Information Notice 1997-76, Degraded Throttle Valves in Emergency Core Cooling System Resulting from Cavitation-Induced Erosion During a Loss-of-Coolant Accident
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UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
October 30, 1997
NRC INFORMATION NOTICE 97-76: DEGRADED THROTTLE VALVES IN EMERGENCY
CORE COOLING SYSTEM RESULTING FROM
CAVITATION-INDUCED EROSION DURING A LOSS-
OF-COOLANT ACCIDENT
Addressees
All holders of operating licenses for pressurized-water reactors except those who have
permanently ceased operations and have certified that fuel has been permanently removed
from the reactor vessel.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert
addressees to potential problems caused by degradation of emergency core cooling system
(ECCS) throttle valves in the intermediate-head safety Injection pump hot-leg and cold-leg
flow paths and in the charging pump (high-head safety Injection) cold-leg flow paths during
certain loss-of-coolant-accident (LOCA) scenarios. 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 are not NRC
requirements; therefore, no specific action or written response is required.
DescriDtion of Circumstances
On January 11, 1996, Westinghouse Issued Nuclear Safety Advisory Letter (NSAL)96-001,
"Erosion of Globe Valves in ECCS Throttling Applications," which alerted Westinghouse plant
owners to the potential cavitation and erosion of throttling valves used in high- and
Intermediate-head ECCS. The valve erosion occurs as a result of high differential pressure
and flow that may occur during a LOCA. As a result of the valve erosion, the high- and
intermediate-head safety injection pumps may exceed their run out limits, thus forcing the
operator to secure the pumps before the time established in the licensing basis for pump
operability. This issue was originally Identified at Sequoyah Nuclear Plant through the
corrective action program generic review of a problem at the Watts Bar Nuclear plant in
1993.
Using plant-specific calculations, the Tennessee Valley Authority (TVA) evaluated the service
conditions for the ECCS throttle valves during post-LOCA long-term recirculation (more than
100 days) at Sequoyah and Watts Bar. The licensee concluded that the valves might be
operating under conditions that could result in erosion-induced damage and eventual failure.
A Westinghouse evaluation of the ECCS throttle valves confirmed TVA's analysis and
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<-IN 97-76 October 30, 1997 recommended that (1) the throttle valves be replaced with valves better able to operate for
extended periods under the analyzed conditions, (2) orifice plates be Installed in series with
the throttle valves to reduce the pressure drop across the valves, or (3) testing be performed
to better define the wear rate and determine appropriate corrective actions. At both
Sequoyah and Watts Bar, orifice plates will be installed.
The Westinghouse evaluation identified two operational effects caused by degradation of the
ECCS throttle valves during a LOCA. The first effect is reduced throttling capability of the
valves. This occurs through erosion of the valves as a consequence of cavitation
engendered by high differential pressure and flow across the valves. As a result of the
erosion, In approximately 12 days the valves would be unable to adequately throttle ECCS
flow. The second effect is increased pump flow in that valve erosion could increase the
valves' throttling area and result in increased pump flow. In this increased flow condition, the
ECCS pumps could reach run out conditions, resulting in pump damage.
Discussion
The subject throttle valves are located in ECCS flow paths for both the high-head and
Intermediate-head safety injection pumps and are designed to balance Injection flows
between loops while maintaining total Injection flow within normal parameters.
NRC requirements and guidance for the ECCS are provided In 10 CFR 50A6, "Acceptance
Criteria for Emergency Core Cooling Systems for Light-Water Nuclear Power Reactors," and
10 CFR Part 50, Appendix A, "General Design Criteria for Nuclear Power Plants," Criteria 33,
34, and 35. Section 50.46 requires, In part, that "after any calculated successful Initial
operation of the ECCS, the calculated core temperature shall be maintained at an acceptably
low value and decay heat shall be removed for the extended period of time required by the
long-lived radioactivity remaining in the core."
To meet this requirement during a LOCA, the high-pressure ECCS Injects water into the
reactor coolant system (RCS) cold legs to provide cooling flow to the reactor core. Following
the initial injection phase, the ECCS pumps may be aligned for cooling In the recirculation
phase.
During a LOCA, the ECCS may experience a large pressure drop across the throttling valves, thereby causing cavitation and valve erosion. As the valves erode, their throttling capability
Is reduced. This reduction, In turn, Increases ECCS pump flow. If the ECCS pumps reach
run out conditions as a result of the continued valve erosion, the pumps may have to be
stopped before the plant can utilize the residual heat removal (RHR) system for core cooling.
At Sequoyah and Watts Bar, the valves that could experience this type of erosion are
stainless steel, plug guided globe valves with stellite plugs and seats. Valves for this
application are often supplied by the utility andlor the architect engineer. Thus, there may be
other valve designs used In this application.
Westinghouse analyzed long-term operability of throttle valves during both a small-break
LOCA (SBLOCA) and a large-break LOCA (LBLOCA). During an SBLOCA, the pressure
IN 97-76 October 30, 1997 drop across the valves, and the resulting valve erosion, Is smaller than that experienced
during an LBLOCA. Relatively little valve erosion would occur before the plant's emergency
operating procedures (EOPs) direct the operator to secure the pumps. As a result, if the
pumps are secured In accordance with the plant EOPs following an SBLOCA, minimal valve
erosion will occur. This will allow the valves to perform their throttling function and will
prevent pump run out and subsequent pump damage during the SBLOCA.
