United States Nuclear Regulatory Commission - Protecting People and the Environment

Safety Concerns Associated with Pipe Breaks in the BWR Scram System (Generic Letter No. 86-01)


                                January 3, 1986 

TO ALL BWR APPLICANTS AND LICENSEES 

Gentlemen: 

SUBJECT:  SAFETY CONCERNS ASSOCIATED WITH PIPE BREAKS IN THE BWR SCRAM 
          SYSTEM (GENERIC LETTER 86-01) 

On April 10, 1981, the NRC staff sent a generic letter to all BWR applicants
and licensees requesting them to provide their pl ant specific responses 
addressing the concerns identified in Draft NUREG-0785, "Safety Concerns 
Associated with Pipe Breaks in the BWR Scram System." On August 31, 1981 the
staff sent Generic Letters 81-34 to BWR licensees and 81-35 to BWR license 
applicants, wherein it was stated that plant specific responses conforming 
to the guidance contained in NUREG-0803, "Generic SER Regarding Integrity of
BWR Scram System Piping" would satisfy the request for information in the 
April 10, 1981 letter. In Generic Letter 81-35, the staff further stated 
that pipe failure in the BWR scram system is not a safety issue for the Mark 
Ill containment designs. 

The NUREG-0803 guidelines essentially addressed the need for improvement in 
procedures, periodic inservice inspection and surveillance for the scram 
discharge volume (SDV) system, and environmental qualification for essential
equipment needed for mitigation of the consequences of staff-postulated pipe
failures in the SDV piping system. These guidelines were developed to 
address the consequences of a postulated leakage crack in the SDV piping and
resulting large leakage (up to 550 gpm) downstream of the system isolation 
valves. Such a leak would have the potential to cause degradation of the 
needed mitigation equipment. At the time they were developed, these 
conservative assumptions and guidelines were based on 1) lack of generically
identifiable failure mechanisms for the SDV piping system, 2) scarcity of 
available data for the system including uncertainty regarding the 
operability of mitigation equipment in a possibly harsh environment, and 3) 
lack of adequate guidance in the BWR Owners Group (BWROG) Emergency 
Procedure Guidelines (EPGs) for handling reactor building and environmental 
problems that may arise as a consequence of such an accident. 

Based on its review of BWROG and General Electric Company (GE) supplied 
generic information (NEDO-22209, BWROG-8325 and BWROG-8420) and staff 
generic analyses of the SDV piping system integrity, the staff has concluded 
that in accordance with Branch Technical Position (BTP) MEB 3-1, Position 
B.2.e in Standard Review Plan 3.6.2, through-wall leakage cracks instead of 
breaks may be postulated in the piping of those fluid systems that qualify 
as high-energy fluid systems (temperature greater than 200 degrees F or 
pressure greater than 275 psig) only for short operational periods (about 2 
percent of the time) but quality as moderate energy fluid systems 
(temperature less than or equal to 200 degrees F and pressure less than or 
equal to 275 psig) for the major operational period. Furthermore, 



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the staff has concluded that, based on its classification and low stress 
threshold, the SDV piping system satisfies BTP MEB 3-1, Position B.2.c(1) in
that a through-wall leakage crack need not be postulated. 

Since the SDV piping system fulfills the above criteria, breaks and 
through-wall cracks in the SDV piping need not be postulated. In addition, 
the staff has concluded that, even if a staff-postulated through-wall flaw 
is initially present in the SDV system, it will grow negligibly and will not
propagate into a break under the staff defined piping loads. Further, 
leakage from such a flaw will be small (less than or equal to about 5 gpm) 
and, therefore, a harsh environment over large areas of the reactor building 
which could affect redundant safety-related mitigating equipment will not 
result. Thus, the potentially exposed safety-related equipment need not be 
qualified for operation in a harsh environment associated with an SDV break. 

The staff has also concluded that the revised BWROG Emergency Procedure 
Guidelines for secondary containment control (NEDO-24934), together with 
normal plant procedures and the proposed periodic visual verification of the
scram system piping integrity (BWROG-8420), provide sufficient measures for 
detecting and mitigating the consequences of leakage which, may occur in the
SDV piping system. The design basis of the SDV piping system has considered 
transient forces resulting from the worst case control rod drive (CRD) 
system actuation. Although water hammer has been analytically postulated and
hydraulic instabilities have been experienced in the CRD system, no events 
have been experienced of a severity significant enough to constitute a water
hammer. Therefore, water hammer is not considered a contributing factor in 
potential SDV pipe breaks. 

Accordingly, this completes our review of the safety concerns associated 
with pipe breaks in the BWR scram system. No OMB clearance is required since 
no information is requested. 

This information is being provided to BWR applicants and licensees with Mark
III Containments for informational purposes only. 


                              Robert M. Bernero, Director  
                              Division of BWR Licensing 

Enclosure: Staff Safety Evaluation 
Page Last Reviewed/Updated Thursday, June 27, 2013