Generalized procedures for the use of plane strain and elastic-plastic fracture mechanics options in the modernization of nuclear standards

In the design of nuclear power plants and the selection of required structural materials, the assurance of reliability in operation is an essential consideration. The need for analytical criteria for defining the adequacy of fracture toughness is particularly acute for pressure vessel materials. The 1972 revision to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Section III) has adopted linear elastic fracture mechanics (LEFM) methods as a means of assuring fracture-safe operation for nuclear vessels and components. It is noted that LEFM can be used to analyze only the behavior of metals subject to plane strain constraint (i.e. brittle behavior), while many steels when used in structural applications will behave in a ductile fashion. Thus, in the modernization of nuclear codes, there is the additional need to include the full range of fracture mechanics options for system design, that is, elastic-plastic and fully plastic fracture mechanics as well as the linear elastic procedures. The choice of a particular toughness regime for application of the metal (e.g., plane strain or elastic-plastic) can then be made by the designer or regulatory body. It follows that this decision will have a major implication on the selection of nuclear structural materials.This paper describes recent developments in the means for defining the full range of plane strain, elastic-plastic, and plastic fracture mechanics options available to the designer. Comparisons are made between these options and the fracture toughness requirements of the ASME Nuclear Code (Section III). Exisiting dynamic plane strain, K_Id, data for structural metals are analyzed in concert with dynamic tear (DT) test trends. The limited temperature region of K_Id applicability for these materials is shown to presage the elastic-plastic regime through which sharply increasing stress is required for fracture propagation whereby a leak-before-fail condition is ultimately attained. This phenomenon highlights the need to extend the analytical capabilities for fracture assurance into the non-brittle regime. The DT test is an effective engineering tool which, like the crack opening displacement (COD) concept, can be used to define the elastic-plastic and plastic-constraint transitions. The DT test procedure is fully rationalizable in terms of section size parameters and can be used independently or together with the K_Id-temperature trend to predict the onset of the elastic-plastic and plastic regimes as a function of temperature and section thickness.     

Evaluation of effect of hydrogen on toughness of Zircaloy-2 by instrumented drop weight impact testing

Hydride-assisted degradation in fracture toughness of Zircaloy-2 was evaluated by carrying out instrumented drop-weight tests on curved Charpy specimens fabricated from virgin pressure tube. Samples were charged to 60 ppm and 225 ppm hydrogen. Ductile-to-brittle-transition behaviour was exhibited by as-received and hydrided samples. The onset of ductile-to-brittle-transition was at about 130 °C for hydrided samples, irrespective of their hydrogen content. Dynamic fracture toughness (K_ID) was estimated based on linear elastic fracture mechanics (LEFM) approach. For fractures occurring after general yielding, the fracture toughness was derived based on equivalent energy criterion. Results are supplemented with fractography. This simple procedure of impact testing appears to be promising for monitoring service-induced degradation in fracture toughness of pressure tubes.     

The Weibull stress parameters calibration upon the toughness scaling model between cracks having different constraint

An extensive investigation has been carried out on the sensitivity parameters determination describing the fracture behaviour of the body with a crack with respect to the character change of the true stress–strain curve with the dominant region of Lueders deformation. This paper presents the consideration on the change judgement of the J-integral and the constraint as the base parameters of two-parameter fracture mechanics. The Weibull stress model for cleavage fracture originally proposed by Beremin group requires calibration of two micromechanics parameters (m, σ_u). The Weibull stress σ_w seems to be a parameter for the prediction of cleavage failure of cracked bodies and the study is focused on the assessment of the effects of constraint loss on the cleavage fracture toughness (J_c). To quantify the effects of the constraint variation on the cleavage fracture toughness the form of the toughness scaling model based on the Weibull stress σ_w is investigated. Local material parameters have been calculated from Beremin approach and the calibration is performed on various approaches. Methods are based on the weakest link assumption and the incremental fracture probability, which depends not only on the maximum principal stress, but also on the equivalent plastic strain. The fracture resistance has been assessed using data from static tests of three point bend specimens.     

