Transmission electron microscopy study on the liquid metal corrosion mechanisms of the 1.4970 austenitic stainless steel fuel cladding for MYRRHA

Charalampopoulou Evangelia

Promoter

Schryvers Dominique, (UA), nick.schryvers@ua.ac.be

SCK•CEN Mentor

Delville Rémi
remi.delville@sckcen.be
+32 14 33 31 65

SCK•CEN Co-mentor

Lambrinou Konstantza
konstantza.lambrinou@sckcen.be
+32 14 33 31 64

Expert group

Fuel Materials

PhD started

2015-04-15

Short project description

Lead bismuth eutectic (LBE) is a possible coolant for fast nuclear reactors and spallation sources. Its low melting point, high evaporation point, good thermal conductivity, low reactivity and good neutron yield make LBE-cooled reactors and spallation sources prime candidates for the Gen-IV nuclear reactor concept [1,2]. In particular, the multipurpose hybrid research reactor MYRRHA to be built in Belgium [3] relies on a LBE cooling system. The main disadvantage of LBE is that it is a corrosive medium for most steels. However, when the dissolved oxygen concentration in the liquid LBE is sufficient, protective oxide layers will form on the surface of the exposed steels, preventing further corrosion. Better understanding the structure and growth mechanism of these oxides, as well as their immunity to liquid metal attack are essential steps in assessing their effectiveness as corrosion barriers for the MYRRHA steels in the reactor operating conditions. This is particularly critical for thin-walled cladding tubes, which are the first barrier against the release of radioactive fission products to the environment. The fuel cladding tubes are exposed to relatively high temperatures, high neutron flux, fast-flowing LBE on the outside and fuel-induced corrosion on the inside. This makes them particularly vulnerable to liquid metal corrosion which, if not mitigated by a protective oxide layer, will rapidly lead to their failure. In addition, it is important for the safe design of the reactor to be able to model the evolution of the oxide scale composition and thickness over time, since it determines the thermal conductivity properties of the cladding tubes.

Cladding tubes for MYRRHA are made of Ti-stabilized DIN 1.4970 stainless steel, a highly radiation resistant and creep resistant grade that was developed during the sodium fast reactors program in Germany.Their behavior under irradiation at high temperature in a sodium environment is relatively well-known but there is very limited knowledge with respect to their behavior in an LBE environment. A batch of 500 DIN 1.4970 cladding tubes has been manufactured at great cost and has been delivered to SCK-CEN in the summer of 2013. The production of cladding tubes from such steel, the first in almost 20 years, has been a success since the results are in conformity with the very strict nuclear-grade specifications. The results of a series of tests on these cladding tubes will be a key part of MYRRHA fuel qualification program. The most critical tests concern the corrosion resistance of this steel since the cladding is subjected to the highest temperature in the reactor at level where steel dissolution has been observed with similar grade. Of all the materials foreseen in the MYRRHA reactor, the corrosion resistance of DIN 1.4970 represents therefore the greatest challenge.

Recent research on the corrosion resistance of 1.4970 and similar steel grades in liquid LBE has revealed a complex oxide layer structure, the formation of which is not yet fully understood. What was believed to be simply a double oxide layer (outer magnetite Fe3O4 layer and inner Fe-Cr spinel layer) was shown to comprise at least 3 layers with an heterogeneous structure, varying grain size and various defects (pores, inclusions, cracks) [4,5]. The growth of the oxide layer is driven by the counter-diffusion of elements (Fe, Ni, Cr, O, Pb-Bi) along the nano/micro-structures originally present in the steel (grain boundaries, inclusions) or formed during the corrosion process (pores, cracks). Understanding the details of this process and the new phases formed underway holds the key to an accurate model describing the steel oxidation mechanism. It must also be mentioned that the type and density of the formed oxides depends on the amount of dissolved oxygen in the liquid LBE [6]. Research towards this goal is performed at several labs [5,7-10] but many questions that could be tackled during a PhD work still remain unanswered.

