FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
& The Patent Rules, 2003
COMPLETE SPECIFICATION
1. TITLE OF THE INVENTION:
OPTIMISED METHOD FOR LOADING RADIOACTIVE ELEMENTS INTO A PACKAGE
2. APPLICANT:
Name: TN INTERNATIONAL
Nationality: France
Address: 1 rue des Hérons, F-78180 Montigny Le Bretonneux, France.
3. PREAMBLE TO THE DESCRIPTION:
The following specification particularly describes the invention and the manner in which
it is to be performed:
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DESCRIPTION
TECHNICAL FIELD
The invention relates to the field of loading radioactive elements into a
package.It particularly concerns loading of radioactive elements located in a pool, into a
package under a gaseous atmosphere, preferably ambient air.
The invention is preferably applicable to loading radioactive elements
such as wornrod cluster control guides, also known as “RCC guides”, in a nuclear reactor,
but it can also be applied to any other radioactive elements as such irradiated fuel
assemblies.
STATE OF PRIOR ART
RCC guides in a nuclear reactor have to be replaced when they are
worn.To achieve this, there are firstly taken out of the reactor and placed close to the
reactor vessel, without being taken out of the pool.These RCC guides are then extracted
from the pool from the Upper Internals (UI), and then transferred directly and individually
through a transfer hood to a transport and/or interim storage container located in
ambient air.
To achieve this, the transfer hood open at its two opposite ends is
placed flush with the surface of the pool above a RCC guide.The RCC guide is then
extracted from the pool and is inserted in the transfer hood using a handling system
connected to the RCC guide.After this operation, the RCC guide is surrounded by this
hood that forms a radiological protection.The assembly obtained is then moved above
the container including a packagecontainingaRCC guide storage basket.While the hood is
held in place above the container, the RCC guide is moved downwards to bring it into one
of the basket housings.
This sequence of operations is thus repeated once for each RCC guide to
be placed in the container, there are usually between ten and twenty.
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Therefore this solution is not entirely optimised because it does not
enable fast operation due to the large number of operations to be carried out to
completely load all RCC guides in the container.Furthermore, even if the function of the
transfer hood is to provide operators with radiological protection as each RCC guide
passes between the pool and the container, there is still significant exposure to radiation.
Similar disadvantages are observed regardless of the nature of the
radioactive elements to be loaded, once they have been taken out of their pool to be
inserted in a package under a gaseous atmosphere, usually ambient air.
PRESENTATION OF THE INVENTION
Therefore the purpose of the invention is to at least partially overcome
the disadvantages mentioned above related to embodiments according to prior art.
To achieve this, the purpose of the invention is a method of loading
radioactive elements in a package comprising the successive following steps:
(a)placement of a plurality of radioactive elements in a storage basket
provided with radiological protection means, in the pool;
(b)extractionof the basket containing the radioactive elements out of
the pool;and
(c)loading said basket containing the radioactive elements in the
package.
The invention thus deviates from standard practice in which the
radioactive elements under water are transferred one by one into a dry package.This is
made possible by the use of a radiological shielding basket that contains several
radioactive elements when the radioactive elements are still under water, this basket
then being taken out of the pool to be placed in the package ready for interim storage
and/or transport of these radioactive elements.
Conventionally, "basket"refers to an open structure that is absolutely
not configured to confine the radioactive elements placed in it.On the contrary, the
basket is configured to allow water to flow out of it as it is extracted from the
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pool.Therefore, the basket preferably has no closing system, since radioactive elements
are confined solely by the package.
Since the basket is used to transfer radioactive elements between the
pool and the package, there is no longer any need for the transfer hood as in prior
art.This thus reduces the cost of implementing the loading method.These costs also
optimise the work time that is shorter because several radioactive elements are loaded
simultaneously in the package, but also there is no longer any need to dock the hood on
the package.This reduction in application time also reduces the exposure time of
operators to radiation.
