**2. Basic principles of seismic safety**

The fundamental safety objective of design and operation of nuclear power plant is to protect human life and environment in case of any malfunctions, failures of the plant systems, structures and components which may occur during the plant lifetime including those caused by rarely occurring earthquakes. The generic approach for ensuring this safety objective is the application of the concept of the defence in depth. In accordance with this concept, the following requirements are applicable:

	- a. for leading the plant to a safe shutdown state, i.e.
		- for the maintaining the sub-criticality in the reactor and spent fuel pool and,

Seismic Safety Analysis and Upgrading of Operating Nuclear Power Plants 83

Operability of NPPs should be ensured after the moderately frequent and not severe earthquakes. The operational base earthquake (OBE or SL-1 level according to the IAEA terminology) level is defined as a design level for continuous operation. The OBE was usually defined as an event with frequency of 10-2/a, or a ground motion with maximum horizontal acceleration equal to a given fraction of the maximum acceleration value of the SSE. Through the years the concept of designing for two earthquakes has radically changed. Nowadays, the OBE is interpreted as an operational limit and inspection level rather than an obligatory design level. The definition of the OBE level is subject of design, operational, economic considerations; see the IAEA NS-G-3.3 Safety Guide (IAEA, 2002). Design for lower level is not required if the OBE PGA is equal or less than 1/3rd of the SSE PGA, see in Appendix S of the 10 CFR Part 50. Instead of OBE PGA, new criteria for the exceedance of operational limit/inspection level are introduced. The changes of the terminology in the German regulation demonstrate the changes in design concept: the former terms SSE *- Sicherheitserdbeben* and OBE *- Auslegungserdbeben* were replaced by the terms design base earthquake and inspections earthquake, i.e. *Bemessungserdbeben* and *Inspektionserdbeben*.

**3. Tasks for seismic re-evaluation and upgrading of operating NPPs** 




The tasks are determined by the objective of the project as it has been shown above, i.e.

Generic objective of the seismic safety programmes is to ensure the basic nuclear safety



The functions have to be maintained for the earthquakes within the design basis envelope and with some extent for the earthquakes with parameters exceeding the

shutdown the reactor and maintain the sub-criticality after the earthquake,

resolution of qualification issues, ensuring the design basis compliance, etc.

**3.1. Objective and scope of the seismic safety programmes** 


of radioactive substances into the environment.

Major tasks of the seismic re-evaluation and upgrading projects are


for the analyses and performance of the analyses

measures

earthquake.

functions, i.e.

design basis one.


The seismic safety is ensured by the following complex activities:


The basic safety functions, i.e. shut down, cooling and containment, have to be maintained for the earthquakes within the design basis envelope and with some extent for the severe beyond design basis earthquakes.

Traditionally the design of the nuclear facilities adapted the two-level concept: design for safety, using a high-level seismic excitation for design basis and design for production, using a moderate level of seismic excitation for operational limit.

The design base earthquake has to be defined with quite low probability of exceedance during operating time. This earthquake is the Safe Shutdown Earthquake (SSE) as per U.S. terminology; see U.S. NRC 10CFR Part 50, Domestic Licensing of Production and Utilization Facilities (NRC, 1956). It is called *Sicherheitserdbeben*, i.e. safety earthquake in German Nuclear Safety Standards 2101 (Kerntechnische Ausschuss [KTA], 1990), it is the maximum design earthquake (MRZ) according to the Russian-Soviet terminology and it is called SL-2 earthquake level by the IAEA guideline NS-G-1.6 (IAEA, 2003b) [4]. Here the term of Design Base Earthquake (DBE) will be used. According to the international practice the annual probability of exceedance of the DBE is usually 10-4/year in case of nuclear power plants. The lower limit of the peak ground acceleration (PGA) of the DBE is set for 0.1g regardless of the site (article 2.7 of NS-G-1.6). The shutdown and cool-down of the reactor, the continuous heat removal from the irradiated fuel (in the reactor core and spent fuel pool), and the limitation of releases have to be ensured in this limit state. SSCs required for basic safety function have to sustain the earthquake loads without loss of function.

Operability of NPPs should be ensured after the moderately frequent and not severe earthquakes. The operational base earthquake (OBE or SL-1 level according to the IAEA terminology) level is defined as a design level for continuous operation. The OBE was usually defined as an event with frequency of 10-2/a, or a ground motion with maximum horizontal acceleration equal to a given fraction of the maximum acceleration value of the SSE. Through the years the concept of designing for two earthquakes has radically changed. Nowadays, the OBE is interpreted as an operational limit and inspection level rather than an obligatory design level. The definition of the OBE level is subject of design, operational, economic considerations; see the IAEA NS-G-3.3 Safety Guide (IAEA, 2002). Design for lower level is not required if the OBE PGA is equal or less than 1/3rd of the SSE PGA, see in Appendix S of the 10 CFR Part 50. Instead of OBE PGA, new criteria for the exceedance of operational limit/inspection level are introduced. The changes of the terminology in the German regulation demonstrate the changes in design concept: the former terms SSE *- Sicherheitserdbeben* and OBE *- Auslegungserdbeben* were replaced by the terms design base earthquake and inspections earthquake, i.e. *Bemessungserdbeben* and *Inspektionserdbeben*.
