Evaluation of Mechanical Properties of Primary Circuit Components Using SPT Technique

Abstract

The paper deals with the evaluation of mechanical properties of safety-related components of the primary circuit of nuclear power plants (NPPs). During a long-term operation of NPPs, changes of mechanical properties occur. To ensure the safe operation of NPP it is necessary to monitor and evaluate these changes. One possibility how to solve this problem is a direct sampling and the assessment of the actual mechanical properties using the small punch test (SPT) technique. By the SPT technique it is possible to evaluate the basic tensile properties such as the ultimate tensile strength and the yield stress of the tested materials as well as the transition temperature. The authors describe model examples of surface sampling using the special Rolls Royce equipment SSam™-2. The SPT specimens were prepared from the removed samples by application of the wire electrical discharge machining and fine grinding. The prepared specimens were tested and the obtained data were evaluated. Original pieces of primary tubes with diameter 500 mm made of stainless steel type 08Ch18N10T (AISI 321) were used for these model examples. Another experimental material used was a block of the reactor pressure vessel wall cut from NPP Greifswald Unit 7 which contains a weld joint. Those experiments were a part of the preparatory activities for the planned sampling and the assessment of the actual mechanical properties of primary components such as the main primary coolant piping, the steam generator shell and the reactor pressure vessel.

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

“…SPT practice requires a small volume of tested materials what substantially reduces the total activity of the individual irradiated specimen. 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].…”

“…Using this specimen holder, the specimens are prevented from cupping upward during punching, and therefore, plastic deformation is concentrated in the region below the punch rod. Using relatively simple system with recorders of the load and deflection values, we can obtain the following data of basic mechanical properties [6,7]:…”