IEEE Spectrum August, 2010 - 30

Manufacturer: Toshiba
HQ: Tokyo
Type: Liquid-sodium-cooled fast reactor
Power: Thermal, 30 MW; electric, 10 MW
Fuel: Uranium enriched to about

toSHiBA 4S The four
S's in the name stand for super,
safe, small, and simple. Think
of the reactor as a nuclear
battery with a 30-year life

19.9 percent (just below the 20 percent
weapons-usable threshold); the uranium is
mixed with zirconium and clad in steel.

Refueling: The reactor is sealed and never
refueled. When its fuel is exhausted after
30 years, the entire reactor core would be
returned to the manufacturer for disposal,
and another one could take its place.

How it woRkS:
The core, which is long and skinny [1],
has a ring-shaped reflector that moves
up slowly over time. This shield keeps
neutrons contained in a focused area of
the core, where the chain reaction takes
place. As the ring rises, it slowly burns
up the nuclear fuel. For harvesting the
heat, the 4S reactor is configured with
three loops. In the first, liquid-sodium
metal circulates to cool the reactor
core. Liquid sodium also circulates in
the second loop, which transfers heat
to a third, this one containing water and
steam to drive a turbine. In the first and
second loops, convection makes the
liquid metal flow. To improve safety,
electromagnetic pumps-with no
moving parts-help with circulation
[2]. The reactor is designed as a sealed
cylindrical vault, which could be buried
30 meters underground to ensure safety
against tornadoes and terrorists.

Coolant: Liquid sodium
Moderator: No moderator (it's a fast reactor)
Waste: Spent fuel remains sealed in the core.

[1] The reactor's
core is buried underground, while the
heat exchangers
and steam turbine
are aboveground.

wave of the future?

generation
iv Reactors
the most exotic designs, the Generation
IV reactors, use new kinds of fuel and
moderators. And fast-reactor designs do away
with the moderator altogether. As a result,
they require fuel with higher concentrations
of fissile material (plutonium or uranium
235) than do light-water reactors. And a few
promise to do something unprecedented-
burn not just fuel but also the longer-lived
nuclear waste products that have plagued
nuclear energy since its inception. The Next
Generation Nuclear Plant, a Generation IV
design being considered by a consortium
of U.S. companies, will likely be cooled by
helium and moderated by graphite. The
specific technology and leading companies
will be announced early in 2011. Other, even
more radical reactors include TerraPower's
traveling-wave reactor, which the company
hopes to build and test in a little over a decade.
30

NA * iEEE SpEctrum * AuguSt 2010

[2] Electromagnetic
pumps help
circulate
the liquidsodium
coolant.

Radial
shield

AdvAntAgeS:
The reactor can sit mostly unattended
for up to 30 years, in part because the
liquid-sodium coolant doesn't corrode
the metal pipes and vessels of a reactor
the way superheated water does. Also,
because the reactor does not have to
be pressurized, a pipe rupture would
not be explosive. Instead, the molten
sodium would merely seep. Like any fast
reactor, the 4S could fission some of the
longer-lived isotopes in the spent fuel,
which would reduce to some degree the
quantity of isotopes and also the overall
volume of waste.

diSAdvAntAgeS:
Neutron
reflector

Sodium is extremely volatile and explodes
on contact with water. One of the claimed
benefits of this design-the fissioning of
long-lived isotopes in the spent fuel-
might actually be a weakness. Though
the volume of waste product is reduced,
the waste itself is much more radioactive
and could conceivably be used to create
dirty bombs.

tiMe fRAMe:
In the United States, Toshiba has had
preliminary meetings with the NRC and
has submitted preapplication technical
reports. The company expects to
submit its design for review in late
2012. The NRC will not estimate when
it could be approved. The 4S already
has some interested parties, including
the western Alaska city of Galena
(population 599), which plans to apply
for a construction license as soon as
the NRC grants its approval.
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Table of Contents for the Digital Edition of IEEE Spectrum August, 2010
