Safety

Seismic safety under scrutiny in Taiwan

29 May 2009



After two major earthquakes shook nuclear power plants in Asia, the seismic safety of Taiwan’s existing nuclear power plants was re-examined. The analysis examined at US-orientated seismic designs/regulations from a probabilistic risk point of view. It also looked at earthquake shutdown criteria, especially the CAV parameter and its threshold value. By Ting Chow, Yuan Chieh Wu, and Yun Chau Gau


On 26 December 2006, two consecutive strong earthquakes both with local magnitude of 7.0 hit the most southern part of Taiwan, Hengchun village, where the Maanshan nuclear power station is located. This is was the strongest earthquake ever experienced by Taiwan’s existing nuclear power units, and it raised public concerns about the seismic safety of the nuclear power plant operation.

More recently, on 16 July 2007, in Japan, where the earthquake focal mechanisms are very similar to those in Taiwan, all seven reactors at the Kashiwazaki-Kariwa site were struck by a more devastating earthquake: the Niigataken Chuetsu-oki (NCO). Because the earthquake design model for all the nuclear units was exceeded during the event, the assurance of good seismic design and the appropriatness post-earthquake actions in Taiwan became even more urgent.

There are four nuclear power plants in Taiwan, each with two reactor units. Three are operating: Chinshan (GE/BWR4/Mark I containment); Kuosheng (GE/BWR6/Mark III containment); and Maanshan (W/PWR prestressed containment). Lungmen (GE/ABWR/RCCV) is under construction. The map shows the locations of these nuclear installations.

Taiwan is located at a complex juncture between the Eurasian Plate and Philippine Sea Plate, where earthquakes occur frequently. Due to the collision of these two plates, the eastern part of Taiwan moves toward northwest at a rate of about 2.5-8.0cm/year, based on GPS measurements since 1993. Hence, the seismic design of structures, systems and components (SSCs) of nuclear power plants in Taiwan is an important issue.

Although Taiwan is prone to earthquakes, up to now, the country has not experienced an earthquake strong enough to challenge the seismic design of its nuclear power plants. Even the biggest earthquake in 100 years, the magnitude 7.3 Chi-Chi quake on 21 September 1999, had no impact on the three existing plants due to the great hypocentre distance (more than 175km below the earth’s surface).

The only earthquake to affect nuclear power plants in Taiwan was the 2006 Hengchun earthquake, which occurred in the southwestern seabed. Figure 1 and 2 show earthquake data recorded at Maanshan (34km away from the epicentre). From the response spectrum comparison, it can be seen that the striking earthquake (dark blue line) almost reached the operating basis earthquake level (light blue line).Points where the earthquake approached design tolerances are circled in red.


fig1
Figure 1: Response spectrum 26 December 2006, 20:26:32

fig2
Figure 2: Response spectrum 26?December 2006, 20:34:28

During the earthquake, the operator in unit 2 manually shut down the plant due to high vibration of main turbine, while the operator in unit 1 decided to wait and did not take any actions

Seismic designs and regulations

Since all the Taiwan’s current 6+2 nuclear units are of US origin, the regulatory rules and guides used for design and review are all based on the US Nuclear Regulatory Commission (NRC) related documents. There are two design earthquakes, the operating basis earthquake (OBE) and safe shutdown earthquake (SSE). OBE is very similar to S1 and SSE is very similar to S2 of Japan, although they are not the same. All the SSCs that are designed against the OBE must be in the elastic range. The safety-related SSCs should be designed against SSE without losing the intended functions by allowing plastic deformation of the structural materials. The SSE is the maximum potential earthquake of the selected site and the OBE will be decided accordingly. Usually OBE will be half of the SSE although there is a conditional relaxation to the OBE/SSE ratio. The determination of design earthquake of the nuclear power plant is based on the requirement set forth in 10 CFR 100 Appendix A [1]. Table 1 is the summary of the design earthquakes for Taiwan’s nuclear power plants. It should be noted that control point of the design earthquake is the free surface of the reactor foundation.

