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2012 | Buch

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The Effect of Hydrogen and Hydrides on the Integrity of Zirconium Alloy Components

Delayed Hydride Cracking

verfasst von: Manfred P. Puls

Verlag: Springer London

Buchreihe : Engineering Materials

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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Über dieses Buch

By drawing together the current theoretical and experimental understanding of the phenomena of delayed hydride cracking (DHC) in zirconium alloys, The Effect of Hydrogen and Hydrides on the Integrity of Zirconium Alloy Components: Delayed Hydride Cracking provides a detailed explanation focusing on the properties of hydrogen and hydrides in these alloys. Whilst the emphasis lies on zirconium alloys, the combination of both the empirical and mechanistic approaches creates a solid understanding that can also be applied to other hydride forming metals.

This up-to-date reference focuses on documented research surrounding DHC, including current methodologies for design and assessment of the results of periodic in-service inspections of pressure tubes in nuclear reactors. Emphasis is placed on showing how our understanding of DHC is supported by progress in general understanding of such broad fields as the study of hysteresis associated with first order phase transformations, phase relationships in coherent crystalline metallic solids, the physics of point and line defects, diffusion of substitutional and interstitial atoms in crystalline solids, and continuum fracture and solid mechanics. Furthermore, an account of current methodologies is given illustrating how such understanding of hydrogen, hydrides and DHC in zirconium alloys underpins these methodologies for assessments of real life cases in the Canadian nuclear industry.

The all-encompassing approach makes The Effect of Hydrogen and Hydrides on the Integrity of Zirconium Alloy Component: Delayed Hydride Cracking an ideal reference source for students, researchers and industry professionals alike.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Introduction

Abstract

Zirconium alloys are used in nuclear reactors because of their combination of high strength, high corrosion resistance, and low neutron absorption cross-section. Their most demanding applications in nuclear reactors are as fuel cladding and in CANDU, RBMK, and other Pressurized Heavy Water (PHW) reactors as pressure tubes containing the fuel bundles. It is important for the safe and economic operation of these reactors that these components maintain their integrity throughout their design life. However, during their residence in the reactor these components are subject to aging mechanisms resulting from thermal- and pressure-driven changes, fast neutron bombardment, and corrosion at the water/metal interface, the latter resulting in a small fraction of the released hydrogen produced during the corrosion reaction being absorbed in the zirconium alloy. When the hydrogen concentration in the material exceeds the Zr–H solvus composition, zirconium hydrides are formed. These hydrides, which are less ductile than the surrounding metal matrix, can have deleterious effects on the mechanical properties of these components when present at sufficiently high volume fraction. Their deleterious effects are exacerbated by increases in yield strength and decreases in fracture toughness of the zirconium material. These changes are produced as a consequence of the production of dislocation loops and other microstructural changes during fast neutron bombardment in the reactor core.

Manfred P. Puls

Chapter 2. Properties of Bulk Zirconium Hydrides

Abstract

This chapter provides details of the crystallography and mechanical (including fracture) properties of bulk zirconium hydrides. The chapter highlights a conclusion by Beck concerning the role of the γ-hydride phase as a suppository for excess hydrogen atoms when thermodynamic conditions drive the δ hydride phase to reduce its hydrogen composition. The values of the partial molar volumes of hydrogen in the γ- and δ-hydride phases are calculated and an analysis is given of the dependence of this volume in the δ-hydride phase on temperature over the temperature range of practical interest for nuclear reactors.

Manfred P Puls

Chapter 3. Hydride Phases, Orientation Relationships, Habit Planes, and Morphologies

Abstract

This chapter deals with the morphology, habits, and orientation relationships of single, or groups of, γ- and δ-hydride precipitates in zirconium and its alloys in stressed and unstressed material. It is shown that individual hydride precipitates have acicular or plate-like shapes, forming offset arrays of precipitates driven through an autocatalytic nucleation process by the hydrides’ transformation strains. In Zr-2.5Nb pressure tube alloys their habit planes are always the near-basal α-zirconium planes regardless of the orientation of the hydride cluster array as it appears in optical micrographs. From analyses of the hydride’s habit plane and its shape and cluster configuration it is concluded that in Zr-2.5Nb pressure tube material hydride formation is via an invariant plane strain transformation in which almost all of the volumetric transformation strain is oriented in the direction normal to the invariant plane.

