Hydride Embrittlement Research Articles

Effect of hydrogenation conditions on the microstructure and mechanical properties of zirconium hydride

Precipitation of brittle zirconium hydrides deteriorate the fracture toughness of the fuel cladding tubes of light water reactor. Although the hydride embrittlement has been studied extensively, little is known about physical properties of the hydride due to the experimental difficulties. In the present study, to elucidate relationship between mechanical properties and microstructure, two δ-phase zirconium hydrides and one ε-phase zirconium hydride were carefully fabricated considering volume changes at the metal-to-hydride transformation. The δ-hydride that was fabricated from α-zirconium exhibits numerous inner cracks due to the large volume change. Analyses of the neutron diffraction pattern and electron backscatter diffraction (EBSD) data show that the sample displays significant stacking faults in the {111} plane and in the pseudo-layered microstructure. On the other hand, the δ-hydride sample fabricated from β-zirconium at a higher temperature displays equiaxed grains and no cracks. The strong crystal orientation dependence of mechanical properties were confirmed by indentation test and EBSD observation. The δ-hydride hydrogenated from α-zirconium displays a lower Young's modulus than that prepared from β-zirconium. The difference is attributed to stacking faults within the {111} plane, for which the Young's modulus exhibits the highest value in the perpendicular direction. The strong influence of the crystal orientation and dislocation density on the mechanical properties should be considered when evaluating hydride precipitates in nuclear fuel cladding.

Thermal Analysis Capability of UNF-ST&DARDS

In the United States, approximately 2500 casks are loaded with commercial spent nuclear fuel (SNF) that has transitioned from wet storage (spent fuel pools) to dry storage. The number of loaded dry storage casks is increasing by approximately 200 each year. Over time, cask designs have evolved to enhance safety and to accommodate more fuel and higher heat loads. Also, higher burnup fuel is being transitioned into dry storage. The SNF is being stored in dry casks for longer times than specified in the original certification period. Several degradation mechanisms related to fuel assemblies and canisters are affected by temperature. For the cladding, temperature-dependent phenomena include creep and annealing, hydride reorientation and embrittlement, and the ductile-to-brittle transition. Temperature can also influence phenomena that affect the long-term integrity of the storage system, including deliquescence, corrosion, and stress-corrosion cracking. Therefore, accurate determination of the temperatures of various components is needed to evaluate potential safety-related issues during transportation after extended storage and to ensure SNF retrievability. The Used Nuclear Fuel-Storage, Transportation & Disposal Analysis Resource and Data System (UNF-ST&DARDS) is being developed for the U.S. Department of Energy Office of Nuclear Energy to streamline analyses for the waste management system [Nucl. Technol., Vol. 195, p. 124 (2017)]. The thermal analysis capability within UNF-ST&DARDS and example results are discussed herein.

Some problems of hydrogen in reactor structural materials: A review

The review concerns some problems of hydrogen in the main reactor structural materials used in the core of nuclear reactors. Zirconium alloys, steels, and vanadium alloys, as well as the hydrogen and helium synergetic effect exerted on the radiation resistance, are considered. The main sources resulting in the accumulation of hydrogen isotopes in reactor materials are discussed. The causes and consequences of hydride embrittlement of zirconium alloys at relatively low temperatures are analyzed. It is shown that hydrogen can induce the embrittlement of vessel steels through the weakening interatomic bond forces and a stabilization of radiation-induced defects. It is demonstrated that hydrogen in the presence of helium behaves like a gas that enhances the irradiation effect on the microstructure and properties of materials in many cases. The irradiation with simultaneous introduction of hydrogen and helium causes, in particular, a catastrophic swelling of chromium steels and vanadium alloys, whereas in the case of austenitic steel the effect is less pronounced.

Effect of hydrogen isotope content on tensile flow behavior of Zr-2.5Nb pressure tube material between 25 and 300 °C

Tensile properties of autoclaved Zr-2.5Nb pressure tube material containing hydrogen isotope between 5 and 200 wppm were evaluated between 25 and 300 °C using specimens with its axis oriented along longitudinal direction of the tube. Analysis of tensile test results showed that both YS and UTS of this alloy decreased linearly with increasing test temperature. The uniform and total plastic strain decreased marginally with increase in test temperature. At all test temperatures, before necking tensile properties were unaffected by hydrogen isotope concentration whereas hydrogen isotope had clear effect on post-necking tensile properties especially at 25 and 100 °C. Post-necking ductility showed a transition behavior at 25 and 100 °C and it was able to capture the effect of hydride embrittlement in this material.

