High Neutron Flur
Rate (Power Range High Selling) •
High Reactor
Pressure
Emergency Core Cooling
Sywtem (ECCS) Actuation
Steam Generator Waler Level
*1
High Seismic
Acceleration
—
Manual ■
High
Neutron
Flux
(Power Range Low Setting)
High
Neutron Flux
(Neutron
Source Range and Intermediate Range)-, •
Low
Reactor Pressure •
High
Pressurizer Waler Level •
Low
Reactor
Coolant
Flow Rate
Low
Reactor Coolant Pump
Power Voltage *2
Low
Reactor
Coolant Pump Frequency
Turbine
Trip •
Losie
Circuit
Trip
Reactor
Coolant Teiperature
Difference AT
Reactor
Coolant Average Teiperature T avg
Upper
and LowerNcutron
Flux Devialion
Penalty Function
f(Aq)—•
Over
Poser AT High
Reactor
Coolant Teiperature
Difference AT
Seactor
Coolant Average Teiperature
T avg •
Pressurizer
Pressure P
Over
Teiperature
AT High
‘1:
Recent plants
are slandardized
to have a low sleam
generator waler level.
‘2:
Some plants use low reactor coolant pump speed
Figure
3.6.4 Reactor protection system block diagram
High
Head Safety
RHR
Pump Actuation
Containment
Isolation Annulus Air Cleanup Fan Actuation Containment Ventilation
Isolation Auxiliary Feedwater Pump Actuation Feedwater isolation (
Reactor Trip ) (Emergency Diesel Generator Startup)
Steam
Line Isolation
High-high
Containment Pressure
.Manual
Containment Spray Actuation
Containment
Spray Actuation
Containment
Isolation
Figure
3.6.5 Block diagram Illustrating engineered safety feature actuation
NSRA,
Japan
3
'
66High Neutron Flux {Power Range High Selling)
Sleam Generator Feedaler Flow Level tl *
Interlock
Reactor
Turbine Load Neutron Flux Level
Injection Pump Actuation
Chapter
3 Systems of PWR Nuclear Power Plants
Table
3.6.3 Function of reactor trip signals |
Function |
(1) High Neutron Source Range Neutron Flux |
To protect reactor against rapid increase of neutron flux at reactor start-mp and shutdown. To trip reactor when measured value of ex core source range proportional counter exceeds the set value. |
( 2) High Intermediate Range Neutron Flux |
To trip reactor when measured point of ex-core ionization box exceeds the set value. |
(3) High Power Range Neutron Flux |
To trip reactor at high setting neutron flux during high power operation and at low setting neutron flux value during low power operation in order to assure integrity of fuel. |
(4) High Power Range Neutron Flux Change Rate |
Core flux distribution could be distorted beyond design value in case of rod ejection and drop of control rod clusters. Power range neutron flux detectors detect such sudden change of neutron flux and trip reactor to prevent fuel damage. |
(5) Over Temperature A|THigh |
This signal protect fuel cladding from PNB by tripping reactor when the minimum DNBR in the core approches the design minimum low limit. (The minimum DNBR is not directly measured, but can be protected using Tavg, AT and P) |
(6) Over Power AT High |
This signal protects fuel from exceeding the maximum linear power by tripping reactor when measured AT corresponding to core power exceeds the permitted value. AT corresponding to core poweri s corrected with Tavg signal compensating for the dependecy on low coolant temperature. |
(7) Low Reactor Pressure |
To trip reactor when pressurizer pressure drops below the low pressure range of DNBR correlation of ''over temperature A T High” and also prevent excess boiling of primary coolant |
( 8) HighReatctor Pressure |
To trip reactor when pressurizer pressure exceeds the set value in order to limit the protection range of DNB by “over temperature AT high” and also prevent over pressurization of the reactor system before safety valves are opened. |
( 9) Pressurizer later Level High |
To prevent pressurizer from being filled with water and discharging liquid phase water through either safety valve or relief valve, although any direct damage is not expected by high level water. |
(10) Reactor Coolent Flow Rate Low |
To trip reactor during high power operation to prevent fuel damage, since low flow rate lowers heat removal capacity and causes DNB. |
(11) Reactor Coolant Pump Power Voltge Low |
Drop of power voltage to reactor coolant pump causes similar phenomena as low flow rate as described in item (10) abeve. |
(12) Reactor Coolant Pump Power Frequency Low |
Drop of power frequency causes low flow rate of reactor coolant pump and invite the similar phenomena as described in item (10) abeve. |
(13) Actuation of Emergency Core Cooling System |
To trip reactor by the actuation signal of emergency core cooling systme to be described later. |
(14) Steam Generator rater Level Low (Note) |
Drop of water level in steam generator may decrease heat removal capacity of primary coolant system. |
(15) Steam Generator Feedwater Flow Rate Low (Note) |
To trip reactor since low feedwater flow rate invite drop of water level in steam generator and may decrease heat removal capacity of primary coolant system. |
(16) Turbine Trip |
To trip reactor to prevent excess temperature rise and over pressurization of primary coolant system due to turbine trip. |
(17) Seismic Acceleration High |
To trip reactor when either horizontal or vertical seismic acceleration exceeds the set points. |
(18) Manual |
To trip reactor by manual operation at the central control beard when operator needs emergency shutdown of reator. |
[Note]
Both are combined into "Steam Generator rater Level Low”
signal in the recent plants.
