Information Notice 1992-36, Intersystem LOCA Outside Containment
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
May 7, 1992 NRC INFORMATION NOTICE 92-36:
INTERSYSTEM LOCA OUTSIDE CONTAINMENT
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice
to alert addressees of potential plant vulnerabilities to intersystem loss-of- coolant accidents (ISLOCAs).
It is expected that recipients will review
the information for applicability to their facilities and consider actions, as
appropriate.
However, suggestions contained in this information notice are not
NRC requirements; therefore, no specific action or written response is required.
This information notice provides information gathered during a concerted NRC
staff effort to study plant vulnerabilities to ISLOCAs.
The staff gathered
this information by performing (a) detailed evaluations of operating events, (b) inspections of a limited sample of pressurized water reactors (PWRs), and
(c) extensive analyses of the sample PWRs.
The information may be of use in
recipients' individual plant examination (IPE) programs.
Background
The ISLOCA is a class of accidents in which a break occurs in a system con- nected to the reactor coolant system (RCS), causing a loss of the primary
system inventory. This type of accident can occur when a low pressure system, such as the residual heat removal (RHR) system, is inadvertently exposed to
high RCS pressures beyond its capacity.
ISLOCAs of most concern are those
that can discharge the break flow outside the reactor containment building, primarily because they can result in high offsite radiological consequences but
also because the RCS inventory lost cannot be retrieved for long-term core
cooling during the recirculation phase.
In the "Reactor Safety Study," (WASH-1400), published in 1975, and in
NUREG-1150, "Severe Accident Risks: An Assessment for Five U.S. Nuclear Power
Plants," the NRC described the ISLOCA outside containment as an event of low
core damage frequency, but as one of the main contributors to plant risk.
In
those studies the NRC referred to the ISLOCA as "Event-V." Most probabilistic
risk assessments (PRAs) have also shown that the ISLOCA is very unlikely.
However, these PRAs typically have modelled only those Event-V sequences that
include only the catastrophic failure of check valves that isolate the RCS from
92050_ 45 AMA
IN 92-36 May 7, 1992 low pressure systems. These PRAs included little consideration of human errors
leading to an ISLOCA. Also, most existing PRAs have given little or no credit
for operator actions to terminate an ISLOCA or to mitigate its radiological
consequences if core melt were to occur.
On January 22, 1992, the Virginia Electric Power Company, licensee for the
North Anna Power Station, reported that the RHR relief valves would not pass
the design-basis flow to relieve an overpressurization of the RHR system when
the latter is aligned to the RCS.
The function of these relief valves is
important when the RCS is water solid and therefore susceptible to overpressur- ization events, such as from a charging-letdown flow mismatch or a temperature
change.
The licensee made this report after conducting an engineering evaluation to
respond to a notification by the nuclear steam supply vendor, the Westinghouse
Electric Corporation. In February 1990, Westinghouse reviewed the RHR relief
valve design basis for the Westinghouse Owners Group and recommended that its
customers review the following three items:
The adequacy of the RHR relief valves for protecting against cold
overpressure events
Discharge capability of relief valves for probable back pressures
Design basis commitments for valve specifications, commitments in the
final safety analysis report, and technical specifications
The NRC has issued several information notices to discuss certain operational
events regarding ISLOCAs. In IN 90-05, "Inter-system Discharge of Reactor
Coolant,"
the staff. discussed an event during which about 68,000 gallons of
reactor water was discharged outside the containment.
The staff has also
analyzed operational experience and documented its findings in augmented
inspection team (AIT) reports.
On October 23, 1990, the staff issued AIT
Report 50-456/90-020 on an event at Braidwood that resulted in primary water
leakage outside the containment and in the contamination of three personnel, one of whom received a second degree burn. Table 3 is a selected list of
information notices and AIT reports that the staff has issued on ISLOCAs and
related events.
Discussion
Although no ISLOCA has caused core damage, accumulated operational experience, both in the United States and abroad, indicates that ISLOCA-like events have
occurred at a rate higher than expected. In conducting this study, the staff
defined an ISLOCA-like event, or an ISLOCA precursor, as an event that results
from the failure, degradation, or inadvertent opening of the pressure isolation
valves (PIYs) between the RCS and lower pressure systems. An ISLOCA precursor
may become an ISLOCA if it occurs during different plant conditions, or if some
of the failures occur together.
