The FuTure oF nuclear Fuel cycle - MIT Energy Initiative

Advanced High-Temperature Reactor
Abstract—In the last decade, a new reactor concept has been proposed: the Advanced High-
Temperature Reactor (AHTR) that uses liquid fluoride salts as coolants and the coated-particle
fuel developed for gas-cooled high-temperature reactors. It is also called the fluoride-salt
high-temperature reactor (FHR). It has potentially promising economics because of the compact
primary systems that operate at low pressures with large thermal margins and sufficiently
high coolant temperatures to enable use of higher efficiency power cycles. Unlike other reactors,
it naturally uses a combined uranium-thorium fuel cycle in a once-through mode and may
have a conversion ratio near unity if operated with a closed fuel cycle. In the context of fuel
cycles it is a radical departure because one variant can use flowing pebble-bed fuel to enable
three dimensional optimization of the reactor core with time that creates new fuel cycle options
that are today only partly understood. The reactor does not have any single technical issue that
determines technical viability when operated at temperatures below 700 °C, but rather there
has been insufficient work to date to understand the potential capabilities and limitations.
Since the coolant freezes at several hundred degrees C, maintaining such high temperatures at
all times in the coolant circuit is important to reliability.
The AHTR is a new reactor concept [1] that uses the traditional gas-cooled high-temperature
reactor fuel and a liquid salt coolant. The fuel is a coated-particle fuel incorporated
into a graphite matrix. The graphite matrix can be in the form of prismatic fuel blocks, or
fuel assemblies, or pebbles—spheres several centimeters in diameter. Test and prototype
helium-cooled high-temperature reactors have been built with (1) prismatic fuel blocks in
the U.S and Japan and (2) pebble-bed fuel in Germany and China. China is currently constructing
a prototype helium-cooled pebble bed reactor. The AHTR, like high-temperature
gas-cooled reactors, has a thermal to intermediate neutron spectrum.
Several liquid salt coolants are being considered for the AHTR. The leading candidate is
a mixture of 7 LiF and BeF 2 . The coolant exit temperatures would be ~700°C resulting in a
reactor with a high thermal-to-electricity efficiency. The coolant is the same fluid used in
the molten salt reactor (MSR) except unlike the MSR with fuel dissolved in the coolant,
the AHTR uses a clean coolant and thus avoids the corrosion challenges associated with
earlier concepts. Two small molten salt test reactors were built. One was for the Aircraft
Nuclear Propulsion Program in the 1950s. The second was for the Molten Salt Breeder Reactor
program of the 1960s. MSRs can be sustainable reactors with conversion ratios near
unity using a thorium fuel cycle. The pebble bed AHTR in some respects can be considered
a solid-fuel variant of the MSR. (More recently, the French have initiated an R&D program
for a fast-spectrum MSR [2]. This is a more advanced reactor concept than the concepts described
in this chapter with the unusual characteristics of a very low fissile fuel inventories,
a fast-spectrum reactor with a large negative void coefficient, and a long-term candidate for
efficient burning of fissile materials).
The pebble-bed AHTR may enable a modified thorium-uranium 235 fuel cycle for an open
cycle with improved uranium utilization or potentially a closed thorium-uranium 233 fuel
cycle with a conversion ratio near unity. The unusual fuel cycle options are a consequence of
(1) the pebble fuel and (2) liquid cooling efficiency that avoids local hot spots. In a pebblebed
reactor, the fuel consists of pebbles several centimeters in diameter. These pebbles move
through the reactor core over a period of a few weeks producing power inside the reactor
core. As pebbles exit the reactor, radiation detectors determine their burnup. Pebbles with
low burnup are recycled back to the reactor core. Pebbles with high burnup become SNF.
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