United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 81-21: Potential Loss of Direct Access to Ultimate Heat Sink

                                                             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. 
.

                                                              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.) 
.

                                                              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. 

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