Effect of contact conditions on embrittlement of T91 steel by lead-bismuth

The T91 martensitic steel is a candidate structural material for the liquid lead-bismuth eutectic (LBE) MEGAPIE spallation target. This paper first reviews some results on Liquid Metal Embrittlement (LME) of martensitic steels by liquid metals. It appears that LME of steels can occur provided a few criteria are fulfilled simultaneously. Intimate contact between liquid metal and solid metal is the first one. Usually, it is impossible to avoid the oxide film formation on the steel surface even after short exposure to air. This explains the difficulty arising when one would like to determine the susceptibility to LME of T91 steel whilst put into contact with lead-bismuth. Later, we report on different methods of surface preparation in order to remove the oxide layer on the T91 steel (PVD, soft soldering fluxes) and the resulting susceptibility to LME.     

Liquid metal embrittlement of the martensitic steel 91: influence of the chemical composition of the liquid metal.: Experiments and electronic structure calculations

In previous works [Scripta Mater. 43 (2000) 997; J. Nucl. Mater. 296 (2001) 256], we showed that the martensitic steel 91 is prone to liquid metal embrittlement (LME) by liquid lead provided that some metallurgical conditions are fulfilled. In this work, we report results of LME of the steel 91 in contact with Pb-Bi and other low melting temperature metals such as Sn and Hg. Our experimental results can be interpreted within the framework of the surface energy reduction models for LME. To account for the experimental observations, we performed electronic structure calculations to assess the chemical interaction between low melting temperature metal atoms and iron surfaces. Our results allow to establish a simple criterion that can give trends on the embrittlement power of a liquid metal in contact with iron.     

On the kinetics of LME for the ferritic-martensitic steel T91 immersed in liquid PbBi eutectic

Use of heavy liquid metals (HLM), such as the Lead-Bismuth eutectic (LBE), is foreseen for targets and coolants in the newly designed accelerator driven systems (ADS). However, most of the structural materials suffer severe damage when in contact with the HLM. In particular, the simultaneous presence of load and environment can induce a phenomenon named Liquid Metal Embrittlement (LME) in the reference material (ferritic-martensitic steel T91). This phenomenon could give rise to high crack growth rates and the abrupt collapse of the structural material.In this study the effect of LBE on the crack growth rate of the T91 steel was evaluated. The effect of pre-exposure to flowing LBE was considered and assessed in terms of crack growth rates and fracture surface appearance.It was concluded that if conditions for wetting were reached during exposure in LBE, e.g. by dissolution or rupture of protective oxide layers, LME was likely to occur under stress, at 300 °C, regardless of the LBE chemistry.     

The influence of environment on the formation of fluting microstructures during fracture of Zircaloy

Fluting microstructures are commonly-occurring features on Zircaloy fractures produced under stress-corrosion cracking (SCC) and liquid (or solid) metal embrittlement (LME) conditions and they are generally closely associated with areas of cleavage cracking. Although it is widely believed that flutings are a consequence of localised plastic deformation near the cleavage cracks and that the aggressive chemical agents do not contribute directly to fluting formation there is little clear experimental evidence to support that view. Consequently the small programme of tests, described here, was performed in order to ascertain whether flutings can be generated in Zircaloy during exposure to stress in the absence of a SCC or LME agent.The implications of the results are discussed in terms of the currently proposed mechanisms for fluting formation in particular, and crack propagation generally, in Zircaloy SCC fracture.     

Compatibility of ferritic-martensitic steel T91 welds with liquid lead-bismuth eutectic: Comparison between TIG and EB welds

The 9 wt.% chromium ferritic-martensitic steel T91 is being considered as candidate structural material for a future experimental accelerator driven system (XT-ADS). This material and its welded connections would need to be used in contact with liquid lead-bismuth eutectic (LBE), under high irradiation doses. Both unirradiated tungsten inert gas (TIG) and electron beam (EB) welds of T91 have been examined by means of metallography, scanning electron microscopy (SEM-EDX), transmission electron microscopy (TEM), Vickers hardness measurements and tensile testing in both gas and liquid lead-bismuth environment. The TIG weld was commercially produced and post weld heat treated by a certified welding company while the post weld heat treatment of the experimental EB weld was optimized in terms of the Vickers hardness profile across the welded joint. The mechanical properties of the T91 TIG and EB welds in contact with LBE have been examined using slow strain rate tensile testing (SSRT) in LBE at 350 °C. All welds showed good mechanical behaviour in gas environment but total elongation was strongly reduced due to liquid metal embrittlement (LME) when tested in liquid lead-bismuth eutectic environment. The reduction in total elongation due to LME was larger for the commercially TIG welded joint than for the EB welded joint.     

