Liquid-metal cooled reactor with practically static intermediate zone

In a fast reactor constituted by an open-topped main vessel containing liquid metal coolant and an inner vessel mounted within the main vessel, a transverse skew wall forming an inner vessel extension is associated with a baffle which extends above the skew wall. A space formed between the baffle and the skew wall and containing a practically static volume constitutes a thermal shield between the hot liquid metal located within the inner vessel above the baffle and the cold liquid metal located between the inner vessel and the main vessel beneath the skew wall.

This invention relates to a nuclear reactor, especially a liquid-metal 
cooled fast reactor, and is primarily concerned with a particular 
arrangement of a first vessel or so-called inner vessel which contains the 
reactor core. Said inner vessel has a lateral skew wall which forms a 
separation between the volume of hot liquid metal discharged from the 
reactor core and collected within said inner vessel and the liquid metal 
which is cooled as it passes through heat exchangers arranged in spaced 
relation around the core. After cooling, said liquid metal is returned 
into the space located between said inner vessel and the wall of a second 
vessel or so-called main vessel which surrounds the inner vessel and 
contains all the internal reactor components. 
The general structural characteristics of a liquid-metal cooled fast 
reactor corresponding to an integrated reactor-block arrangement are well 
known in the technique. In this type of reactor, which includes the liquid 
metal fast breeder reactor of French design known as Super Phenix, the 
reactor core is formed by an array of fuel assemblies maintained in the 
vertical position by engagement of their bottom end-fittings in a core 
support diagrid. An inner vessel which contains the reactor core is 
surrounded by an open-topped main vessel and this latter is suspended from 
a reactor vault roof of substantial thickness. The cavity or reactor vault 
which is closed by the roof is formed within a shield structure which is 
usually of concrete and forms an outer containment for the installation. 
The main vessel contains a suitable volume of liquid metal which usually 
consists of sodium and is circulated upwards through the reactor core 
within the inner vessel and in contact with the fuel assemblies. The hot 
sodium which has absorbed the heat produced by nuclear fission within the 
fuel assemblies is collected at the top of said inner vessel, then 
directed towards heat-exchanger inlet ports. The heat exchangers are 
suspended vertically from the reactor vault roof in such a manner as to 
extend downwards to a point of immersion below the level of liquid metal. 
After passing through the heat exchangers, the cooled sodium is discharged 
from the lower ends of these latter into the space formed between the 
inner vessel and the main vessel, then distributed between said vessels 
and recycled by circulating pumps which are also suspended from the 
reactor vault roof and spaced at intervals around the reactor core between 
the heat exchangers. Said pumps then return the cold sodium into the core 
support diagrid at a sufficient pressure to permit a further passage 
through the reactor core and thus to produce a continuous circulation. 
In accordance with an arrangement which is conventional in this type of 
integrated reactor, the volumes of hot sodium within the inner vessel and 
of cold sodium between said inner vessel and the main vessel are separated 
by a transverse skew wall constituting an extension of the lateral wall of 
the inner vessel, said skew wall being traversed in leak-tight manner by 
the bodies of components such as the pumps and heat exchangers in 
particular. In the case of these latter, the hot sodium inlet ports are 
located above the skew wall and the cold sodium outlet ports are located 
beneath this latter. 
In accordance with French Pat. No. 2,220,847, the peripheral edge of the 
skew wall is bent-back towards the bottom of the main vessel and joined to 
this latter or to a structure which extends in a direction parallel to the 
main vessel wall, thus forming a total separation between the cold sodium 
region and the hot sodium region. The shape which is thus adopted for the 
edge of the skew wall makes it possible in particular to eliminate the 
presence of zones of stagnant sodium beneath the skew wall while avoiding 
the creation of harmful stresses and at the same time offering a high 
degree of mechanical strength. It is not possible, however, to eliminate 
the effects of thermal shocks produced during variations in operating 
regime, especially at the time of reactor shutdown. 
