### Abstract

This report presents a summary of a three years long
study on the failure probabilities of nuclear power plant
piping components. The focus of the study has been on
effect of initial flaw and load assumptions on nuclear
power plant (NPP) failure potential and risk estimate
changes. The effect of inspections has also been taken
into account. The main tools for the pipe break
probability and risk analyses have been probabilistic
VTTBESIT code and Markov process application, but some
calculations were also performed using Swedish NURBIT
code for comparison. In the analyses, the considered
degradation mechanism was stress corrosion cracking
(SCC). During the three previous years, four
representative NPP piping welds were considered. The
computational part of this report concerns one of them.
The initial flaw sizes have a considerable effect on the
failure probability results. Selected distributions
for the fabrication induced cracks provided the initial
flaw state for all analysis cases, whereas for the cases
with SCC induced initial cracks developed by VTT and with
those in the NURBIT code, SCC flaws initiated during
operation were used as well. The break probabilities
after one year in operation are the smallest for the
cases with fabrication induced cracks alone, whereas
these probabilities are from 0.5 to 2 decades higher for
the cases with the SCC induced initial cracks developed
by VTT and with those in the NURBIT code. The effect of
the initial flaw sizes to break probabilities is most
pronounced in the early phase of the time in operation.
Whereas the maximum break probability values
after 60 years in operation are almost matching for all
cases. The SCC induced initial crack sizes developed
earlier in this study are recommended to be used. The
magnitude of the loading has the largest effect on the
break probability results. The loading is
dominated by the weld residual stresses (WRS). WRSs from
several different sources were included in the study and
differences were large with regard to break probability.
For welds joining NPP pipes of austenitic stainless
steel, the WRSs given in the SSM handbook and SINTAP
procedure are recommended to be used.

Original language | English |
---|---|

Publisher | VTT Technical Research Centre of Finland |

Number of pages | 24 |

Publication status | Published - 2015 |

MoE publication type | D4 Published development or research report or study |

### Publication series

Series | VTT Research Report |
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Volume | VTT-R-00020-15 |

### Keywords

- initial cracks
- inspections
- WRS
- NPP
- piping component
- weld

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## Cite this

Tyrväinen, T. (2015).

*Summary on effect of initial flaw and load assumptions on risk estimate changes*. VTT Technical Research Centre of Finland. VTT Research Report, Vol.. VTT-R-00020-15 http://www.vtt.fi/inf/julkaisut/muut/2015/OA-Summary-on-effect.pdf