Secondary coolant circuit for nuclear-reactors

A secondary coolant circuit for a nuclear-reactor of the liquid-metal cooled type, said circuit comprising at least one intermediate exchanger mounted in the vessel of said reactor, outside said vessel a steam-generator for the exchange of calories between the secondary liquid-metal flowing through said secondary circuit and water-steam, at least one pump for circulating said secondary sodium and one tank for storing said secondary liquid-metal and recovering those products generated by a possible liquid-metal-water reaction in said steam-generator, said liquid-metal being likely to occupy the lowest possible level in said tank, said secondary coolant circuit being characterized in that said tank is situated at the lowest possible level in the nuclear installation, in that the lower extremity of the liquid-metal outlet duct of said steam-generator is directly dipped into said tank, in that, in said tank above the liquid-metal, is maintained an inert gas cover at such a pressure that it balances the liquid-metal pressure in the whole secondary loop, said tank, in addition, acting as the downstream ram-effect preventing tank for said steam-generator and as an expansion tank during the temperature variations of said liquid metal, and in that the rotor of said pump is situated above said lowest level.

The present invention relates to a secondary coolant circuit for 
nuclear-reactors of the liquide sodium cooled type. 
In the present state of the art, fast neutron nuclear power plants resort 
to liquid sodium as coolant. The latter is usually placed in two 
successive coolantcircuit units. In the first circuit, so-called primary 
circuit, the sodium draws off the heat generated by the core fuel-elements 
and transfers said heat into a so-called intermediate heat-exchanger, 
wherein it is cooled and yields its heat to sodium contained in a second 
circuit unit, absolutely independent of said primary circuit, said second 
unit being called secondary circuit. In said secondary circuit, hot sodium 
issuing from the intermediate exchanger transfers heat into a further 
heat-exchanger, viz the steam-generator, in which the sodium yields its 
heat to pressurized water that vaporizes and finally serves to feed the 
current turbo-generator. 
The secondary circuit is usually divided into several independent parallel 
mounted sub-circuits, or secondary loops. Said loops, three or four in 
number, are identical as regards the power transferred and their overall 
arrangement. If need be, it is possible to stop one of said loops and to 
cause the other loops to operate at rated power: the power delivered by 
the power-plant then decreases according to the power of the unused loop. 
The adoption of the secondary circuit in said type of reactor aims at 
securely confining the radioactive primary sodium and at protecting the 
primary circuit from the possible sequels of a leak in the steam-generator 
thermal-exchange surface. 
In fact, in such an occurence, water of high-pressure steam come into 
contact with the sodium. The resulting chemical reaction is highly 
exothermic and it releases corrosive and harmful reaction products 
(caustic soda, hydrogen). Accordingly, it is necessary to protect the 
core, i.e. the primary circuit, from the possible consequences of such a 
sodium-water reaction (ram-effects, pollution by soda). 
In FIG. 1 is shown the usual structure of the cooling secondary loop of a 
fast reactor. The reactor vessel 2 contains the cooling primary circuit. 
In said vessel are to be found, in particular, intermediate exchangers 4 
(two in number, in the present embodiment), the outlets of which are 
connected to steam-generator 6, through conduit pipes 8 and 8', 
respectively. In the upper portion thereof, the steam-generator comprises 
an argon pocket 6a defining a free level N of sodium. In said 
steam-generator 6, as mentioned above, there takes place an exchange 
between the secondary sodium and the water. The outlet of said 
steam-generator is connected, through conduit pipe 10, to the inlet of 
pump 12, the outlet of which are connected to the inlets of the 
intermediate exchangers 4, through conduit pipes 14 and 14', respectively. 
The specific properties of liquid sodium have led to design sodium 
mechanical pump according to a particular technique. In particular, as 
regards the sealing packings across shaft 12a, it is usually resorted to a 
mechanical packing 16, the latter being in direct contact not with sodium, 
but with an inert gas (usually argon) interposed between the sodium and 
the packing. To this end, in the pump casing, it is necessary to provide a 
free level N.sub.1 of sodium surmounted with an argon pocket. Vertical 
drive shaft 12a passes through the free surface and said argon pocket 
prior to reaching packing 16. In addition, it is necessary to take special 
measures in order that the level N.sub.1 of sodium be prevented from 
rising up to the packing. The trick normally used in such cases consists 
in placing the pump in a so-called expansion tank 18, the name of which 
derives from the fact that it is usually large enough for absorbing all 
the possible volume increases of the volume of the secondary loop sodium, 
without drowning the packing. Moreover, by placing said tank right on top 
the circuit, one may be sure that, even in the event of leakage of the 
argon contained in the protective pocket, no drowning whatever of the 
packing would occur between tank 18 and the remaining portion of the loop, 
through an effect of comunicating vessels. In addition, with a view to 
prevent the sodium, should an unforeseen leakage of sodium to the 
atmosphere occur, from being released in the form of a high-pressure jet, 
great care is taken to limit the pressure in the secondary loop as much as 
possible. Considering the above described arrangement of expansion tank 
18, such a restricting step consists in adjusting the pressure of the 
argon pocket of the latter to the smallest admissible value. The latter is 
equal to atmospheric pressure, plus a slight overpressure ensuring that 
any leakages that might take place would not lead to the introduction of 
air into the secondary loop. 
