On the long term estimation of hydrogen embrittlement risks of titanium for the fabrication of nuclear waste container in bentonite buffer of nuclear waste repository
Graphical abstract
Hydrogen permeation efficiency is obtained by extrapolating to the corresponding corrosion rate, according to which hydrogen content in titanium container can be estimated to assess the susceptibility of titanium to hydrogen embrittlement.
Introduction
Nuclear technologies are widely used by many countries in a lot of fields including energy, medical science, manufacture, agriculture and so on [1]. However, it brings with high-level nuclear waste (HLNW) which has extremely long decay time, strong radioactivity and high toxicity. According to data from the International Atomic Energy Agency, there were 380 thousand cubic meters of HLNW to be disposed in the world in 2015. How to handle the HLNW efficiently and reasonably is the most difficult challenge for each country wanting to develop nuclear power [2]. There are a number of solutions that have been proposed for HLNW disposal, such as ice treatment, space disposal, storage under the sea, deep burial. At present, many countries see deep geological disposal as the best choice. A ‘multi-barrier system’ including vitrified nuclear wastes, packing materials (container), buffer materials and surrounding rock is planned for deep geological disposal [[3], [4], [5], [6], [7], [8]]. For the buffer materials, Sweden, Finland and Canada [[9], [10], [11]] all chose bentonite.
As the first barrier, choices of the container materials for blocking HLNW and preventing groundwater access are of great importance. It is well known that titanium has a remarkable corrosion behavior because of the very stable oxide film formed on its surface [12]. Researchers have shown that the low corrosion rates of titanium and its alloys make the competitive container materials for deep geological disposal environment of China [13,14]. Meanwhile, scientists in the USA have considered applying Ti drip shields over Ni alloy containers [15,16]. However, given the extremely long disposal period, it is anticipated that at some point oxygen within the repository will be depleted and the cathodic process that sports corrosion of the container will switch to reduction of protons to form hydrogen. Blackwood et al. noted that if the passive film on titanium cannot be maintained, it is vulnerable under anaerobic conditions, particularly at high temperatures [17].
Researchers in Canada and Japan have studied the corrosion behavior of titanium and its alloy as candidate materials for HLNW containers and found that microbiological and pitting corrosion did not happen for titanium container, but there is a risk of hydrogen embrittlement (HE) and crevice corrosion [[18], [19], [20], [21]]. Titanium is susceptible to HE which is assumed to occur by hydrogen absorption near the crack tip, thus promoting crack formation in a stress corrosion cracking process [22]. For materials stressed in low hydrogen fugacity environment ‘hydrogen-embrittled’ materials can be divided into two categories: (a) non-hydride and (b) hydride forming system, with titanium belonging to the latter [23]. If hydrogen content exceeds the solubility of hydrogen in titanium (20 ppm–150 ppm), hydrides that are liable to cause embrittlement can be formed on the titanium [[23], [24], [25], [26], [27]].
Numakura and Koiwa [28] have found that titanium hydride has a face-centered tetragonal structure (c/a = 1.09) with an ordered arrangement of hydrogen, being isomorphous to y-zirconium hydride. Zhang and Kisi [29] have also reported that the unit cell volume dilation suggests the hydrogenation process begins with the formation of a titanium–hydrogen solid solution followed by the formation of titanium hydride. Meanwhile, because of hydrogen absorption, titanium is transformed from metal (h.c.p._A3, α-phase) to hydride (f.c.c._C1, δ-phase) [30]. Setoyama et al. [30] have studied the mechanical properties of titanium hydride and drawn the conclusion that titanium hydrides have smaller elastic moduli than the metal and the elastic moduli are dependent on the hydrogen content. Schutz [31] thinks that there are three main factors that can cause HE of titanium alloys: (1) there is a mechanism for continuously generating new atomic hydrogen on the surface of titanium (such as corrosion), (2) if the reaction temperature is higher than 353 K, the rate of hydrogen diffusion into α-Ti is more pronounced, (3) the pH is lower than 3 or higher than 12 or the applied potential is more negative than −0.70 V (relative to SCE).
