Observations

Since the 2011 off-the-Pacific-coast-of-Tohoku earthquake, there have been initiatives to improve the seismic safety of nuclear power plants, and the perception about the necessity of an ASTS seems also to be changing.

In the Republic of Korea, their improvement goals include ‘ensuring that reactors are capable of safe shutdown even in an earthquake greater than the design basis level’. The introduction of the ASTS was recommended as a short-term response to be completed by 2012. More specifically, all the reactors will undergo improvement work so that they will automatically shut down upon detection of an earthquake at the level of 0.18 g or higher.

In India, while at present only a limited number of nuclear power plants (RAPS 1-2 and NAPS 1-2) feature a system that automatically shuts down the reactor in an earthquake event, as discussed in Section 2.4.6.3, it has been recommended to introduce the system to all the nuclear power plants.

While European Union nations have conducted stress tests on their nuclear plants since the 2011 off-the-Pacific-coast-of-Tohoku earthquake, some of the countries are considering the improvement of seismic instrumentation systems as a future task. Future tasks cited are outlined below, as an overview of post stress-test national action plans regarding seismic instrumentation.

As part of the European Nuclear Safety Regulators Group (ENSREG) Action Plan regarding the follow-up of the peer review of the stress tests performed on European nuclear power plants.

The ENSREG decided to prepare by September 2012, a consistent compilation of stress test peer review recommendations and suggestions to assist the preparation or review of national action plans by national regulators.

ENSREG summarized the experience of stress tests and peer review of the stress tests performed on the European nuclear power plants [28]. ENSREG recommended installation of seismic monitoring systems with related procedures and training (paragraph 3.1.5 Seismic Monitoring in Ref. [28]).

In the framework of national stress tests and/or in response to ENSREG recommendations the seismic instrumentation of plants has been reviewed and actions have been defined. The actions are of different type as follows:

 Improvement of the seismic instrumentation, ensuring compliance to the national or international requirements (Belgium, Czech Republic, Finland, France);

 Analysis of arguments for automatic seismic scram, its advantages and disadvantages (Hungary, France, Switzerland);

 Considerations for amendment of existing or development of new regulatory requirements related to seismic instrumentation (Finland, Slovenia, Germany);

 Comparative study national versus international practice (France).

Many countries judged the plant seismic instrumentation and procedures for operator post-earthquake actions as appropriate (Bulgaria, Romania, Slovenia, Spain, Sweden and Hungary).

Following the ENSREG recommendation, some countries included into the plant’s seismic instrumentation the regional (micro) seismic monitoring network and official institutions made it available to plant operators the national seismic network monitoring records (Slovakia, United Kingdom).

2.4.8. Future requirements and recommendations 2.4.8.1. Decision making process

A key element in the future decision-making process is the performance of systematic evaluations of the potential benefits and drawbacks of implementing an ASTS. Probabilistic safety assessment approaches for generic nuclear power plant types, but treating specific sites/plants, are recommended.

2.4.8.2. DIP application

Many of the ASTSs currently in use employ seismic switches, but there is a possibility that a system integrating a digital SDAS and an ASTS will be adopted in the future, with such a suggestion already being addressed. However, it is not necessarily appropriate to use the acceleration values themselves, as recorded with the SDAS, in the trigger settings for the ASTS, without any other evaluation.

In case that a system recording earthquake acceleration time-history is used for the ASTS, there is a need for measures to ensure the activation of reactor scram and preventing unnecessary reactor scrams. This includes introducing redundancy (number and layout of acceleration sensors), ensuring seismic capacity and enforcing the specifications required for quality assurance, as well as paying special attention to spurious scram. Records taken with accelerometers include acceleration noise or acceleration spikes from earthquake motions, however they possibly have no influence on the damage to SSCs. It is therefore necessary to introduce measures for preventing undesired activation through, for example, installation of frequency filters.

The objective of installing an ASTS (especially in Case A of Section 2.4.4) is to shut down a reactor upon detecting earthquake motion severity greater than the pre-defined level, which could lead to the functional damage of safety-related facilities. As discussed later in sections 3 and 4, DIP is more suitable than acceleration as indicator to be used for this purpose.

If an earthquake motion parameter is used as the ASTS trigger signal, the parameter needs to be observed or calculated in real time during an earthquake. Challenges surrounding such real time calculation of the DIPs proposed by this publication are discussed in Section 4.1.2 for the CAV, and in Section 4.1.3 for the Japanese instrumental intensity AJMA (IJMA). The challenges are also related with the data processing capacity of computers to be used for the SDAS.

Through comprehensive considerations on specific system configuration, specifications, etc., The adoption of a DIP-based trigger signal is desirable.

2.4.8.3. Prediction capability

The most desirable concept of an ASTS is that it were possible to predict earthquake motion severity and, in the case significant damage was predicted, to complete reactor scram before the main component of the strong earthquake motions arrived [22].

In Japan, since October 2007, the Meteorological Agency has announced earthquake early warnings for disaster preparedness, recommending that as many possible preparedness measures be implemented before the arrival of the strong motions. This system was developed jointly by the Meteorological Agency and a railway company (Japan Railways East). Japan Railways East's ‘Urgent Earthquake Detection and Alarm System’ successfully initiated the emergency shutdown of Tohoku Shinkansen operations at the time of the 2011 off-the-Pacific-coast-of-Tohoku earthquake, thereby preventing an accident.

In the nuclear field, as described in Section 2.3, there was an example of actually introducing the ASTS from a SAS. However, in these cases, it is necessary to carefully evaluate the balance of the reliability of prediction and that of plant ‘scrammability’, at the anticipated level of earthquake motions, because an unnecessary reactor scram could cause a transient to undermine reactor safety and produce inconvenience to the society.

The earthquake early warning system, currently adopted by Japan Railways East, predicts the earthquake magnitude and hypocentral distance based on the starting gradient of absolute acceleration for P-waves. Discussion needs to be also held on whether experience-based data is applicable to a different region (country) with different seismic characteristics.

From the perspective of prediction reliability, anticipated topics for debate include as follows:

(a) Using international data, as the base data for prediction varies according to seismic characteristics and regions (countries);

(b) Considering the impact of background noise in coastal regions with numerous industrial facilities located, and

(c) Availability of only a small amount of data for countries with a low seismic activity, thereby influencing the accuracy of prediction.

While the time required to recover a train service from the state of shutdown is relatively small, it takes much longer to restore a shutdown reactor. The shutdown could have an extremely large social impact, including the possible discontinuation of electricity supplies. Since the possibility of inadvertent scram undermining reactor safety cannot be denied, actual performance data such as at the Onagawa nuclear power plant, where the system is already used for alarm activation, is expected to be accumulated further for more discussions prior to employing the system in the nuclear field.

3. PARAMETERS INDICATING EARTHQUAKE MOTION INTENSITY AND