Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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