Preferably, each radioactive element is held in place by gravity in its
storage basket.This design facilitates the step to load radioactive elements from the pool
into the basket.
Preferably, each basket has a plurality of housings inside each of which
one of the radioactive elements is placed.
Preferably, steps(a) to (c) are repeated several times such that several
baskets are loaded in said package.The fact that several baskets are provided can reduce
the size of the baskets, and particularly provides a satisfactory solution to operating
constraints related to the small available volume in the pool to accommodate the
baskets.
The baskets housed in the package then preferably define an external
lateral surface that is a approximately complementary to the lateral surface of a cavity of
the package in which they are housed.Thus, once the baskets are placed in the cavity,
they are perfectly held in position in this cavity due to the complementary shape of the
external lateral surface of the baskets and the lateral surface of the cavity.This special
feature may be adopted when a single basket will be housed in the cavity of the package,
regardless of the shape of this cavity, to which the external lateral surface of the basket(s)
may be adapted.
Preferably, radiological protection means are also provided with the
package.Therefore they are additional to the protection means provided with the storage
basket.Naturally, the radiological protection means fitted on the basket and the
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packageare configured to satisfy regulatory transport and/or interim storage criteria for
radioactive elements.
Preferably, the package is closed by a lid after said basket containing the
radioactive elements has been loaded in the package cavity.Therefore this basket is
intended to remain permanently in the package throughout transport and/or interim
storage of the radioactive elements, so as to form a container with it.
Preferably, each storage basket houses between five and ten radioactive
elements.Thus, when it is intended that two baskets will be housed in the same package
cavity, the packagewill beconfigured to contain between ten and twenty radioactive
elements.
Preferably said radioactive elements are worn RCC guides or irradiated
fuel assemblies.Other types of radioactive elements may be concerned, without going
outside the scope of the invention.
Finally, another purpose of the invention is a method of transport
and/or interim storage of radioactive elements includingimplementation of the loading
method disclosed above before said transport and/or interim storage.This transport on
the public highway and/or interim storage on site consequently takes place with the
shielded basket containing radioactive elements housed in the package provided for it.
Other characteristics and advantages of the invention will become clear
in the non-limitative detailed description given below.
BRIEF DESCRIPTION OF THE DRAWINGS
This description will be made with reference to the appended drawings
among which;
- figure 1 shows a sectional view of a part of a nuclear reactor;
- figure 2 shows a first step in a preferred embodiment of the method
according to the invention intended to install RCC guides in two shielded storage baskets,
in the pool;
- figure 3 shows a detailed perspective view of storage baskets in the
previous figure, containing RCC guides;
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- figures 4 to5 show a subsequent step in the method configured to load
the storage baskets in a package;
- figure 6 shows a cross-sectional view taken along plane P in figure 5;
- figure 7 shows a perspective view of a quarter of a container formed
by the package and storage baskets contained in it;and
- figure 8 is an enlarged view of part of the view in figure 7, in which
dimensions have been added.
DETAILED PRESENTATION OF PREFERRED EMBODIMENTS
Firstly with reference to figure 1, the figure shows a nuclear reactor 1 of
the pressurised water type, according to a classical configuration that will be briefly
described below.
The reactor comprises a vessel 2 at the bottom of which nuclear fuel
assemblies 4 are placed forming the reactor core.An upper core plate 6 covers the
assemblies 4 and separates them from a series of RCC guides8 each aligned with a fuel
assembly 4.The guides 8, usually in the form of a sheath, have a two-part design, namely
there is an upper part 8a and a lower part 8b connected to each other by a flange42.The
RCC guides are fixed by this flange 42 to anRCC guide support plate 10 to form a set of
Upper Internals (UI).Finally, a control RCC 12 is associated with each RCC guide and is
controlled by a control mechanism 14 in the part above the lid 16 of the vessel.