Fracture mechanics behavior of the T91 martensitic steel in contact with liquid lead-bismuth eutectic for application in an accelerator driven system

The fracture toughness of the T91 martensitic steel in liquid lead-bismuth eutectic has been measured at 300 °C in plane stress and plane strain conditions. The effect of achieving wetting at the crack tip prior starting mechanical testing is demonstrated to be the key factor for a correct evaluation of the potential effect of LBE on fracture toughness. In plane stress, one observes a serrated fracture mode associated with a reduction of fracture toughness between 20% and 30%. The toughness reduction is higher in plane strain where the cleavage fracture mode prevails. The difference between the two fracture modes is due to the higher plastic deformation level reached at final fracture in plane stress and to the higher crack growth rate in plane strain. These results will be useful for the design of future nuclear systems cooled by LBE planning to use martensitic steels as structural materials.     

Sandia National Ladoratories Cask Drop Test Programme: A Demonstration of Fracture Mechanics Principles for the Prevention of Brittle Fracture

Abstract

Sandia National Laboratories recently completed a cask drop test programme. The aims of the programme were (1) to demonstrate the applicability of a fracture mechanics-based methodology for ensuring cask integrity, and (2) to assess the viability of using a ferritic material for cask containment. The programme consisted of four phases: (i) materials characterisation; (ii) non-destructive examination of the cask; (iii) finite element analyses of the drop events; and (iv) a series of drop tests of a ductile iron cask. The first three phases of the programme provided information for fracture mechanics analyses and predictions for the drop test phase. The drop tests were nominally based upon the lAEA 9 m drop height hypothetical accident scenario, although one drop test was from 18 m. All tests were performed in the side drop orientation at a temperature of ?29°C. A circumferential, mid-axis flaw was introduced into the cask body for each drop test. Flaw depths ranged from 19 to 76 mm. Steel saddles were welded to the side wall of the cask to enhance the stresses imposed upon the cask in the region of the introduced flaw. The programme demonstrated the applicability of a fracture mechanics methodology for predicting the conditions under which brittle fracture may occur and thereby the utility of fracture mechanics design for ensuring cask structural integrity by ensuring an appropriate margin of safety. Positive assessments of ductile iron for cask containment and the quality of the casting process for producing ductile iron casks were made. The results of this programme have provided data to support IAEA efforts to develop brittle fracture acceptance criteria for cask containment.     

Fracture Toughness of DCI Casks and Prediction by Small Specimen Test

Abstract

Spent nuclear fuel transport and/or storage containers (casks) must maintain their structural integrity even when subjected to hypothetical accidents during transport or handling accidents at storage facilities. For ductile cast iron (DCI) to be used as a cask containment boundary material, adequate fracture toughness must be demonstrated at service temperatures and Impact loading conditions of concern. In Japan, comprehensive studies of the fracture toughness of heavy section DCI have been undertaken by a number of research organisations to provide the safety assurance for the DCI casks. In the present study, the fracture toughness data were used to develop a lower bound trend curve for heavy section DCI and to examine the prediction methods by small specimen tests. The fracture toughnesses K_Ic, K_IIc and K_IIIc were also obtained to study the safety assessment of DCI casks under different loading mode conditions.     

Heavy-section steel technology program: Intermediate-scale pressure vessel tests

Eight flawed 990-mm-OD, 152-mm-thick vessels of ASTM A508 class 2 forging steel or A533, grade B, class 1 steel plate have been pressurized hydraulically to burst or rupture. The rupture test of one vessel (V-7) was repeated pneumatically to study the effects of the sustained load thus attainable. Test temperatures ranged from 0 to 91°C so that transitional to upper shelf fracture toughness behavior was observed. Pretest predictions of failure pressure were based on tensile and toughness properties determined from Charpy-size to 102-mm-thick specimens representative of the test vessels. Linear elastic fracture mechanics based on strain and plastic instability analyses were generally adequate for determining the failure pressure, which ranged from 2.15 to 3.28 times design pressure.     

