Note: Descriptions are shown in the official language in which they were submitted.
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CONTAINER FOR- NUCLEAR FUEL;TRANSPORTAT10N
This invention concerns improvements in and relating to fuel
transportation, particularly but not exclusively relating to
enriched nuclear fuels.
Nuclear fuels, such as enriched uranium or mixed oxide forms,
frequently need to be transported between sites, for instance
the enrichment site and the fuel rod production site. The fuel
is normally in the form of pellets or powder at this stage.
International standards apply, requiring certain levels of
thermal insulation and structural strength. A major concern is
criticality control. The mass of enriched fuel within the
transport container must be strictly limited to ensure that a
criticality event does not occur. This single requirement
places astringent limit on the volume of fuel which can be
transported in any given volume of a transport container. In
this regard, the transportation of nuclear fuel differs
significantly from transportation of other radioactive
materials. Radioactive waste is of a far lower enrichment,
thus facilitating transport of greater volumes in proximity
with one another. In assembled fuel rods on the other hand,
the volume of~fuel when compared with the overall fuel rod and
supporting structure volume is very low.
Present systems usually consist of a cylindrical drum provided
with one or more layers of wood on all sides, the wood defining
a central recess into which a single cylinder containing the
enriched fuel is placed.
The fuel containing volume of the inner drum is very low
compared with the volume of the outer drum. As a consequence
the transportation of fuel takes up a considerable amount of
space. The commercial considerations of this apply as they do
. to any transportation procedure. Additionally the cylindrical
nature of the unit presents handling and stability problems.
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According to a first aspect of the invention we provide a
transportable container for nuclear fuel, the container
comprising an outer container provided with internal
insulation, the insulation defining an internal cavity, the
cavity receiving a plurality of fuel containers, wherein the
internal volume of the fuel containers is at least 5% of the
external volume of the outer container.
A container having this level of fuel volume to overall volume
has not previously been achieved. The present invention also
allows this level to be reached whilst meeting the necessary
criticality, insulation and other standards.
Preferably the internal volume of the fuel container is at
least 10% of that of the external volume of the outer
container. A level of at least 15%, 20% or 25% is preferred.
Levels of at least 30%, 35% or even 40% may be reached. Any
increase in fuel volume to overall container volume is
significant in reducing transportation costs and the capital
costs involved in providing the strong fuel containers.
Preferably the outer container is formed with a steel and most
preferably stainless steel skin. The corners and/or edges of
the outer container may be provided with strengthening
elements. These may take the form of L-shaped sections. The
outer container is preferably provided with feet.
The outer container is preferably provided with a lid. The lid
is preferably releasably fastened to the outer container.
Clamps attached to the outer container and releasably
engageable with the periphery of the lid are preferred. The
clamps may also be releasably engaged with the outer container.
The lid may be provided with handles or other forms of
engagement for removal of the lid.
It is particularly preferred that the lid be received within
the perimeter of two or more projections from the outer
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container. The strengthening elements projecting above the top
of the container may define this perimeter.
Preferably the insulating material is provided in a series of
. discrete layers. One or more base layers and/or one or more
wall layers for each wall may be provided. The lid insulation
may be mounted on the metal lid or may be provided separately.
If provided separately a pair of interlinking sections may be
provided.
The insulating layer is preferably thermally insulating and/or
neutron absorbing. Calcium silicate offers a preferred
insulating material. One or more different materials may be
used together or in a sandwich style structure.
Preferably the insulation layer defines the boundaries of a
single internal cavity. A rectilinear cavity is preferred.
The internal cavity is preferably provided with a
correspondingly shaped single unit internal container
comprising four side walls and a base. The internal container
is preferably made of steel, boronated steel, or most
particularly stainless steel.
In one form the internal container is preferably divided up
into a series of chambers. The chambers may be defined by one
or more elements crossing the internal cavity or container.
Preferably the elements are plates spanning the full height, or
at least substantially the full height, of the internal volume.