Liquid metal embrittlement susceptibility of ferritic-martensitic steel in liquid lead alloys

The susceptibility of the ferritic-martensitic steels T91 and EUROFER97 to liquid metal embrittlement (LME) in lead alloys has been examined under various conditions. T91, which is currently the most promising candidate material for the high temperature components of the future accelerator driven system (ADS) was tested in liquid lead bismuth eutectic (LBE), whereas the reduced activation steel, EUROFER97 which is under consideration to be the structural steel for fusion reactors was tested in liquid lead lithium eutectic. These steels, similar in microstructure and mechanical properties in the unirradiated condition were tested for their susceptibility to LME as function of temperature (150-450 °C) and strain rate (1 × 10⁻³-1 × 10⁻⁶ s⁻¹). Also, the influence of pre-exposure and surface stress concentrators was evaluated for both steels in, respectively, liquid PbBi and PbLi environment. To assess the LME effect, results of the tests in liquid metal environment are compared with tests in air or inert gas environment. Although both unirradiated and irradiated smooth ferritic-martensitic steels do not show any or little deterioration of mechanical properties in liquid lead alloy environment compared to their mechanical properties in gas as function of temperature and strain rate, pre-exposure or the presence of surface stress concentrators does lead to a significant decrease in total elongation for certain test conditions depending on the type of liquid metal environment. The results are discussed in terms of wetting enhanced by liquid metal corrosion or crack initiation processes.     

Liquid metal embrittlement of nuclear materials

The phenomenological features of liquid metal embrittlement (LME) are reviewed and the influence of metallurgical factors and testing conditions is described. The process is shown to be similar in many respects to the elastic fracture observed at lower temperatures in some bcc and hcp. metals. An important difference in the case of LME arises from the need for the embrittler to be present at the crack-tip during fracture. This condition imposes a low temperature limit which occurs when the embrittler is no longer mobile. The relationship between susceptibility to LME and alloying characteristics is discussed. The various theories have been considered and it is concluded that a reduction in surface energy leading to lower crack-tip cohesion and hence lower plastic deformation is consistent with most of the experimental evidence.The second section of the review uses this general understanding as a basis for speculation about possible synergy between LME and radiation effects. Those which alter the plastic deformation behaviour, i.e., radiation hardening or radiation annealing, are considered likely to have the same influences on embrittlement as on elastic fracture. Radiation creep will reduce any tendency to slow crack growth under restricted embrittler availability. Radiation-induced transmutation is seen as a possible source of supply of embrittling species, particularly as fission products, but interaction with high temperature creep-cavitation fracture by helium segregation is less likely because LME tends to be more severe at lower temperatures.The review concludes with a brief catalogue of LME-susceptible couples, concentrating as far as possible on nuclear materials (i.e. ferrous and zirconium alloys) to provide an initial source of data for materials selection by designers and operators and for failure analysis.     

Mechanical properties of Eurofer 97 in Pb-16Li and irradiation effect

To be used in a fusion reactor, structural materials, and in particular steels, has to be selected and optimised in their composition to achieve a reduction in the long-term radioactive waste. A reduction in the long-term radioactive inventory could be reached substituting elements like molybdenum, niobium and nickel with other ones like tantalum and tungsten which have the same functions as alloying elements and, if irradiated, do not produce long lived radioisotopes. The martensitic steel belonging to the family of 8-9% Cr Eurofer 97 is considered the reference structural steel for fusion application. However, only few information are available about its mechanical properties in the liquid eutectic alloy Pb-16%Li. Particularly, the problem of liquid metal embrittlement (LME) has not been studied in detail and the effect of neutron irradiation on LME has not been investigated at all so far. This work presents the results obtained irradiating tensile specimens of Eurofer 97 up to 5.9 dpa in lead lithium. Tensile tests of samples have been performed out of pile in the same alloy at the same temperature at which irradiation was carried out.     

Liquid metal embrittlement of T91 and 316L steels by heavy liquid metals: A fracture mechanics assessment

LME of the martensitic T91 and the austenitic 316L steels have been investigated in the CCT geometry in the plane-stress condition. Using such a geometry, premature cracking induced by a liquid metal (PbBi and Hg) can be studied using a fracture mechanics approach based on CTOD, J-Δa and fracture assessment diagram. One is able to measure a reduction of the crack tip blunting and a reduction of the energy required for crack propagation induced by the liquid metal. In spite of some limitations, this qualitative evaluation shows that liquid metals do not induce strong embrittlement on steels in plane-stress condition. Rather, the effect of the liquid metal seems to promote a fracture mode by plastic collapse linked with strain localization. It indicates that the materials, in spite of a potential embrittlement, should still be acceptable in terms of safety criteria.     