Effect of yttrium on nucleation and growth of zirconium hydrides

Addition of yttrium in zirconium causes precipitates of yttrium, which form two types of particles and are oxidized upon heat treatment. One type of particles with sub-micrometer scale sizes has a low population, whereas the other with nano scale sizes has a high population and cluster distribution. Owing to strong affinity of yttrium to hydrogen, the nanoparticles, mostly within the grains of the Zr–Y alloy, attract nucleation of hydrides at the clusters of the nanoparticles and cause preferential distribution of intragranular hydrides. In comparison with that of Zr, additional nanoparticles in the Zr–Y alloy impede further growth of hydride precipitates during hydriding. It is deduced that the impediment of growing hydride precipitates by the nanoparticles is developed during an auto-catalytic nucleation process, which leads to formation of thin and intragranular hydrides, favorable to mitigation of hydride embrittlement.

Effect of yttrium addition on microstructure and orientation of hydride precipitation in Zr-1Nb alloy

The purpose is to investigate the role of yttrium addition in improving hydride embrittlement resistance in Zr-1Nb alloy, because yttrium addition has been widely used to improve oxidation resistance of alloys. The results suggest that 0.2 wt.% yttrium addition can effectively lower hydrogen absorption in Zr-1Nb alloy. The microstructural characterization shows that the precipitated hydrides are interlinked to have a width of 200–600 nm and a shape of lamellar or “dendritic”. The electron backscattered diffraction (EBSD) results show that the crystallographic orientation relationship between precipitated hydrides and zirconium matrix is (0001)α-Zr//{111}δ-H0.62Zr0.38 or (0001)α-Zr//{100}δ-H0.62Zr0.38, depending on the position and morphology of hydrides. The depression of hydrogen absorption is attributed to the reduction of transgranular hydride precipitation by yttrium addition. This study provides an experimental basis for designing new zirconium alloys with enhanced hydrogen absorption resistance.

The influence of hydride on fracture toughness of recrystallized Zircaloy-4 cladding

In this work, RXA cladding tubes were hydrogen-charged to target hydrogen content levels between 150 and 800wppm (part per million by weight). The strings of zirconium hydrides observed in the cross sections are mostly oriented in the circumferential direction. The fracture toughness of hydrided RXA Zircaloy-4 cladding was measured to evaluate its hydride embrittlement susceptibility. With increasing hydrogen content, the fracture toughness of hydrided RXA cladding decreases at both 25°C and 300°C. Moreover, highly localized hydrides (forming a hydride rim) aggravate the degradation of the fracture properties of RXA Zircaloy-4 cladding at both 25°C and 300°C. Brittle features in the form of quasi-cleavages and secondary cracks were observed on the fracture surface of the hydride rim, even for RXA cladding tested at 300°C.

사용후핵연료의 장기 건식 건전성 성능과 주요 열화 기구에 관한 고찰

최근 국내에서도 원전 부지 내에 건설된 습식저장조의 용량이 곧 포화될 것으로 예상되어 사용후핵연료의 건식저장에 관한 논의가 활발하다. 이 논문에서는 앞으로 다양하게 논의될 저장시스템의 안전성과 함께 장기 건식저장 시 발생하는 사용후핵연료의 특성 및 건전성 변화에 대해 이제까지 국내외에서 연구 보고된 내용들을 면밀히 검토하고 향후 추구해야 할 연구방향을 제시하고자 하였다. 조사 결과 건식저장 기간 동안 진행될 수 있는 여러 피복관 열화기구 중에서 가장 대표적인 기구는 크립 변형과 수소화물에 의한 영향이었으며, 이들이 사용후핵연료 장기 건식저장 시 규제기술기준의 주요 근간을 이루고 있는 것으로 분석되었다. 한편 과거에는 피복관의 크립 변형이 가장 중요한 열화기구로 평가되었으나, 최근의 연구 결과를 통해 수소화물에 의한 영향이 더 심각한 것으로 드러났고 이는 미국의 규제기준과 새로운 온도 범위를 제시하고 있는 일본의 규제기준에서 확인할 수 있었다. 그러나, 아직까지 수소화물에 의한 영향이 발생하는 응력과 온도 조건을 명확히 규명할 수 있는 연구 자료가 충분하지 못하며, 나아가 사용후핵연료의 취급 시 거동에 대한 연구도 지속적으로 수행해야 할 부분으로 드러났다. 따라서 국내 사용후핵연료 특성에 맞는 건식저장조건을 수립하기 위해서는 국내에서도 본격적인 연구를 통해 이들 자료에 대한 충분한 생산과 평가 및 분석이 뒤따라야 할 것으로 판단된다. As the capacity of spent nuclear fuel storage pool at reactor sites becomes saturated in ten years, long term dry storage strategy has been recently discussed as an alternative option in Korea. In this study, we reviewed safety-criteria-related research results on spent nuclear fuel performance and integrity under long-term dry storage and proposed the direction and the scope of future domestic research and development. Creep and hydride effect in relation to the embrittlement are known to be the major degradation mechanisms of the spent fuels during the long term dry storage. However, recent research results showed that hydride reorientation and hydride embrittlement are one of the most critical factors to the spent fuel integrity. Accordingly safety criteria of US and Japan for the storage system are basically founded on those mechanisms. However, in Korea, not only in-pile but out-of-pile experimental data have not been generated to understand fuel cladding degradation and to determine the criteria to ensure the safety. In addition, the transient behavior of the spent fuel during transportation also needs to be thoroughly examined. Therefore, various experimental research and development will be required to establish our own safety criteria for future long-term dry storage of domestic spent fuels.

