Note: Descriptions are shown in the official language in which they were submitted.
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CONTAINER FOR STORAGE, TRANSPORTATION AND DISPOSAL OF USED
NUCLEAR FUEL, AND OTHER RADIOACTIVE NUCLEAR FUEL WASTES.
1.0 SUMMARY OF THE INVENTION
This invention is directed to a container for packaging used nuclear fuel and
other
radioactive materials for the purpose of storage, transportation and disposal.
It comprises a
metallic vessel of cylindrical shape including an insert, also of metallic
construction, which is
intended to maintain used nuclear fuel bundles or other radioactive materials
in a specific
geometric arrangement in ail orientations, and to assist in conducting the
heat produced by
the radioactive materials to the periphery of the container. The container's
outer shell and
the container lid, including the lid closure weld, provide a permanent, high-
integrity
containment barrier for the radioactive materials. In addition to providing a
sealed boundary
that prevents leakage of radioactive contaminants, the said container protects
the integrity of
the used nuclear fuel and fuel wastes from the effects of mechanical impact
and vibration
and from degradation by chemical agents.
The said container is designed to be a core component of storage systems,
transportation systems and disposal systems for used fuel and nuclear fuel
wastes.
Accordingly, it will be referred to in this Summary and all associated
documents as the "STD
Module", or simply "the Module".
The suitability of the Module as a core component of storage, transportation
and
disposal systems enables it to serve as the basis for an integrated approach
to managing
the used fuel discharged from CANDU nuclear reactors, right from the time of
its removal
from the irradiated fuel baythrough to its final disposal in a deep geologic
repository. Once
used fuel bundles and fuel wastes are loaded in the Module they will not need
to be
individually handled for re-packaging. The fuel bundles or nuclear fuel wastes
packaged in
the Module can be placed in an on-site storage facility, or placed in a
transportation cask to
be moved off-site, or placed into a container designed for final disposal of
the waste, while
remaining sealed in the STD Module.
The Module capabilities result in substantial economic advantages, derived
from the
increased simplicity of the facilities and processes required for long-term
management of the
nuclear fuel wastes. They Module capabiiities result also in increased safety
of operations,
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since the waste will remain permanently sealed within the Module. They also
result in
increased flexibility for the development and implementation of long-term
plans for nuclear
fuel waste management.
2.0 CURRENT PRACTICE
In CANDU nuclear power stations, the fuel discharged from the reactors is
directly
transferred to irradiated fuel bays where it is kept for a minimum period of
ten years. The
water in the fuel bays provides thermal cooling for the fuel and provides
shielding from the
radiation emitted by the fuel. After the cooling period, current practice is
to remove the fuel
from the bays and place it in dry storage at the reactor sites. Two different
methods are
currently used in Canada for dry storage. At Ontario's commercial power
plants, dry storage
containers (DSCs) as described in CP 2014065 [1 ] are used. At the New
Brunswick and
Quebec utilities and at spent fuel storage facilities owned by Atomic Energy
of Canada
Limited (AECL) the fuel is packaged in fuel baskets, which are stored at the
reactor sites in
fixed shielding structures made of reinforced concrete. These two methods of
storage are
briefly described below.
At Ontario's nuclear utilities, after being discharged from the reactors, used
fuel
bundles are placed in storage trays with a capacity of 24 bundles, which are
stacked inside
the irradiated fuel bay. Prior to being loaded into DSCs, fuel bundles must be
transferred
from the trays into a steel structure called storage module, which holds 96
fuel bundles
within an array of 48 horizontal steel tubes held by a steel frame. The
storage module holds
the fuel bundles inside a DSC but it does not provide a containment barrier,
the fuel bundles
simply sit in the open tubes. Containment during storage is provided by the
DSC inner liner.
2.1 DSC LOADING OPERATION
Loading of storage modules into a DSC is done inside the fuel bay. The DSC is
submerged in the bay water and lowered onto an impact pad placed at the bottom
of the
bay. Then, a crane provided with a special tool loads four modules one by one
into the
DSC. When the loading operation is complete, the DSC is lifted from the bay,
drained, and
vacuum dried. After this, a temporary lid is placed on the container and a
specially designed
vehicle moves the DSC to a storage facility within the reactor site, where the
container is
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vacuum-dried a second time and filled with helium before the permanent lid is
sealed by a
multiple-pas weld. The DSC loading operation is described in further detail in
Reference 2.
The weight of a fully loaded DSC (384 fuel bundles) varies in a range from 64
to 80
tons, depending on the density of the aggregate used for the concrete walls
which provide
shielding for the radioactive load. The DSC is not easily transportable
because of its size
and weight, and it is not suitable for disposal in a deep geologic repository
for several
reasons, but primarily because its containment barrier does not meet the
service life
requirements of a disposal container. Therefore, the fuel bundles stored in
DSCs will need
to be removed and re-packaged at least once before they are placed in a
disposal facility.
This requires cutting open the DSC lid and handling the bundles one by one in
a hot cell
facility.
2.2 STORAGE BASKET LOADING OPERATION
In Quebec and New Brunswick nuclear utilities and at AECL facilities, used
fuel
bundles stored in trays at the irradiated fuel bays are loaded one by one in
storage baskets,
within the fuel bays. After each basket is loaded, it is removed from the bay,
its load is dried
and the lid is welded in place. Then the basket is transferred to a concrete
shielding
structure for storage at the reactor site. The structure providing shielding
for reactor-site
storage of the baskets are either concrete silos or a large volume storage
facility called
CANSTOR, supplied by AECL. The storage baskets and the shielding structures
are
designed for a service life of 50 years. Recent studies, however, have
determined that
under certain conditions the service life of this storage system can be
extended up to 100
years. The storage baskets, however, do not meet the requirements for
transportation of
the spent fuel. Transportation of the fuel bundles in these baskets would
result in damage
to the fuel. This would present a serious problem at a later time, when the
fuel needs to be
re-packaged for disposal.
3.0 PROPOSED INNOVATION
The proposed alternative approach described here to existing methods for long-
term
management of used CANDU fuel consists in loading the used fuel bundles
directly from the
irradiated fuel bay into STD Modules. The STD Modules can be constructed to
have a
service life of hundreds of years, and their design meets the requirements for
transportation
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of used CANDU fuel. They are also compatible with current designs of used fuel
disposal
containers. Therefore, once used CANDU fuel bundles and nuclear fuel wastes
are loaded
into an STD Module they can remain sealed inside the Module throughout the
remaining
steps of the fuel cycle. Once the Module is sealed, it becomes the basic
handling unit for
the nuclear waste. It constitutes a practical modular package providing a
permanent
containment barrier for the nuclear waste, which will not have to be re-
packaged either for
transportation or for disposal in a deep geologic repository.
4.0 ADVANTAGES OF THE INNOVATION
The advantages of the proposed approach for managing used CANDU fuel and
nuclear fuel waste with respect to existing methods are discussed below.
The STD Module constitutes a suitable container for use as a core component of
the
most cost efficient systems currently in use for storing used CANDU fuel, such
as concrete
silos and the CANSTOR system, both designed by AECL. Both, CANSTOR and
concrete
silos provide radiation shielding and a secondary containment for fuel stored
in existing fuel
basket designs. Monitoring of the secondary containment provides indication
throughout the
dry storage period of the status of the primary containment. The benefits of
using the STD
Module instead the existing fuel basket designs are that: a) the used fuel
doesn't need to be
re-packaged for transportation, b) the tighter packaging of the fuel wastes
and better heat
conduction capabilities result in a better fuel temperature profile within the
module, which
helps to prevent fuel degradation and c) the Module has a much longer service
life which
works together with its compatibility with transport and disposal systems,
resulting in
enhanced flexibility for the planning stage and higher efficiency in the
implementation of
long-term management of nuclear fuel wastes.
The Module is suitable for transportation of the fuel, via public roads or
rail, to a
remote facility, either for extended storage or disposal of the nuclear fuel
waste. For
transportation, the STD Module must be placed in either a road or a rail cask
which would
provide radiation shielding as well as protection against impact and fire
during transport.
The primary advantage of the STD Module with respect to used fuel baskets is
that fuel and
fuel wastes will not have to be re-packaged. In contrast, used fuel
transported in existing
fuel baskets, in either vertical or horizontal orientation, would be expected
to sustain
damage, simply because the baskets were not designed to protect the fuel
bundles from
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mechanical damage during transport. Therefore, receiving and handling of fuel
baskets
needs to be conducted in a complex hot cell facility where the baskets are cut
open and the
fuel wastes removed from the baskets and re-packaged, since the baskets are
not suitable
for disposal of the fuel.
The road casks or rail casks used for transport of STD Modules would be
completely
re-usable. Under normal conditions, the Module surfaces will be free of loose
contamination, therefore its removal from the transport cask would be a simple
operation
conducted in a shielded facility. After routine safety checks the transport
cask would be
returned to the point of origin and it would be ready to continue Module
shipments. In
contrast, used CANDU fuel stored in DSCs presents difficulties in the steps
that follow dry
storage. Whether the fuel is transported in DSCs, or re-packaged for
transport, either a
dedicated fuel bay or a hot cell facility would be required for the process of
cutting the DSC
open, removing the fuel storage modules and re-packaging the fuel. DSCs are
essentially
non-reusable, at the end of their service life they become a large mass of
radioactive waste.
The dilemma of whether the fuel is removed from DSCs and re-packaged at the
reactor-site or DSCs are transported first and retrieval and re-packaging of
the used fuel
takes place at a remote facility is an question of economics and logistics. In
either case
complex automated systems operating in large hot cells are required to re-
package the
nuclear fuel waste and large volumes of contaminated waste are generated in
the form of
several thousands of opened DSCs. In contrast, transporting the STD modules,
where the
used fuel bundles and fuel wastes remain permanently sealed is a much simpler
and
cleaner operation. Placing STD Modules in disposal containers is also a
comparatively
simple operation.
Redundant levels of safety, longer service life, elimination of the need for
re-
packaging the fuel, compatibility with re-usable casks resulting in the
elimination of large
volumes of radioactive wastes, elimination of radioactive contamination, and a
dramatic
simplification of the interFaces between fuel storage, transportation and
disposal of the
CANDU fuel wastes are all advantages derived from packaging used CANDU fuel in
STD
Modules at the irradiated fuel bays. In a project of the scale of disposal of
the Canadian
inventory of used nuclear fuel, the savings accrued by packaging the fuel in
STD modules
are in the billions of dollars.
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STD MODULE GEOMETRY
Figure 1 shows the geometry and dimensions of a typical CANDU fuel bundle.
Figure 2 shows a cross-section of the fuel array in a loaded STD Module. The
Module has
essentially a cylindrical symmetry with each of the fuel bundles occupying one
space in a
honeycomb pattern. The hexagonal pattem, which constitutes the geometry of the
Module
insert, provides the most efficient packing of the CANDU fuel bundles. A
series of hexagonal
arrays with increasing numbers of cells would provide even higher packing
efficiency,
however, thermal considerations related to the disposal container make a set
of more than
sixty bundles unsuitable. Smaller bundle arrays, although suitable for the
purpose, result in
poor economics for a deep geologic repository system. Therefore, the 54-bundle
array
illustrated in Figure 2 constitutes the reference cross-section geometry for
the STD Module.
Figure 3 illustrates one embodiment of the STD Module consisting of two layers
of 54
fuel bundles in the geometry described above. The Module can have a capacity
of 54
bundles or, theoretically, any multiple of 54, however, thermal and mechanical
considerations make 54-bundle and 108-bundle capacities the most practical
configurations.
The same principle of using hexagonal cells for packaging CANDU fuel bundles
can
be used for packaging used nuclear fuel or fuel wastes of different sizes and
geometry. The
only geometric constraint that applies is that a Module or a set of Modules
should fit inside
the disposal container. Other constraints that might apply to the detailed
design of the
Module type used for a specific kind of nuclear waste (such as research fuel)
are related to
heat dissipation, criticality and handling.
The Module components that constitute the containment boundary are connected
by
full welds. All of the components, except the lid, are welded at a Module
manufacturing
facility. The lid is piaced in the Module at the irradiated fuel bay and
sealed by an automatic
machine weld inside a shielded enclosure after the Module is removed from the
fuel bay.
Some of the lid features include two vent ports, which may be used to purge
the Module's
inner volume after sealing, and spacers which are needed as interfaces with
other modules
and systems at different stages of its service life.
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MATERIALS OF CONSTRUCTION
The durability of the Module depends on the materials selected for its
construction.
Appropriate materials can be selected to meet the required service life as
well as any
requirements dictated by interfacing systems. For example, an external shell
made of
stainless steel can provide a reliable primary containment with a service life
of hundreds of
years. However, the STD Module design can be implemented using any materials
of the
group specified in the Claims document, to meet specific requirements defined
by the user.
The Module inserts may also contain neutron absorbers to reduce the reactivity
of the used
fuel or fuel wastes and prevent criticality.
REFERENCES
1. Canadian Intellectual Property Office, Patent No. 2,014,065. "Metal-Clad
Container for
Radioactive Material Storage".
2. "Pickering Waste Management Facility Safety Report". An Ontario Power
Generation
Report to the Canadian Nuclear Safety Commission (periodically updated in
compliance
with the requirements of applicable Canadian Nuclear Safety Regulations).