Influence of Zn as a spallation product on the behaviour of martensitic steel T91 and austenitic steel 316L in liquid Pb-Bi

The liquid Pb-Bi alloy is proposed as material for the spallation target in hybrid systems. During the spallation process, several chemical elements are produced in the target which could generate specific liquid metal embrittlement phenomena. Among these species, zinc is known as an element which can promote LME (liquid metal embrittlement). Corrosion tests were carried out in liquid Pb-Bi in isothermal static conditions without and with 80 wppm of zinc at 150 °C, 350 °C and 600 °C up to 6000 h. No modification of the corrosion kinetics of T91 martensitic and 316L austenitic steels was observed for either unstressed or U-bend specimens with zinc in Pb-Bi. Moreover, no sign of embrittlement was observed for any of the samples with and without zinc.     

Investigation of LBE embrittlement effects on the fracture properties of T91

The susceptibility to liquid metal embrittlement (LME) of the T91 steel was studied by performing 3-point bending tests in liquid lead-bismuth eutectic (LBE), and for comparison, in argon (Ar) atmosphere as well. The specimens of T91 with different heat treatments were tested to access the hardening effect on the fracture toughness of the steel after exposure in LBE. The results showed that the fracture toughness of steel was reduced by contacting with LBE. The susceptibility of T91 to LBE embrittlement increased with the hardening of the steel introduced by heat treatments.     

Measurement of Anisotropy of Diffusion Coefficient in Plate Cell

Ion-exchange property and thermal stability were studied on four synthetic hydrous metal oxides from the viewpoint of applicability for the processing of liquid radioactive waste. Hydrous titanium (IV) oxide and hydrous niobium (V) oxide, consisting of metal ions having larger ionic potential, possessed large cation-exchange capacities, and showed high affinity for both the transition metal ions and the representative element ions. Hydrous tin (IV) oxide and hydrous zirconium (IV) oxide, consisting of metal ions having smaller ionic potential, were amphoteric ion exchangers possessing large anion-exchange capacities, and showed much higher affinity for the transition metal ions than for the representative element ions. The selectivity series are understood in terms of the formation of a bond partially covalent in character for the transition metals, and the purely electrostatic interaction between anionic exchange sites and unhydrated or hydrated cations for the representative elements. An examination of the effects of heat treatment in air on the ion-exchange properties indicated that the hydrous niobium (V) oxide was the most stable among the four hydrous metal oxides. Even this material, however, was not stable enough to be used under hydrothermal conditions.     

Influence of surface oxidation on electric potential measurements in MHD liquid metal flows

Experimental investigations for magnetohydrodynamic flows in rectangular ducts are performed using GaInSn as model fluid. Measurements of electric potential on channel walls and inside the flow show reproducible discrepancies compared to analytical results. These discrepancies can be ascribed to the formation of oxide layers causing a contact resistance between the electrically conducting duct walls and the liquid metal. An exact analytical solution for pressure drop, velocity and potential distributions has been derived taking into account the presence of a contact resistance. Analytical results for velocity and potential profiles and for pressure drop are discussed for different values of contact resistance and strength of the applied magnetic field. A comparison of measured potential with data from the analytical solution allows estimating the order of magnitude of the contact resistance in the present experiments.     

乳状液膜技术回收处理含铀溶液中铀的研究

采用乳状液膜技术对含铀溶液中的铀进行回收处理,研究了制乳原料的最佳体积比,制乳时的搅拌速度、温度等因素,及回收处理铀时的温度、pH值、铀初始质量浓度、提取时间等因素对乳状液膜技术提取回收含铀废液的影响;探讨了乳状液膜提取回收铀过程中铀的迁移机理;通过热力学对乳状液膜技术提取回收铀的液膜传输过程进行了分析。结果表明,当制乳原料中P₂₀₄和液体石蜡的体积分数为0.1和0.05,Span80、磺化煤油与P₂₀₄的体积比分别为0.06、0.79,搅拌速度为2000 r/min,内水相盐酸的浓度为4 mol/L时,可制得稳定的油包水型乳状液膜。在常温常压、pH值2.5、铀初始质量浓度小于100 mg/L、含铀废液与乳状液膜的体积比为5时,用乳状液膜对铀废液提取0.5 h,铀的回收率可达到99%以上,Gibbs自由能ΔG<0,说明外水相的铀可自发地向内水相富集。     