Preferably one or more elements span the internal volume in
different directions, most preferably at substantially 90
degrees to one another. Preferably the plates are
substantially vertically provided. It is particularly
preferred that two plates cross the internal cavity in each of
two directions at 90 degrees to one another. Preferably the
internal volume is divided up into nine substantially
equivalent chambers.
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In a second alternative form, the internal cavity may be fitted
with elements such as plates spanning the full height of the
internal volume to define an internal container. The chambers
again being defined by one or more elements crossing the
internal cavity. A base plate may be provided on the base
insulating layer to define a base for the internal volume. A
top plate may also be provided. Side plates may also be
provided to define the sides of the chambers.
One or more of the base, top or dividing elements or plates may
be formed of metal. Steel and in particular stainless steel or
boronated steel.
The base, side and dividing plates or elements of the single
unit internal container is alternatively provided as a separate
unit to the insulating layers and outer container.
In a further alternative form the internal cavity may be fitted
with a plurality of sleeve elements. Preferably the sleeve
elements are adapted to receive fuel containers or pails. The
sleeves may be continuous or substantially continuous.
Preferably the sleeves are of circular cross-section.
Preferably the sleeves internal diameter is substantially
equivalent to the external diameter of the fuel containers or
pails. Preferably the sleeves are rigidly separated from one
another. The sleeves may be rigidly separated by mounting on
a base plate.
Preferably the sleeves are discrete from one another around
their entire periphery. Four or more, and preferably 8 or 9
such sleeves may be provided within the internal cavity.
The base plate may be attached to one or more side plates or
elements. The side plates or elements may form walls
corresponding to the walls of the internal cavity. An internal
container may thus be provided.
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Preferably one or more of the sleeves are at least in part
surrounded by a neutron absorbing material. Preferably one or
more of the sleeves, and most preferably all of the sleeves,
are surrounded by a neutron absorbing material around their
entire circumference. A neutron absorbing material may
optionally be provided around one or both ends of one or more
of the sleeves.
Preferably the neutron absorbing material is a resin based
material. Preferably the neutron absorbing material is fire
resistant. Preferably the resin based material is loaded with,
up to 6.5% boron, or up to 5% boron, and more preferably up to
2.5% boron. Preferably the resin occupies at least 50% of the
non-sleeve volume of the internal cavity. The neutron
absorbing material may fill the entire non-sleeve volume of the
internal cavity or lower density materials may be incorporated,
such as polystyrene.
The internal container is preferably provided with a lid.
Preferably the fuel containers or pails comprise cylindrical
drums. Preferably releasable lids are provided. The release
mechanism for~the lid is preferably contained within the plan
profile of the container in the sealed position to minimise
space.
The fuel preferably occupies at least 50% of the fuel container
and may occupy 60, 70, 80, 90, 95% or any individual % value
over 50%.
The fuel may be provided within the fuel containers in plastic
bags, such as polythene.
The fuel may be in pellet, powder or other form. Unirradiated
enriched uranium may be the fuel. The provision of uranium at
substantially up to 5% enrichment may be used. A density of
around 1.4 g/cm3 may be used. In such a case each individual
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fuel container may have a volume of between 15 and 20 litres,
for instance 17.3 litres.
The boron content of any one of the insulator layers, internal
divisions, sleeves, fuel containers, or remaining chamber space
may be increased to give increased absorbtion.
Fuel containers are preferably provided in more than three of
the chambers or sleeves. The provision of the fuel containers
in peripheral chambers or sleeves and most preferably all the
peripheral chambers or sleeves is envisaged. One or more of
the chambers or sleeves may be provided with a neutron
absorber. Preferably the neutron absorber is provided in a
unit corresponding in dimensions to the chamber or sleeve
receiving it. The provision of polythene as the neutron
absorber is preferred. The polythene absorber may be in a
steel container corresponding to the size and shape of the
chamber or sleeve receiving it. The absorber may also be
provided with a lid corresponding with the chamber or sleeve
into which the absorber is placed in order to assist in
retaining the absorber within the chamber or sleeve. The lid
is preferably of steel.