The control RCCs 12 are used to control the reactor 1.They are formed
by several rods or bars manufactured with neutron absorbing metals, and are housed free
to slide in the RCC guides8.The chain reaction in the reactor produces a surplus of
neutrons, while the RCCs adjust the power.If an incident occurs, they are capable of very
quickly stopping the nuclear reaction by dropping under the action of gravity along the
fuel rod assemblies 4, by sliding through their RCC guides8.
Due to their position above the assemblies 4 in the reactor, the lower
part of the RCC guides 8 is highly activated during operation of the reactor.Consequently,
this part has a high level of gamma radiation.
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In the remainder of the description, we will disclose a preferred
embodiment of the method according to the invention aiming at loading the RCC guides 8
in a package.This method is used when the guides 8 are worn and they have to be
replaced by new RCC guides.
Firstly, it has to be noted that the entire UIhas been previously extracted
from the reactor and placed in a working pool adjacent to the vessel.The guides 8 are
then separated from the support plate10 (shown in figure 1), and are then placed in one
or several storage baskets 30 specific to this invention.These baskets may firstly be used
as storage devices for new RCC guides that will replace the worn guides in the
reactor.Thus, new guides are interchanged with the worn guides 8 as the worn guides are
transferred to the baskets30.
Each storage basket has radiological means of protection essentially
against gamma radiation.In this case, these means are in the form of a thick sidewall 32
made of steel or a similar material, that will cover the activated lower part of the RCC
guides.In this solution with two baskets 30, each of the two walls 32 has a semi-circular
cross-section.
Each basket 30 defines a plurality of housings 34 in each of which anRCC
guide 8 will be housed as can be seen infigure 2.It has a head plate 36 on which all
housings 34 open, this plate also participating in holding the sheaths 38 which define the
housings in place and which extend over practically the entire height of the basket.As
mentioned above, the shielding sidewall 32 in the lower part of the basket 30 surrounds
the sheaths 38 as can be seen in the two near baskets in figure 3.There will be one or
several intermediate plates 40 on these baskets 30 holding the sheaths 38, and a plate 41
closing the sensitive zone assembled on the top end of the thick wall32.This plate 41,
through which the sheaths 38 also pass, also provides radiological protection against
gamma radiation.
Therefore, the worn RCC guides 8 are placed one by one in the shielded
baskets 30, while being still under water.Conventional handling means are used to
achieve this, to extract each guide from the rack 20 and then to place it in one of the
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housings 34 of the baskets.In this regard,each basketdisclosed has seven housings 34 to
accommodate seven worn RCC guides8.
Once each guide 8 has been placed in its basket housing 34, it will only
have its lower part 8b embedded in the basket, and its upper part 8a that is inactive will
remain projecting upwards beyond the head plate36.It has to be noted that the
mechanical junction 42 between the lower part and the upper part of each guide can act
as a stop against this head plate36, so that the guide remains in the basket by
gravity.Alternately, the bottom end of the guide could bear in contact with a bottom
plate in the basket, not shown in the embodiments in figures 2 and 3.In this case, one or
several openings will be formed in the bottom so that the containercan be drained and
then dried.
It has to be noted also that the shielding sidewall 32 does not extend
over the entire height of the lower part 8a of the RCC guides, but only over the most
active lower part.
Once one of the baskets 30 contains the seven RCC guides 8, the basket
will be extracted from the pool still using conventional handling means, and will then be
inserted in the cavity 44 of a package 50.To achieve this, the sheaths 38 of the baskets 30
are configured to allow water to escape during extraction from the pool and to prevent
any harmful water retention zone when drying the package cavity after loading.When the
basket is extracted from the pool, this basket containing the RCC guides is not surrounded
by any elements.In particular, it is fitted with its own radiological protection means, and
therefore does not have to be placed in a package or a similar material to be extracted
from the pool.
The package 50 shown in figures 4 to 7 has a conventional design,
namely it comprises a bottom52, a side body 54 and a lid 56, its opposite ends possibly
being provided with shock absorbing covers 58.The side body 54 also provides
radiological protection against gamma radiation due to the large thickness of the material
at the relevant locations, this material preferably being steel.
