What is the difference between OBE and SSE earthquakes?

The main difference between an Operating Basis Earthquake (OBE) and a Safe Shutdown Earthquake (SSE) in seismic analysis of nuclear components lies in their severity and design considerations:

Operating Basis Earthquake (OBE):

• Represents a more frequent, moderate earthquake. Expected to occur at least once during the lifespan of the nuclear facility.
• Used to design certain systems and components to remain functional and ensure safe operation of the plant during and after the event.
• OBE forces are generally considered half as strong as the SSE.

Safe Shutdown Earthquake (SSE):

• Represents the maximum credible earthquake for the specific site, considering geological and seismic history. Considered a very rare event.
• Used to design critical safety systems in the facility. These systems must be able to withstand the SSE and safely shut down the reactor even under these extreme conditions.
• The goal is to prevent radioactive releases and ensure the integrity of the containment structure.