POWER December 2012 - 75

NUCLEAR FUEL
1. Complex construction project.
The Mixed Oxide Fuel Fabrication Facility at
the DOE's Savannah River Site near Aiken,
S.C., recently entered its sixth year of construction.
Courtesy: Shaw AREVA MOX Services
LLC
the construction activities. " We actually do
about 58% of our subcontracts with small
businesses. To date, we have subcontracted
about $900 million in small business
awards. "
Trice proudly acknowledged the team's
achievement of a new safety milestone over
the summer. " We have also just crossed
11.5 million safe work hours without a lost
work day accident, which is a significant
accomplishment for a project of this size,
complexity, and importance. "
Cold-startup testing is scheduled to
begin in 2016, followed by fabrication of
the first fuel assemblies slated to begin in
2018. The MFFF will be licensed for 20
years and is expected to operate into the
2030s.
The Differences Between Radioactive Materials
Several pairs of radioactive substances are
discussed in this article. Here is a primer
on their different properties.
Highly Enriched Uranium and Plutonium.
There are important differences between
highly enriched uranium (HEU) and plutonium.
HEU can be diluted with other, more
abundant, naturally occurring uranium isotopes
to make low-enriched uranium (LEU),
which cannot sustain the fast-neutron chain
reaction needed for a nuclear explosion. LEU
fuel is used in most of the world's nuclear
reactors. In contrast, plutonium cannot
be diluted with other plutonium isotopes
to make it unusable for weapons. ''Re-enriching "
LEU for weapons requires complex
enrichment technology, while separating
plutonium from other elements and mixing
it with fresh reactor fuel requires only
straightforward chemical processing.
Weapons-Grade and Reactor-Grade
Plutonium. There is substantial worldwide
experience with the use and behavior of
reactor-grade (RG) plutonium, as it is created
during the fission process. U.S. reactors
were not specifically designed to
use weapons-grade (WG) plutonium (see
the table). The primary difference is the
percentage of the plutonium isotopes: WG
plutonium contains more plutonium-239,
while RG plutonium has more plutonium-240.
Weapons-grade plutonium is more
fissionable, but RG is more radioactive.
MOX and Low-Enriched Uranium. MOX
fuel and LEU fuel behave much the same in
reactors. MOX fuel assemblies and LEU fuel
assemblies are essentially identical with
70
Plutonium isotopic composition
comparison. Source: American Nuclear
Society
Plutonium
form
Pu-238
Pu-239
Pu-240
Pu-241
Pu-242
Weapons-grade
plutonium
(weight %)
92-95
5-7
0-0.5
0-0.05
Reactor-grade
plutonium
(weight %)
1-4
50-60
24-27
6-11
5-10
respect to mechanical design. Both MOX
fuel pellets and LEU fuel pellets consist of
sintered ceramic pellets that are predominantly
U-238 dioxide, and the respective
material properties are very similar.
However, the microstructures of the
two types of fuel pellets differ. LEU fuel
is a homogeneous mixture of U-238 dioxide
and U-235 dioxide, whereas MOX fuel
is more heterogeneous, with very small
plutonium-rich particles in a matrix of
depleted uranium oxide. The nuclear characteristics
of MOX and LEU fuel are also
different due to the nuclear cross-section
differences between uranium and plutonium.
Also, the decay heat from MOX is
slightly lower than that of LEU fuel, which
provides a safety benefit.
Fundamentally, MOX fuel is very similar
to LEU fuel. MOX fuel has been demonstrated
to perform well in commercial nuclear
power reactors. Fuel assembly, core, and
plant design practices can effectively accommodate
the minor differences that do
exist between the MOX and LEU fuel types.
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Building Fuel Assemblies
The MFFF is composed of two main process
operations: the aqueous polishing
process to remove impurities, such as americium
and gallium, and the MOX process,
which converts the plutonium and depleted
uranium into fuel pellets, fuel rods, and
fuel assemblies (Figure 2).
The process will begin with incoming
plutonium and depleted uranium received
in their respective shipping containers and
inventoried according to the MFFF material
control and accounting and radiation
protection programs. The material would
then be moved to the aqueous polishing
(AP) area.
Aqueous Polishing Process. Before
combining the surplus WG plutonium with
depleted uranium to produce ceramic pellets
for MOX fuel rods, the plutonium oxide
will be purified using an AP process,
equivalent to the process the French nuclear
industry has successfully used for over
30 years. The AP process removes impurities
such as gallium, americium, aluminum,
and fluorides and consists of three
major steps: dissolution, purification, and
conversion.
Plutonium oxide is dissolved in nitric
acid in the first step. Next, a solvent extraction
process removes impurities and
purifies the material. Then plutonium is
separated from the uranium. The solid and
liquid materials removed are recycled to
reduce waste volume. The final step converts
the purified plutonium stream back
to an oxide powder by precipitation and
calcination. The oxide powder is then homogenized,
sampled, and stored in cans for
future production of MOX fuel pellets.
MOX Process. The MOX process is a
mechanical process and consists of four
major steps: powder master blend, pellet
production, fuel rod production, and fuel
assembly production.
In producing the powder master blend,
polished plutonium oxide is mixed with depleted
uranium oxide and recycled powder/
pellet material. This mixture is micronized
in a ball mill and mixed with additional depleted
uranium oxide and recycled material
to produce a final blend with the required
plutonium content. A lubricant and poreformer
are added to control density.
Next, the final powder blend is pressed
to form " green " pellets, which are then sintered
in a furnace to obtain the required ceramic
qualities. The sintering step removes
organic products dispersed in the pellets
and the previously introduced pore-former.
The sintered pellets are ground to a specified
diameter and then inspected to verify
dimensions, density, markings, and appearPOWER
| December 2012
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POWER December 2012

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