In order to perform such an adjustment, there is provided a sodium make-up 
conduit pipe 20, opening into tank 18. Said pipe 20 comprises a 
circulating pump 19 and a device 21 for purifying sodium. An overflow 
conduit 22 is mounted on tank 18. Finally, an argon inlet 23 makes it 
possible to ajust argon pressure to a suitable value. Conduit 20 
originates in a tank 24 for the storage of sodium and, if need be, the 
recovery of those products due to a sodium-water reaction in the case of a 
leak in steam-generator 6, a free level N.sub.2 being maintained in said 
tank 24 through the introduction of an inert gas, e.g. argon, via duct 26. 
As already mentioned, in a nuclear reactor of such a type, a violent 
sodium-water reaction may happen should there be a leak in the 
steam-generator, and the desire to fully protect the primary circuit 
thereform makes it necessary, in pracice, to provide the greatest possible 
protection of intermediate exchanger 4, constituting the only possible 
point of contact between said primary circuit and the secondary circuit. 
In order to avoid such a risk, the following steps have to be taken: 
(a) installation upstream and downstream of the steam-generator, of 
ram-effect preventing tanks, viz. tanks in direct communication with the 
secondary loop and having a free surface surmounted with an argon pocket. 
Should a sodium-water reaction occur, the pressure waves issuing from the 
steam-generator are largely damped in said tanks before reaching 
intermediate exchanger 4. 
(b) installation, on the generator proper or in the immediate vicinity 
thereof, of large diameter diaphragms 28 rupturing through ram-effect and 
that uncover ports permitting to depressurize the secondary loop to the 
outside. In practice, with a view to avoiding an explosion resulting fron 
the reaction of hydrogen with the oxygen contained in the air and avoiding 
to contaminate the environment with sodium and soda, further diaphragms 
are mounted downstream, recovery tank 24 (described above) acting as a 
separator for the liquid and gas products, connected by duct 30. If need 
be, said tank is, in its turn, extended by a still more efficient second 
separator 32 (e.g. of the cyclone type) and by a stack 34 serving to the 
exhaust into the atmosphere of the gaseous products only (hydrogen, argon, 
steam, still loaded with a few soda aerosols). 
An improvement usually resorted to for simplifying said system and reducing 
the cost thereof, consists in using the expansion tank of pump 18 as the 
downstream ram-effect preventing tank. As regards the upstream ram-effect 
preventing tank, a further improvement consists in making it coincide with 
the upper portion 6a of the steam-generator, in which, under such 
conditions, an argon pocket must be trapped. Finally, another improvement 
consists in using recovery tank 24 as the tank for storing the secondary 
loop sodium, whenever said loop has been stopped and drained. To that end, 
it is of course necessary, between the loop and tank 24, in addition to 
those ducts equipped with diaphragms, to provide a second network of drain 
pipes provided with large diameter valves (one has to be in a position to 
drain the secondary loop very quickly, in the occurrence of a leakage of 
sodium to the atmosphere in any portion of said loop). 
Said network of ducts is constituted, in particular, by duct 36 associated 
to drain valves V.sub.1 and V.sub.2 putting tank 24 in communication with 
the secondary ducts 8 and 8', and by duct 38 associated to valve V.sub.3 
putting duct 10 in communication with said tank 24. 
The thus-constituted installation, including the above-mentioned 
improvements, still comprises costly devices, and, in addition, entails 
obligations as regards its exploitation. Such is the case, in particular, 
as regards diaphragms 28. In practice, it is difficult to prevent the 
calibration of these diaphragms (viz. the pressure at which they are 
caused to rupture) from being altered in the long run (through age or 
because of creep and fatigue). One is thus led to contemplate replacing 
said membranes, e.g. every second or every third year, which, in addition, 
requires a sophisticated removable mounting, likely to induce sodium 
leakages. Finally, it is to be feared that, should a seism occur, the 
ram-effect provided by the latter in all the secondary loops 
simultaneously, would cause all the diaphrams to be ruptured. In such a 
case, the reactor would be deprived of its normal circuits for the exhaust 
of power, in particular of the residual power. In consequences of such a 
juncture would be so serious that it is necessary to provide duplicates of 
the means for exhausting residual power, said duplicates being independent 
of the secondary loops, in order to avoid such a risk. 
FIG. 2 shows such an emergency cooling system according to the prior art. 
Said system essentially comprises an extraexchanger E mounted in parallel 
with steam-generator 6. Said exchanger E is connected to ducts 8, 8' and 
10 by means of ducts 8a, 8'a and 10a, respectively, duct 10a being 
connected to duct 10 by a mixer 10M. A portion of the secondary sodium 
main flow is thus deviated. The secondary portion of said exchanger E is 
constituted by an air-stack E', associated to a fan E'a. 
It makes it necessary to install valves S.sub.1, S.sub.2 of very large 
diameters on the main pipes 8, 8' and 10, and mixers M for mixing sodium 
streams at different temperatures. Such devices are very expensive and 
they are the possible sources of failure and incidents, in particular as 
regards mixers M, the fully reliable operation of which is not yet ensured 
in the present state of the art. For reasons of safety, care is usually 
taken that the emergency exchanger be fed by the circuit pump, but also 
that it be possible to feed said exchanger through a thermosyphon effect 
or through natural convection, which necessitates that exchanger E be 
situated substancially higher than the intermediate exchangers. 
