Page 209 - Managing Global Warming

Current and future nuclear power reactors and plants 171

is utilized, which acts as a buffer between the radioactive sodium—reactor coolant in
the primary loop and the water/steam in the third loop—a steam Rankine power cycle.
Lead is proposed for use in an LFR at pressures close to 0.1MPa. Lead has a
higher melting point (327.5°C) and significantly higher boiling point (1750°C) com-
pared to that of sodium, which significantly impacts on the reactor. Also, it is a more
inert liquid metal than sodium. As a result, the LFR needs only two loops: (1) a pri-
mary loop with lead as a reactor coolant and (2) a secondary loop with water/steam as a
Rankine power cycle.
LBE is an eutectic alloy of lead (44.5%) and bismuth (55.5%) and is being consid-
ered instead of lead as an option for the LFR. One of the main advantages of LBE is its
melting point of 123.5°C, which is significantly lower than that of lead and quite close
to that of sodium. Neither the lead nor the LBE react readily with water or air, in con-
trast to sodium, which allows for the elimination of the intermediate-coolant loop used
in SFRs. Moreover, LBE is not a new technology—it has been proven by years of
reliable experience as a coolant in nuclear-powered submarines operated by the Soviet
Union since the 1970s.
A major advantage of liquid-metal reactor coolants is the low operating pressures
inside a reactor (close to one atmosphere) with a possibility to achieve high temper-
atures. Also, all current liquid-metal reactors use a fast-neutron spectrum, which
allows for more efficient fuel cycles.
Further information on liquid-metal reactor coolants can found in references
[28–32].

4.4.2.4 Molten-salt coolants
Molten-salt fluorides, which are proposed as coolants in MSR, have promising
thermophysical and thermohydraulic properties. Molten salts, similar to liquid metals,
have a low vapor pressure even at high temperatures, which is quite attractive com-
pared to water and gaseous coolants. Salts are less chemically reactive than sodium. In
addition, salts can provide moderation due to their light-element composition such as
F, Li, and Be in FLiBe.
Table 4.13 lists the main thermophysical properties of a number of coolants. The
range of temperatures investigated covers the operating ranges of the corresponding
reactors.

4.4.3 Thermophysical properties of proposed Generations
II, III, III+, and IV reactor coolants
The basic thermophysical properties are shown within a wide range of temperatures
(from 250°C to 1000°C) that cover the operating ranges of current and Generation IV
reactors (see Table 4.13 and Fig. 4.39).
The properties of subcritical and supercritical water, carbon dioxide, and helium-4
were obtained from reference [27]. The properties of sodium were taken from refer-
ence [33], and the properties of other coolants were calculated using correlations pres-
ented in reference [34].