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Resolution of Generic Safety Issues: Issue 18: Steam-Line Break with Consequential Small LOCA ( NUREG-0933, Main Report with Supplements 1–35 )


Historical Background

This issue effects all PWR-type reactors (Westinghouse, CE, B&W). The issue as described,23,34,36 concerns postulated accidents resulting from a steam line break which consequentially results in a steam generator tube rupture or a small LOCA in the primary system in PWRs (combined LOCAs). The issue can be divided into two sub-issues: (1) steam-line break with a subsequent small LOCA resulting from failure of partially degraded steam generator tubes, and (2) steam-line break with a subsequent small LOCA, other than a steam generator tube rupture (SGTR), resulting from a stuck-open PORV or safety valve actuated during the primary system transient or resulting from pipe whip or jet impingement from the broken steam line.

Safety Significance

In PWRs, the potential exists for steam-line breaks consequentially leading to a small primary system LOCA. An NRR analysis36 has indicated that the primary pressure and the pressurizer level may vary qualitatively in the same way during a combined LOCA as during a primary break, a steam-line break, or a steam generator tube rupture. The primary temperature and secondary pressure may vary during a combined LOCA qualitatively the same as during a steam-line break. For these latter two parameters, a primary rupture or steam generator tube rupture appear clearly distinct from the behavior of a combined LOCA.

Possible Solution

Information concerning this issue was sent to the Commission on July 13, 1982 (SECY-82-296).432 In this transmittal, the staff informed the Commission that calcuations performed for this issue indicate that, in all analyzed cases, the core remains covered with liquid. Further, the calculations showed that the plants initially stabilize without operator action. If the steam line break is inside containment, the offsite releases are not greater than that from events currently analyzed in the FSARs. If the break is outside containment and cannot be isolated, the radiological consequences are more severe. To decrease the risk, early diagnosis of the SGTR must be assured. This can be done by upgrading the operator instructions. Staff reviews of Emergency Operating Procedures (EOP) are part of TMI Action Plan Item 1.C.1 of NUREG-0737.98 RSB proposes to handle these concerns within the ongoing review of Item 1.C.1.432


The technical resolution of this issue (which is identical to Issue 42) has been identified. Implementation will be in accordance with TMI Action Plan Item 1.C.1 of NUREG-0737,98 as supplemented by NRC Generic Letter No. 82-33.376


0023.Memorandum for Distribution from W. Minners, "Generic Issues Screening Activity," September 30, 1981. [8110190695]
0034.Memorandum for the Commissioners from W. Dircks, "Resolution of Issue Concerning Steam-line Break with Small LOCA," December 23, 1980. [8101150357]
0036.Memorandum for C. Michelson from H. Denton, "Combination Primary/Secondary System LOCA," December 8, 1981. [8201200049]
0098.NUREG-0737, "Clarification of TMI Action Plan Requirements," U.S. Nuclear Regulatory Commission, November 1980, (Supplement 1) January 1983.
0376. Letter to All Licensees of Operating Reactors, Applicants for Operating Licenses, and Holders of Construction Permits from U.S. Nuclear Regulatory Commission, "Supplement 1 to NUREG-0737, Requirements for Emergency Response Capability (Generic Letter No. 82-33)," December 17, 1982. [ML031080548]
0432.SECY-82-296, "Resolution of AEOD Combination LOCA Concern," U.S. Nuclear Regulatory Commission, July 13, 1982. [8207230202]