Finally, theoretical calculations regarding propagation of the ram-effect 
following a sodium-water reaction, indicate that the hydraulic system 
constituted by the secondary loop with generator 6 provided with its 
diaphragm and surrounded by its two argon pockets, does not always permit, 
under the best conditions, to restrict the transmission of substantial 
overpressures to the intermediate exchangers. Moreover, such a hydraulic 
system does not readily prevent from contaminating the secondary loop by 
the reaction products, up to the intermediate exchanger. These facts can 
be diagrammatically and qualitatively explained in the following way: 
whenever a leak is initiated, the overpressure generated in the 
steam-generator induces an oscillatory movement of large amplitude and of 
fairly long period of the sodium between the two ram-effect preventing 
tanks. Therefore, instead of increasing continuously, the pressure at the 
level of the diaphragm is caused to fluctuate and it may happen that the 
amplitude and duration of the first pressure peak be in sufficient for 
rupturing the diaphragm. In such case, one should wait until the 
occurrence of the second peak, or even the third one, for obtaining 
rupture. Now, in the meantime, the leak continues to flow and to store 
pressure energy in the secondary loop. At the moment the diaphragm rupture 
takes place, since the pressure energy to be exhausted is more important, 
it takes more time for depressurizing the system. The concurrence of these 
phenomena tends to result in an increase of stresses at the level of the 
intermediate exchanger and an increased contamination of the circuit. 
(c) as mentioned above, the rapid drainage of the secondary loop requires 
appropriate pipings and valves of large diameter, viz. expensive devices. 
Moreover, the presence of two upper points (the pump and the upper portion 
of the generator) and of two lower points (exchanger 4 and the lower 
portion of generator 6) makes it necessary to provide the drainage device 
at least in duplicates, since, as mentioned in FIG. 1, the presence of two 
parallel mounted exchangers 4, for instance, may render it necessary to 
duplicate the ducts for feeding them. Each of them will have to be 
provided with a duct and with a drain-valve (the latter being often 
duplicated for safety reasons). To the direct cost of those drains must be 
added obligations as regards the operation. Thus, when the circuit is in 
operation, any leak in the flap of these valves will induce a gradual 
drainage of the loop. In order to avoid the necessity of stopping a 
secondary loop, one is led to install the small circulating pump 19 for 
sodium leaks in storage tank 24. In its turn, said pump 19 entails 
obligations: as a protection against the risk of drowning the packings of 
main pump 12, it is necessary to provide a level regulation which, for 
safety reasons, will have to be reinforced by the return of sodium to the 
storage tank by means of overflow device 22. In any case, if the leak of 
the valves is too important, the operation of the loop has to be stopped. 
Quite favorably, circulating pump 19 is normally used for extra purposes. 
In particular, it permits to fill the loop from the storage, after a 
certain down-time. Said pump may also be used for feeding the devices 21 
for purifying sodium and regulating purity (cold traps, plugging 
indicators). During the filling operation, care must be taken that a false 
manoeuvre does not induce the drowning of the pump packing by an effect of 
communicating vessels between the two ram-effect preventing tanks, if the 
pressure therein is not suitable. In order to avoid such a risk, in 
addition to the already mentioned level and overflow adjustments, one 
takes care to place the upper portion of the generator (or the upstream 
ram-effect preventing tank) and the pump in the same horizontal plane. 
With a view to still more safety, the corresponding two argon pockets are 
connected by means of pipes 25 for balancing the levels and pressures. Of 
course, all these devices are very expensive and may be the cause of 
failures and entail obligations as regards exploitation. 
(d) in order that the ram-effect preventing tanks be sufficiently 
efficient, one is led to give same a fairly large volume. Again, for 
certain transient regimes, if it is desired to prevent the sodium level 
from varying to a too large extent, expansion tank 18 must be given a 
large volume. As explained above, it is intended to absorb the thermal 
expansion of sodium without drowning the packing. In addition, it must 
absorb the sodium thermal contractions occurring, e.g. during an emergency 
stoppage of the reactor: a very fast cooling of the sodium and the related 
contraction thereof might proceed until the pump be unprimed through 
unwatering of its suction ports. In such a case, the circulating pump is 
not sufficient for compensating the volume due to the contraction of 
sodium, unless the expansion tank has a sufficient sodium supply. The 
latter tank, in which, in addition, the pump must be installed with 
sufficient clearance for preventing packing 16 from being drowned, as 
explained above, is a large, heavy and expensive tank. 
In FIG. 3 is shown, with more accuracy, the installation of a secondary 
cooling circuit of the type of the one shown in FIG. 1. Similar or 
identical parts are designated by the same reference numbers in both 
figures. FIG. 3 also shows the lower floor 40 of the nuclear plant and the 
wall 42 defining the reactor confinement enclosure. The figure also shows 
the plug 44 of the nuclear reactor vessel 2, and, more diagrammatically, 
the means 12s for supporting pump 12 with its expansion tank 18, the means 
6s for supporting steam-generator 6 and the means 24s for supporting 
storage and recovery tank 24. 
The installation of a secondary loop according to the already described 
design requires a very large volume for the following two series of 
reasons: 
(a) in order that the loop can be drained by gravity, which is the safer 
solution, the lowest point of the main pipes has to be situated fairly 
above the storage tank. It is necessary, indeed, to have enough room for 
containing the drain-valves, the drain-pipes and the expansion forks with 
which said pipes must be provided in order to accomodate temperature 
changes. The assembly constituted by the secondary loop and the storage 
tank therefore occupies much room in height, which entails costly 
supporting means and large buildings. 
