Background
The Swedish Radiation Safety Authority (SSM) and the Swedish nuclear power plant owners have financed Inspecta Technology in Sweden to evaluate the PRO-LOCA code. It is a computer code, developed by Battelle in the USA, in which the leak- and rupture probabilities of piping in nuclear power plants are analysed. The pipe systems may contain the damage mechanisms fatigue and/or stress corrosion cracking. In the report, the results are presented for evaluating PRO-LOCA, version 4.1.9. The report contains a sensitivity study for both PWR PWSCC and BWR fatigue.
Objective
The primary objective has been to make an independent quality check, to detect possible bugs in the code and to suggest improvements. Another objective has been to understand what the key parameters are in this type of probabilistic approach.
Results
- The PRO-LOCA code is capable to predict the leak or rupture probabilities of nuclear piping systems taking into account the whole sequence of crack initiation of a circumferential crack, subcritical growth until wall penetration following by leakage and further crack growth (if undetected) until possible instability of the through-wall crack (pipe rupture).
- Performed benchmark analyses and parametric sensitivity studies are presented for two main cases, a PWR pipe subject to PWSCC degradation, and a BWR pipe under fatigue cracking.
- The main influencing parameters on the predicted probabilities for the PWSCC case are growth rate, weld residual stress, inspection interval, inspection effectiveness and leak rate detection capability.
- The main influencing parameters on the predicted probabilities for the BWR fatigue case are initial defect size, growth rate, inspection interval, inspection effectiveness and leak rate detection capability.
- For BWR fatigue, unrealistic results were obtained for variation of ultimate tensile strength. This should be investigated further.
- Leak rate detection is a powerful way to detect leaks before a pipe break, i.e. the tendency for LBB (Leak Before Break) in the studied cases is quite strong. If a reliable leak detection system is implemented in a pipe system, the only way a break can occur is if the stress state (i.e. system loads and weld residual stresses) are distributed in a way that an initiated circumferential crack will grow almost around the whole circumference before wall penetration. If then eventually a wall penetration occurs, the crack may be immediately unstable and a pipe rupture will occur.