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Fig. 4.34 Molten-salt reactor (MSR): Molten-slat-cooled reactor with outlet temperatures
within 700–800°C (shown with indirect Brayton power cycle).
Courtesy of Generation IV International Forum.

MSR technology was partly developed, including two demonstration reactors, in
the 1950s and 1960s in the United States (Oak Ridge National Laboratory). The dem-
onstration MSRs were thermal-neutron-spectrum graphite-moderated concepts. Since
2005, R&D has focused on the development of fast-spectrum MSR concepts (MSFR)
combining the generic assets of fast-neutron reactors (extended resource utilization,
waste minimization) with those relating to molten-salt fluorides as fluid fuel and
coolant (low pressure and high boiling temperature, optical transparency).
In contrast to most other molten-salt reactors previously studied, the MSFR does
not include any solid moderator (usually graphite) in the core. This design choice is
motivated by the study of parameters such as feedback coefficient, breeding ratio,
graphite lifespan, and U 233 initial inventory. MSFR exhibit large negative-
temperature and void-reactivity coefficients, a unique safety characteristic not found
in solid-fuel fast reactors.
Compared with solid-fuel reactors, MSFR systems have lower fissile inventories,
no radiation-damage constraint on attainable fuel burn-up, no requirement to fabricate
and handle solid fuel, and a homogeneous isotopic composition of fuel in the reactor.
These and other characteristics give MSFRs potentially unique capabilities for acti-
nide burning and extending fuel resources.
MSR developments in Russia on the molten-salt actinide recycler and transmuter
(MOSART) aim to be used as efficient burners of transuranic (TRU) waste from spent