Resolution of Generic Safety Issues: Item B-16: Protection Against Postulated Piping Failures in Fluid Systems Outside Containment ( NUREG-0933, Main Report with Supplements 1–35 )

DESCRIPTION

Historical Background

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.

Safety Significance

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.

Possible Solution

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.

CONCLUSION

This issue has been incorporated into Item A-18, "Pipe Rupture Design Criteria."

REFERENCES

0002. NUREG-0371, "Task Action Plans for Generic Activities (Category A)," U.S. Nuclear Regulatory Commission, November 1978. 0003. NUREG-0471, "Generic Task Problem Descriptions (Categories B, C, and D)," U.S. Nuclear Regulatory Commission, June 1978. 0011. NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants," U.S. Nuclear Regulatory Commission, (1st Ed.) November 1975, (2nd Ed.) March 1980, (3rd Ed.) July 1981. 0018. Regulatory Guide 1.46, "Protection against Pipe Whip Inside Containment," U.S. Atomic Energy Commission, May 1973. [7907100189]