Constraint effects on fracture toughness of impact-loaded, precracked Charpy specimens

Impact-loaded, precracked Charpy specimens often play a crucial role in irradiation surveillance programs for nuclear power plants. However, the small specimen size B = W = 10 mm limits the maximum value of cleavage fracture toughness J_c that can be measured under elastic—plastic conditions without loss of crack tip constraint. In this investigation, plane strain impact analyses provide detailed resolution of crack tip fields for impact-loaded specimens. Crack tip stress fields are characterized in terms of J—Q trajectories and the toughness-scaling model which is applicable for a cleavage fracture mechanism. Results of the analyses suggest deformation limits at fracture in the form of b > MJ_c/σ₀, where M approaches 25–30 for a strongly rate-sensitive material at impact velocities of 3–6 m s⁻¹. Based on direct comparison of the static and dynamic J values computed using a domain integral formulation, a new proposal emerges for the transition time, the time after impact at which interial effects diminish sufficiently for simple evaluation of J using the plastic η factor approach.     

A direct fracture toughness model for irradiated reactor vessel weld material based on reference temperature

The master curve method has opened a new means to acquire a directly measured material-specific fracture toughness curve based on testing a small number of replicate specimens. This process enables, for the first time, the construction of a material-specific fracture toughness curve for an irradiated material directly from fracture tests. Currently, only an inferred fracture model is available through a combination of the ASME Boiler and Pressure Vessel Code and a regulatory guide from the U.S. Nuclear Regulatory Commission. This approach uses the fracture toughness curve of a generic, unirradiated reactor vessel steel that is shifted by a reference temperature (RT_NDT) based on Charpy impact test data. The master curve method yields a key material parameter called reference temperature, T₀, which indicates the location of the transition range fracture toughness curve on the temperature axis. When a small number of pre-cracked Charpy specimens were tested at several different fluence levels, the material specific reference temperatures can be shown as a function of fluence. One such model for the WF-70 weld material is presented in this paper. The irradiated specimen data and analyses from Oak Ridge National Laboratory (ORNL) and the B&W Owners Group (B&WOG) are utilized for this model. This model is based on fracture toughness data, independent of Charpy impact energy levels, percent shear, and most importantly, material properties of unirradiated condition.     

Mechanical integrity analysis of TMI-1 OTSG unplugged tubes

Small I.D. circumferential defects have been identified in many steam generator tubes. The origin of the cracks is known to be chemical, not mechanical. A fracture mechanics evaluation has been conducted to ascertain the stability of tube cracks under steady-state and anticipated transient conditions. A spectrum of hypothetical crack sizes was interacted with tube stresses derived from the load evaluation using the methods of linear elastic fracture mechanics (LEFM). Stress intensities were calculated for part-through wall cracks in cylinders combining components due to membrane stress, bending stress, and stresses due to internal pressure acting on the parting crack faces as the loads are cycled.The LEFM computational code, “BIGIF”, developed for EPRI, was used to integrate over a range of stress intensities following the model to describe crack growth in INCO 600 at operating temperature using the equation (ΔK)^3.5.The code was modified by applying ΔK_Th, the threshold stress intensity range. Below ΔK_Th small cracks will not propagate at all. Appropriate R ratio values were employed when calculating crack propagation due to high cycle or low cycle loading.Cracks that may have escaped detection by ECT will not jeopardize tube integrity during normal cooldown unless these cracks are greater than 180° in extent. Large non-through-wall cracks that would jeopardize tube integrity are not expected to evolve because in axi-symmetric tensile stress fields, cracks propagate preferentially through the tube wall rather than around the circumference. Tube integrity can be demonstrated for mid-span tube regions and for the transition region as well.The as-repaired transition geometry is a design no less adequate than the original. The as-repaired condition represents an improvement in the state of stress due to mechanical and thermal loads as compared to the original.     