Influence of stress field of expanding and contracting plate shaped precipitate on hydride embrittlement of Zr-alloys

The stress fields of expanding (precipitation) and contracting (dissolution) hydride plates were computed by finite element method using Zr–H solid solution and hydride properties at 25, 200 and 400°C for fully and semi-constrained hydride plates. For the first time simultaneous hydride expansion and matrix contraction and vice-versa have been considered in a simulation of hydride precipitation and dissolution, respectively. It was observed that a fully constrained expanding hydride plate exerts a tensile stress field in the matrix close to the edge of the hydride plate while a partially contracting hydride plate exerts a tensile stress field in the hydride plate as well as a large compressive stress in the surrounding matrix close to the edge of the hydride plate. It is suggested that a compressive stress component in the matrix acting normal to a partially shrinking hydride plate could possibly explain an enhanced resistance to hydride embrittlement of Zr-alloy at elevated temperature.

Latent Cracking of Tantalum-Titanium Welds Due to Hydrogen Embrittlement

Establishing electrical interconnects in implantable electronic medical devices frequently requires joining of dissimilar materials. A weld between a tantalum wire and titanium sheet metal on a contact module is presented as an example for dissimilar joining. Latent, brittle cracking was observed in the proximity of the weld upon pull testing. The weld cracking occurs by the mechanism known as hydrogen stress cracking (HSC) and is due to titanium hydride formation. Diffusion facilitated hydrogen transport into the weld area. Diffusing hydrogen accumulates preferably in regions of high stress, causing latent titanium hydride formation and embrittlement of the weld. A broad array of analytical tools such as scanning electron microscopy (SEM), transmission electron microscopy, electron backscattered diffraction, dynamic secondary ion mass spectroscopy, and nanoindentation were utilized to identify the root cause for HSC.

Hydride embrittlement and oxidation resistance of some Zr–Nb–Y alloys

In a recent experiment, a high density of much stronger sites for nucleation of hydrides than available otherwise in a Zr-alloy was created by the authors by a synergistic effect of addition of a small quantity of Y to Zr and microstructural modification of the resulting Zr–Y alloy by quenching. These sites, which are essentially nano-sized Zr–Y–O complexes (<5nm), lead to precipitation of only sub-microscopic hydrides, that are unlikely to embrittle the host matrix. Yttrium has also been used to improve the oxidation resistance of a variety of high temperature alloys and aluminides. In this work, the possibility of achieving the twin benefits of improved resistance to oxidation as well as hydride embrittlement by using Y as an alloying addition in dilute Zr–Nb alloys was explored.

Microstructure and hydride embrittlement of zirconium model alloys containing niobium and tin

To investigate the effects of the addition of Nb and Sn to Zr alloys on hydride embrittlement, experimental alloys with different Nb and Sn contents were prepared and charged with hydrogen up to 850 ppm. When Nb and Sn were added, recrystallization was delayed in Zr alloys, mainly due to β-Nb precipitates in Nb containing alloys and Sn solute atoms in Sn containing alloys, respectively. Among the two alloying elements, Sn was more effective in delaying recrystallization. Tensile test results showed that both Nb and Sn strengthened the Zr alloys, and tensile strengths were nearly independent of the absorbed hydrogen content. While resistance to hydride embrittlement was significantly improved with Nb addition to Zr alloys due to increased hydrogen solubility and delayed recrystallization, no effect of Sn addition was observed.

