Leak-Before-Break Evaluation
SIB-96-131

What is Leak-Before-Break (LBB)?
The concept of LBB implies that any crack or defect which develops in a component will grow to a through-wall configuration, and can be detectable by plant monitoring systems before reaching a size that would significantly reduce margins to component rupture.

When is LBB Applied?
The original application of LBB was for the elimination of protective structures such as pipe whip restraints, jet impingement shields, etc., which provide protection against high energy line breaks. Recently, LBB has been used as part of overall safety assessments for leaving real or postulated flaws in service for some operating period.

LBB Methodology
LBB analyses are based on advanced fracture mechanics techniques and include the following aspects:

• critical flaw size evaluation
• leakage calculation
• crack propagation analysis
• ultrasonic flaw detection/sizing
• leak detection
• service experience.

Representative results of an LBB evaluation are shown in the following figure:

NUREG-1061, Vol. 3 provides the evaluation methodology for LBB analysis. The following provides a summary of the methodology:

• Address all of the prerequisites regarding applicability of LBB to the particular piping system under consideration.
• Calculate through-wall critical flaw sizes for the various locations of interest using Pressure + Deadweight + Thermal + Seismic Load Combination.
• Calculate leakage through half of the critical flaw size (leakage flaw size) using Pressure + Deadweight + Thermal Load Combination.
• Calculate the critical stress for the leakage flaw size and ensure a safety factor of 2.
• Ensure that the leakage through the leakage flaw size can be detected by the plant's leak detection system.
• Perform a fatigue crack growth evaluation to demonstrate that a part through-wall flaw that will be acceptable by ASME Section XI standards will not grow significantly during service.

The application of LBB evaluation to plant operating components is shown below:

Applications and Benefits
LBB methodology has been successfully applied in a number of nuclear power plants. The driving force for such evaluations has been the realization of technical and economic benefits. Some representative applications and their associated benefits are summarized below:

• In the past, LBB analysis has been used in lieu of retrofitted pipe break protection for older plants evaluated under the NRC Systematic Evaluation Program.
• LBB analysis can justify the removal of pipe whip restraints for enhanced accessibility and inservice inspections (e.g., when performing induction heating stress improvement of stainless steel piping).
• LBB analysis can justify the elimination of pipe break restraints and shields during the installation of new or replacement piping systems.
• LBB analysis can demonstrate reduced safety significance of pipe cracking concerns, such as BWR stainless steel piping, cast pipe, valves and pumps, or low toughness weldment materials.
• LBB analysis can reduce equipment qualification requirements for the environmental effects of postulated pipe breaks.

Structural Integrity Associates (SI) has performed LBB analyses for both pressurized water reactors (PWRs) and boiling water reactors (BWRs) for a variety of components including the following:

• Pressurizer surge and accumulator lines
• Feedwater, main steam and reactor water cleanup lines
• Recirculation systems
• Valve castings
• Numerous small diameter piping systems.

Additionally, SI has developed special in-house computer programs to facilitate LBB evaluations based on the requirements of NUREG-1061, Vol. 3.

If you would like to obtain additional information regarding LBB analysis or SI's capabilities in this area, please contact SI.