Important considerations for automatic seismic trip systems

In principle, there may be two cases of ASTSs trigger motion level for an ASTS, considered in relation with the objectives addressed in Section 2.4.1, and depending on the ‘scrammability’

of each nuclear power plant.

 Case A: higher trigger level

 ‘Scrammability’ of nuclear power plant is proved for the SL-2 earthquake motion level

 Objectives: to confirm integrity of safety-related SSCs after an earthquake

 Trigger Level: SL-1 or SL-2, according to the design earthquake motion level of the engineered safety features

 Case B: lower trigger level

 ‘Scrammability’ of nuclear power plant not yet proved for the SL-2 earthquake motion level

 Objectives: to scram the reactor with larger margins during an earthquake

 Trigger Level: predicted level of probable damage to SSCs

In Japan, as Case A, a trigger level slightly lower than SL-1, the elastically dynamic design earthquake ground motion (Sd), is recommended based on the following considerations:

(a) The engineered safety features are normally in a stand-by condition; therefore, it is difficult to prove their full functionality after the observed earthquake without functional tests. On the other hand, their integrity can be assured for earthquake motion levels smaller that the design earthquake with elastic design limit.

(b) ‘Sd’ is large enough to postulate some damage in conventional facilities, such as electric transmission lines. Therefore, automatically shutting down the reactor will have a small influence on the electricity supply (demand). In addition, the possibility of a spurious scram is small at the Sd level, considering the reliable system logic.

FIG. 11. Principle and exterior view of a mechanical seismic switch within an ASTS (example in Japan).

In Japan, the first commercial-use reactor introduced from overseas (Tokai Unit 1: Gas-cooled reactor) had two types of ASTSs. One of them had the trigger level as low as 50 Gal for vertical motions. This was introduced for ‘prediction’, with the intent of sending the reactor into an early scram with the detection of P-waves. It is therefore an example of Case B.

2.4.4.2. Seismic category and qualification

The decision to shut down a reactor automatically when the observed earthquake level exceeds the predetermined threshold needs to be fulfilled without failure by means of an ASTS. This is the reason why an ASTS is installed as reactor protection system or reactor protection system equivalent, accompanied by relevant requirements for reliability. Consequently, since plants are strictly controlled with the ASTS, the quality of the SAS and the SDAS is left up to the discretion of licensees, giving them freedom in data management.

Japan’s Technical Specifications on Safety Protection Features state that the nuclear power plant needs to incorporate safety protection features that, in the event of an abnormal transient during operation or a disruption of the reactor operation due to an occurrence of earthquake, keep the reactor below the allowable fuel damage limit, in conjunction with the reactor shutdown systems and the engineered safety features.

2.4.4.3. Number of seismic switches installed and scram logic circuit

It is necessary to ensure that the ASTS is activated whenever necessary with minimal errors.

This is the reason why the ASTS needs to have redundancy, which determines the number of seismic switches to be installed. The ASTS needs to have a high level of reliability when it is installed as part of the reactor protection system. For this reason, similarly to other reactor protection system devices, its control logic is either ‘two out of four’, ‘double one out of two’

or ‘two out of three’ depending on the plant safety design philosophy. Fig. 12 shows ‘double one out of two’ and ‘two out of three’ examples. This way, at least three or four seismic switches are installed at locations that are perceived to have identical conditions.

2.4.4.4. Installation locations of seismic switches

According to the redundancy requirement discussed in the previous section, seismic switches are installed in at least three or four locations on the same plane (floor), depending on each plant’s safety design logic. The Japanese seismic design guideline recommends the following locations for installing seismic switches [27].

The location for the seismic trigger of the earthquake-detecting equipment needs to be determined by considering the object for which the seismic motion is to be detected; and the selected location(s) need to be easy for maintenance/inspection and need to be able to ensure high reliability.

More specifically, in a building which contains equipment important to safety, the seismic switches are set on the lowest elevation of the building to detect the seismic motion input to the building. In some cases, seismic switches are also set on a typical floor among the upper floors.

In the United States of America, as shown in Fig. 13, three triaxial seismic switches are installed at an equal interval on the foundation mat of a reactor containment structure, which is the same approach as that of Japan (in case of scram logic: ‘two out of three’).

2.4.4.5. Other topics

Similarly to the case of SAS systems, it is desirable to take into account the following matters:

 Maintenance and testing.

Similarly to SAS, it is recommendable to consider accessibility when selecting the installation locations for ASTS, and to facilitate on-site calibration of the seismic switches as much as possible.

 Operability

In order to ensure their activation when needed, it is advisable to install seismic the switches themselves and their connection cables in locations free of interference from surrounding structures even in case of an earthquake. Also, to install protective covers and implement waterproofing measures.

 Main control room notification

Appropriate notification of initiation of the ASTS signal needs to be announced in the main control room.

2.4.5. Lessons learned and other observations

In the United States of America there have been reported cases of automatic reactor shutdown caused by spurious signals from ASTS at testing/research reactors and at the Diablo Canyon nuclear power plant in its early stages of operation. In contrast, no inadvertent activation of ASTS has been reported in Japan.

The prevention of spurious trigger signals is a particularly important issue for the ASTSs with relatively low scram settings. Spurious signal prevention measures are the same as those for SAS, with the basic measures shown below (See Section 2.3.3.1 for details):

 Installing band-pass filters to prevent acceleration noise;

FIG. 12. Logic circuit examples of a seismic switch for an ASTS.

FIG. 13. Layout of seismic switches for ASTS at Diablo Canyon nuclear station Unit 1, United States of America. (Courtesy of Pacific Gas & Electric Company)

 Installing protective covers to prevent external impact noise;

 Multiplexing sensors for redundancy.

In Japan, the seismic switches for ASTS are installed in rugged protective boxes, and mounted with waterproofing and anti-flooding measures, as shown in Fig. 14.

2.4.6. Status of automatic seismic trip systems in nuclear power plants