Page 26 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Introduction to liquid metal cooled 1

reactors

F. Roelofs
Nuclear Research & Consultancy Group, Petten, The Netherlands

1.1 Nuclear energy and fast reactors

Nuclear energy today is one of the most important sources of electricity worldwide
with a small ecological footprint and low carbon emissions. Nuclear reactors use ura-
nium (or alternatively thorium) as natural resource to produce energy. The identified
resources and additional exploitable resources of uranium are sufficient to support
continued use and significant growth of nuclear energy production for well over
300 years. On top of that, there is proof that uranium can be “mined” from seawater.
Today, this is economically not viable, but if natural resources get scarce and more
research is put into the economic efficiency of uranium extraction from seawater,
in time, this will become economically viable.
However, the amount of uranium effectively used in the widely spread water-
cooled nuclear reactors can be improved a lot: in these thermal reactors, only a very
small amount of the uranium is actually split into fission products and producing
energy. By switching to fast reactors, uranium can be used much more efficiently. This
requires switching to different, less common, types of coolant. Already since the dawn
of nuclear energy production, this was recognized and investigated. The first reactor to
produce electricity, the Experimental Breeder Reactor I (EBR-I), was in fact such a
fast reactor. It was not cooled by water but by a mixture of sodium and potassium. At
the time, the known reserves of uranium were limited, which gave a strong incentive to
search for reactors that could use the uranium in an efficient way. These reactors are
often referred to as “breeding” reactors, since in the reactor not only uranium is split
and energy is produced but also plutonium is formed from nonfissile uranium isotopes,
which as such again can be split and produce energy. By changing the design of the
reactor core, the same reactor can be used to transmute long-lived radioactive ele-
ments into fission products that are much less long-lived and less radiotoxic. In this
way, the amount and radiotoxicity of nuclear waste can be significantly reduced.
History taught us that the water-cooled reactors matured earlier and succeeded in
conquering the nuclear energy production market. Other types of reactors, among
which the fast reactors, did not get a chance to mature that rapidly, even though quite
a few fast reactors were constructed and operated as will be explained later in this
chapter. As fast reactors operate with fast neutrons inducing fission reactions, they
cannot be cooled by water that would slow down (or moderate) the neutrons. Alter-
natively, another coolant has to be used. Liquid metals form a category of promising

Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors. https://doi.org/10.1016/B978-0-08-101980-1.00001-6
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