IEEE Spectrum August, 2010 - 31

[1] Uranium
dioxide particles
are coated with
carbon and
silicon carbide.

Manufacturer: Two
designs, commissioned
by the U.S. Department
of Energy under the
Next Generation
Nuclear Plant program,
are currently under
development, one
by a consortium led
by General Atomics, in
San Diego, and another
by a consortium led
by Westinghouse,
in Cranberry
Township, Pa.

Fuel particle
0.5 mm diameter
[2] Coated particles
are mixed with carbon
and pressed into cylinders or spheres.

Type: High-temperature
gas-cooled reactor

Coated fuel particle
0.92 mm diameter
Fuel compact
39 mm tall

Power: Thermal, 250 to
600 MW; electric, 112 to
270 MW

Fuel sphere
60 mm diameter

Prismatic
block
580 mm tall

[3] Fuel
goes into
prismatic
blocks that
are stacked
to form the
core.

[4] Many
thousands
of fuel
spheres fill
the core
vessel.

Prismatic configuration

Pebble-bed configuration

neXt geneRAtion nucleAR plAnt This Generation IV reactor
is designed to produce electricity and also heat for industrial applications

How it woRkS:
This reactor's fuel consists of uranium
particles known as tristructural
isotropic, or TRISO, particles [1, 2].
Blocks or spheres containing the TRISO
fuel and carbon are arranged into a
nuclear core [3, 4], producing a chain
reaction. Helium circulates through the
core, removing heat. The helium drives a
turbine directly, or it can generate steam
to turn the turbines. Because the core
consists of helium and graphite, it can
withstand temperatures of 900 °C and
even higher; pressurized-water reactors
operate at about 300 °C.

spectrum.ieee.org

AdvAntAgeS:

diSAdvAntAgeS:

The reactor's higher temperature
means it could provide heat to industrial
processes such as petroleum refining,
chemical manufacturing of plastics and
fertilizers, and hydrogen production,
thus helping to reduce carbon emissions
and use of oil and natural gas. Despite its
higher temperature, the reactor operates
at one-fifth the power of a PWR. That
reduced power density enhances overall
safety. What's more, additional heat
naturally increases the carbon's ability
to absorb neutrons, so the carbon acts
as a passive safety mechanism capable
of shutting down the core. Fuel particles
can withstand up to 1600 °C, so even in
an accident the fuel would remain intact
and limit fallout. High-temperature
gas-reactor prototypes have been
demonstrated in Germany and China.

Much testing is yet to be done. Fuel
pellets need to be evaluated under
heavy neutron bombardment, and
so does the graphite that would
form the core. The high-temperature
materials to hold the helium-nickelbased superalloys-also must be fully
characterized and tested in extreme
conditions. Reactor physics must be
simulated and validated. The reactor's
core would contain a lot of graphite that
ideally would be recycled, a process that
might prove costly.

tiMe fRAMe:
In early 2011, the U.S. Department of
Energy expects to choose between
proposals from General Atomics and
Westinghouse. Further design work
should take 8 to 10 more years.

Fuel: Microscopic particles of uranium dioxide coated with carbon
and silicon carbide.
These spheres, known as
tristructural isotropic,
or TRISO, particles,
are then mixed with lots
of graphite and pressed
into one of two possible
geometries: spheres
the size of tennis balls
(the pebble-bed design)
or sticks the size of a
piece of chalk that are
inserted into hexagonal graphite blocks (the
prismatic design).
Refueling: The spent
fuel is continuously
replaced without
shutting down the
reactor. In the pebblebed type, TRISO balls
are removed from the
bottom to have their
ﬁssion levels measured,
and new balls are
added to the top. In
the prismatic reactor,
thousands of hexagonal
blocks are stacked and
their TRISO fuel sticks
replaced periodically.
Coolant: Helium
Moderator: Graphite
Waste: The spent fuel
consists of balls (in the
pebble-bed reactor) and
sticks (in the prismatic
reactor) containing
leftover uranium that
didn't undergo ﬁssion
and other radioactive
material; the waste
would be stored in metal
casks on-site.

AuguSt 2010 * iEEE SpEctrum * NA

31
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Table of Contents for the Digital Edition of IEEE Spectrum August, 2010
