Transactions, SMiRT-25 Charlotte, NC, USA, August 4-9, 2019 Division IV
CURRENT STATUS AND COUNTERMEASURES FOR KOREAN NUCLEAR POWER PLANTS AFTER GYEONGJU & POHANG EARTHQUAKE
Jaemoon Kim1, Jungmook Lim2, Heonwoo Kwon3
1 Senior Manager, Korea Hydro & Nuclear Power co., Ltd., Gyeongju, S. Korea ([email protected]) 2 General Manager, Korea Hydro & Nuclear Power co., Ltd., Gyeongju, S. Korea ([email protected]) 3 Vice President, Korea Hydro & Nuclear Power co., Ltd., Gyeongju, S. Korea ([email protected])
ABSTRACT
Korea Peninsula was accepted as a relative safety zone for earthquakes because of its intra-plate location and low seismicity records. However, Gyeongju(M5.8) and Pohang(M5.4) earthquakes occurred in 2017 and 2018 respectively.
On January 2017, after the Gyeongju earthquake, Korean government and utility established comprehensive countermeasures including 21 safety assessments, and Korea Hydro & Nuclear Power co. Ltd. (hereinafter “KHNP”) has finished 12 assessments by 2018. In addition, Korean government will re-evaluate seismic design of NPPs based on investigation of active faults in the earthquake area. According to the schedule of government investigation and re- evaluation, KHNP will develop seismic enhancement methodology for operating NPPs in Korea based on cooperation with the authority and government.
This paper is to explain the results of safety inspection after Gyeongju and Pohang earthquakes, detailed information regarding comprehensive seismic countermeasure and future plan.
INTRODUCTION
Korea Peninsula is located in the intra-plate region, 600km far from the Pacific earthquakes zone, which means that there is low probability of large earthquake in Korea unlike neighbouring country, Japan. Nevertheless, Nuclear power plant should be prepared for the worst earthquake scenario. Like NPPs in other nations, site investigation of 320km radius, qualified seismic design and strict construction management were conducted for Korean nuclear power plants in accordance with international regulation guide and Korean nuclear safety act.
The maximum seismic value on NPP site was set up by deterministic method with historical document review, observed seismic records and geological evaluation. The seismic design value for Korean NPPs was determined as 0.2g with the maximum value plus margin. For new model called APR1400, the seismic design value is upgraded to 0.3g for overseas business.
The design value is generally accepted as relatively high in South Korea because earthquake magnitude of 5.0 or more occurred only once every ten years. Also the highest earthquake magnitude before Gyeongju earthquake was 5.2 in 1978 and 2004. However, the magnitude 5.8 Gyeongju earthquake occurred on September 12, 2016. This was the highest earthquake record in Korea peninsula ever since it
25th Conference on Structural Mechanics in Reactor Technology Charlotte, NC, USA, August 4-9, 2019 Division IV was monitored. The epicenter was close to Gyeongju city (8.7km away) and nuclear power plant (28km away). In addition, the magnitude 5.4 Pohang earthquake occurred in Pohang city neighbouring Gyeongju on November 15, 2017.
Although the earthquakes did not affect the safety of nuclear power plants, they damaged other buildings and facilities. In addition, Gyeongju earthquake was felt in almost every part of the peninsular, which was enough to raise awareness of the earthquake and weaken public acceptance of NPP in Korean society. Korean government and KHNP developed seismic comprehensive countermeasures in four major fields; earthquake/geology, seismic performance, emergency response and others (Lim, 2017). Through the countermeasures, KHNP will re-evaluate existing seismic qualification of NPPs from the bases. Moreover, The Korean government is conducting an investigation of active faults in east-south part of the peninsula and will re-evaluate seismic design value of NPPs. In preparation for the result of government investigation and occurrence of beyond design basis earthquake, KHNP will develop seismic enhancement methodology based on cooperation with the authority and government.
Table 1: Design Ground Acceleration Value.