(b) main pipes 8, 8' and 10, in their turn, must be provided with expansion 
forks or with appropriate devices for compensating expansion. As regards 
the forks, it can be shown that the length of high temperature pipes to be 
mounted between two devices (more precisely, between two fixed points) is 
proportional to power 3/2 of the distance between said fixed points (said 
length is also proportional to the square root of the pipe-diameter). It 
can thus be seen from FIG. 3 that three fixed points have to be connected 
according to the three sides of triangle ABC. All the above enumereted 
obligations regarding installation contribute to lessen the possibility of 
reducing the length of the triangle sides by a large amount; thus, while 
it is possible to modify side BC, it is more difficult to simultaneously 
reduce AB and AC (coincident levels of the argon pocket, steam-generator 
supported preferably in the lower position, necessity of a drainage by 
gravity . . . ). Accordingly, the overall length of the pipes will have to 
be great, as shown, e.g. in FIG. 3, failing which numerous or large 
devices for compensating expansion will have to be provided. 
Moreover, it can be seen that pump 12 is under poor suction hydraulic 
conditions, since it has a low NPSH coefficient (NPSH standing for "net 
position suction head"). In order to avoid cavitation in that pump, one is 
led to adopt a low velocity of rotation, therefore a large diameter wheel 
and a slow driving motor. The whole assembly is very expensive since, as 
is well known, the price of a pump increases according to the square of 
the diameter thereof. 
In addition, the whole unit constituted by the motor-pump and the expansion 
tank is heavy. Since it occupies a high position in the installation, it 
requires important supporting means, in particular to avoid possible 
seismic stresses that tend to increase in proportion to the distance with 
respect to the groundlevel. No wonder therefore that, in various fast 
neutron power-plant designs, the overall price of the motor-pump, the 
expansion tank and the related supporting means, constitute a significant 
portion of the boiler overall cost. 
In brief, the investment and working cost of a secondary loop according to 
the above described prior art, is unfavorably influenced by a few 
parameters bound to the usual design of said loops. 
To sum up, the major drawbacks are as follows: 
the pump is in high position; it has a poor NPSH coefficient; it rotates 
too slowly and it is therefore heavy and costly; 
the expansion tank wherein the pump is normally installed, is heavy and 
bulky; 
the above two assemblies, occupying high positions, lead to expensive 
supporting frameworks (in particular, in view of seisms); 
the piping network is very long because of the existence of three fixed 
points to be connected, allowing few degrees of freedom for bringing same 
nearer to one another. 
protection against water-sodium reaction is obtained exclusively by means 
of rupturable costly diaphragms, entailing obligations as regards safety 
(exhaust of the reactor residual power in case in an unforeseen rupture) 
and as regards maintenance (periodical changes). Such a protection is far 
from perfect (oscillatory movements of sodium); 
the presence of valves and of large diameter drain-pipes that are expensive 
and are likely to induce failures (flap leaks); 
the presence of various pipes for performing various functions connected 
with the above mentioned obligations: filling, overflow, level balance, . 
. . ); 
unfavorable influence of the above factors on the sodium volume of the loop 
and, accordingly, on the size of the sodium tanks that are at least two in 
number (expansion, storage); 
unfavorable influence of the above on the volume occupied in the building 
(area at the ground level, height); 
unfavorable influence of the above on the electrical devices for 
pre-heating pipes and the tanks and on the control. 
The object of the present invention is precisely to provide a cooling 
secondary loop for fast neutron nuclear-reactors cooled by a liquid metal 
(sodium or a mixture of salts of liquid metals of the same type, 
obviating, or at least substantially decreasing the above mentioned 
drawbacks. In particular, the secondary loop forming the object of the 
present invention permits to achieve a substantial decrease of the space 
required for its installation; said secondary loop permits to cause the 
secondary pumps to operate under better conditions; it also permits, 
either to eliminate the safety diaphragms in the case of an explosive 
sodium-water reaction, or at least to render the action thereof less 
inmportant, through the addition of circuits adapted to ensure, in 
addition or exclusively, the exhaust of those products resulting from such 
a reaction, in order to protect the intermediate exchanger or exchangers 
of the secondary loop. 
With a view to providing the above mentioned results and other results to 
be explained later on, the present invention relates to a secondary 
coolant circuit for a nuclear reactor of the liquid-metal cooled type, 
said circuit comprising at least one intermediate exchanger mounted in the 
vessel of said reactor, outside said vessel a steam-generator for the 
exchange of calories between the secondary liquid-metal flowing through 
said secondary circuit and water-steam, at least one pump for circulating 
said secondary sodium and one tank for storing said secondary liquid-metal 
and recovering those products generated by a possible liquid metal-water 
reaction in said steam-generator, said liquid-metal being likely to occupy 
the lowest possible level in said tank, said secondary coolant circuit 
being characterized in that said tank is situated at the lowest possible 
level in the nuclear installation, in that the lower extremity of the 
liquid metal outlet duct of said steam-generator is directly dipped into 
said tank, in that, in said tank above the liquid metal, is maintained an 
inert gas cover at such a pressure that it balances the liquid metal 
pressure in the whole secondary loop, said tank, in addition, acting as 
the downstream ram-effect preventing tank for said steam-generator and as 
an expansion tank during the temperature variations of said liquid metal, 
and in that the rotor of said pump is situated above said lowest level. 
It can thus be seen that the recovery of the sodium-water reaction products 
can be achieved directly without the absolute need of diaphragms and, in 
addition, that the storage tank has also the function of a downstream 
ram-effect preventing tank for the steam-generator, whereby is eliminated 
a large volume tank in the secondary loop. 