The NRC staff conducted root cause analyses of ISLOCA precursors, extensive
plant inspections, and detailed analyses of a sample of PWRs.
These analyses
IN 92-36 May 7, 1992 included thermal-hydraulic analyses, fragility analyses to determine the likely
sizes and locations of a break, and human reliability analyses.
The staff used
the results of these analyses in PRAs to gain insights about the significant
contributors to ISLOCA risk.
The staff directed the studies described in this information notice towards
finding vulnerabilities of PWR plants to ISLOCAs, since the primary pressures
present in PWRs are greater than those found in boiling water reactors (BWRs),
while the design pressures of low pressure systems are about the same in both
PWRs and BWRs. However, BWR licensees also may find this information to be
relevant to their plants.
Upon conducting these studies, the staff made the following observations on the
ISLOCA risk at nuclear power plants:
1. The estimated core damage frequency caused by ISLOCAs could be greater
than was estimated in PRAs for some plants.
The ISLOCA risk depends on both the accident initiators and the capabili- ties for recovery. These factors vary from plant to plant.
The main
contributors to ISLOCA initiation and/or recovery include (a) human errors
and (b) the effects of the accident-caused harsh environment on plant
equipment and recovery activities. Both factors have significant uncer- tainties.
Existing PRAs have provided little or no treatment of these
factors. Plants that are particularly vulnerable to either of these two
factors could have a higher ISLOCA risk than indicated by existing PRAs.
2. Most plants lack contingency plans to provide backup water supplies that
can be transferred readily to provide long-term core cooling after an
By examining a plant's emergency procedures, a licensee can find
insights for improving the plant's features to address the concerns for
both ISLOCAs and other accidents.
3. The root cause analyses of operational events indicate that ISLOCA precur- sors most likely would be initiated by human errors, notably during
testing and maintenance or because of procedural deficiencies. This may
be attributed to the general lack of awareness of the possibility or
consequences of an ISLOCA.
Licensees may significantly reduce the probability of ISLOCA precursors
by improving the ability of operators and maintenance personnel to recog- nize ISLOCAs, mechanisms that can cause them, actions to prevent them, and
methods to manage them if they occur.
4. Most observed ISLOCA precursors have low public risk consequences. However, an ISLOCA precursor can require a shutdown or extension of a shutdown, require radioactivity cleanup operations, and cause personnel injury.
IN 92-36 May 7, 1992 Table 1 presents the staff's observations from root cause analyses and plant
inspections. Table 2 presents insights gained from the ISLOCA PRAs.
The staff is completing its ISLOCA research program under Generic Issue 105,
"Intersystem Loss of Coolant Accidents in Light Water Reactors."
Upon complet- ing this research, the staff may issue further generic correspondence to
licensees.
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.
es ERossi, Drectr
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts:
Kazimieras Campe, NRR
(301) 504-1092
Sammy Diab, RES
(301) 492-3914
Gary Burdick, RES
(301) 492-3812 Attachments:
1. Table 1. "Observed Plant Vulnerabilities to ISLOCA Precursors"
2. Table 2. "ISLOCA Risk Insights"
3. Table 3. "A Selected List of ISLOCA Reports and References"
4. List of Recently Issued NRC Information Notices
Attachment 1
IN 92-36 May 7, 1992 Table 1. Observed Plant Vulnerabilities to ISLOCA Precursors
(Obtained from root cause analyses of ISLOCA precursors and plant inspections)
1. Lack of awareness of the nature or consequences of ISLOCAs
2.
Inadequate emergency procedures for ISLOCA outside containment, especially
for non-power operational modes
3. Poor or incorrect valve labels
4. Different nomenclature used for the same equipment in the same plant
5. Poor coordination between concurrently run tests
6. Miscommunications between the control room operators and auxiliary opera- tors ("get the valve" is meant as "crack open then close," but understood
to mean "open")
7. Poor shift turn-over communications
8.
Poor post-maintenance testing or operability checks
9.
Inadequate application of independent verification
10.
Tendency not to check diverse instrument indications
11. Tendency to commit personnel to extensive overtime work, especially during
shutdown and startup operations, thus increasing the fatigue level and the
likelihood of errors
Attachment 2
May 7, 1992 Table 2. ISLOCA Risk Insights
1. The staff's studies suggest that the core damage frequency caused by an
ISLOCA could be substantially greater than previous PRA estimates for some
plants. This is primarily caused by the effects of operator errors and
harsh environments caused by the accident.