In a particularly preferred form the container comprises an
outer container with a removable lid, the outer container being
provided with an insulating layer on each wall and base, a
further removable insulating layer being provided between the
lid and the internal cavity of the container in use, the
internal cavity being divided into a plurality of chambers, a
fuel container being provided in at least three of the chambers
and at least one of the chambers being provided with a neutron
absorbing material.
In an alternative particularly preferred form the container
comprises an outer container with removable lid, the outer
container being provided with an insulating layer on each wall
and the base, a further removable insulating layer being
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provided on the lid, the insulating layers defining an internal
cavity of the container, the internal cavity being provided
with a plurality of sleeves, a fuel container being provided in
at least 3 of the sleeves and the sleeves being at least
- partially surrounded by a neutron absorbing material.
It is preferred that only one fuel container or pail be
provided in each chamber.
A particularly preferred arrangement provides a rectangular
plan aperture divided into nine chambers, three chambers by
three chambers. Preferably the fuel containers are provided in
the peripheral chambers. A neutron absorbing material may be
substituted in the central chamber and/or one or more of the
other chambers as required.
In a further particularly preferred arrangement a rectilinear
plan internal cavity may be provided with nine sleeve elements,
in a three by three sleeve element arrangement. Preferably
fuel containers are provided in all the periphery sleeves and
most preferably in all of the sleeves. A neutron absorbing
material may be substituted in one or more of the chambers.
Various embodiments of the invention will now be illustrated,
by way of example only, and with reference to the accompanying
drawings in which:-
Figure 1 shows a perspective view of a container according
to a f first embodiment of the invention cut away to show the
fuel containers in the container;
Figure 2 shows a cross-sectional side view of Figure 1;
Figure 3 shows a pail load in plan view;
Figure 4 shows a side view of the container of Figure 1;
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Figure 5 shows a plan view of a closed container according
to the ffirst embodiment of the invention partly cut away to
show the fuel containers in the container of the invention;
Figure 6 shows one embodiment of a fuel container or pail
for use in the present inventions outer container;
Figure 6A shows a plan view of a fuel container or pail of
Figure 6;
Figure 7 shows a perspective view of the container
according to a second embodiment of the invention, cut away
to show the fuel containers in the container;
Figure 8 shows a pail load in plan view; and
Figure 9 shows a cross-sectional side view along axis XX of
Figure 8.
The container as illustrated in Figure 1 has the general form of
a rectangular box. The container 1 is defined by four vertically
arranged walls 2 and a base wall 3. The walls are provided at
the corner joins with strengthening elements 4 in the form of L-
shaped strips. The vertical strengthening elements 4 have
portions 6 which extend beyond the lid 8 of the container. Feet
are provided on each corner of the base and engage with the
portion 6 for easy and stable stacking.
The outer skin forming the walls 2, base 3 and separate lid 8 are
made of stainless steel.
A peripheral flange 12 is provided around the container. The lid
8 is dimensioned to be slidably received within the boundaries of
the L-shaped elements 4. The lid 8 has a flange 16 which
. corresponds with the peripheral flange 12 of the container.
Handles 14 on the lid aid in its removal and insertion.
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In the closed and retained position shown the lid 8 is retained
by a series of quick release nuts and bolts 18 which engage
corresponding openings in the flange 16 of the lid 8. The lid is
provided with suitable seals to prevent any ingress of water.
Next to the steel skin the container is provided with a
substantial thickness of a thermal insulator 20 formed from
calcium silicate. This layer is provided in a series of
sections, see Figure 2. The materials provision in solid
sections ensures accurate positioning during assembly and use.
A single base layer of insulator 22 and four wall sections 24
line the container itself. When the container is loaded, as
described below, a two piece insulating top layer is applied.
These two pieces 26, 28 are shaped to interconnect with one
another.
The rectangular box defined by the interior surfaces of the
insulating layers receives an internal container 30A having four
walls and a base and also made of boronated steel or stainless
steel. This container 30A is also provided with a lid 31 as
shown in Figure 1. As seen in Figure 3 the container consists of
a series of interlocking vertical walls 30 made of boronated
steel/stainless steel. The container 30A has two pairs of
internal walls 30 at 90 degrees to one another defining nine
chambers 32 within the pail load.