According to a preferred embodiment, the circulating pump can be of any 
type, and the inlet of said pump is connected to the outlet duct of said 
steam-generator upstream of said tank, whereas the outlet of said pump is 
directly connected to the intermediate exchanger or exchangers, the said 
pump being outside the said tank. 
It can thus be seen that the pump is not dipped in the tank. In addition, 
it can be seen that the secondary circuit does not actually go through the 
storage tank. The sodium circulates directly from the exchanger to the 
pump. However, the circuit communicates with the tank. Said tank therefore 
does perform its "ram-effect preventing function", but there is normally 
no sodium flowing throughout the tank. 
According to a second embodiment, the said circulating pump is of the free 
level type and the said pump is situated in said tank, the inlet of said 
pump being directly dipped into the liquid metal contained in said tank, 
whereas the outlet of said pump is directly connected to said intermediate 
heat-exchanger or to each of said intermediate exchangers. 
According to a third embodiment, said pump is of the electromagnetic or 
"frozen seal" type, and said pump is situated just above said tank, the 
inlet duct of the pump being dipped into the liquide metal of said tank, 
whereas the outlet duct is directly connected to the intermediate 
exchanger or exchangers. 
According to a fourth embodiment, said pump is integrated to said 
steam-generator and situated in the upper portion of its casing or 
envelope, said steam-generator being provided with a central stack through 
which flows the secondary metal after it has passed through the exchange 
bundle, the said central stack constituting the inlet duct of said pump, 
the outlet of said pump being directly connected to said intermediate 
exchanger or exchangers. 
It can thus be seen that, in this latter case again, there is no liquid 
metal flowing through the storage tank. 
However, when the circulating pump is of the mechanical level type, there 
arise problems in cases where that pump is brought to a standstill whether 
voluntary or unforeseen. More precisely, the problem involved is that of 
the introduction of the covering gas into the sodium circuit, when a pump 
of that type is being unprimed. The same problem arises even in those 
cases where the pump is situated in the storage tank since it is then a 
stub shaft pump, viz. the rotor or wheel of the mechanical pump is 
situated above the lowest level of sodium in the tank. 
For a better understanding of that problem, FIG. 3' shows a free level 
mechanical pump. 
The object of that figure is mainly to show the various possible types of 
sodium operational leaks likely to occur in such a pump. 
The figure shows a supporting sleeve 120, passing through the wall of 
storage tank 24' and closed by a cover 122 provided with a sealing packing 
12'a for pump shaft 12"a (that packing, represented at 12'a in FIG. 1, is 
not shown in FIG. 2). Sleeve 120 supports the pump flange 126 provided 
with its output ports 128, connected to the duct or ducts 10' of FIG. 1. 
Pump 12' also comprises a sodium sucking axial port 130, connected to 
suction duct 130' dipped into tank 24' under the lowest sodium level 
N.sub.2. At the lower end of shaft 12"a is to be found the pumpwheel 132. 
About wheel 132 is a diffuser 134, in one piece with pump casing 136. Pump 
shaft 12"a penetrates into said casing 136 through a hydrostatic bearing 
138. In the vicinity of said bearing 138, are to be found a chamber 140 
for feeding said bearing and a chamber 140' into which penetrates a 
portion of the flow originating from the hydrostatic bearing (the other 
portion being directed towards the suction port of the pump via orifices 
in the wheel upper flange). With a view to providing a certain tightness, 
labyrinth seals 142 or seals with adjusted segments are to be found 
between casing 136 and sodium suction tubing 130' and between said casing 
136 and the supporting sleeve 120. The object of such semi-tight devices 
is to restrict leaks F.sub.1 between the static portions 136 of the pump 
and the supporting sleeve 120, so as to permit an easy dismounting from 
above of the whole assembly constituted by pump casing 136, wheel 132 and 
shaft 12"a. Other labyrinth joints, or joints with adjusted sealing rings, 
are provided between casing 136 and pump wheel 132, respectively. The 
corresponding leaks are designated by arrows F.sub.2. Finally, leaks 
designated by arrows F.sub.3 are mentioned in the figure, said leaks being 
related to the flow into and from hydrostatic bearing 138. 
In addition, it should be stated that sleeve 120 for supporting the pump 
casing is provided with upper vents or ports 146, adapted to ensure the 
balance of argon pressure between the inside and the outside of said 
sleeve 120, said port 146 being situated higher than the highest level 
N.sub.3 of sodium in tank 24' and, if need be, a second series of vents or 
ports 148 adapted to ensure the balance of sodium levels between the 
inside and the outside of sleeve 120, the ports of said second series 148 
being situated preferably under the sodium highest level N.sub.3. 
It will be cleary understood that, when, taking account of the storage tank 
height, the pump shaft is long enough to allow the pump wheel to be under 
the lowest level of sodium, the possible operational leaks are by no means 
an inconvenience. On the other hand, when the pump shaft is very short 
(the wheel being above the lowest level), which is the case in the present 
invention, and when the pump comes to a stop in an unforeseen manner or 
when the speed thereof decreases under a given value, some of these 
operational leaks stop ejecting sodium and are even reversed so that argon 
penetrates into the pump casing, then from said casing into the very loop. 
