Nuclear Plant Journal, July-August 2014 NuclearPlantJournal.com
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recirc phase of the event). Following
transfer of the low head injection pumps
to the sump, the intermediate head and
high head injection pumps must be
aligned to take suction from the discharge
of the low head injection pumps. This
operating mode for the intermediate
and high pressure pumps is known as
high pressure recirc, and it is necessary
because the intermediate and high head
pumps cannot take suction directly from
the sump due to net positive suction head
(NPSH) limitations.
Transferring to high pressure recirc
successfully following a small break is
a risk-significant scenario for McGuire
and Catawba. In the original plant design,
with all pumps taking suction from the
RWST, it takes place approximately 30
minutes following the initiation of the
postulated accident to reach high pressure
recirc. Following the high pressure recirc
alignment, the containment spray pumps
would continue to deplete the RWST
to the low-low level alarm setpoint,
where the spray pumps would be
stopped, the suction source transferred
to the containment sump, and the pumps
restarted. Another nuance of the original
plant design basis was a requirement to
direct one train of low head injection flow
to auxiliary containment spray headers
50 minutes after accident initiation in
order to ensure that containment design
pressure is not exceeded following the
melting of all of the ice. This alignment,
known as residual heat removal (RHR)
spray, essentially redirects a significant
portion of the injection to the upper
containment instead of core injection for
core cooling.
Over the years since initial plant
licensing, Duke Energy developed a
comprehensive in-house capability for
ice condenser containment analysis using
the GOTHIC and RELAP5 computer
codes. RELAP5 is used for the system
mass and energy release calculations and
GOTHIC is used for the containment
response analysis. The GOTHIC methods
were validated against ice condenser
performance data developed by Pacific
Northwest Laboratories under the
sponsorship of Duke Energy, TVA and
American Electric Power, the United
States utilities that operate plants with
ice condenser containments. The Duke
Energy RELAP5/GOTHIC methodology
is mechanistic and represents a significant
improvement over the vendor methods
that were part of the original plant design
basis.
Primary Benefits
1. Significant
improvement
in
plant safety as measured by core
damage frequency (CDF).
The
changes result in approximately a 16
percent CDF reduction for Catawba
and an 18 percent CDF reduction
for McGuire. The main source of
this reduction is the small break
LOCA “high pressure recirc” failure
described below.
ECCS Water Management changes
reduced the probability of needing
to transfer to high pressure
Recirc for Small Break Loss of
Coolant Accidents (SBLOCAs),
thereby reducing the possibility
of the operators or plant failing to
successfully make the transition.
The plant response to a small break
LOCA is a function of break size
and location. For some break sizes,
the plant will depressurize enough
before RWST depletion that core
cooling can then be provided directly
from low head injection. ECCSWater
Management extends the time of the
injection phase, so more break sizes
depressurize fast enough to avoid the
need for high pressure recirc. The
additional time is available primarily
because the containment spray
pumps would not automatically
actuate, and because the minimum
RWST inventory increased and the
low RWST level alarm setpoint was
reduced.
The significantly increased amount
of time available before transfer to
sump recirc for all LOCAs reduces
operator burden during an accident.
The benefit varies with the accident
scenario, but the amount of time
available is at least double the amount
before the ECCS Water Management
changes. As a result, the operators
have additional time to diagnose the
event and prepare for time-critical
evolutions such as transition to sump
recirc and, if necessary, high pressure
recirc.
2. Reduction in the debris loading on
the containment sump strainers.
The ECCS Water Management
Project reduces the total sump flow
rate (as recommended in NRC
Bulletin 2003-01,
Potential Impact
of Debris Blockage on Emergency
Sump Recirculation at Pressurized-
Water Reactors
) and thereby
decreases the transport of debris
to the containment sump screen.
Also, later initiation of sump recirc
provides increased settling time.
The net effect is additional margin
relative to Generic Safety Issue
(GSI)-191, “Assessment of Debris
Accumulation on PWR Sump
Performance.”
3. Increased RWST inventory for
core cooling injection.
As noted in
#1 above, this is due to increased
initial RWST inventory and lower
RWST low level setpoints. The
lower RWST level setpoints were
justified by taking credit for the
lower injection flow rates and refined
vortex formation correlations.
4. Reduction in diesel generator
loading during the initial portion
of the accident.
Prior to ECCS
Water Management there was
relatively little margin in the
emergency diesel generator loading
sequence. Removing the electrical
load of the containment spray pumps
from the initial sequence provides
considerable margin.
5. Eliminated the transfer to sump
recirc for secondary system breaks
(main steam line breaks and main
feedwater line breaks) inside
containment.
Without the ECCS
Water Management changes, high
energy line breaks in containment
would actuate containment spray
and deplete the RWST to the point
at which transfer to sump recirc
would be required. The actuation of
containment spray complicates the
operator response to the transient
and adds time pressure. The operator
response is simplified with the ECCS
Water Management changes.
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