Fast neutron nuclear reactor

Fast neutron nuclear reactor comprising on the inside a main vessel having a vertical axis suspended by its upper open part on a thick protective slab which seals a cavity surrounding said vessel, a volume of a liquid cooling metal and also positioned within said main vessel an inner vessel having a cylindrical wall containing the core of the reactor submerged beneath the liquid metal level, the intermediate space between the main vessel and the inner vessel having heat exchangers and pumps which pass below the level of the liquid metal, wherein the upper end of the inner vessel is covered by an inverted annular bell cap and is placed under the internal pressure of a neutral gas in order to prevent in normal operation direct communication between the hot liquid metal within the inner vessel and the cold liquid metal in the annular space between the main vessel and the inner vessel, said bell cap covering an auxiliary exchanger which externally surrounds the inner vessel in such a way that if there is a fault in the reactor and after reducing the neutral gas pressure beneath the bell cap as a result of natural convection a circulation of the liquid metal takes place beneath the bell cap of the inner vessel towards the annular space and passes through the auxiliary exchanger.

BACKGROUND OF THE INVENTION 
The present invention relates to a fast neutron nuclear reactor where the 
cooling of the core by extracting the calories released by nuclear fission 
is brought about by circulating a liquid metal. 
The invention applies more particularly in the case of a reactor of the 
above type, a so-called integrated reactor, in which the volume of liquid 
metal, generally sodium, necessary for cooling the core is contained in an 
open vessel having a vertical axis called the main vessel suspended 
beneath a horizontal thick slab which closes a cavity containing said 
vessel and located in a concrete protective enclosure. The core of the 
reactor comprising juxtaposed fuel assemblies rests on a girder which also 
supports a second vessel or inner vessel having its upper end open and 
surrounding the core and mounted in the main vessel. In the annular space 
defined between the side walls of the main vessel and the inner vessel are 
provided heat exchangers and circulating pumps, so-called primary pumps, 
traversing vertical passages provided in the upper slab and which pass 
into the sodium volume. The hot sodium from the core, after passing 
through the latter from bottom to top, is collected in the inner vessel. 
It then leaves the latter and penetrates the exchangers where the calories 
obtained are transferred to a secondary fluid, the cold sodium emanating 
from the exchangers in the space between the two vessels being taken up by 
the circulating pumps and is then returned under a suitable pressure 
beneath the core girder and then again passes through the core. 
In a special construction of a fast integrated reactor of the type 
indicated hereinbefore, on leaving the core located in the inner vessel 
the hot sodium is caused to circulate between the core and the side wall 
of said vessel and is then removed from the latter by tubes directly 
connected at the base of each exchanger with a central duct provided in 
the latter. Advantageously an arrangement of this type can be in 
accordance with that described and claimed in French patent application 
760,982 of Mar. 29, 1976 entitled "Fast neutron nuclear reactor" filed in 
the name of the Commissariat a l'Energie Atomique. 
However, in the case of the constructions described hereinbefore the hot 
sodium collected in the upper part of the inner vessel partly stagnates 
due to the fact that the tubes are located in the lower part of said 
vessel supplying the sodium to the exchangers. If a fault or irregularity 
occurs in the reactor due to the unavailability of the secondary circuits 
of the exchangers or the unavailability of both said secondary circuits 
and the primary pumps, it is necessary to be able to remove the residual 
heat from the reactor. 
BRIEF SUMMARY OF THE INVENTION 
The invention relates to an improvement made to the inner structures of a 
fast neutron nuclear reactor of the type indicated hereinbefore, which 
makes it possible to ensure in the case of a fault in the reactor that 
said residual heat is removed through the provision of a liquid metal 
circulation by natural convection. 
Therefore, according to the present improvement the upper end of the inner 
vessel is covered by an inverted annular bell cap and is placed under the 
internal pressure of a neutral gas in order to prevent in normal operation 
direct communication between the hot liquid metal within the inner vessel 
and the cold liquid metal in the annular space between the main vessel and 
the inner vessel, said bell cap covering an auxiliary exchanger which 
externally surrounds the inner vessel in such a way that if there is a 
fault in the reactor and after reducing the neutral gas pressure beneath 
the bell cap as a result of natural convection a circulation of the liquid 
metal takes place beneath the bell cap of the inner vessel towards the 
annular space and passes through the auxiliary exchanger. 
According to another feature of the invention, the auxiliary exchanger is 
carried by a cylindrical ring which is coaxial to the inner vessel and 
suspended beneath the slab. 
According to a first embodiment, the auxiliary exchanger comprises a 
plurality of straight parallel vertical tubes connected at their upper and 
lower ends to two annular manifolds which are common to all the tubes, one 
manifold serving for the admission and the other for the discharge of a 
secondary fluid. 
According to another embodiment, the auxiliary exchanger comprises two 
groups of separated vertical tubes joined independently in each group to 
two independent manifolds for the admission and discharge of a secondary 
fluid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, 1 is a fast nuclear reactor known under the name integrated 
reactor shown in the form of an axial section. Within a cavity 2 in a 
protected enclosure 3 with thick concrete walls the installation comprises 
a first vessel 4 called the main vessel and having a generally cylindrical 
shape with a vertical axis and a substantially hemispherical base, said 
vessel being suspended in its upper open part beneath a horizontal sealing 
slab 5 having on its periphery a circular flange 6 which ensures both its 
positioning and support on a shoulder 7 of enclosure 3. The main vessel 4 
which is externally duplicated by a second vessel 8 having parallel walls 
called the safety vessel and which like the first is suspended beneath 
slab 5 contains a suitable volume 9 of a liquid cooling metal, generally 
sodium. Within vessel 4 and submerged in the sodium volume is located the 
core 10 of the reactor which is formed by the juxtapositioning of not 
shown fuel assemblies positioned vertically on a lower supporting girder 
11 which itself rests on the base of vessel 4 via a flooring 12. 