This invention relates to a liquid-metal cooled nuclear reactor comprising 
an open-topped main vessel having a vertical axis and containing the 
liquid metal, an inner vessel mounted within the main vessel, and an inner 
vessel extension in the form of a transverse skew wall provided with a 
downwardly bent edge joined to the main vessel or to a structure connected 
to said main vessel. The shape and arrangement of the skew wall are such 
as to achieve enhanced mechanical strength and also to ensure continuous 
thermal protection of said skew wall by producing an appreciable drop in 
temperature. This accordingly permits of a considerable reduction in the 
stresses developed in the skew wall at the different operating regimes. 
To this end, the reactor under consideration is distinguished by the fact 
that the skew wall is associated with a baffle which extends above said 
skew wall and delimits with this latter a space containing a practically 
static volume which forms a thermal screen between the hot liquid metal 
located within the inner vessel above the baffle and the cold liquid metal 
located between the inner vessel and the main vessel beneath the skew 
wall. 
As an advantageous feature, the skew wall has the shape of a portion of a 
torus of revolution about the axis of the main vessel and is joined by 
means of conical walls on the one hand to the inner vessel and on the 
other hand to the main vessel or to the structure which is connected to 
said main vessel. 
In a first embodiment of the invention, the baffle which is placed above 
the skew wall is horizontal and rests on stationary bearing members. In a 
first alternative form of this embodiment, the baffle consists of a single 
unit which is supported on stationary bearing members by means of sliding 
contacts. In another alternative form, the baffle is constituted by 
adjacent sectors in juxtaposed relation and provided successively with 
overlapping edges for ensuring continuity of the baffle, each sector being 
joined to one of the cylindrical sleeves through which a pump or heat 
exchanger is intended to pass. Preferably, the baffle is provided with 
circumferential ribs for facilitating the absorption of thermal stresses 
during operation. 
In accordance with a second embodiment, the baffle is self-supporting and 
inclined towards the axis of the main vessel, said baffle being provided 
with an extension in the form of a lateral and vertical bearing shell 
placed within the inner vessel. Depending on requirements, the sloping 
side portions of the baffle are bent downwards or raised in a direction 
parallel to the axis of the main vessel in order to ensure confinement of 
the space in which the liquid metal is conveyed between the skew wall and 
the baffle as well as insulation of said space with respect to the hot 
liquid metal within the inner vessel. 
Finally, in another alternative form, the baffle has a horizontal surface 
which rests freely on stationary bearing members and is provided on its 
internal and external peripheries as well as at the point of penetration 
by each cylindrical sleeve with a downwardly-extending side portion which 
is immersed in the liquid metal and traps a blanket layer of neutral gas 
beneath the horizontal surface.

In the example shown in FIG. 1, the reference numeral 1 generally 
designates the lower portion of a fast nuclear reactor which is cooled by 
a liquid metal. Said reactor comprises in particular a main vessel 2 which 
is open at the top portion (not shown) and constituted by a lateral 
cylindrical shell 3 which terminates in a substantially hemispherical 
bottom wall 4. The main vessel 2 is surrounded externally in known manner 
by a second vessel 5 having a parallel wall or so-called safety vessel. 
The reactor core 6 is placed within the main vessel 2 beneath the level 
(not shown) of the liquid metal contained within this latter and rests on 
a core support diagrid 7 which is applied against the bottom wall 4 of the 
vessel 2 by means of a support structure 8. The reactor core and diagrid 
support structure 8 are completely immersed in the liquid metal contained 
in the main vessel 2 and usually consisting of sodium. This volume of 
liquid metal is also supplied through holes 9 formed in the base of the 
diagrid support structure 8 to a narrow annular space 10 formed first 
between the bottom wall 4 of the main vessel 2 and a sheet metal member 11 
which is parallel to this latter, then extends opposite to the lateral 
cylindrical shell 3 by means of two parallel walls 12 and 13 which define 
two spaces 14 and 15. During reactor operation, the main vessel 2 is 
cooled by the flow of liquid metal which is circulated upwards within the 
space 14, then downwards within the space 15. At the bottom of the space 
15, this coolant flow is discharged through holes 16 formed in the wall 13 
in order to return to the volume contained within the main vessel 2. 