Valve alignment errors during
transition between operating modes can be particularly important.
2. Equipment qualified for a harsh environment is likely to survive the
adverse ISLOCA temperature and humidity, but not the possible submersion
caused by flooding.
3. Multiple system failures may result from the ISLOCA harsh environment or
flooding, depending on the size and location of the break in relation to
affected equipment, the separation of redundant trains, and the effect of
fire sprays on flooding.
4.
ISLOCA recovery is limited by harsh environments, which may damage essen- tial equipment thus complicating long-term cooling, and the rate of loss
of reactor water outside the containment. If the water is not quickly
replenished, an ISLOCA may lead to core damage, even after the leak has
been isolated.
5. Symptom-based procedures may lead the operator to realize that an ISLOCA
has occurred. However, unless the emergency procedures refer to plant
provisions for conserving and replenishing water, the operator may have
difficulty managing the accident.
6. Most observed ISLOCA precursors have low risk consequences, primarily
because of the presence of one or more of the following conditions:
small
leak size, redundant means of detecting and isolating a leak, and low
power or shutdown conditions.
Attachment 3
IN 92-36 May 7, 1992 Table 3. A Selected List of ISLOCA Reports and References
Identification
Title or Subject
Potential for Common-Mode Failure of
HPSI Pumps or Release of Reactor Coolant
Outside Containment During a LOCA
Inter-system Discharge of Reactor Coolant
Potential Overpressurization of Low
Pressure Systems
Date
10/04/90
01/29/90
11/01/89
10/23/90
05/08/89 AIT Report
50-456/90-20
AIT Report
50-293/89-80
An assessment of the 10/4/90 Braidwood
loss of reactor coolant inventory and
personnel contamination and injury
An assessment of the 4/12/89 Pilgrim
overpressurization event, which occurred
during the conduct of the RCIC logic test
Inspection
Report
50-382/90-200
Inspection
Report
50-413,414/90-200
Inspection
Report
50-346/89-201
ISLOCA Program Inspection
plant
ISLOCA Program Inspection
plants
ISLOCA Program Inspection
Besse plant
of the Waterford
of the Catawba
of the Davis
09/14/90
06/11/90
12/21/89 Audit
Report
Docket No. 50-213 NUREG/CR-5745 NUREG/CR-5744 NUREG/CR-5604 NUREG/CR-5124 NUREG/CR-5102 Haddam Neck ISLOCA Audit Report: July 24 -
August 4, 1989, Enclosure to Memorandum
from Frank J. Congel, NRC, to
Steven A. Varga, NRC*
Assessment for ISLOCA Risks -
Draft Methodology and Application:
Combustion Engineering Plant
Assessment for ISLOCA Risks -
Draft Methodology and Application:
Westinghouse Four-Loop Ice Condenser Plant
Assessment for ISLOCA Risks -
Draft Methodology and Application: Babcock
and Wilcox Nuclear Power Station
Interfacing Systems LOCA, Boiling
Water Reactors
Interfacing Systems LOCA, Pressurized
Water Reactors
09/20/89 June 91 Feb 91 Feb 91 Feb 89 Feb 89
-A COpy OT this report is available in the NRC Public Document Room,
2120 L Street, N.W., Washington, DC.
Attachment 4
May 7, 1992 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
Information
Date of
Notice No.
Subject
Issuance
Issued to
92-35
92-34
92-33
92-32
92-31
92-30
Higher Than Predicted Ero- sion/Corrosion in Unisol- able Reactor Coolant Pres- sure Boundary Piping Inside
Containment at A Boiling
Water Reactor
New Exposure Limits for
Airborne Uranium and
Increased Instrument
Response Time When
Pressure Dampening
Devices are Installed
Problems Identified with
Emergency Ventilation
Systems for Near-Site
(Within 10 Miles) Emer- gency Operations Facili- ties and Technical Support
Centers
Electrical Connection
Problem in Johnson
Yokogawa Corporation
YS-80 Programmable Indi- cating Controllers
Falsification of Plant
Records
Spent Fuel Pool Re- activity Calculations
05/06/92
05/06/92
04/30/92
04/29/92
04/27/92
04/23/92
04/22/92
All holders of OLs or CPs
for nuclear power reactors.