In its central portion slab 5 also has a system of rotary buttons 
permitting a handling and control member 13, shown diagrammatically in 
FIG. 1, to be positioned above the reactor core 10 in order to control the 
operation of the latter and ensure the requisite handling operations of 
the fuel assemblies within the latter. 
Once again in conventional manner, core 10 is surrounded by an inner vessel 
15 coaxial to the main vessel 4 and resting on girder 11, said vessel 15 
having an open upper end 16 positioned at a lower level than that of the 
liquid sodium in vessel 4. The upper slab 5 also has a series of passages 
17 permitting the mounting of pumps 18 and heat exchangers 19 within the 
main vessel 4 between the latter and inner vessel 15. Said apparatuses are 
regularly distributed about the core axis and ensure respectively the 
circulation of the sodium and the extraction of the calories produced 
during the passage through the core. 
The liquid sodium 9 contained in vessel 4 traverses core 10 from bottom to 
top, is heated when in contact with the fuel assemblies and is collected 
in vessel 15. The hot sodium is then caused to travel between the side of 
the core and the surface of the inner vessel in accordance with arrows 20 
and is then discharged via tubes 21 issuing into vessel 15 at its lower 
end on the one hand and connected directly to heat exchangers 19 on the 
other. To this end each tube 21 is connected to a central duct 22 in each 
exchanger 19, whereby the thus channelled sodium enters the exchanger via 
intake ports 23. The hot sodium then circulates in the exchanger in 
contact with a group of straight tubes 24 where the calories produced are 
transferred to a secondary fluid. Once cooled, the sodium leaves the 
exchanger by ports 25 provided at its lower end and is collected in 
annular space 26 defined by vessels 4 and 15. The cold sodium is then 
taken up by circulating pumps 18 and is forced under an appropriate 
pressure through pipes 27 which have a large cross-section into a manifold 
28 located beneath girder 11 before again passing through the core, where 
it is again heated and so on. 
According to the invention, the upper open end 16 of inner vessel 15 is 
associated with an inverted bell cap 29 which covers said vessel and makes 
it possible to insulate the hot sodium volume contained in vessel 15 from 
the cold sodium volume contained in space 26 between vessels 4 and 15. To 
this end the inverted bell cap 29 is placed under an appropriate pressure 
of a neutral gas, specifically argon, which makes it possible to maintain 
the sodium level 30 below the upper edge of end 16. To this end bell cap 
29 has a base 32 to which are connected two parallel cylindrical rings 33 
and 34 whereby the first passes into the hot sodium within vessel 15, and 
the second extends into the cold sodium in space 26. 
According to a first embodiment of the invention illustrated in FIG. 1, 
ring 31 also serves to support an auxiliary exchanger 35 located around 
vessel 15 on the outside of the latter beneath bell cap 29. This auxiliary 
exchanger has a series of parallel straight tubes 36 which are regularly 
distributed around the vessel and joined respectively at their lower and 
upper ends to manifolds 37 and 38 for the admission and discharge of a 
suitable secondary fluid. 
When the reactor is operating normally the pressure of the neutral gas 
beneath bell cap 29 makes it possible to maintain the sodium level 30 
beneath the edge of end 16 and insulates the hot sodium volume contained 
in vessel 15 from the cold sodium volume in space 26. However, in the case 
of an accident and particularly if the pumps 18 are damaged which causes 
the circulation of sodium in exchangers 19 to be stopped, the residual 
heat in vessel 15 must be evacuated. To this end, the pressure of the 
neutral gas below bell cap 29 is decreased, sodium circulation taking 
place directly by natural convection above the edge of the upper end 16 of 
the inside of vessel 15 towards the outside, thus circulating in contact 
with tubes 36 of auxiliary exchanger 35. This leads to an adequate cooling 
of the sodium to permit an appropriate removal of the residual heat. 
Moreover, it should be noted that the solution proposed above does not 
take up a large amount of space in the radial direction making it possible 
to locate the exchanger beneath the bell cap, it merely being necessary to 
slightly increase the diameter. Sealing by means of an inverted bell cap 
makes it possible to obtain a good thermal insulation between the space 
which collects the cold sodium and the inside of the inner vessel which 
collects the hot sodium, resulting in a reduction of losses and a 
limitation under normal operating conditions of the heating of the upper 
part of said space. 
According to another embodiment illustrated in FIG. 2, the inverted bell 
cap 29 covering the upper end 16 of vessel 15 is associated with an 
auxiliary exchanger 50 having two groups of separated parallel tubes, 
respectively 51 and 52, joined in the first case to two toroidal manifolds 
53 and 54, and in the second case to two other adjacent manifolds 55 and 
56, whereby pipes 57, 58 on the one hand and 59, 60 on the other are 
joined to said manifolds to ensure the circulation of the secondary fluid 
in tubes 51 and 52. This solution increases the reliability of the 
assembly due to the existence of two independent groups of tubes. 
The invention is not limited to the embodiments described and represented 
hereinbefore, and various modifications can be made thereto without 
passing beyond the scope of the invention.