Page 29 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

4 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

In Asia, the development of nuclear power production started later. Japan started
the operation of the experimental Joyo sodium-cooled reactor in the mid-1970s. Later,
they constructed and operated in phases the prototype Monju reactor. However, due to
some technical and societal issues, recently, it was decided to stop this reactor and also
limit further developments. China only recently embarked on the operation of liquid-
metal-cooled reactors with the experimental CEFR. They have ambitious plans to fur-
ther develop their sodium and their lead and lead-bismuth experience with several
designs for experimental and prototype reactors. Finally, India, which for a long time
was more or less isolated from the rest of the world with respect to nuclear develop-
ments, started considering fast reactors for the efficient use of their scarce uranium
resources. They constructed and operated the experimental sodium-cooled FBTR
since the 1990s and are currently commissioning the PFBR sodium-cooled reactor.

1.4 Benefits and drawbacks of liquid metals as coolants

From a thermal hydraulic point of view, the use of liquid metals as coolants brings
new challenges and the need for advanced tools compared with dealing with coolants
like water and gas. The benefits and drawbacks of using liquid metals are listed and
shortly described later. More elaborate explanations can be found, for example, in
IAEA (2012) or in GIF (2002) or GIF (2014).
The following benefits can be identified related to the application of liquid metal as
a coolant in nuclear reactors:
l The neutronic characteristics of liquid metals are such that neutrons created by fission in the
fuel are not moderated, and a sufficient amount of fast neutrons remains at disposal to keep
the nuclear chain reaction going.
l Metals are liquid at the operating temperatures of the nuclear reactor with sufficient margin
toward the evaporation point. Therefore, the system can be operated without pressurization
in contrast to water-cooled reactors, and the reactors can be operated at low pressure.
l Liquid metals typically possess good heat transport characteristics and high heat capacity
allowing efficient transport of heat generated in the core with relatively small systems and
providing grace time in case of accident situations.
l The high density of liquid metals relaxes the conditions for establishing natural circulation
cooling loops in accident situations.
l The high boiling point of liquid metals, at least above 850°C for sodium, mitigates issues
with core voiding. For lead, the very high boiling point of about 1750°C practically prevents
voiding in the core possibly leading to a clad failure, because the clad itself will have failed
before the boiling point of lead is reached.
A high efficiency for electricity production can be achieved by the application of liquid metal
l
because of the relatively high operating temperatures that can be achieved.
Compared with all other advanced nuclear reactor concepts, there is relatively large oper-
l
ating experience with liquid-metal-cooled reactors, especially with sodium-cooled reactors.
The application of lead or lead-alloys as coolant allows integration of steam generators in
l
the reactor vessel. For sodium alternative, secondary cooling options based on gas are being
investigated.
l The heat transport characteristics of lead and lead-alloys allow large fuel rod pitches that
result in low pressure drops and enable application of natural circulation.