Design Ground Acceleration(g) / NPP Site Note Number of units
Kori 0.2 / 5 units 0.3g for Shin-kori 3, 4, 5, 6 (4 units)
Wolsong 0.2 / 6 units ※ 0.3g units are under construction without shin-kori #3(in operation) Hanbit 0.2 / 6 units
Hanul 0.2 / 6 units 0.3g for Shin-hanul 1, 2 (2 units)
GYEONGJU & POHANG EARTHQUAKES AND RESULTS OF SAFETY INSPECTION
The magnitude 5.8 Gyeongju earthquake occurred on September 12, 2016, was recorded as 0.0981g at Wolsong NPP which is the nearest NPP from the epicenter. The observed value was less than half of the design value, which did not affect the safety of the nuclear power plant. However, Korea Hydro & Nuclear Power Co., Ltd. (hereinafter KHNP) stopped the Wolsong Unit 1 ~ 4 manually according to the standard operating procedure because the observed value exceeded the OBE (Operating Basis Earthquake) response spectrum at certain periods.
Table 2: Observed Value & Distance for Gyeongju and Pohang Earthquake.
Earthquake Kori Hanbit Hanul Wolsong
Observed value for Gyeongju 0.0537 / 51 0.0045 / 254 0.0057 / 148 0.0981 / 27 earthquake(g)/distance(km)
Observed value for Pohang 0.0032 / 88 0.0045 / 276 0.0087 / 108 0.0134 / 45 earthquake(g)/distance(km)
25th Conference on Structural Mechanics in Reactor Technology Charlotte, NC, USA, August 4-9, 2019 Division IV KHNP conducted safety inspection and walk-down after the earthquake for Wolsong Units 1 to 4. The Authority also conducted safety inspection for Wolsong Units 1 to 4, and confirmed the integrity of the power plant and KHNP’s proper response according to the procedure and regulation. Wolsong NPP was approved for re-operation on December 5, 2016.
The magnitude 5.4 Pohang earthquake that occurred in neighbouring Pohang on November 15, 2017. It was recorded as 0.0134g at the nearest Wolsong NPP. The observed value was about 7% of the design value, which had no effect on the safe operation.
ESTABLISHMENT AND ENFORCEMENT OF EARTHQUAKE SAFETY ENHANCEMENT MEASURE
Taking 9.12 Gyeongju earthquake as a momentum, KHNP reviewed the safety of all nuclear power plants from the basis. Based on the results of the review, KHNP has established comprehensive seismic countermeasures including four major fields; earthquake / geology, seismic performance, emergency response and others. In the countermeasures, there are 20 detailed safety assessments which are currently in enforcing. 12 assessments are finished by 2018. Major safety enhancement measures are as follows
First, to ensure reliability of seismic design in the earthquake/geological field, KHNP is investing active faults near Wolsong NPP site apart from the current fault-investigation conducted by the government, undertaking a probabilistic seismic hazard analysis, including the Gyeongju and Pohang earthquake and re- evaluating hazard of all NPP. Through the earthquake / geological field, KHNP will verify the safety of the site and follow up measures such as seismic reinforcement if necessary.
Second, for the seismic reinforcement field, KHNP has completed improvement of seismic performance of core system and equipment of operating NPP to 0.3g or more from 0.2g. In order to confirm the safety against extreme external events, KHNP is conducting stress test for all nuclear power plants and plans to complement the vulnerability according to the test results.
In addition, KHNP has completed the optimization of the seismic monitoring instrument and will improve the coping ability by installing flood protection equipment (water-tight doors) for core equipment even in any external events of a disaster such as earthquake, tsunami and fire.
Third, for the field of emergency response, KHNP has revised the emergency response procedure and the manual which include the step-by-step action procedure and the training scenario, and will construct a complex disaster center equipped with the seismic isolated facility to guarantee the disaster coping ability in case of earthquake and nuclear accident. Additionally to respond to complex events, the information delivery system has improved and systemized in the event of a disaster.
Fourth, in the field of others, KHNP has strengthened the organization for seismic response such as seismic engineering office in headquarter and structural engineering team in NPPs. In order to verify the safety of nuclear power plants, we have received safety inspection from specialized international agencies such as WANO and IAEA. These inspections secure the objectivity of nuclear safety and enhance public acceptability of nuclear power plants.
Moreover, by installing an earthquake response research center specializing for nuclear power plants, we built and are operating a systematic response infrastructure based on diverse opinions from government, industry, and academia on the nuclear and seismic safety.
25th Conference on Structural Mechanics in Reactor Technology Charlotte, NC, USA, August 4-9, 2019 Division IV
Table 3: 20 Safety Assessments of Comprehensive Seismic Countermeasures (Lim, 2017).