If the steel exposure conditions (esp. the liquid metal oxygen concentration and temperature) are such that either no protective layer can be formed on the steel surface or the existing oxide scale is rendered insufficiently protective, the high solubility of certain elements (esp. Ni and Cr) in the liquid LBE will unavoidably lead to steel dissolution, a phenomenon that can cause the premature failure of a thin-walled component, such as a cladding tube. The removal of Ni, Mn and Cr, and to a less extend of Fe creates vacancy clusters and pores that become filled with LBE; moreover, the removal of the austenite stabilizers Ni and Mn can lead to superficial steel ferritization. In addition, locally enhanced dissolution has been observed in 1.4970 steels  [8]. Even though no clear justification has so far been provided in literature for the occurrence of locally enhanced dissolution, possible causes might be the presence of steel impurities/inclusions (e.g. MnS or other particles) or local inhomogeneities in the liquid metal flow pattern. The work performed in the framework of this PhD thesis is expected to help elucidating the causes of enhanced localized dissolution in 1.4970 steel, the processing of which could then be appropriately modified to minimize the occurrence of this corrosion mechanism.

 

Objective

The objective of the proposed research work is to investigate the nano/micro-structures in DIN 1.4970 stainless steel cladding tubes and in the corrosion layers after exposure to LBE. This work is expected to shed light on the oxidation and dissolution mechanisms operating in these steels as a result of their contact with liquid LBE. The student would have to follow closely the corrosion research program currently on-going at SCK•CEN and from which samples will be obtained.

The main tool of investigation would be analytical transmission electron microcopy (TEM). It is proposed to perform this PhD work in collaboration with one of the leading European TEM laboratories, EMAT (Electron Microscopy for Material Science), at the University of Antwerp. Their expertise in the field of TEM and sample preparation will be a great support for this PhD work. At SCK•CEN, a JEOL JEM3010 Scanning Transmission Electron Microscope equipped with an Energy dispersive x-ray spectrometer (EDS) will be used to investigate the samples with conventional TEM, high resolution TEM and electron diffraction to determine the crystal structure, the defect structure and morphology of the different phases both in the corrosion-affected zones and in the 1.4970 steel bulk down to the atomic level. The EDS-system can be used to determine the local chemical composition of the different phases. Other TEM techniques such STEM HAADF, STEM EELS and EFTEM may be applied at the EMAT laboratory. Complementary analyses using SEM-EDS or EPMA (Electron Probe MicroAnalyser) devices available at SCK•CEN may also be considered during this PhD work.

Results obtained from this in-depth study of the liquid metal corrosion behavior of 1.4970 stainless steels will help to acquire a thorough understanding of the operating corrosion mechanisms and might be used to develop models describing the oxidation/dissolution behavior of 1.4970 steels. This PhD work should, therefore, bring an important contribution to the qualification of the MYRRHA fuel cladding.

 

 

[1]        Murty, K.L. and Charit, I., Structural materials for Gen-IV nuclear reactors: Challenges and opportunities. Journal of Nuclear Materials, 383 (2008) 189-195.

[2]        Zhang, J.S. and Li, N., Review of the studies on fundamental issues in LBE corrosion. Journal of Nuclear Materials, 373 (2008) 351-377.

[3]        Aït Abderrahim, H., et al., MYRRHA, a Multipurpose hYbrid Research Reactor for High-end Applications. Nuclear Physics News, 20:1 (2012) 24-28.

[4]        Hosemann, P., Dickerson, R., Dickerson, P., Li, N., and Maloy, S.A., Transmission Electron Microscopy (TEM) on Oxide Layers formed on D9 stainless steel in Lead Bismuth Eutectic (LBE). Corrosion Science,  (2012).

[5]        Hosemann, P., et al., Characterization of oxide layers grown on D9 austenitic stainless steel in lead bismuth eutectic. Journal of Nuclear Materials, 375 (2008) 323-330.

[6]        Steiner, H., Schroer, C., Voss, Z., Wedemeyer, O., and Konys, J., Modeling of oxidation of structural materials in LBE systems. Journal of Nuclear Materials, 374 (2008) 211-219.

[7]        Müller, G., et al., Results of steel corrosion tests in flowing liquid Pb/Bi at 420-600 °C after 2000 h. Journal of Nuclear Materials, 301 (2002) 40-46.

[8]        Muller, G., et al., Behavior of steels in flowing liquid PbBi eutectic alloy at 420-600 degrees C after 4000-7200 h. Journal of Nuclear Materials, 335 (2004) 163-168.

[9]        Rivai, A.K., et al., Effect of Cold Working on the Corrosion Resistance of JPCA Steel in Flowing Pb-Bi at 450ºC, in IWSMT10 2010: Beijing-China

[10]      Johnson, A.L., et al., Spectroscopic and microscopic investigation of the corrosion. of D-9 stainless steel by lead-bismuth eutectic (LBE) at elevated temperatures. Initiation of thick oxide formation. Journal of Nuclear Materials, 376 (2008) 265-268.