REPoUSSES

Project summary

Most of the currently operating nuclear power plants in the Nordic countries have reached the state where Long Term Operation is a reality and as such the need for consistent and realistic models for understanding the ageing of systems structures and component have increased. This is particularly true for the large pressurized components such as the Reactor Pressure Vessel. Ensuring safe operation and the durability of the RPV is at utmost importance. The most pronounced ageing mechanisms of the RPV is the irradiation-induced embrittlement and material degradation, which cause an increase in the Ductile to Brittle Transition Temperature. The DBTT must have a safety margin in respect to the lowest possible temperature in the RPV e.g. at an emergency cooling scenario, as this could initiate a catastrophic brittle fracture. The ageing and embrittlement is monitored through the life-time of the reactor through surveillance programs which represent the RPV material and are irradiated with higher dose rate. The surveillance specimens are mechanically tested and the RPV behaviour can be predicted.

The representativeness between the surveillance specimens and component material has been a concern due to the difference in irradiation conditions. The study of comparison between material from a real component and material used for lifetime assessment will give a unique results on how these two correlate, which is the basis for the lifetime assessment. Verification of this correlation improves the overall safety assessment of Nuclear Power Plants. Characterisation of through-thickness properties gives understanding of the attenuation effect on the material properties, as at different depths of the RPV wall the material is subjected to different doses. The investigated materials in question are from Swedish NPPs.

In order to support the continued operation of the current fleet of nuclear rectors, the understanding of the root causes of material deterioration and weakest links in the microstructure improves the nuclear safety. NKS has supported a previous program, BREDA, that had the objective to harvest and perform mechanical testing of an as-aged/irradiated reactor pressure vessel, RPV. The results are now available to assess the correctness of the employed programs to analyze the ageing effects on the RPV. The RPV weld metal has been thoroughly investigated in terms of mechanical and microstructural properties and brittle fracture initiation. The fundamental structural integrity assessment on RPV metal includes systematic characterisation of the mechanical and microstructural properties. The methodology has proven to be successful and advanced microscopy techniques are implemented to deepen the understanding of the microstructural features. The weld metal properties in irradiated and/or thermally aged state has been compared to the behaviour of surveillance material and non-aged baseline state.

In the investigated component RPV welds, the brittle fracture initiated from relatively large inclusions which are considered as weakest links. Evidences of ageing effects have been seen in the initiation features but the verification is ongoing. Instrumented impact toughness testing and fracture toughness testing using miniature compact tension specimen were used to study the fracture properties. The brittle specimen were studied by fractography using electron microscopy. Further microstructural analysis of the initiation sites are investigated implementing Focused Ion Beam and Transmission Electron Microscopy, where the inclusions are fully investigated. The results are expected to improve the predictive modelling of the ageing-induced brittle fracture initiation. The through-thickness investigation mainly focuses on base material, where the material at different depth locations is tested for mechancial properties and microstructural features affecting brittle fracture initiation or features affected by irradiation. The fracture mechanical testing and electron microscopy is conducted at VTT, complementing analyses such as Atom Probe Tomography are done by Chalmers, and modelling development by KTH.

Lead organisation

Royal Institute of Technology – KTH

Contact person

Pål Efsing

KTH, Engineering Mechanics/Solid Mechanics, Teknikringen 8D, 10044 Stockholm

efsing@kth.se

+46-706-254514