KTA 3206: Leak-before-Break (LBB) Analysis

Seven-step Leak-Before-Break analysis per KTA 3206 (2014-11) Annex A/B using the plastic limit load method (PGL/FSK)

KTA 3206: Leak-Before-Break (LBB) Analysis

KTA 3206 is the German nuclear safety standard for showing "fracture exclusion (Bruchausschluss)" in pressure-bearing pipes of nuclear power plants. This calculator runs the seven-step leak-before-break procedure of Annex A 2 for a circumferential crack (Umfangsriss) in a pipe under internal pressure and bending. The idea is to show by analysis that a crack will produce a detectable leak before it can grow to a fracture, so the monitoring system raises an alarm in time.

From the pipe geometry, material data and loads, the analysis runs these steps:

1. Set a postulated initial crack (step 1).
2. Take the end-of-life crack size a_e / 2c_e from an external fatigue analysis (step 2).
3. Find the critical through-wall crack length 2c_{krit} with three limit-load methods, keeping the smallest (most conservative) value:
   • PGL — plastic limit load;
   • FSK/MPA — flow-stress concept (Annex B 2.1);
   • FSK/KWU — flow-stress concept (Annex B 2.1).
4. Compute the crack opening area and leak rate (Annex B 3.1 / B 3.2) to get the detectable crack length 2c_{L\ddot{U}S}.
5. Pass the LBB check when the detectable length is shorter than the allowable through-crack length (2c_{L\ddot{U}S} < 2c_{zul}).

Scope and limits:

• Pipes only (not vessels).
• Circumferential cracks under pressure plus bending; an axial crack runs the limit-load check only.
• Step 2 fatigue crack growth is supplied as an external input (a_e, 2c_e), because the standard does not give a closed-form solution to do the fatigue integration here.
• All calculations use SI units.
• The flow-stress methods assume the material is in the ductile upper-shelf range.

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