In accordance with an arrangement which is also conventional, the reactor 
core 6 is placed within an inner vessel, the lateral wall 17 of which has 
a substantially conical contour in the example of construction under 
consideration in order to be joined tangentially to a portion of torus 18 
which extends annularly around the axis of the reactor core and of the 
main vessel. This portion of torus 18 which forms a skew wall is extended 
by a second conical portion 19 which is bent downwards and joined to the 
wall 13 of the cooling structure which forms an internal jacket for the 
lateral cylindrical shell 3 of the main vessel. Under these conditions, 
the skew wall 18 and its conical extensions 17 and 19 separate the volume 
of liquid metal within the main vessel 2 into two regions 20 and 21 
respectively which are located in one case above said skew wall and in the 
other case below this latter. 
The nuclear reactor shown in FIG. 1 corresponds to a general arrangement 
known in the technique as an integrated design. Provision is accordingly 
made for a series of heat exchangers 22 and circulating pumps 23 placed 
within the interior of the main vessel 2 and disposed at suitable 
intervals around the reactor core 6 in such a manner that the bodies of 
said heat exchangers and of said pumps extend vertically through the skew 
wall 18 which forms a separation between said regions 20 and 21. 
Each heat-exchanger body 22 is provided with inlet ports or windows 24 
located in the region 20 above the skew wall 18 and outlet windows 25 
provided beneath said skew wall in the region 21 between the inner vessel 
17 and the main vessel 2. The skew wall 18 is traversed by each body of 
the heat exchangers 22 or pumps 23 through wells each constituted by a 
cylindrical sleeve 26 which surrounds the heat-exchanger or pump body and 
is welded to the skew wall. In the case of the heat exchanger, said sleeve 
is in turn covered by a bell-cap 27 connected to the heat exchanger and 
forming a space 28 in which is trapped a suitable quantity of neutral 
blanket gas. The levels of liquid metal inside and outside the sleeve 26 
are in communication respectively with the regions 20 and 21 and are 
represented in the drawing by the references 29 and 30. 
In accordance with the invention, the baffle 18 together with its conical 
extensions 17 and 19 towards the inner vessel and the main vessel is 
associated with a baffle 31 designed in the form of a single and 
substantially horizontal sheet metal plate 32 as shown in the example of 
construction of FIG. 1. Said baffle is provided with sliding contacts or 
shoes 33 which rest on bearing members of the L-section type, for example, 
these latter being rigidly fixed either to a support bracket 35 extending 
from the outer surface of the cylindrical sleeve 26 or provided at the top 
of the lateral neutron shield 36 which surrounds the reactor core 6 within 
the inner vessel. The plate 32 is provided at its periphery with a 
bent-back edge 37 which leaves a small clearance space with respect to the 
wall 13. Finally, the plate 32 is advantageously provided with 
circumferential ribs 38 for absorbing thermal shocks and especially for 
reducing stresses within the baffle at the time of variations in operating 
regime. During reactor operation, the liquid metal which has passed 
upwards through the reactor core 6 is collected within the region 20 
within the inner vessel above the skew wall 18, then penetrates into the 
heat-exchanger bodies 22 through their inlet windows 24. After cooling, 
said liquid metal is discharged from said heat exchangers through the 
windows 25 and collected within the region 21 beneath the skew wall 18, 
between the inner vessel and the main vessel. In this region, the cooled 
liquid metal is recirculated by the pumps 23. After suction through the 
diffusers 39 of said pumps which are supported by beams 40, the liquid 
metal is returned into the diagrid 7 through large-section ducts 41, then 
undergoes a further passage through the reactor core 6, thus maintaining a 
continuous circulation. 
By positioning the baffle 31 above the skew wall 18, there is thus defined 
with this latter an internal region 42 which is capable of constituting an 
effective thermal screen during operation by virtue of the quantity of 
liquid metal which is contained within this region and remains practically 
static. Furthermore, the use of sliding bearing members permits of free 
expansion of the baffle whilst the ribs 38 formed in this latter ensure a 
reduction of thermal stresses. Finally, the solution which is contemplated 
offers great simplicity of construction and is of very limited overall 
size. 
