Document: NUREG-0800
Document ID: b694ca5b-4dcc-4098-be04-431e0137a936
Document Type: srp
Title: - 15.5.2
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0708/ML070820081.pdf
Revision Date: 2023-06
Chapter: 15
Section ID: 15.5.1
CFR Part: 
CFR Title: 

Content:
fore, can lead to a LOCA, which may be considered the an AOO, a postulated accident, a Condition II event, or a Condition IV event, depending upon break size and the event categorization scheme in the licensing basis (see SRP Section 15.0). 15.5.1-15.5.2-7 Revision 2 - March 2007 Typically, design basis accident analyses show that an AOO cannot become a more serious event, by demonstrating that the pressurizer does not become water-solid at any time during the transient, and therefore, a PORV cannot ever relieve water [7]. The event ends when the charging flow is terminated by the operator. The analysis objective is to show that the pressurizer does not become water-solid before the operator can terminate the transient, usually at about ten minutes (or longer) after the event begins. If the plant is equipped with PORVs that are (1) safety-related equipment and (2) qualified for water relief, then they may be assumed to reseat properly after having relieved water. The pressurizer safety valves, too, may be assumed to reseat properly after having relieved water; but only if such valves have been qualified for water relief. It is conservative to assume that PORVs open and relieve steam in order to limit the RCS pressurization, and thereby increase the charging flow rate (and the resulting pressurizer fill rate). This shortens the time available to the operator to terminate the charging flow before the pressurizer fills. Unlike the inadvertent operation of the ECCS, the CVCS malfunction that increases reactor coolant inventory (see SRP Section 15.5.2), a related AOO, does not lead directly to a reactor trip. The reactor is tripped automatically, from a signal that is generated during the transient, e.g., high pressurizer pressure or level. Since power is being generated prior to reactor trip, the event could cause DNB to occur. However, this is not likely, since (1) core pressure increases, and (2) the reactor protection system automatically trips the reactor when it