POWER March 2015 - 52

FUELS
Nuclear Industry Pursues New
Fuel Designs and Technologies
Enriched uranium, the most commonly used fuel in commercial nuclear reactors
worldwide, has many well-known advantages; however, recent
events have underscored its disadvantages. Can new fuel technologies be
developed and proven effective fast enough?
Kennedy Maize
L
ate last year, Japanese engineers and
technicians accomplished a major milestone
nearly four years after the most
damaging light-water reactor accident in
history at the Fukushima Daiichi nuclear station.
They removed the last of the undamaged
fuel rods from the devastated Unit 4 reactor
building, some 1,500 assemblies stored in a
pool above the reactor (Figure 1). Hydrogen
explosions shortly after the Mar. 11, 2011,
earthquake and tsunami nearly destroyed that
reactor, while it did destroy three others.
Removing the fuel from reactor 4 was the
easiest task related to the radioactive fuel at
the plant. Units 1, 2, and 3 suffered complete
fuel meltdowns. The New York Times commented,
" These reactors were so damaged-
and their levels of radioactivity remain so
high-that removing their fuel is expected to
take decades. Some experts have said it may
not be possible at all, and have called instead
for simply encasing those reactors in a sarcophagus
of thick concrete. "
Among the advantages of nuclear power,
fuel is one of the more important. Uranium
packs a lot of punch and is widely available.
Atomic power plants offset high capital
costs with low fuel costs. But nuclear fuel
presents big challenges because of the enormous
amount of energy-and heat-packed
into a small package. Even when nuclear
fission stops, as happens in conventional
light-water reactors during a loss-of-coolant
accident, enormous heat remains. Nuclear
fuel rods can melt, in the worst case into a
glowing, radioactive blob in the bottom of
the reactor vessel.
The Pros and Cons of Zirconium
Cladding
Conventional fuel-enriched uranium oxide
pellets in long rods encased in metal-offers
the first line of defense in a reactor accident.
Metal cladding housing the uranium is designed
to protect the fuel's integrity while
52
emergency cooling equipment removes residual
heat. AREVA explains on its website,
" Zirconium cladding is the reactor's primary
safety barrier. Zirconium is the leading material
for nuclear fuel assemblies used in light
water reactors . . . because it is transparent
to neutrons, it has good temperature performance,
and it withstands corrosion. "
But experience has shown, first at Three
Mile Island (TMI) near Harrisburg, Pa., in
1979 and again, in spades, at Fukushima, that
this first line of defense can fail.
At TMI, as the events of a small loss-ofcoolant
accident progressed, General Public
Utilities operators were confident that the
fuel would not melt. As fuel damage became
obvious, plant officials consistently underestimated
the extent of the damage. According
to the written material accompanying the
TMI exhibit at the Smithsonian Institution's
Museum of American History, plant officials
always took an optimistic view when trying
to understand the unfolding picture of the
fuel damage. In the end, it was clear that a
majority of the fuel had melted down and the
accident completely destroyed the core. Yet,
the conventional wisdom about the accident
is that the reactor experienced a euphemistic
" partial meltdown, " because less than 100%
of the fuel melted.
At Fukushima, there is no challenge to the
observation that the three reactors suffered
complete fuel melting. But that may not have
been the worst event in the accident. A 2013
news release from the Massachusetts Institute
of Technology (MIT) observes that the
zirconium alloy cladding of the Fukushima
fuel played a major role in the events at the
multi-reactor site. Noting the series of spectacular
hydrogen explosions at the site, the
MIT release says that " hydrogen buildup was
the result of hot steam coming into contact
with overheated nuclear fuel rods covered by
a cladding of zirconium alloy, or 'zircaloy'-
the material used as fuel-rod cladding in all
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POWER | March 2015
water-cooled nuclear reactors, which constitute
more than 90 percent of the world's
power reactors. When it gets hot enough,
zircaloy reacts with steam to produce hydrogen,
a hazard in any loss-of-coolant nuclear
accident. "
The Hunt for New Fuel Assembly
Options
MIT researchers are working on a ceramic
cladding for enriched uranium fuel pellets to
offer characteristics similar to zircaloy while
reducing the risks of hydrogen evolution " by
roughly a thousandfold. " MIT's focus is on
silicon carbide (SiC). Mujid Kazimi, the
TEPCO (Tokyo Electric Power Co., owner
and operator of the Daiichi plant) professor
of nuclear engineering at MIT, who is lead1.
Old fuel, new pool. This Nov. 22,
2013, photo shows a fuel rod removed from
the destroyed Unit 4 at the Fukushima Daiichi
nuclear station being moved to the common
pool elsewhere on the site. Courtesy: Tokyo
Electric Power Co.
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POWER March 2015

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