Experiences in Examining and Dispositioning Flaws in Zr-2.5Nb Pressure Retaining CANDU Reactor Components

CANDU reactors use Zr-2.5 Nb alloy pressure tubes, as the primary pressure boundary within the reactor core. These components are subject to periodic inspection and material surveillance programs. Occasionally, the inspection program uncovers a flaw, whereupon the flaw is assessed as to whether it compromises the integrity of the pressure-retaining component. In 1998, such a flaw was observed in one pressure tube of a reactor. Non-destructive techniques and analysis were used to form a basis to disposition the flaw, and the component was fit for a limited service life. This component was eventually removed from service, whereupon the destructive examinations were used to validate the disposition assumptions used. Such a process of validation provides credibility to the disposition process. This paper reviews the original flaw and its subsequent destructive evaluation.

The Development of Structural Integrity Procedures for the Safe Working of Ceramics Industry Ball Mills with Cast Iron Ends

A ball mill at a ceramics materials company in North Staffordshire exploded in the spring of 2000. Luckily, nobody was injured as a result. Even so, the damage was so extensive that many in and around the ceramics industry rapidly concluded that the industry must introduce suitable operating practices so that the risk of a similar explosion was made acceptably small. As a result, a group of engineers, industry representatives and safety professionals has developed a set of Structural Integrity procedures for adoption in this industry. These mills and their operation are very different from the structures in those industries that normally draw upon Fracture Mechanics-based Structural Integrity principles. Hence, the development of these SI procedures for the ceramics industry is a good example of how their rapid acceptance could benefit traditional industries. Ceramics materials are ground wet, and so the mill must be sealed when running, to keep the resultant slurry inside. Since grinding produces a large amount of heat, the pressure in the mill rises as the water vapor and air inside attempt to expand. The mill therefore becomes a pressure vessel if it is left running long enough for the internal pressure to reach significant values. There are more than 250 ball mills in the UK industry. At least one half of those in the ceramics industry have cast iron ends, as did the mill that exploded. They are cylindrical, and rotate around a horizontal axis. Their construction typically involves a steel cylindrical wrapper, fastened, in different ways, to the two end plates. The end plates are therefore the critically sensitive components. The paper describes the work to assess the sensitivity of the cast iron end plates to the presence of cracks, and how this can be related to regular operating practice in the industry. The structure of the industry makes it heavily constrained against large capital expenditure, so effective SI procedures must be affordable by each company. The practicality of the procedures depends crucially upon non-critical flaws being observable by eye for a long period of mill running. As a result, the planned SI procedures can be implemented through a programme of education and training of all plant personnel and their supervisors and managers. This is an excellent example of how SI practice depends upon effective working of all relevant staff in an industry.

A Simplified Method for Calculating Crack Opening Area

In the context of the formulation of a leak-before-break case for a component in a pressurized system, this paper is concerned with the quantification of the crack opening area associated with a two-dimensional crack that is subjected to tensile stresses. We present a simple method, based on the strip yield representation of plastic deformation, for calculating the area. The method is validated against the known result for the ease of an isolated crack in a uniformly stressed infinite solid. It is then used for a general stress distribution, as might arise from a combination of pressure induced and weld residual tensile stresses, with the considerations being focussed on the case where plastic deformation is limited, as is usually appropriate for normal operating situations; application of the method is then especially simple.

Inspection and Evaluation Guidelines for Light Water Reactor Core Internals in Japan

There have been many cracking experiences of light water reactor (LWR) core internals worldwide in the past. Thermal and Nuclear Power Engineering Society in Japan (TENPES) has organized a committee to prepare technically reasonable and appropriate inspection and evaluation guidelines (I&E guidelines) for core internals. This committee consists of scholars and representatives from electric utilities and nuclear plant vendors in Japan. I&E guidelines, which cover a rational inspection plan, structural integrity assessment and repair methods, have been developed considering nuclear safety function and structural strength of each core internal component. For BWR reactors, the development of I&E guidelines cover major core internal components like shroud support, core shroud, top guide, core plate, ICM and CRD housing, core spray piping and sparger, jet pump etc. For PWR reactors, the development of I&E guidelines cover baffle former bolts, barrel former bolts, core barrel weld, bottom mounted instrumentation, etc. The I&E guidelines will be completed by the end of March 2002. The basic concept of the guidelines, and a guideline for shroud support of a BWR as an example, are shown in this paper.

Crack Initiation Behaviour in Small Root Radius Zr-2.5Nb Pressure Tube Specimens Under Monotonic and Cyclic Loading Conditions at Ambient Temperature

To evaluate crack initiation from small radius flaws in Zr-2.5 Nb pressure tubes in operating CANDU reactors, a number of crack initiation tests under both monotonic and cyclic loading in notched tension specimens have been carried out at ambient temperature. Test specimens were machined with the tensile axis parallel to the circumferential or transverse direction in both as-fabricated and ex-service pressure tube materials. The test specimens were first pre-conditioned by subjecting them to a notch-tip creep stress relaxation cycle, and a subsequent hydride formation cycle whereby notch-tip hydrides are produced. Some specimens with no notch-tip hydride were also tested. Test specimens were instrumented with acoustic emission (AE) and DC potential drop (PD) monitoring systems for detection of crack initiation and growth. Test results indicated the following trends: (a) the existence of notch-tip hydride lowers the crack initiation stress, with the hydride effect being much stronger under monotonic loading than under cyclic loading; (b) the greater the hydrogen concentration, the lower the notch-tip hydride crack initiation stress under monotonic loading; (c) for a given alternating elastic peak stress, the smaller the root radius the greater the resistance to crack initiation; (d) as expected, the number of cycles to crack initiation is a strong function of the alternating elastic peak stress, for a given specimen geometry. An analysis of the test results and their impact on flaw evaluation methodology for Zr-2.5 Nb pressure tubes is described.

