Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
September 14, 1988
Information Notice No. 88-74: POTENTIALLY INADEQUATE PERFORMANCE OF
ECCS IN PWRs DURING RECIRCULATION OPERATION
FOLLOWING A LOCA
Addressees:
All holders of operating licenses or construction permits for Westinghouse (W)
and Babcock and Wilcox (B&W)-designed nuclear power reactors.
Purpose:
This information notice is being provided to alert addressees to potential
problems that could result in inadequate performance of the emergency core
cooling system (ECCS) during the recirculation phase of operation following a
loss-of-coolant accident (LOCA). It is expected that recipients will review
the information for applicability to their facilities and consider actions, as
appropriate, to avoid similar problems. However, suggestions contained in
this information notice do not constitute NRC requirements; therefore, no
specific action or written response is required.
Description of Circumstances:
The NRC has received two licensee event reports on potential deficiencies in
performance of the ECCS during the recirculation phase of operation following
a small-break LOCA. These events were reported by the licensees of the Oconee
and Turkey Point facilities after engineering analysis for each facility had
identified the potential problems.
Oconee Nuclear Station
During a technical review of nuclear safety-related mechanical calculations,
the licensee discovered that the design analysis had not been adequately per-
formed for the recirculation phase of operation following a small-break LOCA.
Subsequent engineering analysis indicated the following design deficiencies:
(1) Under certain small-break LOCA scenarios, when the borated water storage
tank (BWST) is depleted and the reactor coolant system (RCS) pressure
remains high (greater than the low-pressure injection (LPI) pump shut-off
head), a piggyback-type operation is required to ensure core cooling.
This operation entails taking suction from the reactor building sump,
through the LPI pumps, through the high-pressure injection (HPI) pumps,
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. IN 88-74
September 14, 1988
Page 2 of 3
and then to the RCS. However, because of (1) the high flow resistance in
the cross-connect piping between the HPI and LPI pumps, (2) the potential
for partially closed LPI discharge throttle valves, and (3) the high flow
rate downstream of the LPI pumps, there may not have been adequate net
positive suction head (NPSH) at the suction inlets of the HPI pumps.
Inadequate NPSH would render the HPI system inoperable. The high flow
rate would have occurred when the reactor building spray pumps and HPI
pumps take suction simultaneously from the LPI pump discharge.
The licensee has corrected the problem by revising the operating pro-
cedures to provide guidance for maintaining adequate NPSH to the HPI
pumps operating in the piggyback mode. The revised procedures provide
for (1) HPI flow limitations when the HPI and reactor building spray
pumps are piggybacked off the LPI pumps, (2) precautions on the use of
reactor building spray, and (3) precautions on the throttling of the LPI
discharge valves LP-12 and LP-14. (A simplified Oconee ECCS and reactor
building spray system are shown in Attachment 1.)
(2) A single failure of the "TD" ECCS switchgear during loss of offsite power
would have prevented the remote alignment of the LPI system to the HPI
system in the piggyback mode because (1) valves LP-9, LP-15, and LPI pump
B are powered by ECCS switchgear bus "TD" but their backup power supplies
are classified as load shed power supplies, and (2) valves LP-9 and LP-15
were in the normally closed position.
The licensee has corrected the problem by (1) changing valve LP-9 to a
normally open position so that it will not be required to operate when
the LPI system is being aligned with the HPI system in the piggyback mode
of operation, and (2) revising the emergency operating procedure to
ensure time is available for operators to locally, manually open valves
LP-15 and LP-16 (if the valves cannot be opened remotely) before LPI
suction must be swapped from the BWST to the reactor building sump.
