core to about two-thirds of the core height in the static water head
upon a recirculation line break. Once the core is reflooded, the
LPCI system compensates only for the water lost by evaporation due
to the decay heat. 'Therefore,
one LPCI loop is sufficient even for a guillotine break of the
recirculation line. The other two loops are manually switched to the
containment spray systems (Section 2.7.3 (3)).
High pressure core spray (HPCS) system
The HPCS system is composed of one motor driven pump, spargers,
piping, valves, and the I&C systems (Figure 2.7.5). This system
injects coolant into the core at any RPV pressures under accident
conditions (from high to low). In a large pipe break accident, the
HPCS system cools the core, together with the LPCS system and the
LPCI system. In a small or medium pipe break, the HPCS system cools
the core by itself.
This system is actuated by a signal “low reactor water level
(level 2)” or “high
drywell pressure”. The system cools the core by spraying water of
the condensate storage tank or the suppression pool atop the fuel
assemblies through the nozzles of the spray sparger header set up
above the core (Figure 2.7.5). The “high reactor water level
(level 8)” automatically halts the spary. The HPCI system uses the
water of the condensate storage tank as its first water source. When
the water level of the condensate storage tank falls below the
pre-specified value or the water level of the suppression pool
exceeds the pre-specified value, the HPCI system water source is
automatically switched to the suppression pool as its ultimate water
source.
Automatic depressurization system (ADS)
The ADS cools the core, together with the LPCI system or the LPCS
system, when the HPCS system is not actuated in a small or medium
pipe break accident.
This system is actuated 120 s after it receives both signals of “low
reactor water level (level 1)” and “high drywell pressure”.
This system promptly depressurizes the RPV pressure by releasing the
steam of the reactor to the suppression pool, and this enables the
LPCI system or the LPCS system to start injecting water into the
core for cooling.
Table 2.7.1 explains how the ECCS network meets the requirements of
redundancy and independency mentioned in the design policies.
Thus, even in a large break of the recirculation piping with a
concurrent single failure of an active component and a
loss-of-offsite-power, an appropriate combination of systems
composing the ECCS network can sufficiently achieve its functions:
depressurization, spray cooling, core reflooding and long term
cooling (Table 2.7.2).
An example of the ECCS network of an ABWR is shown in Figure 2.7.6.
A unique characteristic of the ABWR is that it adopts an RIP system
instead of the conventional external recirculation system.
This design change excludes the large diametric recirculation piping
from the ECCS design pipe break. The new design pipe break is a
small or medium pipe break or a main steam line pipe break above the
core. The ECCS of an ABWR is. designed accordingly as in the
following.
i) ECCS functions
The ECCS of an ABWR has the functions, as in a BWR-5, in order to
ensure core coolability upon a LOCA: core cooling, depressurization
and long
Figure
2.7.5 HPCS system configuration
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Eccs configuration of an abwr
Chapter
2 Systems of BWR Nuclear Power Plants
Table
2.7.1 ECCS functions (BWR-5) |
design philosophy |
redundancy |
independency |
system |
|
a. core cooling |
(i) spray cooling |
2 independent core spray lines, each having sufficient cooling capacity. |
O |
O |
HPCS LPCS |
(ii) reflooding cooling |
3 independent LPCI loops with reflooding capability. Each core spray system has the reflooding capacity equivalent to 1 LPCI loop. 3 loops together have sufficient injection capacity for reflooding, even in the most severe pipe break. |
o |
o |
LPCI HPCS LPCS |
|
b. reactor depressurization |
(i) cold water injection |
1 core spray loop works in the high pressure condition for depressurization. |
o |
o |
HPCS |
(ii) steam relief |
An automatic depressurization valve system having sufficient depressurization capacity under 1 valve failure assumption. |
ADS |
|||
c. Long term decay heat removal |
(i) water injection to core |
Either 1 CSS line or 1 LPCI loop has sufficient reflooding capacity. |
o |
o |
HPCS LPCS |
(ii) pressure suppression pool water cooling |
Each of 2 LPCI loops has 1 heat exchanger, each loop has sufficient capacity for pool water cooling. |
o |
o |
RHR |
|
term decay heat removal. Unique features of the ABWR
ECCS for core cooling are the following.
Core cooling function
The spray cooling method is replaced by the water injection method
to keep the core reflooded and cooled. This is because no pipe break
in an ABWR uncovers the core.
To strengthen the high pressure system redundancy, the core cooling
function of the ECCS is added to the reactor core isolation cooling
(RCIC) system.