All licensees whose opera- tions can cause airborne
concentrations of uranium
and thorium.
All holders of OLs or CPs
for nuclear power reactors.
All holders of OLs or CPs
for nuclear power reactors.
All holders of OLs or CPs
for nuclear power reactors.
All holders of OLs or CPs
for nuclear power reactors
and all licensed operators
and senior operators.
All holders of OLs or CPs
for nuclear power reactors.
92-21, Supp. 1 OL = Operating License
CP = Construction Permit
IN 92-36 May 7, 1992 Table 1 presents the staff's observations from root cause analyses and plant
inspections.
Table 2 presents insights gained from the ISLOCA PRAs.
The staff is completing its ISLOCA research program under Generic Issue 105,
"Intersystem Loss of Coolant Accidents in Light Water Reactors." Upon complet- ing this research, the staff may issue further generic correspondence to
licensees.
This information notice requires
you have any questions about the
of the technical contacts listed
Reactor Regulation (NRR) project
no specific action or written response. If
information in this notice, please contact one
below or the appropriate Office of Nuclear
manager.
Original Signed by
Charles E Rei
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts:
Kazimieras Campe, NRR
(301) 504-1092
Sammy Diab, RES
(301) 492-3914
Gary Burdick, RES
(301) 492-3812 Attachments:
1. Table 1. "Observed Plant Vulnerabilities to ISLOCA Precursors'
2. Table 2. "ISLOCA Risk Insights"
3. Table 3. "A Selected List of ISLOCA Reports and References"
4. List of Recently Issued NRC Information Notices
Document Name: IN 92-36
- See previous concurrence.
C/OGCB:DOEA:NRR D/DOEA:NRR
- CHBerlinger
04/24/92 RPB:ADM
- TechEd
04/09/92 D/DSIR:RES
- WMinners
04/15/92 C/RPSIB:DSIR:RES
- KKniel
04/14/92 RPSIB:DSIR:RES
- GBurdick
04/13/92 C/EIB:DSIR:RES
- RLBaer
04/13/92 OGCB:DOEA:NRR
- CVHodge
04/08/92 SC/RAB:DREP:NRR
- KCampe
04/09/92 C/RAB:DREP:NRR
- WBeckner
04/09/92 D/DREP:NRR
- FCongel
04/09/92 EIB:DSIR:RES
- SDiab
04/13/92
IN 92-XX
April xx, 1992 Table 1 presents the staff's observations from root cause analyses and plant
inspections.
Table 2 presents insights gained from the ISLOCA PRAs.
The staff is completing its ISLOCA research program under Generic Issue 105,
"Intersystem Loss of Coolant Accidents in Light Water Reactors."
Upon complet- ing this research, the staff may issue further generic correspondence to
licensees.
This information notice requires no speci
you have any questions about the informat
of the technical contacts listed below or
Reactor Regulation (NRR) project manager.
fic action or written response. If
ion in this notice, please contact one
the appropriate Office of Nuclear
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts:
Kazimieras Campe, NRR
(301) 504-1092
Sammy Diab, RES
(301) 492-3914
Gary Burdick, RES
(301) 492-3812 Attachments:
1. Table 1. "Observed Plant Vulnerabilities to ISLOCA Precursors"
2. Table 2. "ISLOCA Risk Insights"
3. Table 3. "A Partial List of ISLOCA Reports and References"
4. List of Recently Issued NRC Information Notices
Document Name:
C/OGCB:DOEA:NRR D/D1 CHBerling~* fj, CER
04/21/92gq"'
04/
RPB:ADM
D/D
TechEd J7Hh9q
W"
04/ q/92
04/
OGCB:DOEA: RR
SC/I
CVHodge US9 KCai
04/od/92
04/
ISLOCA REV 2
DOEA: NRR
tossi l
g kS
15~/ 92 RAB: REP:NRR
imp
I9/9 C L
SI R:RES
04/A//92 C/RAB:DREP:IER
WBeckner Xyt
04/A /92 R
Q
DSIR:RES
GB
k
04//3/92 D/DREP:N
FCongel
04/9 /92 C/EIB:D IRRES
RLBaerXiF'
04//3/92 EIB:DSIR L>
SDiab
04/,3/92/