Turkey Point Nuclear Station
As part of its ongoing design-basis reconstitution program, the licensee dis-
covered a condition of potentially inadequate NPSH for the containment spray
and safety injection (SI) pumps during the recirculation mode of operation
following a LOCA. Under these conditions, the low head residual heat removal
(RHR) pumps are providing flow to the containment spray pumps and the SI
pumps. A throttle valve in each RHR discharge line is normally throttled to
30 percent. However, an analysis to support this valve throttling position
had not been performed. To resolve this discrepancy, the subject valves were
locked in the fully open position until an engineering evaluation showed that
the original configuration (30 percent open) was acceptable.
. IN 88-74
September 14, 1988
Page 3 of 3
Discussion:
Following a small-break LOCA, the piggyback mode of operation may be required
(depending on the size and location of the break) to sustain injection flow
when the BWST (B&W plant) or refueling water storage tank (Westinghouse plant)
has been depleted and RCS pressure remains above the shutoff head of the low
head pumps. As previously described, two mechanisms could prevent this in-
tended piggyback mode from operating: (1) insufficient NPSH at HPI (or SI)
pump suction inlets, or (2) inadequate operating procedures combined with
system design deficiencies for a loss-of-offsite-power condition. Although
the low head pumps provide relatively high discharge pressure, the intervening
cross-connect piping between the high head and low head systems and/or
partially closed low head pump discharge valves could result in excessive
hydraulic resistance and cause insufficient NPSH at the HPI (or SI) pump
suction inlets. Insufficient NPSH could also be caused by running multiple
pumps off the dis-charge of a single pump that may increase the flow above the
design flow rate and consequently reduce the available pressure at the low
head pump discharge. These are safety concerns because of the reliance on HPI
(or SI) for mitigating small-break LOCAs, and in some cases, the
piggyback-type operation is used for hot leg recirculation even for
large-break LOCAs.
There are no redundant systems to perform the function of the HPI (or SI)
system in the piggyback mode. Although the reactor vessel high-point vents
and the power-operated relief valve(s) could be used to depressurize the RCS
to low head pump operating pressures, the related safety analysis had never
been performed - thus, their effectiveness was not ensured.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the
technical contacts listed below or the Regional Administrator of the
appropriate regional office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: S. Israel, AEOD
(301) 492-4437
J. Thompson, NRR
(301) 492-1175
P. Wen, NRR
(301) 492-1172
Attachments: 1. Figure 1, Simplified Oconee Emergency Core Cooling
and Reactor Building Spray Systems
2. List of Recently Issued NRC Information Notices
. Attachment 2
IN 88-74
September 14, 1988
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
88-73 Direction-Dependent Leak 9/8/88 All holders of OLs
Characteristics of or CPs for nuclear
Containment Purge Valves power reactors.
88-72 Inadequacies in the Design 9/2/88 All holders of OLs
of dc Motor-Operated Valves or CPs for nuclear
power reactors.
88-71 Possible Environmental 9/1/88 All holders of OLs
Effect of the Reentry or CPs for nuclear
of COSMOS 1900 and power reactors,
Request for Collection fuel cycle
of Licensee Radioactivity licensees, and
Measurements Attributed Priority 1
to That Event material
licensees.
88-70 Check Valve Inservice 8/29/88 All holders of OLs
Testing Program or CPs for nuclear
Deficiencies power reactors.
88-69 Movable Contact Finger 8/19/88 All holders of OLs
Binding in HFA Relays or CPs for nuclear
Manufactured by General power reactors.
Electric (GE)
88-48, Licensee Report of Defective 8/24/88 All holders of OLs
Supplement 1 Refurbished Valves or CPs for nuclear
power reactors.
88-68 Setpoint Testing of Pres- 8/22/88 All holders of OLs
surizer Safety Valves with or CPs for nuclear
Filled Loop Seals Using power reactors.
Hydraulic Assist Devices
88-67 PWR Auxiliary Feedwater Pump 8/22/88 All holders of OLs
Turbine Overspeed Trip or CPs for nuclear
Failure power reactors.
88-66 Industrial Radiography 8/22/88 All NRC industrial
Inspection and Enforcement radiography
licensees.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
