Nuclear Plant Journal, March-April 2016 NuclearPlantJournal.com
19
somewhat in the pathways available for
probe access. The EPRI team’s goal is
to develop generic robots that will work
with most casks and can carry various
inspection tools for emerging cask
inspection standards. Plans include more
tests on cask mockups and, by 2018, the
first-ever inspection of a canister loaded
with spent fuel using probes able to detect
early signs of metal degradation.
"How and Where" of
Aging
For decades, EPRI has conducted
research in aging management for
multiple aspects of plant operations.
“EPRI research has helped form the
technical basis for aging management of
significant reactor internal components,”
said Randy Stark, EPRI’s director of
Fuels, High-Level Waste, and Chemistry.
In April 2015, NRC asked the
American Society of Mechanical
Engineers (ASME) to develop a consensus
guideline for spent fuel storage cask
inspections, and EPRI agreed to support
the ASME efforts. “Our charter is to
identify whether degradation mechanisms
exist in the stainless steel canisters, and
which locations are most susceptible
to these potential mechanisms,” Stark
said. “Once this is understood, aging
management programs for these canisters
can be better developed.”
EPRI had already started examining
longer-term storage safety in 2009 when
it established the Extended Storage
Collaboration Program with industry and
government experts. “The program has
grown to more than 400 members from
16 countries, all seeking to answer the
same question: What needs to be done to
ensure the safe, extended storage of spent
fuel?” said EPRI Senior Technical Leader
Keith Waldrop.
Aging management for storage casks
should be far simpler than for reactors,
Machiels said, because casks have no
moving parts. A typical cask consists of
a basket of fuel assemblies, sealed with
a chemically inert gas inside a shielded
stainless steel canister, which in turn is
placed in a concrete overpack. Small air
vents in the overpack carry off residual
heat from the fuel. Casks range from
about 18 feet to more than 20 feet high,
have diameters of 7 to 12 feet, and can
weigh more than 150 tons with fuel
inside. Canisters are stored vertically or
horizontally.
But cask inspection is not a simple
matter. Casks were not designed to be
inspected; at the time, operators expected
a repository to open in a couple of decades,
according to EPRI Program Manager
Richard Reid. “They were designed to be
sealed and transferred to DOE,” he said.
“Getting inspection equipment inside
the overpack to the canister surface is a
serious technical challenge.” Heat vents
can be used for access, but they measure
just a few inches across. The space
between the canister and the overpack
is at most a few inches wide, and some
designs have vertical fins that restrict
movement in that space. “There is a need
for not only small probes to examine the
canister surface, but also small robotic
devices to carry them there,” said Reid.
EPRI showed progress on this front in a
recent demonstration.
In 2014, researchers made their first
on-site inspections—including visual
scans, temperature measurements, and
surface samples—of two casks that had
been in service for more than 15 years at
Exelon’s Calvert Cliffs Nuclear Power
Plant, on the shore of the Chesapeake
Bay. The two canisters exhibited no
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