KTA 3206: Leak-before-Break (LBB) Analysis Alpha · 2026-09-23

Seven-step leak-before-break check of a circumferential pipe crack per KTA 3206 (2014-11) Annex A/B

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

KTA 3206 is the German nuclear safety standard for showing "fracture exclusion (Bruchausschluss)" in pressure-retaining pipes of nuclear power plants. This calculator runs the seven-step procedure of Annex A 2 for a circumferential crack (Umfangsriss) in a straight pipe under internal pressure and bending. It shows by analysis that a crack will leak at a detectable rate before it can grow to a fracture.

The analysis runs these steps:

  1. Set the postulated initial surface crack a_a, 2c_a from the material and wall thickness.
  2. Take the end-of-life crack a_e, 2c_e from an external fatigue crack growth analysis.
  3. Find the critical through-wall crack length 2c_{krit} with three limit-load methods and keep the smallest value:
    • PGL — plastic limit load (Annex B 2.1.3.2);
    • FSK/MPA — flow-stress concept, MPA method (Annex B 2.1.3.3 a);
    • FSK/KWU — flow-stress concept, Siemens-KWU method (Annex B 2.1.3.3 b).
    The flow stress of each method follows Table B 2.1-1 (material, nominal diameter DN, steel grade, crack in base metal or weld).
  4. Find the critical depth a_{krit} of a surface crack of length 2c_e and the allowable depth a_{zul} = \min\{0.75\,s;\ a_{krit}\}.
  5. Compute the leak area (Annex B 3.1) and leak rate (Annex B 3.2) of a through-wall crack under operating load, and find the detectable crack length 2c_{L\ddot{U}S}.
  6. Integrity check: a_e \le a_{zul} and 2c_e \le 2c_{zul}.
  7. LBB check: 2c_{L\ddot{U}S} < 2c_{zul}.

Scope and limits:

  • Straight pipes with a circumferential crack only; not vessels, bends, nozzles or axial cracks.
  • The limit-load methods assume the material is on the upper shelf of the impact energy curve.
  • The fatigue crack growth of step 2 is an input; the calculator does not integrate crack growth.
  • The leak rate uses the single-phase subcooled liquid equation (B 3.2.2.2). It needs p > p_S; two-phase outflow is not covered.
  • The bulging factor is valid for a shell parameter \lambda \le 5. Outside this range, or when the loads exceed the limit load of the uncracked pipe, the calculator stops with a message instead of giving a number.
  • With cladding, the leak calculation uses the full section including the cladding, while the critical crack length ignores the cladding (as in Annex D 2).

Engineering data

Loading the interactive calculator…

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