Resolution of Generic Safety Issues: Item B-16: Protection Against Postulated Piping Failures in Fluid Systems Outside Containment ( NUREG-0933, Main Report with Supplements 1–34 )
SRP11 Section 3.6, issued in 1975, addressed pipe breaks outside containment by combining limited design basis breaks for mechanistic protection with unlimited breaks for non-mechanistic protection. Prior to this, Regulatory Guide 1.46,18 issued in 1973, addressed pipe breaks inside containment based on a limited number of design basis breaks. Because of this inconsistency of postulating pipe breaks inside and outside containment, protection against postulated piping failures in fluid systems outside containment was given a priority Category B designation and included in NUREG-04713 issued in 1978.
The combination of postulated pipe rupture criteria with more conservative seismic criteria has resulted in piping systems design with higher calculated pipe stress values in newer plants. To counteract these higher stress values, increasing use is being made of dynamic event pipe supports such as snubbers and pipe whip restraints. This practice has produced piping systems that are significantly more rigid during normal operations. Under such conditions, these dynamic event supports have the potential for deleterious interactions with the piping they support.
The solution proposed for Item A-18 in NUREG-03712 was the combination of existing pipe rupture design criteria for inside containment with that for outside containment in order to develop a consistent regulatory position for current licensing needs in both areas.
This issue has been incorporated into Item A-18, "Pipe Rupture Design Criteria."