EPRI COMPREHENSIVE EXAM 2026 QUESTIONS AND
ANSWERS GRADED A+
✔✔High Pressure Core Spray - ✔✔The high pressure core spray system (HPCS)
provides emergency core cooling for small breaks in the reactor coolant pressure
boundary that do not depressurize the reactor vessel. The system assists in removing
decay heat from the reactor and is used as a backup for the reactor core isolation
cooling system (RCIC)
✔✔High Pressure Coolant Injection - ✔✔High Pressure Coolant Injection (HPCI - BWR
1-4 designs)-The high pressure coolant injection system (HPCI) is used to provide
emergency core cooling for small breaks in the reactor coolant pressure boundary that
do not depressurize the reactor vessel. The system will assist in removing decay heat
from the reactor pressure vessel and is used as a backup for the reactor core isolation
cooling system (RCIC)
✔✔Low Pressure Coolant Injection (LPCI) - ✔✔The low pressure coolant injection
system (LPCI) is used to restore and maintain the reactor pressure vessel water level
after a LOCA event where the reactor vessel has been depressurized. This system is
part of the residual heat removal system (RHR). (Long term accident cooling)
✔✔Core Spray (CS - BWR 2-4) or Low Pressure Core Spray (LPCS - BWR 5&6)- -
✔✔The low pressure core spray system (LPCS) is used to restore and maintain reactor
pressure vessel water level after a LOCA event where the reactor vessel has been
depressurized
✔✔Automatic Depressurization System (ADS)- - ✔✔The automatic depressurization
system is used to depressurize the reactor after a pipe break in the reactor core
pressure boundary. The system is used when the pressure cannot be reduced and the
ECCS cannot maintain the water level above the very low level mark. This system will
only depressurize the vessel and will not replace any reactor pressure vessel water
✔✔Four conditions must be present for natural circulation to occur-PWR: - ✔✔Used
when in IMMEDIATE shutdown mode and lose recirculation pumps (or RCP).
1.1.1. PWR
Four conditions must be present for natural circulation to occur:
· A heat source (the reactor core)
· A heat sink to which the secondary fluid transfers its heat (the steam generators or
Feedwater (BWR))
· The heat sink (steam generators) must be at a higher elevation than the heat source
(reactor core)
· A continuous, unobstructed flowpath (the reactor coolant system piping)
Once these conditions are met and natural circulation is taking place, the continuous
flow of coolant without the aid of a pump will remove heat from the fuel.
, The following indications determine if natural circulation is taking place in a standard
Westinghouse PWR (these indications may vary based on PWR design):
· RCS hot leg temperatures are stable or decreasing
· RCS core exit thermocouple temperatures are stable or decreasing
· Steam generator pressures are stable or decreasing
· RCS cold leg temperature is at saturation temperature for existing S/G pressure
· RCS loops have at least 30ºF of subcooling
Natural circulation can be retarded if boiling occurs or if gaseous voids or non-
condensable gases accumulate in the system's high points, such as in the steam
generator tubes or reactor vessel head
✔✔Identify mechanisms required for natural circulation to occur-BWR: - ✔✔Used when
in IMMEDIATE shutdown mode and lose recirculation pumps (or RCP).
1.1.1. BWR- Natural circulation readily takes place in a BWR since the cold water
column is outside the shroud, and the heated water column is inside the shroud and
throughout the reactor core. Water heated in the core is displaced by cooler water
entering from the downcomer region at the bottom of the core. As the water is heated
and rises through the reactor, it reaches the spill-over point at the steam separators
where the water flows into the downcomer region
The water level must be maintained above the bottom of the dryer skirt to ensure
enough head pressure is generated to force water through the steam separators. The
process of natural circulation can happen in a BWR, but is not the normal mode of
operation.
✔✔Describe DBA-LOCA, including symptoms and indications - ✔✔A Design Basis
Accident (DBA) is typically a worst-case accident of interest used to design plant
components. DBAs often bound (have more severe consequences than) other
accidents so the latter do not need to be analyzed further. For example, the Design
Basis Large Break Loss of Coolant Accident (LOCA) bounds (has a higher containment
pressure than) other LOCAs with slightly smaller break areas.
A DBA-LOCA can occur in both Pressurized and Boiling Water reactors.
✔✔Pressurized Water Reactor (PWR) DBA-LOCA - ✔✔A PWR DBA-LOCA is defined
as a double-ended guillotine piping rupture with a simultaneous loss of off-site power.
The single worst case active failure is also assumed. This accident has major
implications on Nuclear Steam Supply System (NSSS) design, particularly the ECCS.
