Information Notice 1988-31, Steam Generator Tube Rupture Analysis Deficiency
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D. C. 20555
May 25, 1988 NRC INFORMATION NOTICE NO. 88-31:
STEAM GENERATOR TUBE RUPTURE ANALYSIS
DEFICIENCY
Addressees
All holders of operating licenses or construction permits for Westinghouse
(W)- and Combustion Engineering (CE)-designed nuclear power reactors.
Purpose
This information notice Is being provided to alert addressees to potential
problems resulting from a non-conservatism in the safety analysis for rupture
of a steam generator tube which may increase offsite dose consequences. 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
Following the steam generator tube rupture at North Anna Unit 1 on July 15, 1987, the Virginia Electric and Power Company (VEPCO) modified the flow resistance of
the steam generator downcomers at North Anna by the addition of flow baffle
plates.
This modification necessitated the reanalysis of certain design basis
events including rupture of a steam generator tube.
The new analysis utilized
a revised Westinghouse method for calculating steam generator water mass and
indicated that during the event, the water level on the secondary side could
fall below the top of the steam generator tubes for a 10-minute period at the
beginning of the event.
Steam generator tube uncovery is significant because, If the break location
becomes uncovered, a direct path might exist for fission products contained
in the primary coolant to be released to the atmosphere without partition
with the secondary coolant. VEPCO and Westinghouse reanalyzed the design
basis steam generator tube rupture accident for Surry using the revised
methods and determined that the steam generator tubes at Surry could also
become uncovered even though the Surry plants were not modified by the ad- dition of flow baffle plates.
8852006ZA
IN 88-31 May 25, 1988 The licensee further concluded that the offslte dose consequences exceeded
those calculated in the Surry Updated Final Safety Analysis Report (UFSAR)
because tube uncovery could produce a direct path for fission product release.
Based on the Surry results, the analysis of steam generator Inventory during
a steam generator tube rupture at other plants may show that the steam generator
tubes may uncover. Thus, for those plants where the steam generator tubes were
thought to remain covered following tube rupture, the previously calculated
safety analysis offsite doses might be exceeded and since the primary coolant
activity limit in Technical Specifications is based upon the occurrence of this
accident, the allowable technical specification limit may be too high.
Discussion
A postulated steam generator tube rupture Is one of the design basis accidents
analyzed in plant Safety Analysis Reports (SARs).
Using conservative assump- tions of single failure and loss of offsite power, it must be shown that the
offsite dose consequences will be limited to the guideline doses of 10 CFR 100
or a fraction of the guideline doses depending on the assumptions made for
iodine spiking.
The iodine in the reactor coolant may be previously dissolved
from allowable operational fuel failure or may result from an Iodine spike which
is the sudden increase In coolant iodine concentration produced by the transient
conditions during the accident. Mechanisms for transport of the iodine that
exits the reactor system to the atmosphere are discussed in Standard Review Plan
(NUREG-0800) Section 15.6.3. In determining the amount of iodine that is trans- ported to the atmosphere, credit may be given for "scrubbing" of iodine-contalned
in the steam phase- andL4n-the atomized-primary -coola t-droplits suspended in the
steam phase for release points which are below the steam generator water level.
The Surry UFSAR assumed that the break is always covered with water so that 99%
of the iodine would remain within the steam generator coolant and only 1% would
be released through the atmospheric relief valves.
The break location is as- sumed to be always covered in the UFSAR calculations because an Initial steam
generator water mass that may be non-conservatively large was assumed in order
to conservatively account for the possibility of overfill and because steam
generator tube failures were thought only to occur close to the tube sheet.
The North Anna tube rupture demonstrated that steam generator tube failures
can occur near the top of the tube bundle.
The revised steam generator water
mass calculations by Westinghouse with the assumption that the break occurs at
the top of the tube bundle led to the conclusion that the break could be un- covered for a significant period of time. Tube uncovery occurs because of the
level shrink that accompanies reactor trip/turbine trip during the tube rupture
event. The tubes would be recovered by the flow of auxiliary feedwater into
the ruptured steam generator and by the reactor coolant which would be added
due to the ruptured tube; however, the amount of iodine released from the
ruptured steam generator could be larger than that previously calculated.
.V
IN 88-31 May 25, 1988 The NRC staff is evaluating the impact that these non-conservative assumptions
may have on calculated offsite doses and the need for further generic action.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical con-;
tact 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 Contact:
Walton L. Jensen
(301) 492-21190
Attachment: List of Recently Issued NRC Information Notices
VI
Attachnent
May 25. 1988
Page 1 of I
LIST OF RECENTLY ISSUED
NRC INFORMATI0N NOTICES
Information
Date of
Notice No.
