Document: NUREG-0800
Document ID: 621763b9-bd48-4ded-8c42-b942a23a06d4
Document Type: srp
Title: – 15.2.5
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0520/ML052070680.pdf
Revision Date: 2023-06
Chapter: 15
Section ID: 15.2.1
CFR Part: 
CFR Title: 

Content:
he steam generator to increase. The latter effect, in turn, results in an increase in reactor coolant temperature, a decrease in coolant density, an increase in water volume in the pressurizer, and an increase in reactor coolant pressure. For a PWR with an integrated control system, reactor power can be run back to a lower level on 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 be tripped off the line by abruptly stopping steam flow to the turbine. This is different from the loss of electrical load condition described above in that fast closure of the turbine stop valves (TSVs) is initiated. The TSVs have faster (0.1 sec) closure times than the turbine control valves, 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 previously discussed loss of electrical load. 3. Loss of Condenser Vacuum A loss of condenser vacuum event is one of the malfunctions that can cause a turbine trip. The remarks in item 2, above, thus apply to this transient event. 4. Main Steam Isolation Valve Closure The main steam isolation valve (MSIV) transient closure event for BWRs can be initiated by 8 various steam line or reactor system malfunctions and by various operator actions. As the MSIVs close, position switches initiate a reactor scram when the valves in three or more of the steam lines are less than 90% open, the reactor pressure is above 600 psi 4140 kPa (600 psi),9 and the reactor mode switch is in the RUN position. The effect of MSIV closure is to limit steam flow to the