Investigation of the sensitivity to EAC of steel T91 in contact with liquid LBE

The ferritic-martensitic steel T91 is one of the most promising material for application in the generation IV type reactors. However, there are critical issues, such as the susceptibility to damage of the steel in contact with the heavy liquid metals and their effect on the mechanical properties of structural materials. In this context, it was initiated a study of the boundary conditions, necessary to ascertain the sensitivity of the T91 to environmentally assisted cracking when loaded in contact with the liquid lead-bismuth eutectic. A series of tensile tests were carried out in a cell where the specimens were immersed in static LBE. Results showed that at high temperature the steel in contact with the liquid metal had a slight decrease of yield and UTS value and a marked increase in the elongation to rupture. However, at low temperature the elongation to rupture and the reduction of area decreased, indicating the sensitivity to EAC.     

Influence of oxide layer morphology on hydrogen concentration in tin and niobium containing zirconium alloys after high temperature steam oxidation

The influence of the oxide layer morphology on the hydrogen uptake during steam oxidation of (Zr,Sn) and Zr-Nb nuclear fuel rod cladding alloys was investigated in isothermal separate-effect tests and large-scale fuel rod bundle simulation experiments. From both it can be concluded that the concentration of hydrogen in the remaining metal strongly depends on the existence of tangential cracks in the oxide layers formed by the tetragonal - monoclinic phase transition in the oxide, known as breakaway effect. In these cracks hydrogen is strongly enriched. It results in very local high hydrogen partial pressure at the oxide/metal interface and in an increase of the hydrogen concentration in the metal at local regions where such cracks in the oxide layer exist. Due to this effect the hydrogen uptake of the remaining zirconium alloy does not depend monotonically on temperature. Differences between (Zr,Sn) and Zr-Nb alloys are caused by differences in the hydrogen production due to different oxidation kinetics and in the crack forming phase transformation in the oxides as well as in the mechanical stability of the oxides.     

Bending tests on T91 steel in Pb-Bi eutectic, Bi and Pb-Li eutectic

The influence of specific liquid metals on the behaviour of the ferritic/martensitic steel T91 are investigated to understand better the processes taking place at the metal surface. Of special interest is particularly if there is a penetration of selected elements out of the melt along grain boundaries. Metallurgical investigation, SEM and EDX analyses analysis were performed on bent T91 specimens exposed to static LBE, Bi and Pb-17Li at 300 °C for 1000 h. Steel T91 was used in the standard and specially annealed conditions. It is shown that the heat treatment of the steel has a strong influence on the corrosion resistance. Additionally the strain affected on the component is responsible for the occurrence of LME.     

Thermodynamic Analysis for Molten Corium Stratification Test MASCA with Ionic Liquid U-Zr-Fe-O-B-C-FPs Database

The molten corium stratification tested in the OECD MASCA project was analyzed with our thermodynamic database and the database was verified to be effective for the stratification analysis. The MASCA test shows that the molten corium can be stratified with the metal layer under the oxide when sub-oxidized corium including iron was retained in the lower head of the reactor vessel. This stratification is caused by the increased density of the metal layer attributed to a transfer of uranium metal that was reduced from uranium oxide by zirconium. Thermodynamic equilibrium calculations with the database, which was developed for the corium U-Zr-Fe-O-B-C-FPs system using the ionic two-sublattice model for liquid, show quantitative agreements with the MASCA test, such as the composition of each layer, fission product (FP) partitioning between the layers and B₄C effect on the stratification.     

New integration of the Gibbs-Duhem equation and application of phase relations and thermodynamics to fuel reprocessing

The application of phase relations and thermodynamic properties to the design of fused salt and liquid metal processes for the separation of fission product elements from nuclear fuel materials is described. Processes can be designed to extract minor components from a fuel material without drastically altering its character or state of aggregation. It is possible to effectively transport solutes from one liquid metal or alloy to a second liquid metal or alloy by bringing the two in contact with an appropriate fused salt. Alternatively it is possible to transport solutes between two liquid salt phases by bringing them in mutual contact with an appropriate alloy. A process for extracting uranium from a thoria-urania solid solution powder by reactions in a fused salt medium is described. The value of the activity coefficient of any particular solute at infinite dilution is an important parameter in determining the ultimate degree of separation to be expected in a given process. Interpolation functions for evaluating the properties of dilute solutions are examined and a new integration of the Gibbs-Duhem equation for binary alloys is developed. The method described lends considerable versatility to the types of curves or functions which can be integrated to yield data for a second component in a binary solution.