Probabilistic Evaluation of Brittle Failure Design Criterion of DCI Cask

Abstract

As a cask material, ductile cast iron may be susceptible to failure in a brittle manner under certain temperature and load conditions. A design criterion for ductile cast iron casks against brittle failure due to drop tests, has been proposed by Central Research Institute of Electric Power Industries. This design criterion includes a safety factor which presents the extent between the detectable flaw size and the critical flaw size and may be interpreted as ‘uncertainty factor’ as to the uncertainties regarding stress prediction, fracture toughness and so on. In this report, to verify the proposed design criterion, probabilistic evaluation was performed according to a series of drop tests using a full scale cask and material tests, and it is confirmed that the proposed design criterion is applicable and reliable. Furthermore, applicability of the safety design method described in the IAEA-TECDOC-717 published in August 1993 was investigated.     

Pressure vessel steel fracture toughness in the regime from room temperature to 400°C

The fracture toughness of steels that are susceptible to dynamic strain aging shows a minimum at temperatures higher than the upper shelf starting temperature. This phenomenon is caused simultaneously by strain aging and plastic deformation. The first aim of the present work is to analyze the effect of dynamic strain aging on the fracture toughness values of three pressure vessel steels in the temperature range between room temperature and 400°C. Fracture mechanics tests were carried out on A533 GB, A516 G70 and 304L steels to obtain the following parameters: J_IC, CTOD_m and the J-R curves. These values were compared against those available in the present references, and good agreement was found. Charpy V notch tests were also carried out on A516 G70 steel at the same test temperatures as for the fracture mechanics tests to analyze the effect of the strain rate. The critical wide stretch zones of the 304L steel specimens were also measured to verify another author's hypothesis about a toughness drop at the upper shelf temperature.     

The use of fracture mechanics and non-destructive testing to reduce the risk of catastrophic failure

Structural integrity assessments involve the use of fracture mechanics together with appropriate design, quality assurance and inspection techniques. Recent application to nuclear pressure vessels has led to improvements in the fracture toughness data base, in methods for measuring fracture toughness and in the use of elastic/plastic and J?R curve concepts. Fatigue crack growth studies in water of realistic flow rate and oxygen content have shown that the effect of a PWR water environment is not as severe as previously reported and has related this to show strain rate cracking. The role of the pressure test has beenn examined, throwing emphasis on the importance of effective non-destructive inspection to detect and characterise flaws. Recent developments to improve and to validate very high levels of effectiveness of NDT are summarised.     

Application of fracture mechanics methods in safety analysis of piping components in subcreep and creep behavior