Earthquake shutdown criteria and ASTS

According to Japan’s Ministry of International Trade and Industry Ordinance 62, every Japanese nuclear power plant unit installed an automatic seismic trip system (ASTS). Unlike Japan, Taiwanese law does not require all nuclear reactors to have an ASTS installed. Every Taiwan nuclear power plant, following the US rules, has to be shut down in an orderly manner when an earthquake strike exceeds the OBE. The plant has four hours to determine whether the OBE has been exceeded. Once the plant has been shut down in this manner, it can take at least three months, or even longer, to return the plant to the power grid after the inspection and review process, based upon US and Japan seismic shutdown experiences. To avoid unnecessary plant shutdown experiences in the USA, a parameter called cumulative absolute velocity (CAV) that is calculated from the earthquake acceleration histogram had been introduced and adopted as one of the seismic shutdown criteria [2,3]. The criterion uses two checks of earthquake data in the free-field. The first check compares the recorded response spectra to that of the OBE, and the second check compares the computed CAV of free-field response against a 0.16g-sec threshold value.

In the USA, only two plants in California – Diablo Canyon (DCPP) and San Onofre (SONGS) – have installed the ASTS. There, the two ASTS implemented were not the result of NRC regulation, but were remnants of advisory committee for reactor safety (ACRS) concern about the use of equipment beyond the level of qualification in an active seismic region.

In Taiwan, it was the disastrous Chi-Chi earthquake that prompted Taiwan’s nuclear authority to request Taiwan Power Company to install the ASTS in all six existing nuclear units. But it was the Hengchun earthquake, and recently experiences of Japan’s magnitude 6.8 Niigataken Chuetsu-oki earthquake in July 2007, that really pushed for the installation of the ASTS.

There are three sub-units in Taiwan’s ASTS: seismic sensors, data processors, and trip control module. Seismic sensors are installed on both the basemat floor and the operating floor in major (reactor/auxiliary) buildings. The number of seismic sensors on each floor depends on plant’s reactor protection system (RPS) trip logic: four on each floor for BWRs and three for PWRs. The trip threshold value is OBE’s zero period acceleration (ZPA) value minus 0.05g. Once the seismic sensors pick up the seismic wave filtered through a low pass filter of 10Hz that is higher than the threshold value, and passed the trip logic tree, then the RPS will be initiated and shut down the reactor immediately. Installations and tests of ASTS were completed for all six operating units in November 2007 and have been put into service since then.

The design response spectra at free ground surface and the associated CAV threshold values in Taiwan’s nuclear power plants are yet to be determined. So the Taiwan regulator’s interim position for the OBE transgression criteria calls for both the ASTS to be activated, and the PSA values in various floors to exceed the design value.

In 1985, the NRC issued a severe accident policy statement, and as a result, all US nuclear power plants were required to conduct an individual plant examination of external events (IPEEE). The seismic portion of the IPEEE addressed the fundamental question: “How large an earthquake can a specific plant sustain, and then safely shutdown?” Seismic probabilistic risk assessment (PRA) turned out to be a solution to the question and has been used together with installation of ASTS. For example, seismic PRA and installation of ASTS was an acceptable solution to the discovery of the Hosgri fault after completion of the DCPP in California.

Seismic hazard analysisin Taiwan

Taiwan launched its first full scope level 2 PRA in 1982, and completed all three existing nuclear power plant PRAs in 1985, 1987 and 1991, respectively. In addition, living PRA for all three plants and shutdown PRA for Maanshan were also accomplished in 1995 and 1996, respectively. The seismic PRA adopted so-called Zion method, which mainly relied on the existing seismic design/qualification documents and generic earthquake test/experience fragility data. Before the Hengchun earthquake, the determined maximum potential earthquake in the subzone to which Maanshan belongs was 5.9. The Hengchun earthquake rewrote this number to 7.3. But the result of the preliminary seismic hazard reanalysis, based on Taiwan’s earthquake catalogue from 1900 to 2007, and using the same Campbell’s attenuation equations and same zoning scheme, showed little difference in the seismic risk to Maanshan.