In use within each of the eight peripheral chambers a fuel drum
or pail 36 is received. The central chamber 32A is provided with
a polythene neutron absorber 38. The absorber 38 is itself
provided in a steel container (not shown) which corresponds with
the shape of the chamber 32 into which it is to be fitted. A lid
is provided on the top of the absorber to retain the absorber in
place in the chamber 32A.
Once the internal container 30A has received all eight fuel drums
36, and the container 1 is sealed by applying the lid 31, the
insulating top layer 26, 28, arid the external lid 8. The lid 8
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is secured to the container 1 by the quick release nuts and bolts
18.
The fuel containing drum 36, as illustrated in Figure 6, consists
of a stainless steel cylinder wall 40 with a base plate 42 and
releasable lid 44. The lid 44 is provided with a standard
internal lever clamp band 46 which enables the lid to be secured
to the fuel drum 36. The provision of the internal lever clamp
band 46 within the outline of the drum 36 is important to
minimise the space taken up. In the closed state the drum 36 is
water tight avoiding any water ingress.
The fuel 55 in either powder of pellet form is contained within
polythene bags. The polythene bags filled with fuel are placed
in a larger polythene bag which is placed in the drum. Once the
larger bag is full this is then closed. The drum is then sealed
with the lid 44. The fuel may typically be enriched uranium
destined to form fuel rods.
In the second embodiment of the invention illustrated in Figure
7 the container 100 is once again in the form of a rectangular
box. The external container 100 is defined in a similar manner
to the container of the first embodiment by vertically arranged
side walls 102 and a base wall 103. Other equivalent elements
are numbered with reference numerals corresponding to those used
in the first embodiment increased by 100.
Thus the strengthening elements, feet, peripheral flange, lid
fixing and lid alignment are provided in a similar manner.
The container 100 is also provided with substantial thickness of
thermal insulator 120 provided by a base section, wall sections
and a section optionally mounted on the lid in a similar manner
to the first embodiment of the invention.
The arrangement within the internal cavity defined by these
insulating layers differs, however.
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The cavity is provided with a series of stainless steel sleeves
150 which are rigidly mounted on a bottom plate standing on the
base layer insulation. The cylindrical sleeves are hollow and
have an internal dimension configured to snugly correspond to the
external dimensions of the fuel containers 152 shown inserted in
the sleeves 150. Nine sleeves 150 are used in a three by three
arrangement with a fuel container 152 being positioned in each in
use.
The fuel containers are generally of the type illustrated in
Figure 6 and 6A above, but include external fasteners projecting
beyond the plan of the fuel containers.
As shown in Figures 7, 8 and 9 the sleeves 150 are surrounded by
a neutron absorbing material 158. This material is introduced to
the volume surrounding the sleeves during the manufacture of the
portion of the assembly filling the internal cavity by pouring in
a liquid resin which is then allowed to harden. A resin tight
unit is preferred as defining this cavity. The resin is loaded
with boron preferably to a level of 2 o to provide the desired
neutron absorbing capability. A boron loading up to 6.5 wt%
and/or a lead loading up to 15 wt% may be provided. The material
offers between 1 x 1022 and 1 x 1023 hydrogen atoms/cm3.
To reduce the cost and weight of the neutron absorbing material,
typically 1.68g/cm3 lighter materials such as polystyrene can be
incorporated in portions where the neutron absorbing volume of
material would otherwise be excessive. Thus at locations 162
between sets of 4 sleeves and externally at the corner locations
164 and locations 166 between the pairs of sleeves the neutron
material may be replaced with the lighter material. This does
not affect the neutron absorbing capability of the container.
The fuel containing drums 152 and the manner in which the fuel,
as powder or pellets is provided within them is as described
above for the first embodiment of the invention.
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The present invention allows approximately 20% - 40% of the outer
container volume to be occupies by fuel 55 and yet still meets
the necessary standards. This compares favourably with prior art
systems. An increased payload is thus provided successfully.
The use of stainless steel and the modular nature of the assembly
assists in refurbishment and any cleaning stages required such as
decontamination.