A survey and review program for the application of fracture mechanics methods in elevated temperature design analysis and safety evaluation was initiated in December 1976. The first report [1] surveyed and assembled the material for a critical review of the theories of fracture and the application of fracture mechanics methods to life prediction and safety analysis of piping components. The second report [2] provided the basic concepts and a review of the problem areas associated with the development of analytical and experimental programs for a systematic evaluation and comparison of the currently available fracture mechanics theories. The basis for such an evaluation was described in terms of a series of benchmark problems which accurately specify conditions of geometry, loading and environment characteristic of large diameter piping systems in nuclear service.The objective of this third report is to establish a data base and detail the additional analytical techniques needed to confirm the validity of existing analytical methods and improve the state of the art in current problematic areas effecting the interpretation and extension of safety evaluation methods. The need for such a program in the elevated temperature field has been demonstrated by a number of independent surveys on various safety aspects of LMFBR related structural analysis methods and matetials problems. The results of this program, however, will be applicable not only to reactor plants operating at elevated temperatures, but will also lead to improvements of light water reactor evaluation methods for operating and accident conditions.The current state of elevated temperature reactor design technology is embodied in the standards and codes which provide guidance and minimum requirements for systematic design and evaluation procedures. These, however, do not necessarily provide specific absolute values which, if satisfied in the course of design, will guarantee thirty to forty years of uninterrupted life. There are numerous assumptions and approximations embodied in these standards concerning materials behavior, damage mechanisms, and failure modes at elevated temperature. There are also numerous areas of uncertainty and conflicting opinion in the interpretation of the existing test data and in the analysis and evaluation methods. Furthermore, the standards and codes leave some areas to the judgement of the designer, some of which require explicit justifications, but no standards or rules are provided.The overall safety therefore lies, at the present time, in the combination of rigorous enforcement of current standards, judicious application of experience with high temperature equipment even if not in nuclear service, and the surveillance of actual operating conditions. In the past, one criterion proposed for elevated temperature design has been that the time for crack initiation should exceed the design life. However, due to the complexities of the piping structures and the nature of the stress history during service, the evaluation of initiation times is difficult and often leads to uneconomical designs. In addition defects may exist in the component before it enters service. Hence, the knowledge of the growth rates of cracks and the residual strength of the components containing cracks is important in a realistic design evaluation. For more brittle materials and lower temperature applications where plasticity is restricted, linear elastic fracture mechanics methods have been developed. For more ductile materials where the plastic zones near the cracks are larger, linear fracture mechanics methods are not directly applicable, but in these nonelastic cases the opening displacement and J integral methods of assessment have been proposed. In the complex situation encountered in nuclear power plant design, the analysis must also account for cyclic thermal strains, time dependent creep, and the effect of harmful environments which are not explicitly treated in the above-mentioned methods. In this report an in-depth review is presented in sufficient detail to illustrate the degree of agreement between the theoretical and empirical methods available in the literature and indicate the scope of the additional analyses and experimental work needed for the development of reliable safety evaluation methodology.For pure cylindrical bending, cracks perpendicular to the load start to grow when reaches a critical value which is generally larger than the corresponding critical uniaxial tension value. There appears to be a thickness effect in the bending case which is probably due to interference from the compressive sides of the crack.For a circular plate with lateral pressure and small lateral displacements, results agree with the bending data when using the nominal bending stress . For larger displacements when bulging occurs, the results agree with the tensile data when the nominal tensile stress is used.For curves surfaces, such as a cylinder under internal pressure, the data agree with the expression developed by Folias both for axial cracks under hoop stress σ and for circumferential cracks under axial stress σ Generally, the expressions were accurate up to , showing a tendency to be lower than the experimental data at higher values of the parameter. The parameter is a promising one.To study the influence of cracks at different angles to the applied load, analysis and data are available including the stress component parallel to the crack in the stress field around a crack tip. This, together with the concept of a critical circumferential stress at a critical distance (α = 0.1) ahead of the crack provides improved correlation with fracture predictions for both the angle of fracture and the critical stress intensity factor for the angled cracks in flat plates.For a hollow cylinder under torsion with angled cracks, the best correlation was given by the same analysis although the results were not as conclusive as for the flat plate. From elastic theory useful curves for the variation of K₁, K₂, and K₃ around the border of an elliptically shaped crack are available.In a plane stress fracture the addition of a biaxial stress produces an increase in the apparent fracture toughness compared with the uniaxial case. However, there is as yet no evidence to show that there would be the same increase in a plane strain situation. Hence, in the absence of biaxial information the uniaxial fracture data may be the most conservative for flat plates. However, for shells there will also be a curvature effect.In an analogous manner, fatigue crack propagation rates appear to be less rapid under biaxial stresses than under uniaxial stress. However, this shift is not great and generallly will be masked by other effects such as environment and temperature service situations.The analysis of cracks in weldments with residual stress effects are also available. In the case of a crack in a weld the estimated residual stress distribution agreed reasonably well with some experimental data for elastic conditions. Results indicate that there can be a tensile stress intensity factor even when the original residual stress distribution has changed to compressive. A point to remember is that residual stresses near welds can be beyond yield.An analysis based on Lagrangean mechanics is useful for indicating the different effects of liquids and gases as pressurizing media in hollow pipes. The results show that whereas gases maintain their pressure as a crack begins to propagate, the pressure in the liquid can quickly decrease so that subsequent catastrophic failure is less likely even in large diameter piping.     