✔✔PWR-LOCA initial symptoms and indications include: - ✔✔· Unusual increase in
containment pressure
· Decreased reactor coolant system pressure
· All ECCS pumps start
· Reactor may empty and depressurize
ANSWERS GRADED A+
✔✔High Pressure Core Spray - ✔✔The high pressure core spray system (HPCS)
provides emergency core cooling for small breaks in the reactor coolant pressure
boundary that do not depressurize the reactor vessel. The system assists in removing
decay heat from the reactor and is used as a backup for the reactor core isolation
cooling system (RCIC)
✔✔High Pressure Coolant Injection - ✔✔High Pressure Coolant Injection (HPCI - BWR
1-4 designs)-The high pressure coolant injection system (HPCI) is used to provide
emergency core cooling for small breaks in the reactor coolant pressure boundary that
do not depressurize the reactor vessel. The system will assist in removing decay heat
from the reactor pressure vessel and is used as a backup for the reactor core isolation
cooling system (RCIC)
✔✔Low Pressure Coolant Injection (LPCI) - ✔✔The low pressure coolant injection
system (LPCI) is used to restore and maintain the reactor pressure vessel water level
after a LOCA event where the reactor vessel has been depressurized. This system is
part of the residual heat removal system (RHR). (Long term accident cooling)
✔✔Core Spray (CS - BWR 2-4) or Low Pressure Core Spray (LPCS - BWR 5&6)- -
✔✔The low pressure core spray system (LPCS) is used to restore and maintain reactor
pressure vessel water level after a LOCA event where the reactor vessel has been
depressurized
✔✔Automatic Depressurization System (ADS)- - ✔✔The automatic depressurization
system is used to depressurize the reactor after a pipe break in the reactor core
pressure boundary. The system is used when the pressure cannot be reduced and the
ECCS cannot maintain the water level above the very low level mark. This system will
only depressurize the vessel and will not replace any reactor pressure vessel water
✔✔Four conditions must be present for natural circulation to occur-PWR: - ✔✔Used
when in IMMEDIATE shutdown mode and lose recirculation pumps (or RCP).
1.1.1. PWR
Four conditions must be present for natural circulation to occur:
· A heat source (the reactor core)
· A heat sink to which the secondary fluid transfers its heat (the steam generators or
Feedwater (BWR))
· The heat sink (steam generators) must be at a higher elevation than the heat source
(reactor core)
· A continuous, unobstructed flowpath (the reactor coolant system piping)
Once these conditions are met and natural circulation is taking place, the continuous
flow of coolant without the aid of a pump will remove heat from the fuel.
, The following indications determine if natural circulation is taking place in a standard
Westinghouse PWR (these indications may vary based on PWR design):
· RCS hot leg temperatures are stable or decreasing
· RCS core exit thermocouple temperatures are stable or decreasing
· Steam generator pressures are stable or decreasing
· RCS cold leg temperature is at saturation temperature for existing S/G pressure
· RCS loops have at least 30ºF of subcooling
Natural circulation can be retarded if boiling occurs or if gaseous voids or non-
condensable gases accumulate in the system's high points, such as in the steam
generator tubes or reactor vessel head
✔✔Identify mechanisms required for natural circulation to occur-BWR: - ✔✔Used when
in IMMEDIATE shutdown mode and lose recirculation pumps (or RCP).
1.1.1. BWR- Natural circulation readily takes place in a BWR since the cold water
column is outside the shroud, and the heated water column is inside the shroud and
throughout the reactor core. Water heated in the core is displaced by cooler water
entering from the downcomer region at the bottom of the core. As the water is heated
and rises through the reactor, it reaches the spill-over point at the steam separators
where the water flows into the downcomer region
The water level must be maintained above the bottom of the dryer skirt to ensure
enough head pressure is generated to force water through the steam separators. The
process of natural circulation can happen in a BWR, but is not the normal mode of
operation.
✔✔Describe DBA-LOCA, including symptoms and indications - ✔✔A Design Basis
Accident (DBA) is typically a worst-case accident of interest used to design plant
components. DBAs often bound (have more severe consequences than) other
accidents so the latter do not need to be analyzed further. For example, the Design
Basis Large Break Loss of Coolant Accident (LOCA) bounds (has a higher containment
pressure than) other LOCAs with slightly smaller break areas.
A DBA-LOCA can occur in both Pressurized and Boiling Water reactors.
✔✔Pressurized Water Reactor (PWR) DBA-LOCA - ✔✔A PWR DBA-LOCA is defined
as a double-ended guillotine piping rupture with a simultaneous loss of off-site power.
The single worst case active failure is also assumed. This accident has major
implications on Nuclear Steam Supply System (NSSS) design, particularly the ECCS.
✔✔PWR-LOCA initial symptoms and indications include: - ✔✔· Unusual increase in
containment pressure
· Decreased reactor coolant system pressure
· All ECCS pumps start
· Reactor may empty and depressurize