SubJect
Issuance
Issued to
88-30
Target Rock Two-Stage
5/25/88
All holders of OLs
SRV Setpoint Drift
or CPs for nuclear
Update
power reactors.
88-29 Deficiencies In Primary
5/24/88
All holders of OLs
Containment Low-Voltage
or CPs for nuclear
Electrical Penetration
power reactors.
Assemblies
88-28 Potential for Loss of
5/19/88
All holders of OLs
Post-LOCA Recirculation
or CPs for nuclear
Capability Due to
power reactors.
Insulation Debris Blockage
88-27 Deficient Electrical
5/18/88
All holders of OLs
Terminations Identified
or CPs for nuclear
In Safety-Related
power reactors.
Components
85-35, Failure of Air Check
5/17/88 All holders-of OLs
Supplement I
Valves to Seat
or CPs for nuclear
power reactors.
88-26 Falsified Pre-Employment
5/16/88
All holders of OLs
Screening Records
or CPs for nuclear
power reactors and
all major fuel
facility licensees.
88-25 Minimum Edge Distance for
5/16/88
All holders of OLs
Expansion Anchor Bolts
or CPs for nuclear
power reactors.
88-24 Failures of Air-Operated
5/13/88
All holders of OLs
Valves Affecting Safety- or CPs for nuclear
---
t lted System-
-
power- reactors.-
-
- Operating License
- Construction Permit
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D.C. 20555
OFFICIAL BUSINESS
PENALTY FOR PRIVATE USE. $300
FIRST CLASS MAIL
POSTAGE & FEES PAID
PERMIT No. 0-7
IN 88-31 May 25, 1988 The NRC staff Is evaluating the impact that these non-conservative assumptions
may have on calculated offsite doses and the need for further generic action.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical con- tact 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 Contact:
Walton L. Jensen
(301) 492-21190
Attachment:
List of Recently Issued NRC Information Notices
- see previous concurrence
EAB:NRR
WJensen:db*
4/7/88 RSB:NRR
WHodges*
5/17/88 EAB:NRR
CRPRB:NRR
RLobel*
LCunningham*
4/7/88
5/5/8 TECH:ED
C:G B: R
CHBerlinger
/
/88
.f/p'e/88 C:EAB:NRR
WLanning*
D
.R4 CERossi
.i70'/88
IN 88- May , 1988 The NRC staff is evaluating the impact that these non-conservative assumptions
may have on calculated offsite doses and the need for further generic action.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical con- tact 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 Contact:
Walton L. Jensen
(301) 492-21190
Attachment: List of Recently Issued NRC
Information Notices
- see previous concurrence
EAB:NRR
WJensen:db*
/
/88 RSB:NRR
WHodges*
/
/88 EAB:NRR
RLobel*
/ /88 CRPRB:NRR
LCunningham*
/
/88 WLa~ ing
SJ /7' 88 D: OEA:NRR
CERossi
/
/88 TECH:ED
C:GCB:NRR
CHBerlinger
/
/88
/
/88
4 IN 88- May , 1988 The NRC staff is evaluating the impact that these non-conservative assumptions
may have on calculated offsite doses.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical con- tact 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 Contact:
Walton L. Jensen
(301) 492-21190
Attachment:
List of Recently Issued NRC
Information Notices
- see previous concurrence
EAB:NRR
WJensen:db*
/
/88 EAB:NRR
RLobel*
/ /88 CRPRB:NRR
LCunningham*
/
/88 C: X RR
, //'9 88 D:DOEA:NRR
CERossi
/
/88 RSB:NRR
WHodges*
/
/88 TECH:ED
C:GCB:NRR
CHBerlinger
/
/88
/
/88
4 IN 88- April
, 1988 The NRC staff is evaluating the impact that these non-conservative assumptions
may have on calculated offsite doses.
No specific action or written response is required by this information
notice. If you have any questions about this matter, please contact the
Regional Administrator of the appropriate regional office or this office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact:
Walton L. Jensen
(301) 492-21190
Attachment:
List of Recently Issued NRC
Information Notices
EAB: NR
WJensen:db
4 /1 /88 RSB RRq
WHodges
5 /(I /88 LL
EAB:NRR
RLobel
A /.i/88 LCunningham
S /
g/88 C:EAB:NRR
WLanning
/
/ 88 C:GCB:NRR
CHBerl inger
/
/88 TECH: ED
AThomas
/
/88 D:DOEA:NRR
CERossi
/
/88