If the stoppage is maintained, the pump is finally unprimed and the loop 
is in a position to be gradually drained. The same thing takes place when 
the mechanical pump is outside the tank. It may happen that such a 
drawback be tolerated, since it does not impair the normal operation of 
the installation but merely gives rise to extra intricacies during 
incidents such as e.g. an unforeseen stoppage of the pump. However, that 
drawback constitutes an obligation that does not exist in normal circuits. 
In addition, it can entail some risks as regards the safety of the 
installation, in particular if the secondary loops are used under every 
circumstance for evacuating the reactor residual power, as is the practice 
in various fast neutron power-station. 
The simplest way to obviate that drawback without modifying the basic 
principle of the secondary circuit according to the invention consists in 
giving the argon circuit fairly large dimensions. Following an unforeseen 
stoppage of the pump, as argon bubbles penetrate into the loop and are 
driven by gravity to the argon pocket of the steam-generator, thus 
inducing the gradual drainage of the secondary circuit sodium, the level 
regulation of the generator argon pocket controls an equivalent 
introduction of argon into the storage tank, which thus permanently 
compensates for the flow escaping from that tank. The drawback of the 
method mainly lies in the bad consequencies of a permanent flow of argon 
through such a device. Indeed, the experience gained from a large number 
of liquid sodium circuits and from the related argon circuits indicates 
that it is of prime importance to prevent transfers of hot argon, loaded 
with steam and sodium aerosols, since they cause the pipes and the devices 
submitted to such gas flows to be frequently choked. A rule in the art 
therefore consists in restricting the movements of argon to no more than 
necessary. 
A solution to that problem might consist in using a pump with a long shaft, 
in which case the pump wheel is situated under the sodium lowest level in 
the storage tank inspite of the height of the latter. Thus, the 
above-mentioned problems disappear. With such an arrangement, the pump 
operation no longer sets any problem of the type of those arising in the 
usual secondary circuits. However, with such a solution, the advantage 
that can be drawn from the fact that the NPSH coefficient is high, may be 
partially or wholly counterbalanced by an increase of the costs due to the 
fact that the pump shaft is much longer than in the usual devices and in 
view of the extra expenses resulting from the necessity of maintaining a 
sufficient supply of sodium at the tank bottom in order to drown the pump. 
Such a solution must therefore be rejected. 
That is why, according to the present invention, improvements are provided 
that are connected to the installation of pumps with a short shaft in the 
storage tank or to the case of free level pumps situated outside of the 
storage tank, permitting to obviate the problem of undrowning the pump, 
even in case the latter is stopped, without substantially complicating the 
circuit. 
According to those improvements, the secondary circuit in which the liquid 
metal occupies, in the storage tank, a lower level whenever said secondary 
circuit is filled with said liquid metal and a higher level whenever said 
secondary circuit is empty, is characterized in that the wheel of said 
mechanical pump is mounted, at the extremity of the shaft thereof, at an 
intermediate level between said lower level and said higher level, in that 
the said suction duct of the pump opens lower than said lower level and in 
that said pump is provided with protective means serving, when the liquid 
metal is at said lower level, to prevent the gas surmounting said liquid 
metal from penetrating into the back-flow duct of said pump, should the 
latter happen to be stopped. 
According to a first embodiment, the said protective means consist in the 
fact that the said back-flow duct forms an elbow and has, in said storage 
tank, a low point situated under said lower level. 
According to a second embodiment, the secondary circuit comprising a 
purification dependent circuit provided with its own pump through which 
the liquid metal flows, the inlet of said dependent circuit opening into 
the storage tank under said lower level, is characterized in that the 
protective means consist in that the outlet of said dependent circuit 
opens into the pump casing above said wheel, the flow-rate of said 
purification dependent circuit being greater than the flow-rate of the 
operational leaks of said pump, when the latter is stopped. 
Of course, it is possible to combine the two types of protection. 
According to another embodiment of the improvements applicable to the two 
ways of mounting mechanical pumps (inside or outside the storage tank), 
the system for avoiding the introduction of gas bubbles consists in that 
the back-flow duct or ducts connecting the outlet of said pump to said 
intermediate exchanger comprises an upper point, the slope of the portion 
of said conduit between the pump outlet and said upper point is sufficient 
and suitably directed to allow the gas bubbles likely to penetrate into 
said pump to be drained and return to said upper point by gravity, said 
upper point is provided with an orifice or vent permitting said bubbles to 
escape and means are provided for introducing an equivalent amount of gas 
into said tank so as to maintain said pressure. 
Quite obviously, in cases where the pumps are contained in the storage 
tank, the latter protective means may be combined with the previously 
described protective means.

The secondary loop, such as shown in FIG. 4a, comprises the reactor-vessel 
2 with intermediate exchangers 4 and its steam-generator 6 connected to 
the intermediate exchangers via ducts 8 and 8'. Steam-generator 6 is 
surmounted with its argon pocket 6a as in FIG. 1. There is also a storage 
tank 24' as in FIG. 1, but in a modified form. There is also a duct 10' 
corresponding to duct 10, connecting circulating pump 12' to the inlet of 
intermediate exchangers 4. With respect to FIG. 1, corresponding to the 
prior art, it must be noted that the outlet tubing 6b of steam-generator 6 
opens directly into tank 24', and that pump 12', in the present case of 
the free surface type, is mounted inside tank 24'. The latter rests on the 
ground and therefore constitutes the lowest point of the nuclear plant. 