However, for LBLOCAs, the rate and amount of valve erosion are greater because of the
greater differential pressure across the valves. As a result, the valves may erode to the point
at which high- and intermediate-head pump flows could reach run out conditions, thus
necessitating the premature shutoff of the pumps.
Another problem associated with erosion of this type of valve concerns preventing boron
precipitation in the RCS. Without adequate flow through the core, boron will concentrate in
the reactor vessel. When the boron concentration reaches the solubility limit, boron could
plate out in the core and inhibit flow. If boron plate out occurs, the safety injection pumps
and the RHR pumps can be realigned to inject into the RCS hot legs to provide hot-leg
recirculation and prevent boron accumulation. However, because the valves in the
recirculation lines for the safety injection pumps can also experience the same valve erosion
that occurs in the cold-leg injection line, the RHR pumps may have to be used to provide hot- leg recirculation. Depending on the system design, in this mode of operation a single failure
could occur which could Interrupt hot-eg recirculation flow, leading to possible boron
precipitation In the core. This could occur if the single motor-operated crossover valve fails.
Westinghouse, as part of oA's Justification for Continued Operation (JCO), analyzed four
scenarios related to the effect of an LBLOCA on core cooling and boron precipitation. These
scenarios constituted the worst-case challenges to establishing and maintaining core cooling
and hot-leg recirculation. Each scenario is discussed below.
(1) Large Cold-Leg Break With No FaIlure of the RHR Hot-Leg Motor-Operated
Crossover Valve
In this scenario, RHR flow would be realigned to the RCS hot legs approximately
12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> after the LBLOCA (in accordance with the EOPs). With the break In the cold
leg and the RHR realigned to the hot leg, a flow path through the core is established
and maintained while preventing boron buildup In the core. In this scenario, flow from
one RHR pump would be sufficient to maintain core cooling and prevent boron
precipitation.
(2) Large Cold-Leg Break With Failure of the RHR Hot-Leg Motor-Operated Crossover
Valve
In this scenario, an attempt would be made to realign RHR flow to the RCS hot legs
approximately 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> after the LBLOCA (in accordance with the EOPs). However,
IN 97-76 October 30, 1997 realignment would be unsuccessful due to the postulated failure of the motor-operated
crossover valve. RHR flow would then need to be realigned to the cold legs, resulting in
cooling water bypassing the core and flowing out the break. Boron concentration would
Increase, ultimately causing boron precipitation in the core.
(3) Large Hot-Leg Break With No FaIlure of the RHR Hot-Leg Motor-Operated
Crossover Valve
In this scenario, RHR flow would be realigned to the RCS hot legs approximately
12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> after the LBLOCA (in accordance with the EOPs). Because the break is in the
hot leg, Injection flow would transit above the core and out the break without substantial
mixing In the core. For this scenario, adequate flow through the core is available If the
hot-leg Injection flow exceeds 3.3 times the boil off. This requirement is met with one
RHR pump.
(4) Large Hot-Leg Break With Failure of the RHR Hot-Leg Motor-Operated Crossover
Valve
In this scenario, an attempt would be made to realign RHR flow to the RCS hot legs
approximately 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> after the LBLOCA (in accordance with EOPs). However, realignment would be unsuccessful because of the postulated failure of the motor- operated crossover valve. In this case, RHR flow would be realigned to the cold legs, thus allowing one RHR pump to provide adequate flow through the core and preventing
boron buildup in the core.
Westinghouse evaluated these scenarios and determined that In each case, with the
exception of scenario 2, the flow from the RHR pumps was sufficient to mitigate the accident.
In scenario 2, as there Is only a single flow path for hot-leg Injection using an RHR pump, a
failure of the motor-operated crossover valve in this flow path could cause a loss of hot-leg
injection during the long-term recirculation cooling mode. This scenario could result In
blockage of the reactor coolant flow path to the core because of boron precipitation once the
boron solubility limit was reached. It should be noted that in scenario 2, the review took
credit for recirculation flow through the hot-leg nozzle gaps in a forward flush path through
the core in accordance with a methodology described In Westinghouse NSAL-94-016,
"Recriticality During LOCA Hot-Leg Recirculation," dated July 25, 1994. This methodology
has not been reviewed or approved by the NRC. Therefore, depending on plant-specific
components and design, the conclusions may or may not be valid.
IN 97-76 October 30, 1997 This information notice requires no specific action or written response. If you have any
questions about the information in this notice, please contact one of the technical contacts
listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
W. Roe, Acting Director
%sion of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Chu-Yu Liang, NRR
301-415-2878 E-mail: cyl@nrc.gov
William F. Burton, NRR
301-415-2853 E-mail: wfbenrc.gov
Attachment: List of Recently Issued NRC Information Notices
.
I
"-u Attachment
October 30, 1997
Page 1 of I
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
Information
Date of
Notice No.
