Document: NUREG-0800
Document ID: 8e754440-b70e-4b8a-9816-b4a64b631220
Document Type: srp
Title: Initial – February 2012
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1136/ML113640138.pdf
Revision Date: 2023-06
Chapter: 8
Section ID: 8
CFR Part: 
CFR Title: 

Content:
st for an onsite or offsite AOT extension will contain a PRA assessment. However, this BTP specifically discusses the defense-in-depth aspects for onsite and offsite power sources from a deterministic perspective. A supplemental power source should be available as a backup to the inoperable EDG or offsite power source, to maintain the defense- in-depth design philosophy of the electrical system to meet its intended safety function. The supplemental source must have capacity to bring a unit to safe shutdown (cold shutdown)1 in case of a loss of offsite power (LOOP) concurrent with a single failure during plant operation (Mode 1). According to NUREG-1784 [Reference 4], considering the changes in electric grid performance post-deregulation, the duration of LOOP events has increased and the probability of a LOOP as a consequence of a reactor trip has increased. This evaluation was done before the August 14, 2003, Blackout in the Northeast. The lessons learned from this Blackout event indicate that restoration of offsite power will take longer than previously considered, indicating that post-deregulation conditions challenge grid reliability. The staff’s objective of requiring an extra (i.e., supplemental) power source for an inoperable EDG or offsite power source is to avoid a potential extended Station Blackout (SBO) event during the period of an extended AOT and to enable safe shutdown (cold shutdown) of the unit if normal power sources cannot be restored in a timely manner. The current design of boiling-water reactor (BWR) and pressurized-water reactor (PWR) safety systems, required for reactor core decay heat removal and containment heat removal, are dependent on alternating current (AC) power. The projected time for restoration of offsite power is now considered to be more than the time previously evaluated for the SBO rule. The staff considers that a replacement (i.e., supplemental) AC power source is needed to back up an inoperable EDG or offsite power source