Perspective

2.1.1. Existing nuclear installations

Paragraph 1.10 of NS-G-2.13 [1] applies to the seismic safety evaluation of

“...existing nuclear installations are those installations that are either (a) at the operational stage (including long term operation and extended temporary shutdown periods) or (b) at a pre-operational stage for which the construction of structures, manufacturing, installation and/or assembly of components and systems, and commissioning activities are significantly advanced or fully completed.”

2.1.2. Reasons to perform seismic safety evaluations

There are many reasons to perform seismic safety evaluations of existing nuclear installations. As stated in para. 2.10 of NS-G-2.13 [1], States may require an evaluation of the seismic safety in the event of any one of the following:

“(a) Evidence of a seismic hazard at the site that is greater than the design basis earthquake, arising from new or additional data (e.g. newly discovered seismogenic structures, newly installed seismological networks or new palaeoseismological evidence), new methods of seismic hazard assessment, and/or the occurrence of actual earthquakes that affect the installation;

(b) Regulatory requirements, such as the requirement for periodic safety reviews, that take into account the ‘state of knowledge’ and the actual condition of the installation;

(c) Inadequate seismic design^[1],generally due to the vintage of the facility;

(d) New technical findings, such as vulnerability of selected structures and/or non-structural elements (e.g. masonry walls), and/or of systems or components (e.g. relays);

(e) New experience from the occurrence of actual earthquakes (e.g. better recorded ground motion data and the observed performance of SSCs);

(f) The need to address the performance of the installation for beyond design basis earthquake ground motions in order to provide confidence that there is no ‘cliff edge effect’^[2], that is, to demonstrate that no significant failures would occur in the installation if an earthquake were to occur that was slightly greater than the design basis earthquake…;

(g) A programme of long term operation [or plant life extension] of which such [a seismic safety] evaluation is a part.”

1 Due to the perception of the seismic hazard being significantly higher than the original design level, or due to evolution of the seismic design methodologies.

2 A cliff edge effect refers to the phenomenon of significant SSC failures occurring when earthquake ground motions slightly greater than the DBE occur at the site.

2.1.3. Margins beyond the design basis for new nuclear installations

NS-G-1.6 [3] applies to the seismic analysis and design of new nuclear power plants. Paragraphs 2.39–2.41 of NS-G-1.6 [3] address the need to provide margins for events beyond the design basis. When necessary, the methodologies for evaluation of existing nuclear installations are being implemented in the design stage for new nuclear power plants to ensure that seismic margins are present in the design. For standard designs of Generation III (or III+) nuclear installations, a beyond design basis earthquake (DBE) event is often considered.

Such consideration is in the design phase for the standard design and for site specific SSCs. In addition to being designed for a DBE ground motion, there is a need to demonstrate capacity to withstand a beyond DBE ground motion with high confidence that a nuclear safety related failure will not occur. The level of the beyond DBE ground motion to be considered is established through applicable requirements. The seismic margin capacity is established during the design process and later verified for site specific seismic hazards and for SSCs designed for site specific conditions. In performing these evaluations of beyond DBEs, acceptance criteria different from those used in the design process may be used. For example, more realistic material properties rather than specified minimum values and an inelastic energy absorption and ductility factor may provide failure margins to some types of SSC.

In the United States of America, certified designs exist for standard nuclear power plants, which are currently designed to broadbanded ground response spectra anchored to 0.3g peak ground acceleration (PGA) in the horizontal direction and companion design ground response spectra in the vertical direction.

These design basis ground motions, known as certified seismic design response spectra, are compatible with the specifications of the utility requirements document in the United States of America. For US certified designs, a plant high confidence of low probability of failure (HCLPF) capacity of at least 1.67 times an SL-2 earthquake³ needs to be demonstrated. The term HCLPF is defined as 95% confidence of less than 5% probability of failure or a mean confidence of less than 1% probability of failure. During the certified design stage, a specific site has not been identified. Therefore, site specific seismic hazards are not known and, consequently, a site specific SPSA cannot be performed. In order to address the issue of unknown sites, SMA procedures are used [4–6]. Specifically, a PSA based SMA has to be performed for certified designs in the United States of America.

3 A seismic level 2 (SL-2) earthquake is defined in NS-G-1.6 [3] and “is associated with the most stringent safety requirements” for which the plant’s design needs to ensure the

The European Utility Requirements Document⁴ (EUR), developed by a group of major European electricity producers specifies, similar requirements for standard designs in Europe: DBEs comprising broadbanded ground response spectra anchored to a PGA of 0.25g and consideration of beyond DBE events in the design phase. The EUR specifies that it needs to be demonstrated that a standard design achieves a plant seismic margin of 1.4 times SL-2. The EUR specifically excludes the performance of an SPSA, which means that the SMA approach will be implemented.

The former nuclear regulatory authority of Japan provided requirements for the seismic analysis and design of nuclear power plants. It recognized the possibility of beyond DBE occurring, denoting it residual risk [7]. This residual risk has to be considered for existing nuclear installations and for new installations.

2.1.4. Margins beyond the design basis for existing nuclear installations Although engineering practice for existing installations was to include design margins, it did not make them as explicit as is now the case for new facilities. To the extent possible, the purpose of a seismic evaluation is to make these margins explicit, quantify them and assess the safety of the installation accordingly. This approach may lead to the conclusion that margins are not sufficient for some SSCs. In such cases, the purpose of seismic evaluation is also to propose engineered upgrading actions to achieve the desired safety level.