Effect of hydride embrittlement on creep life of spent fuel cladding

In this study, we propose a rupture criterion for the creep crack growth in Zircaloy cladding on spent fuel in dry storage. Based on the creep fracture mechanics parameter, C*, and the strain energy density criteria, the relationship between the creep crack growth rate and the fracture mechanics parameter, C*, is established theoretically. The effects of hydride embrittlement, initial crack lengths, and storage temperature profiles on cladding failure are discussed in detail. The results show that the initial crack length and the storage temperature profile play an important role in cladding failure during interim dry storage. We found that the creep life in the presence of hydrides is only slightly shorter than that without the presence of hydrides. The methodology appears to be helpful to the evaluation of spent fuel rod cladding in dry storage.

Effect of Nb on hydride embrittlement of Zr– xNb alloys

The effects of Nb on hydride Zr alloys were investigated. Various Zr alloys with different Nb content up to 2.0% were prepared in a sheet shape and charged with hydrogen up to 850 ppm. It was found that the fraction of recrystallized grains was reduced with increasing Nb content during the heat treatment. Intergrain and intragrain hydrides were tangled in unalloyed Zr, which has fully recrystallized grains. On the other hand, Nb containing Zr alloys that have partially cold-worked grains had only intergrain hydrides precipitated along the rolling direction. In tensile tests, elongation was decreased significantly with increasing hydrogen content for unalloyed Zr. However, the reduction of elongation was less significant for Nb-containing alloys. The softening of the Zr matrix was caused by hydrogen, which was arrested in cold-worked grain. And β-Nb precipitates, which have high solubility of hydrogen, retarded the precipitation of hydrides, thereby enhancing the resistance of hydride embrittlement. Fracture surface of the tensile tested specimen was observed by SEM. From the SEM observation, secondary cracks caused by hydrides were found on the fracture surface of the hydrogen-charged specimens. Cleavage facets were observed on the fracture surface of unalloyed Zr; however, a mixture of dimple and cleavage facets was observed on the surface of the Nb-containing alloys, which indicates that Nb increased the resistance to hydride embrittlement of Zr alloys.

An elasto-plastic fracture mechanics based model for assessment of hydride embrittlement in zircaloy cladding tubes

This paper describes a finite element based fracture mechanics model to assess how hydrides affect the integrity of zircaloy cladding tubes. The hydrides are assumed to fracture at a low load whereas the propagation of the fractured hydrides in the matrix material and failure of the tube is controlled by non-linear fracture mechanics and plastic collapse of the ligaments between the hydrides. The paper quantifies the relative importance of hydride geometrical parameters such as size, orientation and location of individual hydrides and interaction between adjacent hydrides. The paper also presents analyses for some different and representative multi-hydride configurations. The model is adaptable to general and complex crack configurations and can therefore be used to assess realistic hydride configurations. The mechanism of cladding failure is by plastic collapse of ligaments between interacting fractured hydrides. The results show that the integrity can be drastically reduced when several radial hydrides form continuous patterns.

Hydrogen as a working atmosphere for manufacturing permanent magnets based on rare-earth metals

The phase compositions and magnetic properties of permanents magnets of the systems Sm – Co and Nd – Fe – B are analyzed. Features of the hydrogenation–disproportionation–desorption– recombination (HDDR) process in the Nd 2 Fe 14 B intermetallic are considered. Using the Dd – Fe – B system as an example, we assess stages of manufacture of commercial permanent magnets and show the prospect of using hydrogen as a working atmosphere for the manufacture of magnetic powder. It is established that the HDDR of Dd – Fe – B alloys leads to their homogenization, grain refinement to a grain size of 0.2 to 0.5 μm, and an increase in the volume content of the main ferromagnetic phase Dd 2 Fe 14 B. By optimizing such a treatment, we managed to increase the magnetic energy (by 10%) and the lift force (by 25 – 27%) of Dd – Fe – B commercial permanent magnets. In modern materials science, a new trend in chemicothermal treatment of metals that consists of using hydrogen as a working atmosphere is being intensively developed. Developed hydrogen technologies are based on the regularities of its effect on the phase transformations, specifically, atomic ordering and hydride formation, in metals [1 – 3]. Earlier, magnetic materials were treated in a hydrogen atmosphere to clear them from impurities by annealing at high temperatures (1100 – 1300°C) [4]. However, in the manufacture of permanent magnets based on rare-earth metals (REMs), the possibility of using hydrogen both during their milling (hydride embrittlement) and for the change of the phase–structural state arose. Optimizing conditions for such a treatment, one can increase the operating characteristics of magnets (the coercive force Hc and residual induction Br ) and decrease the consumption of energy in their manufacture. Rare-earth magnets based on samarium were first obtained in the early 1970s. Alloys of the samarium–cobalt (Sm – Co) system have high saturation magnetization, coercive force, heat resistance, and corrosion resistance. However, due to the high cost, their use in industry is limited to a range of elevated temperatures (150 – 500°C). At present, magnets based on the Nd – Fe – B system are most extensively used. Of all known ferromagnets they exhibit the highest value of the energy product ( BH ) max (to 50 MG ⋅ Oe), which is five times higher than those of the best Alnico type magnets (Al – Ni – Co) [4]. Moreover, the determining advantage of these magnets over other materials is the relatively low cost calculated per unit magnetic energy (Table 1).

