Fracture Mechanics Analysis on VVER1000 RPV with Different Methodologies
NURETH-14 - 2011 September 25-30


Presented at:
NURETH-14
2011 September 25-30
Location:
Toronto, Canada
Session Title:
O7-1 Pressurized Thermal Shock: General Studies

Authors:
Giuseppe Agresta (University of Pisa, GRNSPG, Italy)
Dino Araneo (University of Pisa, GRNSPG, Italy)
Francesco S. D'Auria (University of Pisa, GRNSPG, Italy)
  

Abstract

This paper describes the main steps of a PTS analysis: system thermal-hydraulic calculation, CFD analysis, stress analysis and the Fracture Mechanics analysis for the RPV of a generic VVER1000. In particular the paper shows the comparison of the results of the fracture mechanics analysis performed with different methodology for the calculation of the stress intensity factor at crack tip (KI). 

 The main component that limits the operational life of the (Nuclear Power Plant) NPP is the Reactor pressure Vessel (RPV) because of the property of carbon steel material change during the operational life due to the different causes: high neutron flux in the welding region, thermal aging etc. This results in an increase of RPV embrittlement level that decreases the safety margin for the crack propagation in case of transients with fast cooling rate due to the emergency systems injection, or increase of secondary side heat exchange. This problem is known as Pressurized Thermal Shock (PTS) and constitutes a relevant problem for the safety of the NPP that are in operation from several years. 

 

Nowadays, the scientific community is trying to change the approach to the PTS analysis toward a “Best Estimate” (BE) scheme with the aim to remove the excess of conservatism in each step of the analysis coming from the limited knowledge of the phenomena in the eighties when the problem has been considered in the safety analysis. This change has been pushed from the possibility to extend the operational life of some plants and this has been possible due to the availability of always more powerful computer and sophisticated computer codes that allows to the analyst to perform very detailed analysis with very high degree of precision of the mixing phenomena occurring at small scale in the down-comer and to calculate the stress intensity factor at crack tip with very refined mesh of millions of nodes. 

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