3-67
NSRA,
Japan
Table
3.6.4 List of engineering safety feature (ESF) actuation signals |
Detector |
Functional Logic |
Interlock |
|
Emegency Core Cooling System (ECCS) Actuation Signal |
a. Low Reactor Pressure abnormally |
Pressurizer Pressure Detector |
2/4 |
Manual Block below (P-11) |
b. Low Main Steam Line. Pressure |
Main Steam Pressure Detector |
1/4 of Low Main Steam Line Pressure (2/4 for Each Line) |
Manual Block below (P-11) |
|
c. High Containment Vessel Pressure |
Containment Pressure Detector |
2/4 |
|
|
d. Manual |
|
1/2 |
|
|
Main Steam Line Isoration Signal |
a. High-high Containment Vessel Pressure abnormally |
Containment Pressure Detector |
2/4 |
|
b. Low Main Steam Line Pressure |
Same as Emergency Core Cooling System (ECCS) Actuation Signal b. |
Same as Emergency Core Cooling System (ECCS) Actuation Signal b. |
Same as Emergency Core Cooling System (ECCS) Actuation Signal b. |
|
c. High Main Steam Line Pressure Negative Rate |
Main Steam Pressure Detector |
1/4 of High Main Steam Line Pressure Negative Rate (2/4 for Each line) |
Automatic Block above (P-11) |
|
d. Manual |
|
1/2 |
|
|
Containment Spray Actuation Signal |
a. High-high Containment Pressure |
Containment Pressure Detector |
2/4 |
|
b. Manual |
|
(2/2) xl/2 |
|
|
Containment Isolation Signal |
a. Emergency Core Cooling System (ECCS) Actuation Signal |
Same as Emergency Core Cooling System (ECCS) Actuation Signal |
Same as Emergency Core Cooling System (ECCS) Actuation Signal |
|
b. Containment Spray Actuation Signal |
Same as Containment Spray Actuation Signal |
Same as Contaminant Spray Actuation Signal |
|
|
c. Manual |
|
1/2 |
|
|
each train are capable to operate the associated equipment, so that
a single component failure will not compromise the safety protection
system function.
The conventional analog and logic circuits of the ESFs actuation
system are nowadays being replaced by a digital control system using
microprocessors for new plants and the conventional systems for
operating plants are being replaced step-by-step.
ESFs actuation signals and their functions
The ESFs actuation signals of a typical PWR plant are given in Table
3.6.4. Also, permissive
logic circuits are provided to allow startup and shutdown operation
conditions without defeating the safety functions of the reactor
system. The permissive
signals associated with the ESFs actuation system are
listed in Table 3.6.5.
Primary coolant discharge in an unlikely event of primary coolant
system rupture, also known as the LOCA, results in a decrease in
both pressurizer pressure and the pressurizer level as well as an
increase in containment pressure. At this condition, a reactor
pressure abnormally- low-signal, or a containment pressure-high
signal causes the ECCS actuation signals.
NSRA,
Japan
3-68
Chapter
3 Systems of PWR Nuclear Power Plants
Table
3.6.5 List of permissive signals of engineering safety feature
(ESF) actuation signals |
Function |
Input Signals |
P-11 |
a.Permission for manual block of emergency core cooling system (ECCS) actuation signal due to abnormally low reactor pressure and low main steam line pressure |
(Note) 2/3 of low pressurizer pressure |
b.Permission for manual block of main steam line isolation signal due to low main steam line pressure |
(Note) 2/3 of low pressurizer pressure |
|
c.Automatic block of main steam line isolation signal due to high main steam line pressure decreesing rate |
(Note) 2/3 of high pressurizer pressur |
[Note]
2/4 instead of 2/3 in the recent
Uncontrolled steam discharge in an event of a steam line rupture,
will result in steam pressure drop. At this condition, pressure-low
signals from each steam line actuate the ECCS and close all steam
line isolation valves to prevent blowdown flow of more than two
steam-generators and maintain the reactor residual heat removal
capability of the secondary system.
Also, either a primary or secondary system component rupture
accident inside the containment structure, will increase the
containment pressure which in turn, provides actuation signals for
the ECCS, the main steam line isolation and the containment spray
system which is provided for removal of iodine and containment
depressurization.
The ECCS and containment spray system actuation signals provide
signals for the containment isolation valves to isolate all systems
other than those important
for accident suppression. Further, each of the above mentioned
actuation signals can be manually caused by the operators from the
main control panel.
The reactor pressure abnormally-low-signal or the steam line
pressure-low signal can be manually blocked by the pressurizer
pressure- low input signal (permissive function P-11) to prevent
unnecessary actuation of the ECCS during the plant cool down and
heat up processes. Further, if the main steam line pressure-low
signal is blocked, isolation of the main steam line is actuated by
the main steam line pressure droprate-high signal.
c. Design philosophy of the safety protection system
High quality and high reliability are necessary
for the safety protection system to meet its performance
requirements. Descriptions of the main design philosophies and
design precedents of the safety protection system are given below.
Redundancy
The safety protection system is designed with sufficient redundancy
so that a single component failure will not compromise the system
function.
Independence
The redundant components of the safety protection system are
designed and arranged to be electrically isolated and physically
separated to achieve maximum independence and separation between
them. Analog channels and logic trains (shown in Figure 3.6.3), as
well as their electrical sources and cable trays, are all
independent and physically separated from each other.
Separation / interference of safety protection system from the
control system
In PWR NPPs, the safety protection system is separated from the
normal control system to the maximum practical extent possible to
increase reliability of the safety protection system. However, if
the control system signals are derived from a part of the safety
system, they are sent through isolation amplifiers installed at
signal branching points (Figure 3.6.6).
Testing and calibrating
The safety protection system is designed to permit routine testing
and calibrating during power operation without system trip, in order
to demonstrate soundness of its functioning. Since its design
provides sufficient redundancy and independence, the instrumentation
channels and logic circuit trains can be tested separately, while
the remaining parts of the system during the testing operation
continue to meet the safety
3
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