Page 30 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Introduction to liquid metal cooled reactors 5

The high density of lead may induce, depending on the type of fuel, that molten fuel floats
l
and thus in the case of a fuel melt near the core outlet moves in the direction of lower or
no power.
A lead or lead-alloy reactor pool ensures a high self-shielding capacity.
l
As always, also in nuclear engineering, there is no free lunch. The following draw-
backs can be identified related to the application of liquid metal as a coolant in nuclear
reactors:
l The typical high mass of liquid-metal-cooled systems (especially lead- and lead-bismuth-
cooled systems) requires special measures for seismic events.
l Corrosion issues may be always present, but they increase especially at temperatures above
600°C. When operating temperatures above 600°C are envisaged, which should in principle
be beneficial especially for lead and lead-alloys, new materials need to be developed to with-
stand the corrosion issues.
l In-service inspection in opaque coolant is significantly more difficult than in transpar-
ent coolants like water and gases as optical inspection methods cannot be applied.
Apart from that, the high density and elevated operation temperature of liquid metals
create higher forces on inspection tools that therefore need to be specially developed
and tested.
l The high melting point of liquid metals requires preheaters and measures against solidifica-
tion of the coolant in case of shutdown, both during normal operation and during accident
situations.
l The high mass of lead and lead-alloys leads to erosion issues in the components of the pri-
mary cooling system. This limits the coolant speed in such systems below 2m/s as a rule
of thumb.
The chemical reactivity of sodium with air and water requires a sealed coolant system and
l
special measures to prevent (nuclear) consequences of such reactions. Typically, this
involves multiple barriers between sodium and the environment. Also, it requires special
care in the heat transfer from the primary sodium circuit toward the eventual energy conver-
sion circuit. Mostly, an intermediate sodium loop is designed to prevent a chemical reaction
between primary sodium and the water-steam loop of the energy conversion circuit.
Obviously, this increases costs and at the same time leads to a loss of efficiency. Therefore,
studies are ongoing to eliminate such an intermediate circuit.
l During irradiation of lead-bismuth, highly radiotoxic polonium is produced that should be
confined at all times.

1.5 European liquid metal reactor designs

As illustrated before, worldwide, quite some effort has been put in the development of
liquid-metal-cooled nuclear reactors. Ongoing developments are mostly related to
design work of future liquid-metal-cooled reactors. The main projects that can be
identified are the further development of the Russian sodium reactor line with the
BN1200, a scaled-up version of the commercial BN800 reactor that was put into oper-
ation recently. Also in Russia, the lead technology is being pushed toward
commercialization by the design of the lead-bismuth-cooled prototype SVBR small
reactor and the lead-cooled larger prototype BREST reactor. In Japan, the design work
on advanced reactors has been put to hold after the tsunami in 2011. However, in