FIG. 2 illustrates an alternative form of the embodiment described in the 
foregoing in which the baffle 31 is no longer designed as a single unit as 
in the previous embodiment but is constituted by adjacent sectors 31a, 
31b, 31c . . . , each sector being joined to a cylindrical sleeve 26 in 
which a pump body or heat-exchanger body passes through the baffle and the 
skew wall. Preferably, these sectors are provided with edges 31'a, 31'b, 
31'c, . . . , which overlap successively in order to ensure continuity of 
the baffle. In this alternative form, there are again shown the 
circumferential ribs 38 in the form of circular undulations which are 
intended to endow the baffle with the necessary degree of flexibility by 
virtue of the inherent elasticity of said ribs. 
In the first alternative form of a second embodiment shown in FIG. 3, the 
elements which were already illustrated in FIG. 1 are again shown 
partially. In this variant, the baffle 51 is self-supporting and has a 
flat portion 52 which is inclined towards the axis of the main vessel and 
rests on a lateral cylindrical shell 53 which is mounted within the inner 
vessel and the lower end of which in turn rests on the diagrid support 
structure 8. At the opposite end which is directed towards the periphery, 
the baffle 51 has a downwardly-extending side portion 54 which leaves a 
narrow clearance space with respect to the wall 13. In order to confine 
the volume within the region 42, the baffle 51 is also provided with a 
flange 55 opposite to each of the skirts 26 through which the bodies of 
the heat exchangers or pumps 22 and 23 traverse the skew wall 18. Both the 
flange 55 and the downwardly-extending side portion 54 extend to the 
bottom level of the baffle in order to prevent circulation of liquid metal 
by natural convection. In this embodiment as in the previous form of 
construction, the liquid metal contained between the skew wall and the 
baffle remains practically stagnant during operation. 
In a second alternative form of the second embodiment shown in FIG. 4, the 
baffle 51 is again self-supporting as in the alternative embodiment shown 
in FIG. 3 and also has a portion 52 which is inclined towards the axis of 
the main vessel. In this variant, the baffle is arranged as indicated 
hereinafter with a view to ensuring leak-tightness between the regions 42 
and 20. The baffle 51 is provided at its periphery with an 
upwardly-directed side portion 56 which extends parallel to the wall 13 to 
the neutral gas atmosphere 60 located above the free level 58 of liquid 
metal. The baffle 51 is also provided with a side portion which is similar 
to the side portion 56 around each of the penetrations (not shown) 
provided in the skew wall 18 for the cylindrical sleeves 26 which surround 
the pump bodies. At the point of penetration of the skew wall 18 by the 
heat-exchanger bodies 22, the baffle 51 is provided with a side wall 57 
which extends upwards and terminates in the neutral gas space 28. The 
foregoing arrangements make it possible to prevent any circulation between 
the region 42 located between the skew wall and the baffle and the region 
20 containing the hot sodium. Equalizing of pressures between the region 
20 and the confined region 42 is obtained by means of orifices 59 formed 
in the lower portion of the cylindrical shell 53 which supports the baffle 
51. 
FIG. 5 illustrates another alternative embodiment in which the baffle 
associated with the skew wall is mounted in a floating arrangement. In 
this alternative embodiment, the baffle 62 has a flat surface 63 which 
extends horizontally above the skew wall 18. This surface rests on 
supports such as those designated by the reference 64 and formed in the 
wall 13 on the side nearest the main vessel or in a cylindrical shell 65 
on the side nearest the reactor core, or alternatively in the external 
surface of the cylindrical sleeves 26 through which the pump and 
heat-exchanger bodies traverse the skew wall. The surface 63 of the baffle 
is provided with downwardly-bent side portions 66 extending beneath the 
level of the liquid metal which is trapped within the supports 64, thus 
confining beneath the baffle a blanket layer 67 of suitable neutral gas 
such as argon or helium. 
By virtue of these arrangements, total leak-tightness is accordingly 
obtained between the volume of hot liquid metal above the baffle and the 
volume of colder liquid metal located beneath this latter, thus permitting 
an appreciable reduction in friction forces at the time of differential 
radial expansions of the baffle. Finally, the presence of the gas blanket 
ensures more efficient thermal insulation and serves to lower the 
temperature of the practically static volume of liquid metal between the 
baffle and the skew wall. 9n