Limit Load Solutions for Surface Cracks in Plates Under Different Loading Types

Limit load solutions of plates containing surface cracks are determined using non-linear finite element analysis. The study covers both shallow and deep cracks with different crack length/crack depth ratios under different loading types. The crack configurations consist of semi-elliptical surface cracks with a/t = 0.20, 0.40, 0.60, 0.80 and l/a = 2, 5, 10. Also studied are plates containing infinite surface cracks with a/t = 0.00, 0.20, 0.40, 0.60, 0.80. The cracked plates are subjected to pure tension, pure bending and combined tension and bending. The finite element results obtained from this study are compared with some published limit load solutions in the literature. It is shown that the exiting solutions are in general overly conservative.

Overview of NESC-IV Cruciform Specimen Test Results

NESC-IV is an experimental/analytical program to develop validated analysis methods for transferring fracture toughness data generated on standard test specimens to shallow flaws in reactor pressure vessel welds subject to biaxial loading in the lower-transition temperature region. It is the fourth major project of the Network for Evaluating Structural Components (NESC). The testing program has exploited material removed from a production-quality reactor pressure vessel. In Part A six clad cruciform specimens containing shallow surface-breaking flaws located in weld material were successfully tested. For Part B a further four beam tests were performed using an innovative test piece design with a simulated embedded flaw. Post-test analysis is now in progress. The implications for current best-practice procedures for evaluation of RPV shallow are also being considered.

Validation of Constraint Based Methodology in Structural Integrity – VOCALIST: Project Overview

VOCALIST (Validation of Constraint Based Methodology in Structural Integrity) is a shared cost action project co-financed by the European Commission under the Fifth Framework of the European Atomic Energy Community (EURATOM). The motivation for VOCALIST is based on the observation that the pattern of crack-tip stresses and strains causing plastic flow and fracture in components is different to that in test specimens. This gives rise to the so-called constraint effect. Crack-tip constraint in components is generally lower than in test specimens. Effective toughness is correspondingly higher. The fracture toughness measured on test specimens is thus likely to underestimate that exhibited by cracks in components. The purpose of VOCALIST is to develop validated models of the constraint effect, and associated best practice advice, with the objective of achieving (i) an improved defect assessment methodology for predicting safety margins; (ii) improved lifetime management arguments. The work is intended to be of benefit to both plant operators and their regulators. The project started in October 2000 and will run for thirty-six months. The main achievements to date are as follows: • Identification and assessment of current issues affecting the application of constraint-based fracture assessment methods in practical situations. • Materials selection and identification of test matrices. • Initiation of characterisation testing. • Detailed definition of analytical programme and alignment with experimental programme. • Definition of test conditions for Benchmark and Features tests. • Procurement and execution of Benchmark tests. • Procurement and execution of scoping Features tests. This paper provides an overview of VOCALIST, describes its various Work Packages, and provides an up-to-date summary of progress as the project reaches mid-term.

LBB Assessment on Ferrite Piping Structure of Large-Scale LMFR

These days, the interest on LBB behavior becomes to be rising in the step of design of LMFR structural components in Japan. The governing service loading of LMFR can be identified thermal expansion stress and thermal transient stress because the operation temperature is higher than that of LWR and the liquid metals contain higher heat transfer coefficient than water. On that reason, the ferrite steel which yields less thermal expansion is considered to be desirable for the piping components of LMFR in the viewpoint of high temperature structural integrity. This study deals with LBB assessment of the primary piping components of LMFR subjected to a series of transients and the results may envision the use of ferrite steel for LMFR components.

Validation of Constraint Based Methodology in Structural Integrity – VOCALIST: Experimental Programme

The aim of VOCALIST (Validation of Constraint-Based Assessment Methodology in Structural Integrity) is to develop and validate innovative procedures for assessing the level of, and possible changes to, constraint-related safety margins in ageing pressure boundary components [1]. An iterative process of experiment and analysis will address this overall objective. The experimental investigations within VOCALIST are performed on three different materials representing the as new state of materials used for components of nuclear power plants as well as a state representing an in service degraded state of RPV materials. Within the experimental programme fracture mechanics specimens with different constraint situations are tested in order to quantify the influence of the constraint on the specimens failure behaviour as a basis for the advanced components integrity assessment. The investigations are performed on small laboratory specimens as well as on biaxially loaded cruciform specimens and large piping components. Within this contribution the experimental programme of VOCALIST is introduced. The investigated materials are characterized in terms of their mechanical properties. Special consideration is given to results of fracture mechanics specimens highlighting the constraint effect via the shallow crack effect and its contribution to a shift of the master curve.