Manfred P Puls

Chapter 4. Solubility of Hydrogen

Abstract

The properties of hydrogen atoms dissolved in α and β zirconium are modeled treating the atoms as misfitting defects. Relationships are derived on this basis for their interactions with each other, with dislocations and with other external and internal sources of stress. Experimental results are given showing that hydrogen is more soluble in the β compared to in the α zirconium phase. The variation of this solubility difference, expressed as a partitioning ratio, with Nb concentration in the β zirconium phase is given. Following Kirchheim, relationships are given for the trapping of hydrogen (as hydrides) at dislocations, using Fermi–Dirac statistics with a density-of-sites-energy approach for hydrogen site occupancy.

Manfred P. Puls

Chapter 5. Diffusion of Hydrogen

Abstract

This chapter deals with the diffusion properties of hydrogen in zirconium as driven by mass, stress, and thermal gradients. The relevant diffusion equations are derived starting from the phenomenological theory of irreversible thermodynamics. A general expression for the flux of hydrogen in mass (concentration), stress, and temperature gradients is derived. The derived general equation forms the basis of specific flux relations for diffusion of hydrogen under thermal gradients to cold spots and under stress gradients to regions of elevated tensile stress. The latter provides the basis for the DHC growth equation derived in Chap. 10. The flux of hydrogen depends on the diffusion coefficient for hydrogen and, for thermal diffusion, also on the heat of transfer. Experimental results for the heat of transfer and the diffusion coefficient of hydrogen in zirconium and its alloys are summarized. A theoretical treatment is given for the effect on the diffusion rate of fast diffusion channels such as β phase stringers in α/β zirconium alloys and of hydrogen/hydride trapping at dislocations.

Manfred P Puls

Chapter 6. Characteristics of the Solvus

Abstract

This chapter deals with the dilute side of metal-hydrogen phase diagrams (referred to as the solvus). It starts out with a review examining in general the sources of hysteresis in first-order phase transitions. From this review it is concluded that, generally, hysteresis is the result of the presence of a macroscopic energy barrier that must be overcome during phase transformation. Such a finite-sized barrier cannot be overcome by thermal fluctuations alone but, when overcome through the application of a sufficiently large externally applied thermodynamic force, results in a finite boundary movement, the energy of which is dissipated by internal entropy production. It is concluded that the internal entropy production produced by this finite phase boundary movement is the main source of hysteresis. In comparison, the earliest models of hysteresis in hydride forming metals reviewed in this chapter all assume that hysteresis is a consequence of plastic relaxation of the accommodation energy barrier during or after the phase transformation. An updated version of the accommodation energy model for hysteresis derived for the Zr–H system is given.

Manfred P. Puls

Chapter 7. Theories of Coherent Phase Equilibrium

Abstract

A general treatment of phase equilibrium of binary substitutional, or interstitial (metal-hydrogen) coherent solids is given in this chapter. In such systems, the total coherency energy between the misfitting species and/or phases includes a contribution from the indirect (non-configurational) interactions that occur in solids of finite size between the misfitting phases or atomic species. It is shown that, as a result of this latter contribution and the associated dependence of the total coherency energy on the square of the concentrations of the misfitting phases or atomic species in the solid, the equilibrium phase relationships for such solids differ significantly from the classical, Gibbsian thermodynamic equilibrium relationships applicable to liquid and hydrostatically stressed (incoherent) solid systems. Two bounding cases of the application of these coherency energy models are reviewed in this chapter.

