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170 Managing Global Warming

transfer, called critical heat flux (CHF), which usually cannot be exceeded during a
nuclear reactor operation.
Supercritical water (SCW) is a coolant in an SCWR concept with an operating pres-
sure of 25MPa, and reactor inlet and outlet temperatures of about 350°C and 625°C
(max), respectively. Specifics of SCW thermophysical properties and heat transfer
are discussed in Appendix A3 of reference [1] and in the following publications:
IAEA-TECDOC-1746 [22]; Gupta et al. [23]; Pioro [24]; Pioro et al. [25]; Pioro
and Mokry [26]; NIST REFPROP [27]; and Pioro and Duffey [17].
Main disadvantage of water as a reactor coolant is that to reach higher thermal effi-
ciencies of NPP greater temperatures are needed, which require application of super-
critical pressures.

4.4.2.2 Gas coolants
For comparison purposes in this Section, it was decided to consider subcritical-
pressure carbon dioxide. Carbon dioxide at subcritical pressures is currently being
used in the most efficient nuclear power reactors—AGRs. In general, carbon dioxide
is not a strong absorber of thermal neutrons and does not become very radioactive.
Other advantages of carbon dioxide are its chemical stability within the operating
range of temperatures (292–650°C). In addition, carbon dioxide does not react with
either the moderator or nuclear fuel.
Using helium as a reactor coolant at high outlet temperatures (850°C and 1000°Cin
GFR and VHTR, respectively) makes it possible to achieve very high thermal efficien-
cies of the plant that are close to those of modern advanced thermal power plants. The
major advantages of helium are: (1) a relatively high thermal conductivity compared
to that of other gases (the exception is hydrogen), which is close to that of liquids; and
(2) its behavior as a noble or inert gas.
In general, the advantages of gaseous reactor coolants compared to water are a pos-
sibility to achieve high, or even very high, temperatures at the reactor outlet using sig-
nificantly lower pressures; and there is no CHF phenomena at gas cooling, which
limits heat transfer in fluid cooling. However, heat-transfer coefficients for gas
forced-convection cooling are usually significantly lower than those for water
cooling.

4.4.2.3 Liquid-metal coolants
Liquid sodium is currently used in the Russian BN-600 and BN-800 reactors—the
only ones operating SFRs so far in the world—and is proposed to be used in Gener-
ation IV SFRs. Sodium is a well-known low-melting-point (97.7°C) alkali metal,
which has the main advantages of high thermal conductivity and low neutron absorp-
tion cross-section. Also, the relatively high boiling point (882.8°C) of sodium allows a
reactor to operate at pressures close to 0.1MPa. In addition, very high heat-transfer
coefficients can be achieved with sodium cooling.
However, sodium is very reactive substance, which requires special precautions,
when used as a reactor coolant. For improved reactor safety, a secondary sodium loop