Therefore, pump 12' occupies a low position in the installation. In 
addition, circuit 20' permits to purify the sodium by picking up a portion 
thereof. Said circuit 20' essentially comprises a purification unit 50 of 
a known type and a circulating pump 52, usually of the electromagnetic 
type. 
FIG. 4a thus represents a secondary loop with a free surface pump 12' 
installed in a low position in a large tank 24' assuming a storage 
function (when the loop is stopped), and an expansion function (for every 
mode of operation) and, in addition, acting as downstream ram-effect 
preventing tank and as a tank for the recovery of the products of a 
possible sodium-water reaction. Several tanks of the usual system are 
replaced by a single tank, the latter moreover occupying a low position. 
Pump 12' sucks sodium from that tank and repels it into exchangers 4. 
Sodium is then introduced through the generator upper extremity, where an 
argon pocket 6a has been provided. From that point, sodium flows downwards 
through the bundle of tubes. At the exit of the generator, the sodium is 
sent back to the single storage-expansion-recovery tank 24', via a duct 
6b, the latter being as short and as upright as possible, in order to 
promote a rapid drainage (in particular, of the soda-contaminated sodium 
in the case of a sodium-water reaction) and to reduce the overall height 
of the system. 
With a view to making up for the piping expansion between the pump and the 
tank wall and between the steam-generator and said tank, it is possible to 
provide expansion compensators 24'a at appropriate places. For reasons 
peculiar to the technique of sodium, said compensators 24'a are usually 
compensators with metal bellows. By placing said compensators in front 
instead of in direct contact with the sodium, as permitted by the 
arrangement of the secondary loop according to the invention, it is 
possible to obviate the usual drawbacks of said sealing devices when in 
direct contact with sodium. The position they occupy renders them 
perfectly reliable and safe and, should they happen to be ruptured, there 
would be no leakage of sodium to the outside. Indeed, the storage tank has 
been selected so as to be large enough to contain all the loop sodium, at 
the highest temperature expected, while maintaining, above free surface 
N.sub.3, an argon pocket sufficient for preventing: 
(1) the drowning of the packing of pump 12'a; 
(2) the drowning of expansion compensators 24'a, if any. 
In order to operate the system, the argon pressure inside tank 24' must 
compensate the head if sodium in the loop pipes and in the 
steam-generator. For a zero flow-rate of the sodium, the absolute pressure 
of the tank argon is therefore equal to the pressure inside pocket 6a of 
the generator (which, as explained above, should be hardly higher than 
atmospheric pressure), plus the pressure equivalent to the height of 
sodium within the loop with respect to the free surface in the storage 
tank. From most of the known generator designs, it can be assumed that 
said height of sodium will not be more than 30 m (100 ft) and, in any 
case, will be in the vicinity of that value: it is a moderate pressure, in 
perfect compliance with the rules concerning pressurized-gas devices for 
that type of circuit. It can be seen that, when the pump is in operation, 
the argon pressure inside the tank is smaller, since it is then decreased 
by the head-loss of sodium through the steam-generator (said head-loss 
being, e.g. of about 1 bar). It follows therefrom that the pressure of the 
tank argon is still smaller than when the loop is full and stopped. 
However, that pressure is still high enough for providing, at the pump 
inlet, a high NPSH coefficient, e.g. of about 2 or 3 absolute bars. Such a 
value is substantially greater than that obtained in the prior art (FIG. 
1), where the pump is at the upper point (e.g. 1 to 1.3 bar). A 
substantial gain can be obtained on the pump rotation velocity and, 
therefore, on the cost thereof. 
In FIGS. 4b and 4c are shown modified embodiments of steam-generator 6. In 
steam-generator 6' (FIG. 4b) argon pocket 6'a is spaced from the 
steam-generator outer envelope and connected to the inlet of said 
generator by means of duct 6'b. In other respects, the steam-generator is 
similar to that of FIG. 4a. In FIG. 4c, steam-generator 6" is of the 
modular type. As in the previous example, there is an argon pocket (6"a) 
spaced from the outer envelope; however, the heat-exchanger proper is 
constituted by a plurality of parallel-mounted modules 6d. In other words, 
the inlets of the exchange modules are all connected to duct 6'b, whereas 
the outlets are connected to duct 6b. In other respects, the two devices 
are similar. 
In FIGS. 6a to 6d are shown various modes of circulation of the secondary 
sodium inside steam-generator 6 or inside one of the exchange modules of 
said steam-generator. In FIG. 6a is to be found the same arrangement as 
described with respect to FIG. 4b. In other words, the inlet tubes for 
secondary sodium are situated at the upper portion of the steam-generator 
outer envelope, and the secondary sodium flows from top to bottom the 
various exchange-tubes containing water, said tubes, designated by hatched 
portion 54, occupying the whole cross section of the heat-exchanger. The 
exit of cooled secondary sodium is through tubing 6b. In the case of FIG. 
6b, the outer envelope of the steam-generator is provided with a central 
stack 56, connected to inlet tubing 8. Deflector 58 directs secondary 
sodium towards annular space 54' containing the exchange-tubes in which 
the water flows. The exit of cold secondary sodium takes place by means of 
a connection with outlet tubing 6b. Here again, therefore, there is a flow 
of secondary sodium from top to bottom. 
In FIG. 6c, the sodium is introduced in the lower portion of the exchanger 
outer envelope, and it flows first in an annular space 60 defined between 
said outer envelope and a baffle 62 containing the whole system of 
exchange-tubes 54". Deflectors 64 are adapted to direct the sodium towards 
the upper portion of the bundle of exchange-tubes 54". The exit of cold 
secondary sodium takes place via outlet tubing 6b. 
