demineralized Water System (DWS)

  15. 3. Details on the Event and Countermeasures • Description of the Related Systems • SCS : During normal power conditions the SCS is isolated from the RCS and not pressurized. During shutdown conditions SCS is operated and pressurized to 395 psig ( 29 kg/cm2 absolute) maximum. Furthermore SCS is operated during the refueling or safety injection. During the long term cooling mode is pressurized to approximately 57 psig (4.5 kg/cm2 absolute) • PASS : The system is designed to operate during post-accident conditions, the sampling pump was operated so far only for some minutes for testing (every 6 months). During normal, shutdown and restart the PASS is not operated but in standby. • DWS : The DWS is in use during all operational modes of the plant. The pressure of the DWS is controlled to 100 psig (8kg/cm2 absolute) at all operational modes.

  16. 3. Details on the Event and Countermeasures • Valve Leakage Tests • The Staff of Unit 5 has performed several valve leakage tests to identify the discharge point of contaminated water into DWS. The tightness of the isolation valves in the PASS interface was verified. For these valve a maximum leak rate of 1.5 cm3/hr under full design pressure (RCS pressure) is acceptable by specification. The leakage tests were performed with different measurement conditions and constellations. • The maximum leakage of 600 cm3/h was measured at collection point A; Isolation valve V041 closed, isolation valve V098 opened, slow pressurization up to 28 hPa (over 17 min). • These tests were attended by the valve supplier and an independent expert organization (KINS).

  17. 3. Details on the Event and Countermeasures • Foreign Material and Valve Design • A belt of foreign material of isolation valve V041 was found. The Schematic Diagram of pilot solenoid valve

  18. 3. Details on the Event and Countermeasures • RMS Alert Response Actions • On Dec. 22 the radiation alert occurred at radiation monitor (RE-054) at TSC. The indicated value reached 0.1 Bq/cm3 (alert: 0.057Bq/cm3; alarm: 0.57 Bq/cm3) • The alert and alarm set point are generally defined to protect workers. Based on 10CFR20 the alert set point are usually a fraction (e.g. 1/10) of the alarm set point value. • The alert was identified by EO at MCR, reported to SRO and finally notified HP. • Response and actions : check RE-054 and RE-255(showed background levels). contradiction to the monitor readings. check the dose rate. check the electrical functions of RE-054. air samples. Change (new) and switch the detector of Unit 6. • No air contamination and opening the ceiling. 10,000cpm near the piping and check the T/B sump. DWS was finally identified as source of the contamination.

  19. 3. Details on the Event and Countermeasures • Monitoring of Uncontaminated Systems • No monitoring of the clean systems was performed prior to the incident. This caused a delay of contamination recognition. • The water treatment plants (general water treatment facility, sewage water facility) are not equipped with radiation monitoring systems. • During the incident and after detection of the contamination, water samples from these plants were analyzed and their further discharge was stopped. The samples showed contamination level between 1 and 15 Bq/L for Co-58 and Mn-54. • As a results of the incident and as prevention against reoccurrence the sanitary and water treatment facility are sampled weekly. The storm (rain water) drainage is sampled monthly.

  20. 3. Details on the Event and Countermeasures • Environmental Monitoring • After identification of the spreading of radio nuclides via water pathways to the environment, the affected water treatment facilities stopped their discharge into the sea. • Water samples were taken and analyzed for gamma emitting radio nuclides. • The amount of treated and discharged wastewater is measured and documented on a daily basis. This data was and still is collected in a manner independent of any incident, so that this data is available for the whole time over which the incident developed. • The amount of discharged water: 3,529 ton via central waste water facility and 1,329 ton via sewage water treatment facility.

  21. 3. Details on the Event and Countermeasures • Environmental Monitoring • The dose from seafish consumption is calculated: 0.131 µSv [The Co-58 data: 79.3kg(mean value of Korea), DCF from ICRP-2001, 1000 (bioaccumulation factor), etc] • The dose from shellfish consumption is calculated: 0.145 µSv [The Co-58 data: 17.6kg(mean value of Korea), DCF from ICRP-2001, 5000 (bioaccumulation factor), etc] • The dose from algae (seaweed) consumption is calculated: 0.261 µSv [The Co-58 data: 15.8kg(mean value of Korea), DCF from ICRP-2001, 10000 (bioaccumulation factor), etc] • The calculated dose for the released Mn-54 is: 0.023 µSv. • The total effective dose over both nuclides and over the three exposure pathways is: 0.56 µSv.

  22. 4. Occupational Dose Assessment • External dose assessment for worker : Radioactivity release to DWS → Contamination of DWS and turbine building ( Liquid Waste System) → Radiation dose control and measurement of contaminated area → Time tracking for RCA and dose assessment (Establishment/set up for temporary RCA at contaminated area, such as Humidifier or T/B, etc) → Dose assessment by calculation based on record and judgement • ☞ The maximum dose is 0.18 mSv(18 mrem) Investigation of radiation dose rate and working condition Investigation of worker and working time (Record, RWP, and Interview, etc.) Occupational dose assessment based on review and onsite investigation Investigation of dose record by ADR

  23. 4. Occupational Dose Assessment • Internal dose assessment for workers • Internal dose assessment based on investigation of Radiation Work Permit (RWP) / Work tracking system (83 persons) and drinking of demineralized water (4 persons) : no internal dose Internally deposited gamma radionuclide measurement using WBC Internally deposited tritium radionuclide measurement using LSC Radioactivity measurement of contaminated water Intercomparison Intercomparison Intake estimation based on air concentration of working area Intake estimation of drunken person of demineralized water Intercomparison and confirmation of intake and committed effective dose

  24. 4. Occupational Dose Assessment • The schematic diagram of internal dose assessment for internally contaminated radionuclide using whole body counter (WBC) Pre-amplifier Report Pre-amplifier High Voltage Unit ○ Main Amplifier ○ MCA Board Assessment of committed effective dose by internal dose computer code Measurement by WBC Internal activity (M) Choice of Metabolic data Intake assessment Whole Body Counter Model : FASTSCAN-2250 Geometry : Stand Type Genic PC : O/S-2 Radionuclide analysis program : Abacos KIDAC/IMBA Program Operating Mode - Intake assessment - Dose assessment

  25. 4. Occupational Dose Assessment • Radioactivity measurement and dose assessment from tritium intake using urine samples and liquid scintillation counter (LSC) Selection/ Request of Urine Submitted Intake and CED assessment / confirmation by dose tracking system

  26. 5. Summary • The incident leads to a contamination of non-nuclear (clean) system, DWS, by reactor coolant in the SCS. • The leakage was caused by incomplete closure due to the deposition of foreign material inside the valve. • The contamination was not recognized immediately due to no RMS at DWS. Only after analyzing an alert of RE-054 the contamination source finally identified and spreading through DWS stopped. • There were time delay due to followings: No direct monitoring of the DWS. Indirect alert and misleading. Only alert signal (no alarm). • Response and actions: Installation of quick disconnect coupling (physical separation). A systematic review of all possible flow path from RCS to secondary system. Valve replacement plan for bidirectional flow isolation. • The complete review of sampling/environmental monitoring program and uncontrolled discharge over the water pathway. • Radiation monitoring improvement (ex, water treatment system) and timely action/measures of contamination.

  27. Thank you very much for your attention !!!

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