Document: NUREG-0800
Document ID: 504a24fe-9056-495a-8541-0e278016ce80
Document Type: srp
Title: in that offsite AC power remains available to operate the station
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0703/ML070300702.pdf
Revision Date: 2023-06
Chapter: 15
Section ID: 15.2.6
CFR Part: 
CFR Title: 

Content:
15.2.1-15.2.5-2 Revision 2 - March 2007 auxiliaries (e.g., reactor coolant pumps). Onsite emergency diesel generators are therefore not required for the loss of external load event. Immediate fast closure of the turbine control valves (TCVs) is initiated for a loss of generator load. For a boiling water reactor (BWR), a fast (0.15–0.20 second) TCV closure causes a sudden reduction in steam flow and results in a reactor pressure surge. For a BWR without select rod insert, reactor scram occurs. For a pressurized water reactor (PWR), there is also a sudden reduction in steam flow causing the pressure and temperature in the shell side of the steam generator to increase. The latter effect, in turn, results in an increase of reactor coolant temperature, a decrease in coolant density, an increase of water volume in the pressurizer, and an increase in reactor coolant pressure; depending on reactor design and the size of load rejection, a reactor scram may occur. For a PWR with an integrated control system, reactor power can run on a lower level upon TCV closure. In all light-water-cooled reactors, sensible and decay heat can be removed through actuation of one or several of the following systems: steam relief system, steam bypass to the condenser, reactor core isolation cooling system (BWR), emergency core cooling systems, and auxiliary feedwater system (PWR). 2. Turbine Trip: In a turbine trip event, a malfunction of a turbine or reactor system causes the turbine to trip off the line by abruptly stopping steam flow to the turbine. This event is different from the loss of external load condition in that fast closure of the turbine stop valves (TSVs) is initiated. The TSV closure times are faster (0.10 second) than those of the TCVs, resulting in more severe transients. For typical BWR and PWR plants, position switches on the TSVs sense the trip and initiate reactor scram. The remainder of this transient is similar to the loss of external load. 3. Loss of Condenser Vacuum: