Fracture Mechanics

In 1993, a network for the evaluation of structural components (NESC) (Rintamaa and Taylor, 2001) was formed, based on a multi-partner collaboration agreement and managed by the Joint Research Centre at Petten. It was composed of utilities, regulators and research organisations and the main objective was to develop and validate structural integrity techniques for assessment. There was a broad representation of countries across Europe, covering countries operating a wide range of nuclear power plants.

There have been four NESC projects.

NESC-1 was the first large-scale project to evaluate the whole process of structural integrity assessment. In particular, the spinning cylinder PTS test was designed to simulate the conditions of an ageing RPV subjected to a severe PTS loading. It demonstrated the beneficial effect of cladding in inhibiting cleavage initiation in the cylinder surface. It was used to validate structural mechanics assessment techniques and to validate no-destructive inspection techniques.

The NESC-2 programme included two large-scale PTS tests on thick wall (200 cm) cylinders with shallow defects. The objective of the tests was to consider brittle crack initiation, the propagation and arrest of shallow cracks in a cladded vessel under PTS loading. The first test, which included a circumferential under-clad notch of depth 8 mm, exhibited a crack growth that was arrested. In the second test, there were two shallow semi-elliptical through clad effects but no growth occurred.

In NESC-3 there is a large-scale test on a dissimilar weld pipe assembly of aged PWR Class 1 piping. It is a benchmark test to demonstrate the load to cause failure at a large defect. The purpose is to quantify the accuracy of assessment procedures for a defect containing dissimilar metal welds, to address issues regarding inspection performance, and to promote best practice.

The NESC-4 test series is to test defect-containing beams, designed to clarify the role of bi-axial stress effects on shallow flaws in RPV weld material.

There are several other EC R&D programmes that have links with NESC. These include exploratory or pilot projects that investigate certain aspects that could lead to further major tests (Tice et al., 1999; Faidy et al., 2000; Leggart et al., 1999) and other collaborations that utilise NESC results (Lidbury et al.).

The European SMILE project (Bezdikian et al., to be published) considers whether the structural margins of aged embrittled RPVs can be improved if a particular potential beneficial effect of load history is taken into account (warm pre-stress). The programme will provide data from representative steels.

In recent observations, different cracks have been discovered in different US and European nuclear power plants (VC SUMMER, RINGHALS, BIBLIS). The issue is the integrity of aged cracked metal welds involving different materials, e. g. ferritic to stainless steel. The extent of crack growth and paths followed by a crack through the weld will be followed under various loads in the ADIMEW project (Faidy et al., to be published).

Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *