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| | Nuclear
Fuel Drying and Storage
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Vacuum
Technology is a vital part of the nuclear industry. Large high-speed
diffusion pumps, baffles, ion and getter pumps, sealing techniques, vacuum
measuring instrumentation and vacuum hardware were largely evolved in
nuclear work.
The
electricity generated by nuclear power emits virtually no greenhouse-gas
causing emissions. The by-product of electricity generated from nuclear
power is nuclear waste, which is managed in a contained and controlled
manner. Vacuum is helpful here.
When the fuel
bundles are removed from the reactors, they are radioactive and need to be
managed safely and responsibly for an extended period of time. The first
step is to cool the fuel bundles under water in specially engineered used
fuel bays in the station.
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Dry
Storage
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The term
“dry storage” comes from the fact that the used fuel is stored in a
dry state, contained and shielded by the concrete and steel in the Dry
storage containers (DSCs) and not by water as in the used fuel bays.
The used fuel bundles have been stored in the water filled bay for at
least 10 years, during which they have cooled and become less radioactive.
The lid of the container is then installed and secured to the base with a
clamp. The container, now holding 384 used fuel bundles is removed from
the bay, drained, decontaminated and vacuum dried.
After the
inside of the container has been vacuum dried, it is filled with helium
gas. The drain port is then seal-welded. The helium gas provides a means
of leak detection for the sealed container and creates an inert atmosphere
for the stored used fuel. Before being placed into storage, the container
undergoes rigorous testing to ensure that it is absolutely leak tight.
Prior to placing the container into storage, safeguard seals are applied
by an inspector from the International Atomic Energy Agency.
Dry
storage containers
(DSCs) are extremely robust and provide an effective barrier against
radiation. Each DSC is made of reinforced high- density concrete
approximately 510 mm (20 inches) thick and is lined inside and outside
with 12.7 mm (half-inch) thick steel plate. |
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Fuel
elements / Fuel rods:
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Fuel elements
are metal tubes filled up with nuclear fuel (Pellets of uranium
dioxide or a mixed uranium/plutonium dioxide) and Helium (to allow
better heat conduction)
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They are
made out of corrosion resistant metals as Zirconium (Zircaloy) and
have the function to prevent that radioactive fission products (such
as noble gases) may enter the cooling water surrounding the fuel
elements
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Their
length and diameter varies, depending on the specific demand of the
power plant. One typical length is 4,17 m with a diameter of 11 mm.
Their typical wall thickness is app 0,6–0,8 mm.
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As the
fuel fissions, the radioactive fission products are also contained by
the cladding, and the entire fuel element can then be disposed of as
nuclear waste when the reactor is refueled. |
Fuel
bundles:
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Fuel
elements are assembled into bundles for handling and to allow good
cooling.
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There are
about 179-264 fuel elements per fuel bundle and about 121 to 193 fuel
bundles are loaded into a Pressurized Water Reactor (PWR) core.
Generally, the fuel bundles consist of fuel rods
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Heavy
water reactor types as the Canadian CANDU reactors use different forms
of Fuel bundles |
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"Castor“ is the abbreviation for „Cask for storage and transport
of radioactive material“. They
are special casks for storage and transport of highly radioactive
substances, e.g. fuel-elements.
The casks are app. 6
m long and have a diameter of app. 2,5 m. The wall thickness is 45 cm. |
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Used” fuel bundles (Which still contain 95% active Uranium) are stored
inside the Castor cask for the transport to their final storage location.
The fuel elements inside the Castor still produce app. 1 kW energy and
have a temperature of app. 60°C.
The containers - called Dry Storage Containers (DSCs) - are engineered to
last at least 50 years and will provide safe, interim storage until a
long-term management pro- gram is in place. This dry storage process is a
proven, safe and regulated technology.
The nuclear reactor core is a lead housing for fuel elements and control
elements
The core is filled with water, which removes the heat of the fission
reaction but also acts to moderate the neutron reactions. |
Before
sealing the cask, the wet fuel bundles need to be dried. For this vacuum systems
are used!
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Drying
of fuel elements / Castor casks
Simplified process description:
1.Pre-Drying
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Rough
drying to remove liquid water
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Pump
down to 6 mbar only (at deeper pressures build up of ice in CASTOR is
possible)
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Total
time: several hours
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Ice
is build up in Condensate lance and particle filter
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2. Fine
Drying
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Direct
connection of vacuum system with vessel
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Pressure
maintained at 6 mbar (at deeper pressures build up of ice in CASTOR is
possible)
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Total
time: 2-5 days
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Total
amount of water removed: 10-60 liter
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Removed
water will be collected, measured and logged
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Sometimes
Helium is introduced for faster and better drying. In this case the
drying pressure is 0,5 mbar. |
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Typical Vacuum equipment:
1.
Conventional Standard System
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2x
SV300 (2nd pump only for redundancy purpose)
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Pump-size
based on customers pump-down requirement for empty vessel
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2. System for Process with Helium Drying step
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1x
WS251, operated with frequency converter
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2x
SV300 (2nd pump only for redundancy purpose)
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Drying
time reduced by app. 1 day
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3. Special System for higher vapor load
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2x
SV300 (2nd pump only for redundancy purpose)
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8
m˛ Inlet condenser, cooled with 7°C cooling water
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Designed
for ≤ 180 kg per drying batch
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For
further information about using vacuum in the nuclear industry, please contact Osanak in our
applications support department; o.mir@vpcinc.ca. |