Lessons learned

Although the follow-up plant walk down survey and seismic hazard reanalysis showed no impact to Maanshan, the free-field CAV is much higher than the proposed shutdown threshold value 0.16g-sec., and the 7.0 earthquake magnitude was much more than original estimation of its maximum capability in that deep subzone. The different response of the operators in two units also raised some concern about the shutdown timing.

The largest earthquake affecting nuclear power plants so far, the Japanese NCO earthquake, caused all seven reactors at Kashiwazaki-Kariwa to shut down. They shut down not only because the devastating earthquake triggered the ASTS but also because it challenged the Japanese design earthquake S2. From the public available information, for example, in [4,5], as well as the private communications during our visit to Japan last March, the following facts of the event should be noted. The location and magnitude of this earthquake has never been expected in the design of the affected nuclear power plant. It was the ASTS that shut down the plant at the earliest timing, and avoided a possible disaster. The inherit safety margin of plant’s SSCs kept the plant in a cold shutdown state after the hit of the earthquake, which exceeded S2. While failures of some non-safety SSCs caused unexpected damages to the plant. The new design earthquake standards are all higher than original design earthquake S2 for all 55 existing nuclear units in Japan.

Both Hengchun and NCO earthquake experiences have taught us a number of lessons.

• The knowledge of geology, seismic tectonic plates and earthquake source mechanisms generally around nuclear power plants, especially in nearby sea regions, were found to be insufficient, and as a result, it is very difficult to predict/determine the possible fault mechanism surrounding a chosen site.

• For a plant’s safety, it is vital to have the ASTS installed.

• PRA methodology provides a good tool to evaluate the impact caused by new finding fault and the residual risk, ie the risk of beyond SSE (or S2).

• The CAV parameter is believed to be a good index for the damage estimation, but the current 0.16g-sec CAV threshold value seemed to be too conservative for OBE transgression criteria. (Our study on the CAV showed that 0.2g RG1.60 response spectrum has roughly a 0.60-0.70g-sec CAV value).

• The vulnerable SSCs, such as the fire water system, HVAC passes, penetrations, etc, and rotating machines such as turbine rotors and pump motors, should be structurally enhanced.

• A well-designed plant should have substantial safety margin.


Author Info:

Institute of Nuclear Energy Research, Atomic Energy Council, P.O. Box 3-21, Lung-Tan, Tao-Yuan, Taiwan, Republic of China. Tel. +886-3-471-1400 ext. 2553, Fax. +886-3-471-3839, E-mail: tchow@iner.gov.tw

Related Articles
Diablo Canyon would survive earthquake from new offshore faultline
Kashiwazaki-Kariwa-7: commercial operation eyed for mid-summer
IAEA report on Kashiwazaki-Kariwa

References

[1] US NRC, Appendix A to 10 CFR 100, "Seismic and Geologic Siting Criteria for Nuclear Power Plants"(1997)

[2] EPRI NP-5930, "criterion for Determining Exceedance of the Operating Basis Earthquake" (1988)

[3] US NRC, RG 1.166,"Pre-earthquake Planning and Immediate Nuclear Power Plant Operator Post-earthquake Actions" (1997)

[4] IAEA, "Follow-up IAEA Mission in relation to the Findings and Lessons Learned from the 16 July Earthquake at Kashiwazaki-Kariwa NPP" (2008)

[5] Tokyo Electric Power Company English Website, http://www.tepco.jp/en/index-e.html



Tables

Table 1 - Design earthquakes of Taiwan's nuclear power plants

map map
fig1 fig1
fig2 fig2
fig3 fig3
Maanshan Maanshan


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