20
NuclearPlantJournal.com Nuclear Plant Journal, May-June 2014
Research &
Development
Dry Storage Canister
EPRI has completed the initial set
of inspections of loaded stainless steel
dry storage canisters at three operational
dry storage facilities. The goals of these
inspections were to demonstrate the
accessibility of the canister surface and to
collect information on the environmental
conditions experienced by critical
canister elements over decades of storage,
especially the welds. This work was
partially funded by the U.S. Department
of Energy through a contract administered
by Idaho National Laboratory.
EPRI collaborated with Constellation
Energy Nuclear Group (Calvert Cliffs),
PSEG Nuclear (Hope Creek), PG&E
(Diablo Canyon), Transnuclear, and
Holtec International to obtain visuals,
temperature measurements, and surface
samples from the spent fuel storage
canisters. While laboratory studies
indicate that stress corrosion cracking
can occur in the materials used for dry
storage canisters when exposed to marine
environments, data from in-service
canisters are needed to evaluate such
behavior in real-world conditions.
Due to differences in the geometry
and design of the storage systems
evaluated, the inspection techniques
used for the first inspection (horizontal
system) differed from those used for the
second two (vertical systems). At Calvert
Cliffs, a high-definition pan-tilt-zoom
camera was lowered through the outlet of
the horizontal storage module (HSM) for
viewing the dry shielded canister surface
and the support structure and interior
surfaces of the HSM. Specially designed
tools were inserted in the gap between the
canister and the HSM for remote access
to the canister surface. Tools included a
thermocouple to measure the canister
surface temperature, a dry scraping tool
to collect surface samples directly from
the canister surface, and a wet collection
device that dissolved the surface deposits
and collected them through a wicking
device.
The systems at Hope Creek and
Diablo Canyon took advantage of the
vertical geometry by inserting a tool in
the outlet vent near the top, lowering it
into the annular gap between the canister
and the concrete overpack to the desired
location, and remotely actuating the tool
to contact the canister surface. Similar dry
scraping tools and a wet collection device
were used for sample collection. Each
tool had a thermocouple for temperature
measurement and an onboard camera to
visually examine the canister surface and
ensure good contact of the surface and
temperature devices.
The inspections revealed a coat of
dust on the upper surfaces of all canisters,
which was thicker for canisters in service
the longest. One canister exhibited a few
small surface rust spots on the base metal
that were believed to be from free iron
contamination, although no signs of rust
were seen on any of the welds in the field
of view. This is significant because the
welds are more prone to stress corrosion
cracking. The measured temperatures
generally
behaved
as
expected,
dependent on initial heat load and
location, with some differences between
measured and predicted. Although there
was a wide range in heat load (4–17
kW) and time in service (2–19 years),
some areas of all canisters were in the
temperature range for “deliquescence”
to occur, which is when the salts on the
surface absorb moisture from the air,
potentially creating concentrated brines
that can lead to corrosion. Measured
temperatures are providing important
benchmark information for best-estimate
temperature calculations to more
accurately predict when deliquescence
may occur. Laboratory analysis of the
collected surface samples indicate that
the chloride concentrations are low and
the cation/anion compositions are more
representative of rainwater, indicating
that the sites inspected are less marine, or
corrosive, than anticipated.
Contact: Keith Waldrop, telephone:
Source: Electric Power Research
Institute’s (EPRI) Nuclear Executive
Update, January 2014.
Refurbishment
Specifications
EPRI has developed several guidance
documents to assist nuclear plant owners
in working with vendors to define and
clarify refurbishment specifications for
equipment such as pumps and motors.
Refurbishing
equipment
may
be preferable to procuring a new
replacement, particularly in cases
where the equipment is obsolete and
replacement with a different model would
require significant resources. Long lead
time and significant engineering effort is
often needed to establish the suitability
of a new item through activities such as
prototype and equipment qualification
testing. A carefully defined refurbishment
specification can help assure a successful,
on-schedule outcome.
The initial urge to attach the
original equipment specification to a
purchase order should be re-considered.
Original specifications are not typically
updated to reflect modifications and
changes made by the plant subsequent
to initial procurement and installation.
In addition, original specifications do
not always identify the quality oversight
activities conducted by the purchaser
during the original procurement, can
include information not relative to a
refurbishment, typically omit information
that is relative to a refurbishment, and
may include references to dated standards
and obsolete materials.
Development of refurbishment
specifications often requires active
communication with the supplier to
ensure the supplier understands the
technical, quality oversight, and schedule
requirements. Generic specifications
developed by EPRI through the Large
Electric Motor Users Group and the
Pump Users Group for the refurbishment
of medium-sized motors and pumps
may provide a good starting point.
Key considerations when preparing
refurbishment specifications include:
• Provisions for design modifications
and changes that were implemented
after the component was provided
in accordance with the original
specification.
• Considerations for wear resulting
from service (original dimensional
information may no longer apply).
• Verification of clearances, specified
tolerances, and alignment to ensure
that the new parts mate and interface
correctly with the original assembly
or component.