Therefore, the package 50 defines the cavity 44 inside which the two
loaded baskets 30 will be housed, and then kept for subsequent transport operations on
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the public highway and/or for interim storage on site.The cavity 44 is then configured to
confine the radioactive elements contained in it, the basket not contributing to
performing this function.Figure4shows loading of the first basket 30 that is preferably
done vertically from the upper opening in the side body 54.Figure 4’ shows the package
with the first basket 30 loaded in the cavity 44, while figure 5 shows the same package
after the second basket 30 has been loaded, also preferably vertically.Once these baskets
30 have been loaded in the cavity 44, they are held on the bottom 52 of the package by
the force of gravity.
As shown diagrammatically in figure 6, once the baskets 30 are in
position in the package, the two walls 32 together define a lateral external surface with a
circular cross-section approximately complementary to the lateral surface 60 of the cavity
44 of the package.This allows the baskets 30 to be held in position in this same cavity,
simply by the complementary shapes.
The cavity 44 is then closed by the package lid 56 as shown in figure 7,
thus enclosing the baskets 30 loaded inside this cavity, so that the worn guides 8 can then
be transported and/or stored, using this container formed by the package 50 and the
baskets 30.
Figure 8 shows the lower part of this container comprising the
radiological protection means provided on the package 50 and the baskets 30.
Starting from the bottom 52 of the package over a heightd1, the thick
sidewall 32 of the baskets and the solid lower part 54a of the side body 54 are radially
superposed providing maximum radiological protection surrounding the active lower part
of the RCC guides.This height may be of the order of 1100 mm for a global shielding
thickness of the order of 215 mm, composed of 85 mm originating from the wall 32 and
130 mm originating from the solid lower part 54a of the lateral body 54.The container
then is prolonged over a height d2 in which the thickness of the sidewall 32 of the baskets
is maintained, but only two concentric shells 54b form the shielding of the lateral body
54.For example, each of these two shells 54b may be of the order of 30 mm thick, and the
annular space between the shells is possibly filled with a foam54c.This configuration with
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two concentric shells is maintained as far as close to the top end of the package, while the
walls 32 stop at the level of the plate 41.
Obviously, those skilled in the art could make various modifications to
the invention disclosed above solely as non-limitative examples.
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We claim:
1. Method of loading radioactive elements (8)in a package (50),
comprising the successive following steps in this order:
(a)placement of a plurality of radioactive elements (8) in a storage
basket (30) provided with radiological protection means (32), in the pool;
(b)extraction of the basket (30) containing the radioactive elements
(8)out of the pool;and
(c)loading said basket (30) containing the radioactive elements (8) in the
package(50).
2. Method according to claim 1, in which each radioactive element(8) is
held in place by gravity in its storage basket (30).
3. Method according to any one of the previous claims, in which steps
(a) to (c) are repeated several times such that several baskets (30) are loaded in said
package.
4. Method according to claim3, in which the baskets(30) housed in the
package (50) together definean external lateral surface that is a approximately
complementary to the lateral surface (60) of a cavity(44) of the package in which they are
housed.
5.Method according to any one of the previous claims, in
whichradiological protection means (54a, 54b) fitting the package (50) are also provided.
6. Method according to any one of the previous claims, in which
thepackage(50) is closed by a lid(56) after said basket(30) containing the radioactive
elements (8) has been loaded in the package cavity(44).
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7.Method according to any one of the previous claims, in which each
storage basket(30) houses between five and ten radioactive elements (8).
8.Method according to any one of the previous claims, said radioactive
elementsare worn rod cluster control guides (8) or irradiated fuel assemblies.
9. Method oftransport and/or interim storage of radioactive elements
(8) including implementation of the loading method according to any one of the previous
claims, before said transport and/or interim storage.