Subject
Issuance
Issued to
97-78
97-77
97-75
97-74
Crediting of Operator
Actions In Place of
Automatic Actions and
Modifications of Operator
Actions, Including Response
Times
Exemptions from the
Requirements of
Section 70.24 of
Title 10 of the Code
of Federal Regulations
Enforcement Sanctions
Issued as a Result of
Deliberate Violations
of NRC Requirements
Inadequate Oversight
of Contractors During
Sealant Injection
Activities
10/23/97
10/10/97
09/24/97
09/24/97
All holders of OAs for
nuclear power reactors
except those who have
permanently ceased
operations and have
certified that fuel has
been permanently removed
from the reactor vessel
All holders of OLs for
nuclear power reactors
All U.S. Nuclear
Regulatory Commission
licensees
All holders of OLs for
nuclear power reactors
except those who have
permanently ceased
operations and have
certified that fuel
has been permanently
removed from the
reactor vessel
OL Operating Ucense
CP = Construction Permit
IN 97-76 October 30, 1997 This information notice requires no specific action or written response. If you have any
questions about the information in this notice, please contact one of the technical contacts
listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
original signed by
Jack W. Roe, Acting Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts:
Chu-Yu Liang, NRR
301-415-2878 E-mail: cyl@nrc.gov
William F. Burton, NRR
301-415-2853 E-mail: wfb@nrc.gov
Attachment: List of Recently Issued NRC Information Notices
Tech Editor has reviewed and concurred on 8/19/97 DOCUMENT NAME: 97-76.IN
- SEE PREVIOUS CONCURRENCE
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1OFFICE
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DATE
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729197
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9/11197
10/10/97
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OFFICIAL RECORD COPY
IN 97-xx
October , 1997 This information notice requires no specific action or written response. If you have any
questions about the information in this notice, please contact one of the technical contacts
listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
Jack W. Roe, Acting Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Chu-Yu Liang, NRR
301-415-2878 E-mail: cyl@nrc.gov
William F. Burton, NRR
301-415-2853 E-mail: wfb@nrc.gov
Attachment: List of Recently Issued NRC Information Notices
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IN 97-xx
October , 1997 This Information notice requires no specific action or written response. If you have any questions
about the information in this notice, please contact one of the technical contacts listed below or
the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
Jack W. Roe, Acting Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts:
Chu-Yu Liang, NRR
301-41 5-2878 E-mail: cyt@nrc.gov
William F. Burton, NRR
301-415-2853 E-mail: wfb~nrc.gov
Attachment: Ust of Recently Issued NRC Information Notices
IT NAME: G:\\WFB\\SEQECCSI
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DOCUMEIN
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IN 97-xx
September , 1997 This information notice requires no specific action or written response. If you have any
questions about the information in this notice, please contact one of the technical contacts
listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
Jack W. Roe, Acting Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts:
Chu-Yu Liang, NRR
301-415-2878 E-mail: cyl@nrc.gov
William F. Burton, NRR
301-415-2853 E-mail: wfbenrc.gov
Attachment: List of Recently Issued NRC Information Notices
DOCUMENT NAME: G:\\WFB\\SEQECCS1 To receive a copy of this document, Indicate In the box:
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August , 1997
Page
5 EOPs, sufficient RHR flow would be available for core cooling. However
e potential for
boron precipitation in the core when using an RHR pump for hot-leg iection would still need
to be addressed.
This information notice requires no specific action or written re onse. If you have any
questions about the information in this notice, please contact ne of the technical contacts
listed below or the appropriate Office of Nuclear Reactor R
ulation (NRR) project manager.
M ,lee M. Slosson, Acting Director
-iision of Reactor Program Management
ice of Nuclear Reactor Regulation
Technical contacts:
William Holland, RI/
404-562-9612 E-mail: weh@nrc.ov
William F. Burt
, NRR
30141 5-285 E-mail: wfb nrc.gov
Chuyu Ling, NRR
30141
2878 E-mai cyl@nrc.gov
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tly Issued NRC Information Notices
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EQECCS1
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CYLiang 7/30/97*
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08/ /97
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v-IN 97-xx
July , 1997 A'
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with the plant EOPs, sufficient RHR flow would be available for core cooling. Hoyvver he
problem of boron precipitation in the core when using an RHR pump for hot-leg Anjet on still
exists.
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This information notice requires no specific action or written response. If y u
ave any
questions about the information in this notice, please contact one of the technical contacts
listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
Marylee M. Sl0
on, Acting Director
Division of Rejctor Program Management
Office of Nu ear Reactor Regulation
Technical contacts:
William Holland, RII
/
404-562-9612
/
E-mail: weh@nrc.gov
William F. Burton, NRR
301-415-2853 E-mail: wfb@nrc.gov
Chuyu Liang, NRR
301-415-2878 E-mail: cyl@nrc.g
Attachment: List of Recently Issued
C Information Notices
DOCUMENT NAME: G:\\WFBISEQECCS
- SEE PREVIOUS CONCURRENCE
To receive a copy of this document, Indicate In he
x: "C" = Copy without enclosures "E' a Copy with enclosures IN" - No copy
OFFICE
Contacts
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