Review of Test Methods for Evaluating Hydrogen Embrittlement Susceptibility of Materials

For several decades, engineers and technicians responsible for safe transport and storage of hydrogen, in both the government and industry sectors, have had to contend with the problem of hydrogen embrittlement. Today, the problem of hydrogen embrittlement must be considered anew with the systems and environments emerging with the developing hydrogen fuel cell industry. This paper discusses several methods to test for the susceptibility of metallic materials to hydrogen embrittlement. The objective of this review is to assist engineers and designers in selecting hydrogen embrittlement test methods to simulate actual manufacturing and/or operating conditions while using appropriate specimen geometries. Reviewed are standard test methods from American Society for Testing and Materials (ASTM) International, British Aerospace Series (BSEN), and General Motors Engineering sources. These include constant load, rising step load, slow strain rate, inclined wedge, bend, disk pressure, and cantilever beam test methods. Information is provided on load and displacement characteristics, uses, a brief description, and required equipment for each test.

Investigation of Constraint Effects on Fracture Toughness for CC(T) Specimens

Within the framework of the European research project VOCALIST, centre cracked tension, CC(T), specimens made of an RPV steel were tested and analysed to quantify the influence of local stress state on fracture toughness. The CC(T) specimens demonstrate a significant loss of crack tip constraint resulting in a considerable increase in fracture toughness as compared to standard fracture mechanics specimens. So, the master curve reference temperature, To, determined on the basis of CC(T) tests performed in this study is about 43°C lower than To obtained on standard C(T) specimens. Finite element analyses of the tests revealed that the above experimental finding is in a good agreement with the empirical correlations between the reference temperature shift and the crack tip constraint as characterised by the T-stress or Q parameter (Wallin, 2001; Wallin, 2004). The results of this work are consistent with a number of other tests performed within the VOCALIST project and contribute to the validation of engineering methods for the crack assessment in components taking account of constraint.

Application of a Weibull Stress Model to Predict the Failure of Surface and Embedded Cracks in Large Scale Beams Made of Clad and Unclad RPV Steel

Within the European Network NESC, the project NESC IV deals with constraint effects of cracks in large scale beam specimens, loaded by uni- or biaxial bending moments and containing surface or embedded cracks. The specimens are fabricated from original US RPV material, being cladded or cladding is removed. All large scale tests have been conducted at ORNL outside the NESC IV project. The outcome and the analyses of these uncladded and cladded beams containing the surface or embedded cracks are shown. By means of the finite element method, local approach methods and the Weibull stress models the specimens are analysed at the test temperatures and the probability of failure is calculated, taking into account constraint effects. For the case of the embedded cracks it turned out that the failure moment of the uncladded beam is 5% lower than the one of the cladded beam. Both crack fronts of the embedded crack are supposed to fail at the same failure moment. The results of the analysis of the cladded beam showed that the upper crack front nearer to the surface fails prior to the lower crack front, which is located deeper in the specimen (the failure moment is 5% lower). The numerical results agree very well with the experiments. The experimental failure moments could be well predicted and the failure scenario (which crack front fails first) could be determined. A theoretical shift in the transition temperature T0 due to constraint effects could be defined for both crack fronts.

Impact of Welding Sequence on the CRDM Nozzle-to-Vessel Weld Stress Analysis

A three-dimensional finite element model is presented to simulate the welding process of the side-hill control-rod-drive-mechanism (CRDM) nozzle to the vessel head. Emphasis is given to how the weld is laid out in the analysis so that accurate residual stress results can be obtained while the required computing time is viable. In the order of complexity, three approaches are examined in this study: a) the simultaneous approach, i.e., the weld bead (therefore the heat associated with it) is put in the model in a uniform fashion; b) the piece-by-piece approach, i.e., the weld is laid out segment by segment; c) the moving-source approach where the analysis is done by simulating the moving heat source. It is found that there is a significant difference between the stress results by the uniform approach and the piece-by-piece approach. While the moving source method gives the closet representation of the welding process, the computing time for such a multi-pass, three-dimensional model is still prohibitive. The natural choice is therefore the piece-by-piece approach, with the number of segments for the weld dependent on the weld parameters and the geometries of the nozzle and vessel head.

Postirradiation Ductility Demonstration Tests of Weapons-Derived MOX Fuel Cladding

The U.S. Department of Energy (DOE) Fissile Materials Disposition Program (FMDP) is pursuing reactor irradiation of mixed uranium-plutonium oxide (MOX) fuel for disposal of surplus weapons-usable plutonium. Since most of the MOX fuel utilization experience has been with reactor-grade plutonium, it is desired to demonstrate that the unique properties of the surplus weapons-derived or weapons-grade (WG) plutonium do not compromise the applicability of this MOX experience base. A related question to be addressed for weapons-derived MOX fuel is that of ductility loss of the cladding. While irradiation induced loss of ductility has long been known and quantified for many cladding materials, the potential synergistic effects of irradiation and the unique constituents (i.e., gallium) of weapons-derived MOX fuel are not known. As part of an extensive fuel qualification research program conducted by Oak Ridge National Laboratory (ORNL), a new test method was developed and validated to measure the room temperature ductility and hoop tensile properties of MOX fuel cladding. The cladding material is a zirconium alloy designated as Zr-4 manufactured by Sandvick Corporation. This paper is a summary of the test method developed and of demonstration test results obtained for MOX cladding irradiated to 21 GWd/MT [7 × 1020 n/cm2 (E > 1 MeV)].

