NRC Generic Letter 86-01, Safety Concerns Associated with Pipe Breaks in the BWR Scram System

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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,

8601090294the 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