Application of Small Punch Testing Methods for Thermal Ageing Monitoring at Primary Circuit Components in Nuclear Power Plant

Abstract

This paper deals with the evaluation of material properties of safety-related components of the primary circuit of nuclear power plants (NPP) such as reactor pressure vessel (RPV), primary piping (PP) or steam generator (SG). The main degradation mechanism of NPP’s components is radiation damage, but these processes are situated only in the core region of the reactor pressure vessel (RPV). The main mechanical loading of all individual parts of NPP’s primary circuit are the influence of high pressure at elevated temperature till 300°C. These processes lead to the fatigue damage of structural materials, which is characterized as the thermal fatigue or thermal ageing. The reactors in Slovakia have been behind several decades in operation. Therefore, to ensure the safe and reliable operation of NPP, it is necessary to monitor and evaluate these changes. While the monitoring of the radiation degradation is standardly performed using surveillance programs during all plant life operation, the monitoring of thermal ageing at primary circuit components was realized only after few years of all NPP units operation. The concept of the thermal ageing assessment was divided into two possible approaches to evaluation. The first is long-term exposition of surveillance specimens. And the second approach is a direct surface sampling of heavy components after several years of operation. The Small Punch Test (SPT) methods are mainly used for evaluation of materials actual state. By the SPT technique it is possible to evaluate the basic tensile properties as the ultimate tensile strength and the yield stress of the tested materials from a very small amount of obtained material. The details of the original VUJE design program of thermal ageing monitoring of NPP primary circuit materials in Slovakia is described in this paper.

Hydrogen pre-charging effects on the notch tensile properties and fracture behaviour of heat-affected zones of thermally aged welds between T24 and T92 creep-resistant steels

“…Moreover, individual regions of the welds, namely the base material (BM), weld metal (WM) and heat-affected zone (HAZ) degrade very specifically which consequently leads to their strongly differing properties [3,4]. The situation becomes even more complex in the case of dissimilar welds, i.e.…”

Interim results of the reactor pressure vessel materials evaluation within the framework of the implemented Advanced Surveillance Specimen Programme

“…There's no need for special hot cells when testing SPT sample, a simple lead brick shielding of a testing machine is adequate [6,7]. Because there is a lack of RPV experimental materials, the great advantage is using the SPT methods for evaluation of their basic mechanical properties [8].…”

The role of small specimen creep testing within a life assessment framework for high temperature power plant

“…Based on the current maturity level of the testing techniques outlined in the section ‘The role and application of small specimen creep testing’, it is proposed that the most practical approach to consider at the moment is the further development and deployment of the impression creep method, particularly since it has been exploited on ageing plant by Brett [13–15,109] for purposes of component risk ranking and condition assessment. The extensive programme of sampling undertaken on Slovakian reactors [79,80,110] is also noteworthy and is another example of the proactive and systematic use of these small specimen testing techniques. It is worth noting that other methods such at the two-bar creep test specimen may provide additional benefits due to the potential to model the tertiary phase of creep, however further work on miniaturisation is likely to be required.…”