Page 137 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

110 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Although not critical for safety, dissolved impurities such as oxygen can play a key role for
l
corrosion; see next section. Depending on the application, it might be necessary to remove or
control the amount of dissolved impurities during operation. Taking advantage of the
temperature-dependent solubility, eventually, a cold trap can be used.

Although LBE does not present safety concerns related to violent exothermic reac-
tions, the toxicity of lead must be taken into account, and the oxygen content plays
a major role. Thus, the above-listed considerations usually apply also to heavy
liquid-metal facilities.

3.3.1.2.3 Corrosion (compatibility with solid materials)
This is a very broad research topic, as the corrosion of solid materials by flowing
liquid metals involves different physical phenomena as in water, for example, dis-
solution, fretting, embrittlement, and accelerated creep. It is beyond the scope of this
work to describe in detail the state of the art on this field; instead, the reader is
referred, for example, to the Heavy Liquid Metal handbook of the OECD (2015).
Some broad guidelines for construction of liquid-metal facilities can be summarized
as follows:
l As the degradation of solid materials becomes more significant with increasing temperature,
it is important to stick to a simple design. The general rule of keeping the temperature not
only as high as necessary but also as low as possible means, for example, placing most com-
ponents (e.g., valves and pump) on the cold leg.
Liquid metals present relatively large solubility limits for some common transition metals,
l
including aluminum (Al), copper (Cu), and silver (Ag) and some steel components such as
chromium (Cr) and nickel (Ni), which could lead to leaching. Moreover, as this limit
increases with temperature, materials dissolved in the hot sections can be later deposited
in the colder parts of the facility. In general, corrosion of steels by sodium is less severe than
by heavy liquid metals, such as LBE.
l Although the general rule would be to avoid the abovementioned materials if possible,
technical solutions are developed for improving the performance of stainless steels. In
particular, controlling the oxygen concentration, a protective oxide layer is formed at
the wall, preventing leaching. Based on the experiences at KIT, satisfactory long-term
service has been obtained with the austenitic steel 1.4571 for flowing LBE (Schroer
et al., 2011).
l The technique of oxygen control is applicable within a certain temperature range, according
to the chemical stability of each oxide species (Schroer and Konys, 2007). For higher tem-
peratures, beyond the envisaged nominal operating conditions of liquid-metal reactors, yet
necessary for investigations such as fuel-coolant interaction, refractory metals (W, Mo, and
Ta), ceramics, and quartz glass can be used if high pressures are not imposed; see, for exam-
ple, Heinzel et al. (2017).
It is important to note that relevant solid materials are not only structures, such as pipes
and vessels, but also all surfaces in contact with the liquid metal. Thus, these consid-
erations apply as well to sealing, instrumentation, and pump impellers, among others.
For special cases, such as thin-walled components, technical solutions based on anti-
corrosive coatings can be adopted.