United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No.80-27 – Degradation of Reactor Coolant Pump Studs

                                                          SSINS No.: 6835  
                                                          Accession No.:   
                                                          8005050068       

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT 
                          WASHINGTON, D.C. 20555  
                                     
                               June 11, 1980  

                                           Information Notice No. 80-27 

DEGRADATION OF REACTOR COOLANT PUMP STUDS 

Description of Circumstances: 

On May 17, 1980, the NRC staff was informed by Omaha Public Power District 
(OPPD) that severe corrosion damage was found on a number of closure studs 
in two of the four Byron Jackson reactor coolant pumps at Fort Calhoun Unit 
1 (PWR). 

At the time, the reactor coolant system was undergoing a routine low 
pressure leak test (180 psig) and visual inspection prior to plant restart 
after a four-month outage for refueling, pipe support modifications and 
scheduled inservice inspection. During the visual inspection, saturated and 
dripping insulation was observed at one of the Byron Jackson reactor coolant 
pump flange regions. Upon removal of the insulation, evidence of coolant 
leakage was found emanating from the seating surfaces between the pump 
casing and the pump cover. Further investigation of the three remaining 
pumps indicated similar coolant leakage past both inner and outer flange 
gaskets on two of the three pumps. After complete removal of the nonmetallic 
insulation, further visual observations revealed three studs located 
side-by-side on one pump and three studs similarly located on the other pump 
had significant corrosion wastage in the shank area next to the lower thread 
section in the pump casing flange. Wastage of approximately 50% of the 
original diameter of the stud giving them an "hour-glass" type appearance 
was observed. The corroded studs were located in the vicinity of a component 
cooling water line on both affected pumps but no direct correlation of this 
fact has been established. Although not confirmed by metallurgical analysis, 
the cause of the stud wastage is thought to be corrosive attack by hot boric 
acid from the primary coolant. 

The pump cover and casing for these pumps are constructed of ASTM A-351, 
Grade CF8M stainless steel. Sealing between the cover and casing is achieved
by two concentric 304 stainless steel flexitallic gaskets. A leak-off line 
installed between the gaskets on each pump was plugged and not instrumented.
The leak-off line was not in use and therefore, no indication of RCS leakage
from the inner seal was available. Each pump has 16 closure studs, 
consisting of ASTM A-193 Grade B7 low alloy steel, which are chrome plated 
in the thread area and phosphate coated in the shank area. The studs are 
approximately 3-1/2 inches in diameter and about 29 inches long. 

No maintenance requiring removal of the pump casing studs had been performed
on the reactor coolant pumps since initial construction. The studs were 
covered 
.

Information Notice No. 80-27                             June 11, 1980  
                                                            Page 2 of 2    

with block type insulation since construction thereby limiting them from 
view. The studs on two pumps were ultrasonically inspected in place in 
accordance with the applicable ASME Section XI code rules. These ultrasonic 
examinations were intended to locate cracks in bolting and were not 
effective in revealing wastage of the studs. 

The three affected pumps will be disassembled for further cleaning and 
inspection of the studs and mating surfaces. Prior to reassembly, all studs 
exhibiting significant corrosion will be replaced. All new, or acceptable 
used studs, will be subjected to ultrasonic, visual and magnetic particle 
examinations. Installation of instrumentation for actively monitoring the 
leak-off lines between the flexitallic gaskets is being performed. Future 
inservice inspections, presently limited to ultrasonic examination, will be 
supplemented with visual examination of the studs installed in the reactor 
coolant pumps. Replacement insulation will be in the form of a removable 
blanket to facilitate visual examination. 

The condition of the studs discovered at Ft. Calhoun raises concerns that 
such severe corrosion, if undetected, could lead to stud failures which 
could result in loss of integrity of the reactor coolant pressure boundary. 
The lack of effectiveness of current ultrasonic examinations in revealing 
wastage emphasizes the need for supplemental visual inspections and use of 
instrumented leak detection systems to preclude unacceptable stud 
degradation going undetected. Licensees should consider that the potential 
for undetected wastage of carbon steel bolting by a similar mechanism could 
exist in other components such as valves. 

This Information Notice No. is provided as an early notification of a 
significant matter that is still under review by the NRC staff. It is 
expected that recipients will review the information for possible 
applicability to their facilities. 

No specific action or written response to this Information Notice No. is 
required. If NRC evaluations so indicate, further licensee actions may be 
required. 

Page Last Reviewed/Updated Tuesday, November 12, 2013