POWER November 2015 - 63

NUCLEAR TECHNOLOGY
ant. Japan's Monju reactor also closed after
similar problems, including a nasty coolant
fire and a utility and government cover-up.
Last May, the governments of France and
Japan announced they would move forward
with Astrid, aimed at developing a demonstration
breeder burning spent nuclear fuel
at CEA's Marcoule site near Avignon on the
Rhone River. In 2011, France put up $900
million to fund Astrid through 2017. Japan
Prime Minister Shinzo Abe and French
President Francois Holland signed a joint
agreement to " intensify their civilian nuclear
research, " while both countries were restructuring
their nuclear programs. Japan is still
recovering from the Fukushima disaster,
which led to the shutdown of all of the country's
nukes, now slowly coming back into
service. France, meanwhile, is aiming to reduce
its dependence on nuclear, from 75% of
its electricity production down to 50%.
According to the World Nuclear Association,
" Astrid is envisaged as a 600 MWe prototype
of a commercial series of 1500 MWe
[sodium fast reactors] which is likely to be deployed
from about 2050 to utilise the half million
tonnes of [depleted uranium] that France
will have by then and also burn the plutonium
in used MOX fuel. Astrid will have high fuel
burnup, including minor actinides in the fuel
elements, and while the MOX fuel will be
broadly similar to that in PWRs [pressurized
water reactors], it will have 25-35% plutonium.
It will use an intermediate sodium coolant
loop, and the tertiary coolant is nitrogen with
Brayton cycle. " CEA says a final decision
on whether to build the Astrid prototype will
come in 2019. As is the case with all advanced
nuclear technologies, the economics of commercial
plants are entirely unknown.
Hot, Hot, Hot
One weakness of LWRs is the low quality of
their steam, which reduces the efficiency of
the plant. Typical steam outlet temperatures
for PWRs are under 400C, which compares
unfavorably to modern ultrasupercritical
coal-fired plants that operate with steam temperatures
above 600C.
Very-high-temperature nuclear reactors
with outlet temperatures around 1,000C are
possible and have actually been demonstrated,
although not commercially in the U.S. The
AEC and the Philadelphia Electric Co. teamed
up on a high-temperature gas reactor (HTGR),
a 40-MW graphite-moderated, helium-cooled
design originated by General Atomics in the
1950s. The unit located at the company's
Peach Bottom site operated well from 1967 to
1974, with a lifetime capacity factor of 75%.
As a result, Public Service Co. of Colorado,
with AEC financial backing, ordered a
300-MW HTGR based on a scale-up of Peach
November 2015 | POWER
Bottom-the Fort St. Vrain plant. But that
plant operated poorly from the day it opened
in 1979 until it shut down just a decade later.
Both Peach Bottom and Fort St. Vrain used
large prism-shaped fuel blocks of enriched
uranium surrounded by a graphite moderator.
Since then, much time, effort, and money
have gone into attempts to revive HTGRs,
mostly focused on a General Atomics design
that uses billiard ball-sized " pebbles " consisting
of many 9-mm uranium fuel spheres
embedded in graphite (the moderator), surrounded
by a ceramic coating. These are
known as " pebble bed reactors. "
According to noted nuclear engineer Andrew
Kadak, an HTGR is fundamentally different
from LWRs. The differences include
higher thermal efficiencies, an inert and
noncorrosive helium coolant, lower water
requirements, the use of gas turbine technology,
and a less-complicated design, because
there is no emergency core cooling system
like those required in LWRs.
Philadelphia Electric, before it was acquired
by Exelon Corp. in the early 2000s,
spent some $20 million for a 12% share in a
joint venture with South Africa's state-owned
Eskom utility to develop a commercial 110MW
pebble bed HTGR, aimed both at South
African and U.S. markets. After Chicagobased
Exelon took over, spending on the
project stopped in 2002. Eskom continued
work but dropped it in 2010, citing " run-away
costs and technical problems. " According to
Kadak, among the disadvantages of HTGRs
are the poor operating history, " little helium
turbine experience, " and " licensing hurdles
due to different designs. "
Yet industry interest in the HTGR concept
remains. A consortium of New York utilities,
the New York State Energy Research and Development
Agency, National Grid, and South
Africa's Pebble Bed Modular Reactor Co. in
2013 pitched the design into the U.S. Department
of Energy's (DOE's) second-round competition
for financial support for small, modular
reactors (SMRs). That bid was unsuccessful,
losing out to NuScale's more conventional
LWR-based SMR technology. NuScale is now
negotiating with the DOE over terms and conditions
for a cooperative funding agreement.
Nuclear reactor designers have produced a
wide variety of fascinating concepts for alternatives
to the light-water technology that is the
world's go-to choice for atomic energy. But
these exotic designs, no matter how elegant, exist
mostly on paper and may not be practical or
economic. For now, LWRs rule the real world. ■
-Kennedy Maize is a frequent contributor
to POWER.
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POWER November 2015

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