An accident sequence outside the design basis, typically involving multiple failures or extreme external events, that may or may not lead to core damage. Such sequences are analysed with best-estimate methods and addressed with additional equipment, portable pumps and generators, and procedures. International safety standards call these design extension conditions and require the design to address selected sequences explicitly.
The portable pumps are for beyond design basis accident scenarios, so they don't count toward the LCO.
Also heard as
- BDBA
- design extension conditions
- DEC
Related terms
Design basis accident
A postulated accident that a nuclear plant is explicitly designed to withstand without exceeding set safety limits.
Severe accident
An accident involving significant damage to the reactor core, with possible fuel melting and large releases.
SBO
Station blackout: complete loss of offsite and onsite AC power at a nuclear plant.
PSA
Probabilistic safety assessment: a systematic model of how accidents could happen and how likely each outcome is.