Project NURBIM (Nuclear RI-ISI Methodology for Passive Components): Benchmarking of Structural Reliability Models and Associated Software

A consortium has been formed, sponsored by the European Commission, to perform a project with the acronym NURBIM. The NURBIM project is focusing on the definition of best practice methodologies for performing risk analyses and establishing a set of criteria for the acceptance of risk quantities that can help Regulatory bodies in Europe to accept Risk-Informed In-Service Inspection (RI-ISI) as a valid tool for managing plant safety. One of the main areas within NURBIM has been Work Package 4 (WP-4) named Review and Benchmarking of Structural Reliability Models (SRMs) and associated software. The results of the benchmark studies are used in the process of formulating criteria which should be fulfilled for a suitable SRM and associated software for application of RI-ISI. Reports of the complete benchmark study can be found in Brickstad (2004) and Schimpfke (2004). The final report of both studies is given by Brickstad et al (2004).

Experimental Four Points Bending Test on a Real Size Bimetallic Welded Pipe: European Project ADIMEW

In July 2003 EDF-R&D performed a 4 points bending test on a real size bimetallic welded pipe. This test (following the BIMET program) is a major part of the European project ADIMEW (Assessment of aged piping DIssimilar MEtal Welds integrity). The mock-up was representative of an Expansion line of the Pressurizers of French N4 Power Plants: it was a 16” pipe including a ferritic part (SA508 Class 3), an austenitic one (316L) and a weld with buttering (309L) and weld metal (308L). An outer circumferential 1/3-thickness defect had been introduced in the first layer of the buttering, parallel to the buttering/ferritic interface (at a distance of 2mm). The test was conducted under displacement control and the partial unloading method was used to control the defect initiation and propagation. Crack initiation occurred at a relatively low bending moment but a long stable propagation was to observe. This proves the ability of such junctions with large defects to overcome very high bending loads without any unstable crack growth. This paper will provide more detailed information about the testing conditions, all important experimental results and a discussion around them.

Structural Integrity of Dissimilar Welds: ADIMEW Project Overview

The objective of the ADIMEW project, that follows BIMET program finished in 2000, is to contribute to the development and verification of analysis methods which describe the behaviour of an external circumferential defect at the surface of a Dissimilar Metal Weld (DMW). The complexity of the problem results from the prevailing mixed-mode loading conditions, the variation in material constitutive equations across the weld zone, and the presence of large residual stress field. Under these circumstances, classic fracture mechanics concepts are difficult to apply. Recently different cracks has been discovered in different US and European plants (VC SUMMER, RINGHALS, BIBLIS). The problem of integrity of aged (cracked) DMW’s remains an important safety issue. This paper presents the new on-going program ADIMEW, with similar organisation than BIMET and complementary objectives. Within the framework of this project, one large test has been successfully done on July 2003. The paper presents a general overview of the program and the major recommendations and conclusions.

ADIMEW Test: Assessment of a Cracked Dissimilar Metal Weld Assembly

ADIMEW (Assessment of Aged Piping Dissimilar Metal Weld Integrity) was a three-year collaborative research programme carried out under the EC 5th Framework Programme. The objective of the study was to advance the understanding of the behaviour and safety assessment of defects in dissimilar metal welds between pipes representative of those found in nuclear power plant. ADIMEW studied and compared different methods for predicting the behaviour of defects located near the fusion boundaries of dissimilar metal welds typically used to join sections of austentic and ferritic piping operating at high temperature. Assessment of such defects is complicated by issues that include: severe mis-match of yield strength of the constituent parent and weld metals, strong gradients of material properties, the presence of welding residual stresses and mixed mode loading of the defect. The study includes the measurement of material properties and residual stresses, predictive engineering analysis and validation by means of a large-scale test. The particular component studies was a 453mm diameter pipe that joins a section of type A508 Class 3 ferritic pipe to a section of type 316L austentic pipe by means of a type 308 austentic weld with type 308/309L buttering laid on the ferritic pipe. A circumferential, surface-breaking defect was cut using electro discharge machining into the 308L/309L weld buttering layer parallel to the fusion line. The test pipe was subjected to four-point bending to promote ductile tearing of the defect. This paper presents the results of TWI contributions to ADIMEW including: fracture toughness testing, residual stress measurements and assessments of the ADIMEW test using elastic-plastic, cracked body, finite element analysis.

Crack Initiation and Arrest of Irradiated RPV-Steels: A National Project on German RPV Weld and Base Materials

In Germany the procedure which has to be applied for the safety assessment of the reactor pressure vessel is based on the RTNDT concept. The Master Curve concept (based on T0) has the advantage compared to the RTNDT concept that the basic tests are fracture toughness tests instead of Charpy impact energy or Pellini tests. By means of the recently initiated German project CARISMA (Crack Initiation and Arrest of Irradiated Steel Materials), a data base will be created on pre-irradiated original materials of the four generations of German nuclear pressurized water reactors, which allows to examine the consequences if the Master Curve instead of the RTNDT concept will be applied.