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Current and future nuclear power reactors and plants 183

Fig. 4.48 Heat-transfer coefficients calculated for flow of coolants in Generation IV reactors,
AGR, and PWR in a bare tube at generic operating conditions [1].

4.4.5 Conclusions
Based on the aforementioned, the following conclusions can be made.
In general, liquid-metal coolants have high thermal stability, high boiling points, and very
l
low saturated-vapor pressures, which distinguish them from other types of nuclear coolants.
The specific heats of Pb and LBE are nearly identical and 10 times lower than those of Na
l
and CO 2 , and are almost 40 times lower than that of He. At temperatures higher than 450°C,
the specific heat of He is even higher than that of SCW.
l As one would expect, the thermal conductivity of liquid metals is significantly higher than
that of gases (50–3000 times). The highest thermal conductivity is for sodium with the value
1
of 60–70W (mK) ).
l The volumetric expansivity of liquid metals is much lower than that of the other coolants
examined, and stays almost constant.
l The thermophysical properties of liquid metals and gases show only small linear changes
with temperature. However, all the properties of SCW go through very rapid changes within
the pseudocritical range.
l The thermophysical properties of LBE, except for the thermal conductivity, are close to the
average values of those of lead and bismuth.
l At high temperatures (>500°C), the Prandtl number of SCW behaves similar to gases.
l One of the least desirable properties of water is its high vapor pressure, which increases
rapidly with temperature. Its relatively low critical temperature ( 374°C) limits the