Manfred P. Puls

Chapter 8. Experimental Results and Theoretical Interpretations of Solvus Relationships in the Zr–H System

Abstract

The equilibrium relationships derived for coherent equilibrium given in Chap. 7 and the conclusions derived from a general review of the origin of hysteresis in first-order phase transformations given in Chap. 6 are assessed here to provide a first attempt at a new semi-quantitative interpretation of the zirconium–hydrogen phase relationships. Numerical estimates are made to determine the stability conditions for the zirconium–hydrogen system on the basis of the coherent phase relationships derived for the polymorphic coherent transformation case given in Chap. 7. A complete quantitative assessment of the foregoing new concepts and their consequences to DHC, however, still remains to be completed. The theoretical assessment is followed by a comprehensive examination of most of the available data for the solvus in zirconium–hydrogen systems. This review has led to some reinterpretation of the meaning and physical significance of some of the solvus data as it applies to DHC.

Manfred P. Puls

Chapter 9. Fracture Strength of Embedded Hydride Precipitates in Zirconium and its Alloys

Abstract

Data for the fracture strength of embedded hydride precipitate clusters in zirconium and its alloys are reviewed and the results analyzed. Fracture strength values are derived from two types of tests. One type involves determining fracture initiation in hydrides in rising load tensile tests of zirconium material of different strengths and microstructures containing a uniform distribution of radial (perpendicular) hydride clusters. In these tests, fracture initiation is determined either by Acoustic Emission (AE) or monitored with a high intensity source of synchrotron X-rays. Another type of test involves growing long hydrided regions from planar surfaces of cantilever beam specimens loaded in bending under a series of constant loads to determine the lowest applied load at which such hydrided regions fracture and DHC is initiated. The lower bound fracture strength value consistent with the results obtained from the complete data set of these two types of tests is 450 MPa. This result is limited to embedded hydride clusters in unirradiated or pre-irradiated Zr–2.5Nb pressure tube material oriented such that their long dimensions are perpendicular to the applied load.

Manfred P. Puls

Chapter 10. Delayed Hydride Cracking: Theory and Experiment

Abstract

A brief overview is presented of the general features of delayed hydride cracking (DHC) in zirconium alloys. The data on DHC growth rate and threshold stress intensity factor \( K_{IH} , \), are examined in relation to theoretical models developed to rationalize these. The dependence of DHC growth rate on temperature, applied stress intensity factor, direction of approach to temperature (hydrogen concentration in solution), yield strength, microstructure, and the limiting conditions for growth are explored. Likewise, the parameters and conditions for \( K_{IH} , \) are identified. Various threshold limiting conditions are rationalized in terms of an explicit theoretical relationship between the applied stress intensity factor and the critical dimensions of the hydrided region formed at the crack tip. These analyses have led to a number of new insights concerning the factors controlling \( K_{IH} , \)

Manfred P. Puls

Chapter 11. DHC Initiation at Volumetric Flaws

Abstract

Threshold conditions for DHC initiation at volumetric flaws—which require a different treatment than is appropriate for DHC initiation at cracks—is summarized in this chapter. The initial approach, based on a peak stress threshold criterion, resulted in this threshold decreasing with the number of reactor shutdown cycles. This was found to be too restrictive on reactor operation. Moreover, the method did not have an explicit dependence on flaw geometry (root radius) nor could it readily be used to assess the DHC initiation potential of sharp secondary flaws at the root of blunt flaws. The hydride process zone model that was developed to eliminate these deficiencies of the peak threshold stress model is described in this chapter. Details of the validation procedures of the engineering process zone model in relation to the experimental data base for DHC initiation from blunt flaws in unirradiated and pre-irradiated Zr–2.5Nb pressure tube material are given.

Manfred P Puls

Chapter 12. Applications to CANDU Reactors

Abstract

This chapter gives examples of how engineering data of DHC initiation, growth rate, and solvus determinations, supported by theoretical models, and associated underlying property measurements and analyses, are applied in integrity assessments of pressure tubes of CANDU reactors. Examples are given of (1) planar and volumetric flaw assessment approaches and (2) reactor core assessments (leak before break analyses).

Manfred P Puls

Titel: The Effect of Hydrogen and Hydrides on the Integrity of Zirconium Alloy Components
verfasst von: Manfred P. Puls
Verlag: Springer London
Electronic ISBN: 978-1-4471-4195-2
Print ISBN: 978-1-4471-4194-5
DOI: https://doi.org/10.1007/978-1-4471-4195-2

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