Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSIN No.: 6835
Accession No.:
810330402
IN 81-21
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
July 21, 1981
Information Notice No. 81-21: POTENTIAL LOSS OF DIRECT ACCESS TO
ULTIMATE HEAT SINK
Description of Circumstances:
IE Bulletin 81-03, issued April 10, 1981, requested licensees to take
certain actions to prevent and detect flow blockage caused by Asiatic clams
and mussels. Since then, one event at San Onofre Unit 1 and two events at
the Brunswick Station have indicated that situations not explicitly
discussed in Bulletin 81-03 may occur and result in a loss of direct access
to the ultimate heat sink. These situations are:
1. Debris from shell fish other than Asiatic clams and mussels may cause
flow blockage problems essentially identical to those described in the
bulletin.
2. Flow blockage in heat exchangers can cause high pressure drops that, in
turn, deform baffles, allowing bypass flow and reducing the pressure
drop to near normal values. Once this occurs, heat exchanger flow
blockage may not be detectable by pressure drop measurements.
3. Change in operating conditions. (A lengthy outage with no flow through
seawater systems appears to have permitted a buildup of mussels in
systems where previous periodic inspections over more than a ten year
period showed no appreciable problem.)
We are currently reviewing these events and the responses of the licensees
to IEB 81-03. We expect licensees are performing the actions specified in
IEB 81-03 such that cooling water flow blockage from any shell fish is
prevented or minimized, and is detected before safety components become
inoperable.
On June 9, 1981, San Onofre Nuclear Generating Station Unit No. 1 reported
that as a result of a low saltwater coolant flow rate indication and an
apparent need for valve maintenance, a piping elbow on the saltwater
discharge line from component cooling heat exchanger E-20A was removed by
the licensee just upstream of butterfly valve 12"-50-415 to permit visual
inspection. An examination revealed growth of some form of sea mollusk such
that the cross-sectional diameter of the piping was reduced. The movement of
the butterfly valve was impaired and some blockage of the heat exchanger
tube sheet had occurred. Evaluation of the event at San Onofre is
continuing. However, the prolonged (since April 1980) reactor shutdown for
refueling and steam generator repair is believed to have caused the problem
since previous routine inspections conducted since 1968 at 18 month
intervals had not revealed mollusks during normal periods of operation.
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IN 81-21
July 21, 1981
Page 2 of 3
Two events at Brunswick involved service water flow blockage and
inoperability of redundant residual heat removal (RHR) heat exchangers,
primarily due to oyster shells blocking the service water flow through the
heat exchanger tubes. On April 25, 1981, at Brunswick Unit 1, while in cold
shutdown during a maintenance outage, the normal decay heat removal system
was lost when the single RHR heat exchanger in service failed. The failure
occurred when the starting of a second RHR service water pump caused the
failure of a baffle in the waterbox of the RHR heat exchanger, allowing
cooling water to bypass the tube bundle. The heat exchanger is U-tube type,
with the service water inlet and outlet separated by a baffle. The
copper-nickel baffle which was welded to the copper-nickel tubesheet
deflected and failed when increased pressure was produced by starting the
second service water pump. The redundant heat exchanger was inoperable due
to maintenance in progress to repair its baffle which had previously
deflected (LER 1-81-32, dated May 19, 1981). The licensee promptly
established an alternate heat removal alignment using the spent fuel pool
pumps and heat exchangers.
As a result of the problems discovered with Unit 1 RHR heat exchangers, a
special inspection of the Unit 2 RHR heat exchangers was performed while
Unit 2 was at power. Examination of RHR heat exchanger 2A using ultrasonic
techniques indicated no baffle displacement but flow testing indicated an
excessive pressure drop across the heat exchanger. This heat exchanger was
declared inoperable. Examination of the 2B RHR heat exchanger using
ultrasonic and differential pressure measurements indicated that the baffle
plate was damaged. The licensee initiated a shutdown using the 2A RHR heat
exchanger at reduced capacity (LER 2-81-49, dated May 20, 1981).
The failure of the baffle was attributed to excessive differential pressure
caused by blockage of the heat exchanger tubes. The blockage was caused by
the shells of oysters with minor amounts of other types of shells which were
swept into the heads of the heat exchangers since they are the low point in
the service water system. The shells resulted from an infestation of oysters
growing primarily in the 30" header from the intake structure to the reactor
building. As the oysters died their upper shells detached and were swept
into the RHR heat exchangers where they collected. Small amounts of shells
were found in other heat exchangers cooled by service water. Most of the
operating BWRs use U-tube heat exchangers in the RHR system. (The heat
exchangers used at Brunswick were manufactured by Perflex Corporation and
are identified as type CEU, size 52-8-144.)
The observed failures raise a question on the adequacy of the baffle design
to withstand differential pressures that could reasonably be expected during
long term post accident operation. However, it should be noted that since
the baffles at Brunswick are solid copper-nickel as are the tubesheets and
the water boxes are copper-nickel clad, the strength of the baffles and the
baffle welds is somewhat less than similar heat exchangers made from carbon
steel. Therefore, heat exchangers in other BWR's may be able to tolerate
higher differential pressure than that at Brunswick without baffle
deflection. (Brunswick opted for copper-nickel due to its high corrosion and
fouling resistance in a salt water environment.)
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IN 81-21
July 21, 1981
Page 3 of 3
The use of differential pressure (dp) sensing between inlet and outlet to
determine heat exchanger operability should consider that baffle failure
could give an acceptable dp and flow, indications and thereby mask
incapability for heat removal. However, it is noted that shell blockage in a
single-pass, straight-through heat exchanger can readily be detected by flow
and dp measurement.
Evaluation of the events at Brunswick is still continuing. Under conditions
of an inoperable RHR system, heat rejection to the ultimate heat sink is
typically through the main condenser or through the spent fuel pool coolers.
This latter path consists of the spent fuel pool pumps and heat exchanger
with the reactor building closed cooling water system as an intermediate
system which transfers the heat to the service water system via a single
pass heat exchanger. These two means (i.e., main condenser or spent fuel
pool) are not considered to be reliable long term system alignments under
accident conditions.
This information is provided as a notification of a possibly significant
matter that is still under review by the NRC staff. The events at Brunswick
and San Onofre emphasize the need for licensees to initiate appropriate
actions as requested by IEB 81-03 for any credible type of shell fish or
other marine organisms; e.g., fresh water sponges, (not only asiatic clams
and mussels). In case the continuing NRC review finds that specific licensee
actions would be appropriate, a supplement to IEB Bulletin 81-03 may be
issued. In the interim, we expect that licensees will review this
information for applicability to their facilities.
No written response to this information is required. If you need additional
information regarding this matter, please contact the Director of the
appropriate NRC Regional Office.
Attachment:
Recently issued IE Information Notices