Resistance to hydride formation in zirconium: An emerging possibility

Fully recrystallized Zircaloy 2 was subjected to hydride formation through gaseous hydrogen charging. Primarily grain boundary hydrides were observed. To describe the relative preference, if any, a hydride preference index (HPI) was proposed: HPI Q = f h,Q / f r,Q , where f h,Q and f r,Q are the respective fractions of Q-type boundary present among hydrided boundaries and present in the total sample. HPI Q values were estimated from 1200 distinctly hydrided boundaries and showed a clear preference for hydride formation and grain boundary nature. Coincident site lattice boundaries were, in general, resistant to hydride formation. Elastically harder grains or orientations also arrested the formation of hydrides. The study indicates a clear possibility: a previously unknown/uncharted possibility of suitably ‘tailored’ zirconium microstructures, microstructures with reduced potential for ‘hydride embrittlement’ and delayed hydrogen cracking.

Mechanical Properties of Zircaloy-4 PWR Fuel Cladding with Burnup 54-64MWd/kgU and Implications for RIA Behavior

Abstract The PROMETRA material testing program is a support program related to the study of high burnup fuel rod behavior under Reactivity Initiated Accidents (RIA) and to the interpretation of the CABRI REP-Na RIA test results. Hoop and axial tensile tests have been performed on fresh and irradiated Zircaloy-4 cladding alloy first at CEA Grenoble hot labs and now at CEA Saclay in order to assess the cladding mechanical behavior during RIA transients. Efforts have been continuously carried out in order to improve the prototipicallity of the tests for RIA studies involving new specimens and new testing techniques. The corrosion level of irradiated specimens reached up to 130 μm of oxide layer thickness. The influence of in-pile oxide layer spallation has also been addressed. High strain-rate material properties of irradiated Zircaloy-4 and the consequences of hydride embrittlement can be derived from the PROMETRA program.

중수로 압력관의 수화물이 LBB평가에 미치는 영향

The aim of this study was to investigate the hydride embrittlement when the LBB evaluation was carried out for the integrity of PHWR Pressure Tubes. The transverse tensile and CCT toughness tests were performed at three hydrogen concentrations while the test temperatures were changed (RT to 30). Both the transverse tensile and the fracture toughness tests showed the hydrogen embitterment clearly at RT but this phenomenon was disappeared while the test temperature arrived at 25. Using the DHC test results, the CCL and LBB time were calculated and compared. The hydride embrittlement at the LBB evaluation made the LBB time short definedly. If the operating temperature, DHCV and LBB deterministic parameters such as A and m were known, LBB time could be estimated without the calculation of CCL.

Hydride embrittlement and irradiation effects on the hoop mechanical properties of pressurized water reactor (PWR) and boiling-water reactor (BWR) ZIRCALOY cladding tubes: Part I. Hydride embrittlement in stress-relieved, annealed, and recrystallized ZIRCALOYs at 20 °C and 300 °C

This series of articles on the hydride embrittlement effect in ZIRCALOYs consists of three parts. Part III deals with the mechanical properties of some hydrides in some ZIRCALOYs at two temperatures (20 °C and 300 °C). The damage and fracture mechanisms were investigated by SEM in-situ testing. Based on the scanning electron microscopy (SEM) in-situ observations and the mechanical modeling, the mechanical behavior of hydrides confined within a matrix was determined. The hydrides can undergo significant plastic deformation under certain conditions. They have higher strength than the surrounding ZIRCALOY matrix. The radiation effect on hydrided ZIRCALOYs and the combined effect were also discussed. Part I is concerned with the general influence of hydrides on the mechanical properties and damage and fracture mechanisms of ZIRCALOYs at two temperatures (20 °C and 300 °C) and for hydrogen content up to 4500 ppm. In Part II, the morphology and crystallographic structure of hydrides in two ZIRCALOYs are examined at different magnifications and by different techniques (SEM, transmission electron microscopy (TEM), and X-ray diffraction) as a function of their pre-heat treatment and their hydrogen content.