System redundancy and independency
The ECCS of an ABWR is designed, as in a BWR-5, to deal with a
single failure of an active component in the ECCS network and a
loss-of- offsite-power supply simultaneously with any pipe breaks
composing the reactor pressure boundary.
The ECCS consists of three independent divisions of high and low
pressure systems with limited capacities, since no large diametric
pipe
break needs to be assumed in an ABWR
Low pressure flooder (LPFL) system
The LPFL system uses the low pressure injection mode of the RHR
system. It collects water from the suppression pool and injects it
into the outer space of the core shroud in the RPV.
The LPFL system keeps the core covered in a LOCA,
together with the high pressure core flooder (HPCF) system, the RCIC
system and the ADS.
The LPFL system consists of three independent loops as shown in
Figure 2.7.6. The LPFL system is automatically actuated by a signal
“low reactor water level Qevel
1)” or “high drywell
pressure”. One LPFL loop injects water through the feed water
piping.
High pressure core flooder (HPCF) system
The HPCF system injects water into the reactor core shroud from the
condensate storage tank as its initial water source and then from
the
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Table
2.7.2 ECCS redundancy for active component single failure
assumption |
effective ECCS |
short term core cooling |
long term core cooling |
|||||||||
ADS |
HPCS |
LPCS |
LPCI |
reactor depressurization |
core spray cooling |
reflooding cooling |
PSP water cooling |
core cooling |
||||
no failure |
(n+1) valves |
1 |
1 |
3 loops |
ADS HPCS |
HPCS LPCS |
HPCS LPCS 3LPCI |
RHR-HX #1 RHR-HX #2 |
HPCS LPCS LPCI#2 |
|||
1 |
IADS valve |
n |
1 |
1 |
3 |
ADS HPCS |
HPCS LPCS |
HPCS LPCS 3LPCI |
RHR-HX #1 RHR-HX #2 |
HPCS LPCS LPCI#2 |
||
2 |
HPCS pump |
n+1 |
0 |
1 |
3 |
ADS |
LPCS |
LPCS 3LPCI |
RHR-HX#! RHR-HX #2 |
HPCS LPCS LPCI#2 |
||
3 |
LPCI#1 pump |
n+1 |
1 |
1 |
2 |
ADS HPCS |
HPCS LPCS |
HPCS LPCS 2LPCI |
RHR-HX #2 |
HPCS LPCS LPCI#2 |
||
4 |
emergency DG#1 |
n+1 |
0 |
1 |
3 |
ADS |
LPCS |
LPCS 3LPCI |
RHR-HX #1 RHR-HX #2 |
LPCS LPCI#2 |
||
5 |
emergency DG#2 |
n+1 |
1 |
1 |
2 |
ADS HPCS |
HPCS |
HPCS 2LPCI |
RHR- HX#2 |
HPCS LPC1&2 |
||
6 |
emergency DG#3 |
n+1 |
1 |
1 |
1 |
ADS HPCS |
HPCS LPCS |
HPCS LPCS ILPCI |
RHR-HX # 1 |
HPCS LPCS |
||
Notes
to this table
PSP: Pressure suppression pool
HX: Heat exchanger
DG: Diesel generator
suppression pool as its ultimate water source.
The HPCF system keeps the core covered with water in a LOCA,
together with the RCIC system, the LPFL system and the ADS. The HPCF
system backs up the RCIC system in recovering the reactor water
level in transients or loss of feedwater accidents and other
associated incidents.
The HPCF system consists of two independent trains (Figure 2.7.6)
and is automatically actuated by a signal “low reactor water level
(level 1.5)” and is automatically halted by a signal “high
reactor water level (level 8)”. The HPCF system injects water
through the sparger header nozzles, differing from the HPCS system
of a conventional BWR-5.
Reactor core isolation cooling (RCIC) system
The RCIC system has an ECCS function to strengthen the high pressure
system redundancy.
The RCIC system of an ABWR keeps the core covered, together with the
HPCF system, the LPFL system and the ADS in a LOCA caused by a small
diametric pipe break. The RCIC system injects water into the reactor
vessel via the feed water line, initially from the condensate
storage tank and eventually from the suppression pool as its water
source.
Automatic depressurization system (ADS)
The ADS is partly different from that of a BWR-5. It is actuated 30
s (cf. 120 s) after it receives both signals of “low reactor water
level (level 1)” and “high drywell pressure”. This system
promptly depressurizes the reactor pressure by releasing the reactor
steam to the suppression pool, and enables the LPFL system to inject
water into the core for cooling.
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