Fields Assessments
Ⅰ. Earthquake / 1. Investigation for active faults/probabilistic seismic hazard analysis in Wolsong
Geology 2. Re-evaluation safety for NPP sites
3. Seismic enhancement for operating NPPs
4. Optimization of the seismic monitoring instrument
5. Installation watertight doors for operating NPPs
6. Stress Test for all NPPs Ⅱ. Seismic 7. Seismic Design/Performance enhancement Performance 8. Review of Seismic enhancement for NPPs under construction
9. Safety integrity for the containment building of Wolsong unit 1
10. Construction of Seismic storage battery room (Wolsong)
11. Construction of seismic Storage building for FLEX
12. Optimization of the emergency response procedure
Ⅲ. Emergency 13. Revision of earthquake response manual
Response 14. Construction complex disaster center equipped with seismic isolated facility
15. Improvement of information delivery system
16. Installation of seismic experimental equipment and infrastructure
17. Establishment of earthquake response research center
Ⅳ. Others 18. Inspection of IAEA SEED for Wolsong
19. WANO Inspection
20. Enhancement KHNP’s organization
25th Conference on Structural Mechanics in Reactor Technology Charlotte, NC, USA, August 4-9, 2019 Division IV
Table 4: Finished Safety Assessments until 2018.
Year Finished Safety Assessments
2017 7 assessments finished; #8, 13, 15, 17, 18, 19, 20 in Table 3
2018 5 assessments finished; #3, 4, 9, 11, 12 in Table 3
SEISMIC DESIGN/PERFORMANCE ENHANCEMENT
The seismic design of domestic nuclear power plants is based on the wide-area survey up to 320 km radius and site-precise geological survey, and the maximum seismic value that can affect the site is calculated to determine the seismic design level by adding technical margin.
The domestic nuclear power plants were designed to withstand 0.2g earthquake, and the design standard for the new nuclear power plants following Shin-kori unit 3 and 4 were designed to withstand 0.3g earthquake. After Fukushima accident, the seismic performance of operating NPPs designed to 0.2g has been upgraded to 0.3g for the core system of the 24 units (Kim, 2018).
In addition, providing for the government’s inspection of capable fault and an earthquake exceeding the design standard, KHNP is conducting seismic design/performance enhancement.
As for the step 1 in this study, overseas design/performance enhancement methodologies of nuclear power plants and examples of beyond design earthquakes were reviewed. In the step 2, the performance- based site specific ground motion response spectrum (GMRS) development methodology for domestic sites will be reviewed. And the step 3 consists of analyzing the seismic probabilistic risk analysis of the power plant based on the GMRS to confirm the NPP safety.
In addition, this year, KHNP has agreed with Kozo Keikaku engineering Inc. which is a Japanese institute specializing in the ground analysis for seismic enhancement technology cooperation. Moreover, KHNP signed in membership with CNEFS (Center for Nuclear Energy Facilities and Structures) of North Carolina State University dealing with seismic qualification, research and development.
Through this project, KHNP will develop an optimal seismic enhancement methodology and determine the seismic performance enhancement method to be applied to domestic nuclear power plants together with regulators and government to prepare for the occurrence of large-scale earthquake and the result of government active fault investigation.
CONCLUSION
KHNP is re-evaluating the existing seismic qualification based on the Gyeongju and Pohang earthquake, strengthening seismic qualification of major structures and equipment, and implementing countermeasures to strengthen seismic safety by building response bases for systematic response in the event of a disaster including developing a seismic enhancement methodology in preparation for the occurrence of a major earthquake to ensure the safety of NPPs in any event of disasters.
25th Conference on Structural Mechanics in Reactor Technology Charlotte, NC, USA, August 4-9, 2019 Division IV
REFERENCES
Kim, J. (2018). “Seismic design of nuclear power plants” Nuclear safety white book, Ministry of Science and IT, Korea. Lim, J. (2017). Detailed plan for comprehensive seismic countermeasures after Gyeongju earthquake. KHNP Report, Korea. Lim, J. (2017). “Seismic Countermeasures on Nuclear power plants” Nuclear Power white book, Ministry of Trade, Industry and Energy, Korea.