Scientific American 201905

May 2019, ScientificAmerican.com 75

A fourth concept, from Lightbridge, a new U.S. market en­

trant, combines uranium and zirconium into a single, less reac­

tive alloy shaped like a licorice stick, a configuration that would

transfer heat better. The uranium would have to be enriched to

higher levels than are allowed today, so U.S. reg ulations would

have to change.

For decades utility owners have had difficulty gaining regu­

latory approval for any type of new fuel, but they are trying

again, sensing a need to compete with inexpensive natural gas

and increasingly abundant solar and wind power. U.S. owners

are getting design and manufacturing help from an extensive

nuclear research and development infrastructure, notably the

National Laboratories. Yet the effort is quickly becoming global.

In July 2018 scientists from the U.S. and the European Union

held a workshop at Idaho National Laboratory to discuss how

to best pool research on both continents. The Organisation for

Economic Co­operation and Development is developing a

framework for testing new fuels. If accident­tolerant fuels per­

form well, nuclear power could regain momentum in Japan,

where debate continues about how much of the nation’s reactor

fleet to restart.

Of course, significant hurdles must be cleared. Considerable

in­core testing of small fuel quantities and computer modeling

of performance, under both normal operating and accident

conditions, have to be done before new fuels are ready for

commercial use. Industry skeptics will have to be convinced

that the new materials will work as promised. More advanced

modeling techniques are coming online to aid this effort. Sim­

ulation technology at the U.S. Department of Energy’s Consor­

tium for Advanced Simulation of Light Water Reactors, based

at Oak Ridge National Laboratory in Tennessee, could sig­

nificantly speed up basic research, engineering development

and commercialization.

If data from trials are convincing, the U.S. fuel­supply chain—

from the fabrication shop to the reactor­refueling floor—would

have to retool, and plant processes and procedures would have

to be adjusted. Regulators would have to approve every step.

Rethinking fuel may be just the beginning of greater change.

Scientists and engineers are designing high­temperature gas­

cooled reactors that would use uranium particles wrapped in

exotic coatings; gumball­like pellets themselves would control

the nuclear reaction rather than control rods commonly in­

serted among fuel rods. Also underway are molten salt reactors,

in which the fuel and reactor coolant can be combined, allow­

ing simple mechanisms to prevent overheating.

The natural gas, solar and wind industries have changed

considerably in just a few years. The nuclear energy industry

may be ready to reinvent itself as well.

MORE TO EXPLORE

Advanced Fuel Pellet Materials and Fuel Rod Design for Water Cooled Reactors.

International Atomic Energy Agency, October 2010.

Accident Tolerant Fuel Concepts for Light Water Reactors. International Atomic

Energy Agency, June 2016.

FROM OUR ARCHIVES

The Fusion Underground. W. Wayt Gibbs; November 2016.

scientificamerican.com/magazine/sa

Add Chromium
Coating the zirconium cladding with a thin layer
of chromium can prevent the cladding from reacting
with water and producing hydrogen—the way
a coating of rust-proofing on metal prevents
oxidation. The coated cladding can also withstand
more heat and last longer. In addition, adding
chromium to the uranium oxide pellet helps to
prevent the fuel from cracking or deforming under
heat, making the entire fuel-rod assembly more
resilient under accident conditions and less likely to
release radioactive material. (Design: Framatome)

Incorporate Silicon

Changing the pellet material from uranium dioxide

to uranium silicide allows it to transfer more heat

to the surrounding water. The fuel can thus operate

at a lower temperature, which makes hydrogen

production and radiation release in an accident less

likely. The zirconium cladding can either be coated

with chromium (left) to reduce potential hydrogen

generation or be replaced with silicon carbide,

which does not react with water and is less likely

to crack or deform, further reducing accident risk

( right ). (Designs: Westinghouse Electric Company)

Twist the Rod

An aggressive redesign eliminates the pellet. The

fuel is uranium zirconium, bound to zirconium

cladding; the materials have similar expansion rates,

so no gap is needed. The fused fuel and cladding is

twisted to create a solid rod with more surface area

for transferring heat to water. (The displacer helps

to distribute heat evenly.) The enhanced transfer

allows the fuel to operate at a lower temperature,

which makes overheating and potential accident

conditions less likely. The uranium has to be en -

riched to 20 percent rather than 5 percent for the

other fuels shown. (Design: Lightbridge)

Fuel rod

Thin
chromium
coating

Chromium-

doped uranium

dioxide

fuel pellet

Helium-filled

gap

Zirconium
cladding

Fuel rod

Thin

chromium

coating

Uranium

silicide fuel

pellet

Helium-filled

gap

Zirconium

cladding

Displacer

Uranium-

zirconium

fuel

Zirconium

cladding

Fused fuel

and rod

Silicon carbide

cladding

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