The HLNW releases heat during decay, which may be sustained for even hundreds of thousands of years. According to the near HLNW container temperature simulation models in various countries [5,9,10,[32], [33], [34]], and considering the concepts of HLNW disposal in different countries, the temperature evolution model of for China’s proposed repository was devised and is shown in Fig. 1 [14]. The temperature will reach its maximum 363 K about 10 years after disposal, and then it gradually decreases. Fig. 1 clearly indicates that there will exist a period during which the temperature near the titanium container is higher than 353 K; the second criterion regarded by Schutz [31] for HE in titanium. Moreover, the dissolved oxygen concentration is also an important factor for the corrosion environment near the HLNW container. Yang et al. simulated the oxygen consumption process trapped in the pore material of the buffer material through solving the water-bio-geochemical coupling model [35]. They acquired the oxygen concentration evolution in various simulated scenarios and showed that the oxygen is almost depleted (less than 2 ppb) in one year at the fastest after the closure of the repository. At this point the cathodic reaction supporting the containers corrosion will switch to hydrogen evolution, which at pH 9 (i.e. the pH of bentonite) means the corrosion potential will fall to below −0.77 V vs SCE. As a result the remaining two criteria outlined by Schutz for HE will be meet. That is to say, at some point during its lifetime the deep geological environment in the repository meets all the conditions of HE for titanium container, meaning a thorough investigation of this risk is required.
If the container fails due to HE, HLNW could migrate into the biosphere with the groundwater; of particular concern are the highly soluble radioactive isotopes of cesium, Cs134 and Cs137 that have half-lives of 2 and 30 years respectively. It is clearly a matter of necessity and severity to study the HE of titanium container in China’s deep geological disposal environment, which has not been touched upon by researchers working on HE. Direct evaluation of the accumulated hydrogen content of titanium containers after a long-term deep geological disposal is not practical, therefore the evaluation of HE has to be done by a combination of accelerated experiments and simulations. In this paper, hydrogen permeation efficiency (HPE) and the accumulated hydrogen content (HA) of titanium container materials were evaluated for different disposal periods by an electrochemical simulation method and metallurgical analysis. Furthermore, based on two aspects that HA and slow strain rate tensile (SSRT) tests, the possibility of titanium container to HE was analyzed comprehensively to assess the safe disposal of container.
Section snippets
Materials and solutions
Grade 2 titanium (UNS R50400), which is nominally a purely alpha phase alloy, along with the low carbon steel (ASTM A283D) were used as the experimental material. And Table 1, Table 2 show their chemical compositions.
Square titanium sheets with dimensions of 5 × 5 mm and a thickness of 0.1 mm were used to measure HPE. Circular titanium specimens with a cross-section radius of 2 mm and a gage length of 25 mm were used for SSRT according to GB 6397-86 ‘Test Specimens for Metal’. Steel specimens
Hydrogen permeation efficiency
For estimating the hydrogen content in titanium container more accurately, an X-ray diffractometer was utilized to identify the constituent phases in the hydride layer formed in deep geological disposal condition. Fig. 3 shows XRD patterns of the charged specimens in infiltrated solution of saturated bentonite at the highest and lowest current densities used; intermediate charging current densities gave very similar XRD patterns. Although there exists the impurity TiN for the original sample,
Conclusions
Hydrogen permeation efficiency (HPE) and its influencing factors for commercial purity grade 2 titanium in simulated deep geological disposal environment at temperature range of 298–363 K were investigated through electrochemical and metallurgical methods. Based on HPE, hydrogen content (HA) and hydride thickness in titanium container were estimated. The conditions under which hydrogen embrittlement (HE) of titanium could occur were also studied. The following conclusions are reached:
- 1)
The lower
CRediT authorship contribution statement
Qichao Zhang: Writing - original draft. Yanliang Huang: Methodology, Writing - review & editing. Daniel John Blackwood: Writing - review & editing. Binbin Zhang: Data curation. Dongzhu Lu: Data curation. Dan Yang: Data curation. Yong Xu: Software.
Declaration of competing interest
The authors declare no competing interests.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China under Grant<No. 51471160> and China Scholarship Council <201804910 646>
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