Information Notice No. 88-31: Steam Generator Tube Rupture Analysis Deficiency
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D. C. 20555
May 25, 1988
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 addition of flow baffle
plates.
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The licensee further concluded that the offsite 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 contained in the steam phase and in the atomized primary
coolant droplets sus-pended 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 assumed 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 uncovered 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.
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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
. Attachment
IN 88-31
May 25, 1988
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION 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 1 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
Related Systems power reactors.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
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