A Survey of Codes and Standards in the USA Relevant to the Prevention of Brittle Fracture in Cask Containment Boundaries

Abstract

The use of non-austenitic materials in cask containment boundaries requires consideration of the potential for brittle fracture under severe loading conditions. In the USA, such guidance for service conditions which containment boundaries must withstand is provided in Title 10 of the Code of Federal Regulations, Part 71, ‘Packaging and Transportation of Radioactive Material’, paragraph 71.73, ‘Hypothetical accident conditions’. The hypothetical accident conditions include a ‘free drop’ of the package at a temperature of ?29°C, ‘…onto a flat, essentially unyielding, horizontal surface…’ from a height of 9 m. Such an event could potentially result in brittle fracture of a non-austenitic material containment boundary. Nevertheless, motivation exists for utilising ferritic materials or titanium alloys for containment applications. US Nuclear Regulatory Commission Regulatory Guide 7.6, ‘Design Criteria for the Structural Analysis of Shipping Cask Containment Vessels’, specifically excludes consideration of brittle fracture in its design criteria. Regulatory Guides 7.11 and 7.12, ‘Fracture Toughness Criteria of Base Material for Ferritic Steel Shipping Cask Containment With a Maximum Wall Thickness of 4 Inches (0.1 m)’, and ‘Fracture Toughness Criteria of Base Material for Ferritic Steel Shipping Cask Containment With a Wall Thickness Greater Than 4 Inches (0.1 m) But Not Exceeding 12 Inches (0.3 m)’, respectively, provide highly conservative criteria for selection of ferritic steel for containment based upon empirical correlations and materials tests suitable only for ferritic steels. Brittle fracture prevention criteria for cask containment based upon fracture mechanics principles remain non-codified in the USA. Methodologies for brittle fracture prevention routinely used in industry have yet to be implemented for cask design due to regulatory reticence. The American Society of Mechanical Engineers (AS ME) has provided models for brittle fracture prevention in Section III (‘Rules for Construction of Nuclear Power Plant Components’) Appendix G (‘Protection Against Nonductile Failure’), and Section XI (‘Rules for Inservice Inspection of Nuclear Power Plant Components’) Appendix A (‘Analysis of Flaws’), of the Boiler and Pressure Vessel Code. The ASME Subgroup on Containment Systems for Spent Fuel and High Level Waste Transport Packagings (NUPACK) has specifically addressed the issue of developing brittle fracture criteria for packages based upon fracture mechanics methodologies in the existing ASME Code but, to date, without resolution. This paper provides a detailed discussion of existing brittle fracture criteria in the USA and a status of new standards development.     

Reactor pressure vessel structural integrity research

Development continues on the technology used to assess the safety of irradiation embrittled nuclear reactor pressure vessels (RPVs) containing flaws. Fracture mechanics tests on RPV steel, coupled with detailed elastic-plastic finite element analyses of the crack-tip stress fields, have shown that (1) constraint relaxation at the crack-tip of shallow surface flaws results in increased data scatter but no increase in the lower-bound fracture toughness, (2) the nil-ductility temperature (NDT) performs better than the reference temperature for nil-ductility transition (RT_NDT) as a normalizing parameter for shallow flaw fracture toughness data, (3) biaxial loading can reduce the shallow flaw fracture toughness, (4) stress based dual-parameter fracture toughness correlations cannot predict the effect of biaxial loading on shallow flaw fracture toughness because in-plane stresses at the crack-tip are not influenced by biaxial loading, and (5) an implicit strain based dual-parameter fracture toughness correlation can predict the effect of biaxial loading on shallow flaw fracture toughness. Experimental irradiation investigations have shown that (1) the irradiation induced shift in Charpy V-notch vs. temperature behavior may not be adequate to conservatively assess fracture toughness shifts due to embrittlement, and (2) the wide global variations of initial chemistry and fracture properties of a nominally uniform material within a pressure vessel may confound accurate integrity assessments that require baseline properties.     

