United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-25: Consideration of Thermal Conditions in the Design and Installation of Supports for Diesel Generator Exhaust Silencers

                                                          SSINS No.:  6835
                                                            IN 85-25  

                               UNITED STATES 
                           WASHINGTON, DC 20555  

                               April 2, 1985 

                                  DESIGN AND INSTALLATION OF SUPPORTS FOR 
                                  DIESEL GENERATOR EXHAUST SILENCERS 

All nuclear power reactor facilities holding an operating license (OL) or a 
construction permit (CP). 


This information notice is provided to alert recipients of a potentially 
significant problem pertaining to the effects of thermal conditions on 
supports for diesel generator exhaust silencers. It is expected that recip-
ients will review the information for applicability to their facilities and 
consider actions, if appropriate, to preclude a similar problem occurring at
their facilities. However, suggestions contained in this information notice 
do not constitute NRC requirements; therefore, no specific action or written
response is required. 

Description of Circumstances: 

Seabrook - An NRC regional inspection (50-443/84-02 and 50-443/84-04) iden-
tified cracks in the concrete supports for the diesel generator exhaust 
silencers that were manufactured by Riley-Beard, Incorporated. Further 
investigation by the licensee determined that the set of concrete pedestals 
supporting the exhaust silencer tank for one of the diesel generators 
cracked during operational testing. The cracking was caused by thermal 
expansion, which had not been accounted for in the pedestal base plate 
design of the steel support saddles of the exhaust silencers. These support 
saddles experi-enced temperatures in excess of 200F and expanded more 
than the anchor bolt clearances allowed at the base plate. Significant 
restraining forces developed in the anchor bolts and in the concrete 
pedestals where the anchor bolts were set. In addition, temperatures above 
200F, which were higher than the maximum recommended sustained 
temperature of the concrete, were reached near the top portion of the 
concrete pedestal. 

River Bend - The NRC Construction Appraisal Team (CAT) inspection at the 
River Bend Nuclear Power Station found that the exhaust silencers for the 
Transamerica Delaval, Inc. diesel generators were installed without any 
sliding connection for thermal expansion. Investigation by the architect/ 
engineer found that the manufacturer's installation drawing was incorrect. 


                                                           IN 85-25  
                                                           April 2, 1985  
                                                           Page 2 of 2 

Shearon Harris - At the Shearon Harris Nuclear Power Station, the NRC CAT 
inspectors found that the diesel generator exhaust silencers, manufactured 
by American Air Filter Company, had grout in and around the slotted sections
of the base plates. These sections were designed to be clear to allow for 
movement caused by thermal expansion. In addition, the concrete support pad 
under the sliding attachment for one of the exhaust silencers was sloped in 
such a fashion as to cause the attachment to wedge itself tight as expansion

Braidwood - At the Braidwood Nuclear Power Station, the applicant identified
cracked concrete pedestals of the diesel generator exhaust silencers 
manufactured by Riley-Beard, Incorporated. The architect/engineer determined 
that the original design did not properly account for thermal expansion. 
During the Braidwood NRC CAT inspection, the CAT inspectors also noticed 
that the bolted connections of the sliding ends of the exhaust silencers may 
have been too tight to permit movement for thermal expansion. Corrective 
action required modifying the sliding joints of the exhaust silencers. 


The deficiencies described above show that there was inadequate 
consideration of thermal conditions in the supports for exhaust silencers at 
four nuclear plant sites. Generally, thermal conditions of the diesel 
generator exhaust silencers were not adequately accounted for in either the 
design or installation requirements for the component concrete pedestals and 
sliding joints. As a result, in two cases the forces created by expansion 
when the exhaust silencers reached operating temperatures caused damage to 
the concrete pedestals. 

If left as they are, the cracked foundations for the exhaust silencers may 
not be able to support imposed loads in a seismic event. Failure of the 
exhaust silencers could reduce the capacity of the diesel generators. 

Similar problems may exist with other mechanical equipment that operates at 
elevated temperatures and that is supported by concrete pedestals with slid-
ing connections. 

No specific action or written response is required by this information 
notice. If you need additional information about this matter, please contact
the Regional Administrator of the appropriate NRC regional office or this 

                                   Edward L. Jordan, Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  John Nemoto, IE 
                    (301) 492-7557 

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