Page 136 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Construction of experimental liquid-metal facilities 109

3.3.1.2.1 Physical properties

The most relevant thermophysical properties of LBE, Na, and water are listed in
Table 3.1. The broad differences in some of these properties have strong effects on
the design, construction, and operation of liquid-metal experimental facilities.
The broad liquid range of LBE and Na has two effects. On the one hand, auxiliary
heating and thermal insulation are necessary to avoid freezing in all operational sce-
narios. On the other hand, operation at low pressure is possible, usually with large
margin to boiling.
Heavy liquid metals such as LBE have a high density, impacting the weight of the
facility, leading to the related static considerations. Moreover, at a given fluid velocity
2
(u), much larger inertial forces (proportional to ρu ) are present than, for example, in
water. This must be taken into account for the analysis of, among others, local pressure
losses and water-hammer effect.
Compared with water, liquid metals have relatively low specific heat capacities
(c p ), also expressed in volumetric terms (ρc p ). Depending on the application and
adjustment of the heating power, volumetric flow rate might be necessary.
The characteristically large thermal conductivity (λ) and diffusivity (a) of liquid
metals implies that much larger heat-flux densities are required in order to obtain tem-
perature differences that can be accurately measured, resulting, for example, in more
compact test sections. On the other hand, the thermal gradients are lower for a given
power input, which can be a practical advantage for the construction of liquid-metal
facilities, as, for example, thermal stresses during transients are reduced.
In general, the coefficient of thermal expansion (β) increases with temperature, and
4
liquid metals at 300°C have a similar value than water at 1bar and 25°C (2.57 10 ).
This property affects mostly the size of expansion vessels and buoyancy consider-
ations for natural convection.
The much larger surface tension (σ) of liquid metals results in a poorer wetting of
solid surfaces. As residual gas bubbles can remain trapped at the wall, purging of the
facility can become important, for example, for heat-transfer tests. For two-phase
flows, large bubble diameters can be expected as a consequence of a larger σ.

3.3.1.2.2 Chemical interactions
Alkali metals such as sodium present exothermic reactions with many common
substances, such as water, air (oxygen and humidity), CO 2 , and some organic liq-
uids as alcohol (Addison, 1984). The interaction with water involves also the pro-
duction of hydrogen, resulting in potentially large energy releases. Thus, sodium
facilities must be constructed in a way that these reactions do not occur. Practical
guidelines for sodium handling have been developed; see, for example, Kottowski
(1981). In particular, the construction of a liquid-metal facility must consider the
following issues:
l A sump tank with capacity for the complete liquid inventory acts as a final vessel for emer-
gency draining of the facility, for example, in case of a leakage in order to reduce its con-
sequences. Accordingly, filling and draining procedures must be defined.
l Avoiding direct contact with air during operation, a cover gas system (e.g., argon) is used.