Auxiliary cooling device for the primary fluid heat exchanger of a nuclear reactor

An auxiliary cooling device for a primary fluid heat exchanger of a rapid neutron nuclear reactor, the exchanger having at least one nest of tubes in which there flows water to be heated to superheated steam and around whose tubes and inside whose casing there flows a liquid alkaline metal constituting the primary fluid. The casing of the exchanger is surrounded by a jacket having an air inlet tube and an air outlet tube at opposite ends. Inside the jacket there are auxiliary heat exchange surfaces such as spikes projecting from the casing of the primary heat exchanger proper. Application is to the absorption of residual power produced by a rapid neutron reactor after it is shut down.

FIELD OF THE INVENTION 
The present invention relates to an auxiliary cooling device for a primary 
fluid heat exchanger of a rapid neutron nuclear reactor. The exchanger 
comprises at least one nest of tubes in which there flows cooling water 
(to be heated to superheated steam) and round whose tubes there flows a 
liquid alkaline metal constituting said primary fluid. 
BACKGROUND 
When a rapid neutron nuclear reactor is shut down, the power supplied does 
not die away instantaneously, but reduces gradually for some time before 
dying completely away. This residual power may be absorbed in special heat 
exchangers, to which the heated liquid alkaline metal is conveyed, instead 
of to the normal operation exchangers. Such special exchangers are however 
relatively expensive and are used for fairly short periods of operation. 
They therefore substantially increase the cost price of nuclear power 
stations using rapid neutron nuclear reactors. 
SUMMARY OF THE INVENTION 
Preferred embodiments of the present invention reduce this disadvantage and 
provide at a relatively low cost an auxiliary cooling device which is able 
to absorb the variable residual power of a rapid neutron reactor. The 
device according to the invention comprises an auxiliary cooling device 
for absorbing residual energy in a primary fluid heat exchanger of a 
nuclear reactor which continues to generate some energy after shut down, 
said exchanger comprising a casing for conveying a flow of primary fluid 
heated in the reactor and a nest of tubes disposed inside the casing for 
conveying a secondary cooling fluid in thermal contact with the primary 
fluid, said auxiliary cooling device comprising a jacket disposed around 
the casing and including inlet means and outlet means for an auxiliary 
cooling fluid, and auxiliary heat exchange surfaces projecting from said 
casing into the space around the casing inside said jacket. 
It also preferably has at least one of the following features: 
The auxiliary cooling fluid is air; 
The auxiliary heat exchange surfaces are spikes; 
The auxiliary cooling fluid inlet and outlet means include means for 
providing a forced flow of auxiliary cooling fluid; 
The auxiliary cooling fluid is free to flow by convection. 
The present invention also provides a heat exchanger including said cooling 
device and a reactor installation including said heat exchanger.

DETAILED DESCRIPTION 
The heat exchanger module, which can be grouped in parallel with other 
modules, includes a jacket or outer casing 1 having a generally 
cylindrical shape, inside which is disposed a coaxial inner casing 2 
surrounding a nest of tubes 3, comprising a large number of parallel 
tubes, not shown. The nest of tubes 3 is connected to a lower tubular 
plate 4 and to an upper tubular plate 5, the tubes forming at their top 
part an expansion-bend 6. The nest of tubes is intended to convey the flow 
of water which is to be heated to superheated steam. The water is admitted 
at the bottom part of the module through tubing 7 and is discharged in the 
state of superheated steam at the upper part through tubing 8. An annular 
space 9 disposed in the upper part of the module between the casings 1 and 
2 is connected to inlet tubing 10 for liquid sodium heated in a rapid 
neutron nuclear reactor. During normal operation, the liquid sodium rises 
between the casings 1 and 2, then flows into the casing 2 around the tubes 
of the nest 3, in opposition to the flow of the water which is to be 
vaporized and superheated. An annular space 11 disposed in the lower part 
of the module between the casings 1 and 2 is connected to tubing 12 for 
discharging the liquid sodium cooled in the exchanger. 
The casing 1 of the module is surrounded over its entire height between the 
annular inlet space 9 and the outlet space 10 with a coaxial outer casing 
or jacket 13 defining an annular space 14 with casing 2. This space 14 is 
connected to an ambient air inlet tube 15 and an ambient air outlet tube 
16. A blower 17 ensures the circulation of air in the annular space 14. 
Horizontal spikes 18 are provided over the entire height of the outer 
surface of the casing 1 that is under the jacket 13. The spikes 18 serve 
to improve the heat exchange coefficient between the liquid sodium and the 
air blown through the space 14 when the nuclear reactor is shut down. 
When a rapid neutron nuclear reactor is shut down, the power which it 
supplies does not die away immediately, but decreases gradually. As the 
flow of water must be stopped on shut down, the residual power must be 
dissipated elsewhere until its supply has died away. This dissipation is 
effected by heat exchange between the sodium and the air flowing in the 
annular space 14 in the direction shown by arrows 19, 20. However, as the 
heat exchange coefficient of air is relatively low, it is improved by the 
use of the spikes 18 so as to ensure the dissipation of the residual heat 
with a moderate air flow. The spikes 18 are immersed in the air flow and 
may have a height which is of the order of half the width of the annular 
space 14. These spikes 18 could be replaced by other auxiliary heat 
exchange means, such as vertical fins. 
When the residual power generated by the nuclear reactor has greatly 
decreased, it is possible to stop the air blower as the natural air flow 
by convection becomes sufficient to dissipate the remaining fraction of 
residual power. 
If the water/sodium heat exchanger is composed of several modules grouped 
together in parallel, it is preferable to provide only one blower feeding 
air to the coverings of the various modules through appropriate tubes. 
Although the structure of the heat exchange module which has just been 
described appears to be preferable, it will be understood that various 
modifications can be made thereto without going beyond the scope of the 
invention, it being possible to replace particular elements described by 
others which fulfill the same technical function.