Mechanism based evaluation of materials behavior and reference curves

The safety assessment of nuclear pressure vessels and piping requires a quantitative estimation of defect growth by stable and unstable manner during service. This estimation is essential for determining whether the defect detected during inspection should be repaired or whether the size of the defect even after its expected growth is small enough to leave the integrity of the vessel unaffected.The most important stable defect growth mechanism is that of environmentally assisted cyclic crack growth. Recent results indicate that it is markedly affected by sulfur content and/or manganese sulfide morphology and distribution. This implies that an essential improvement in component safety has been gained by currently applied steelmaking practices, which result in extra low sulfur content, generally below 0.01 wt%, and in round shape and small size of inclusions, through, e.g., calcium treatment, hence considerably reducing the effect of environment on crack growth rate. This further implies that the ASME Section XI reference curves for environmentally accelerated cyclic crack growth are conservative for steels produced by current steelmaking practices.The ASME Section XI applies predominantly linear elastic fracture mechanics to assess the effects of cracks on the integrity of nuclear power plant components. Unstable linear elastic fracture often propagates by cleavage mechanism. The cleavage fracture process has recently been shown to be of statistical nature in both ferritic and bainitic steels. The carbide size distribution plays a dominant role in controlling the fracture toughness of these steels. A cleavage fracture model has been developed, based on carbide induced cleavage fracture in ferritic and bainitic steels, which can be used to estimate the expected value and probability limits of fracture toughness. This method has been utilized to evaluate the conservatism of the ASME reference fracture toughness curve. For this purpose a microstructural analysis was carried out for the HSST-02 plate material, with which a large amount of K_Ic data has previously been generated for reference curve purpose. The result of the statistical evaluation indicates that based on the 95% survival probability limit some parts of the ASME reference fracture toughness curve are unconservative.     

Dynamic Fracture Mechanics Assessments for Cubic Ductile Cast Iron Containers

Abstract

An improved BAM safety assessment concept for the cask material ductile cast iron (DCI) to cover higher stresses in the cask body, highly dynamic load scenarios, and a broader range of material qualities will require more extensive fracture mechanics analyses based on a combination of material testing, calculation of applied stresses, and inspection standards. As an example, the brittle fracture mechanics assessment of a surface crack in a plate due to the dynamic load from the 5 m drop of a cubic container (not equipped with impact limiters) onto a reinforced concrete target is investigated. The numerically calculated time-dependent stress intensity factor is compared with a previous static solution with the same loading history inserted. For the scenario studied the differences between the curves are negligible because a dynamic load of the cask within a time scale of millisec9nds can be considered as a quasi static load for the crack.     

Development of Assessment Methods For Transport and Storage Containers with a Higher Content of Recycled Metal

Abstract

The mechanical behaviour of transport and storage containers made of ductile cast iron melted with a higher content of recycled metal from decommissioning and dismantling of nuclear installations is investigated. Using drop tests with cubic container-like models, the influence of different real targets on the stresses in the cask body and the fracture behaviour is examined. A foundation for a test stand is suggested, which is simple to manufacture and which greatly improves the reproducibility of the test results. Dynamic fracture mechanics analyses of artificial crack-like defects in the test objects were performed by means of finite-element calculations to uncover safety margins. Numerous test results have shown that containers for final disposal can be built from a ductile cast iron with a fracture toughness of more than 50 per cent less than the lower bound value for the current licensed material. The limits of application of the material are also determined by the opportunities for safety assessment.     

A combined ligament rupture-cleavage model for crack arrest in thick-section steel plate

In the transition regime, plane strain crack propagation in ferritic steels proceeds by a combination of cleavage and ductile rupture processes, the latter being confined to ligaments that are parallel to the direction of macroscopic crack propagation. The paper models crack propagation, and particularly the limiting case of crack arrest, when fracture proceeds via these two modes. An important theoretical result is that, because of the unfractured ligaments which remain behind the crack tip, the plastic zone size is much smaller than when it is predicted for the operative $\text{K}$ values and assuming that there are no ligaments. Linear elastic fracture mechanics procedures may therefore be used to describe the arrest phenomenon at $\text{K}$ values that exceed the normally accepted limits for their validity. The theoretical results are also used to speculate upon the effect of neutron irradiation on the arrest toughness.