In FIG. 6d, hot secondary sodium is introduced through the lower portion of 
the steam-generator by means of ducts 8 and 8'. Said hot sodium flows 
through the bundle of exchange-tubes 54", said bundle, in the present 
instance, forming a ring about a central exhaust stack 66. Once it has 
flowed through said bundle, the secondary sodium is directed towards stack 
66 by deflectors 68. Said stack 66 is connected to outlet tubing 6b. 
Quite obviously, in FIG. 4a, steam-generator 6 might be exchanged for any 
of the steam-generators of FIGS. 6b to 6d. Again, it is possible, without 
going beyond the scope of the invention, to combine the various 
embodiments of FIGS. 6a to 6d with the embodiments of FIGS. 4b and 4c. 
In FIG. 7 are shown the circuits for argon, or more generally for an inert 
gas, permitting to ajust the sodium levels in the various tanks of the 
secondary loop. In said figure, is to be found, first, a duct 70 for rapid 
depressurization, connecting tank 24' with separator 32. Said duct is of 
large diameter and its slope is directed towards the storage tank. The 
temperature is regulated up to the point where said duct opens into 
separator 32. Said duct is provided with a rapid depressurization valve 
V.sub.4, that opens (either under control or automatically) whenever the 
pressure within tank 24' is greater than a reference value. Here again is 
provided an emergency rapid depressurization duct 72, of large diameter 
and the slope of which is, here again, directed towards the storage tank, 
said duct connecting tank 24' with separator 32. The temperature is 
regulated up to rupturable diaphragm M.sub.1. Finally, there is provided 
duct 74 for ensuring the balance of argon pressure between steam-generator 
pocket 6a and tank 24'. Said duct 74 for the return of the condensate is 
of large diameter and its slope towards the tank is constant. The 
temperature is permanently regulated at a value of about 150.degree. C. It 
may be added that duct 72 can be preferably provided with a valve V.sub.5 
that is maintained locked-open in normal operation and closes following 
the rupture of diaphragm M.sub.1 so as to avoid the penetration of air 
into the circuit. The rupture pressure of emergency diaphragm M.sub.1 
mounted in duct 72 is higher than the pressure of automatic opening of 
valve V.sub.4 mounted in duct 70. If desired, valve V.sub.6 for balancing 
pressure between the storage tank and pocket 6a of the steam-generator 
can be automatically controlled when, valve V.sub.4 being open, the 
pressure inside of the storage tank is in the vicinity of the pressure in 
argon pocket 6a. 
At the upper portion of said argon pocket 6a, there is an argon inlet duct 
76, opening into pocket 6a through a three-way valve V.sub.7. The 
controlled or automatic operation of said valve permits to regulate the 
pressure inside argon pocket 6a. A further valve V.sub.8, mounted in duct 
78 for the introduction of argon into storage tank 24', permits to 
regulate the level of sodium in argon pocket 6a. Valve V.sub.9 permits to 
adjust the introduction of argon into separator 32 so as to regulate the 
pressure of argon in said tank. Valve V.sub.10 permits, if desired, to 
pick up some more or less oxidized sodium withdrawn from separator 32. On 
stack 34 is to be found a valve S of large section and low calibration 
pressure, e.g. of from about 0.05 to about 0.1 relative bar. In said 
figure, C.sub.1 designates a sensor mounted in argon pocket 6a and adapted 
to determine the sodium level in said pocket and, accordingly, to control 
valve V.sub.8 through follow-up linkage 79. Finally, C.sub.2 designates a 
pressure sensor mounted in argon pocket 6a and adapted to control valve 
V.sub.7 through follow-up linkage 80. 
The above various argon circuits fulfill the following functions: 
(1) filling the secondary loop with sodium from storage tank 24': valve 
V.sub.6 is closed; the pressure regulation (e.g. 1.1 bar) of the generator 
upper portion is being carried out. By means of valve V.sub.8, the storage 
tank is pressurized, which, by counter-pressure effet, induces the rise of 
sodium in the loop. Once the level determined by C.sub.1 has been reached 
in the steam-generator, the level regulation acts on valve V.sub.8 so as 
to maintain a constant level in said generator. As for valve V.sub.7, it 
keeps on regulating the pressure of argon pocket 6a to the value of e.g. 
1.1 bar. 
(2) starting of pump 12'; operation at full load or at partial load: as 
soon as the pump is in operation (its starting is usually gradual since, 
for other reasons, pumps of that type are driven by a variable speed 
motor), the level tends to change in the steam-generator; the level 
regulation acts on valve V.sub.8 accordingly, in particular in such a 
manner that, at nominal regime, the pressure in the storage tank be 
lessened by an amount corresponding to the loss of head in the 
steam-generator. 
(3) normal drainage: the pump being stopped, the level regulation is 
inhibited and valve V.sub.6 is opened gradually; the pressures tend to 
counter-balance between the storage tank and the generator pocket 6a and, 
accordingly, the level of sodium in the loop is lower and lower as the 
sodium is sent back into the storage tank and is replaced by storage argon 
in the upper portion. The regulation due to valve V.sub.7 by argon 
drainage is operated in such a manner that, when the drainage is over, the 
pressure is uniformly settled at 1.1 bar for instance, or at any other 
value deliberately selected. 
(4) rapid drainage (because e.g. of a sodium leak in the loop): both valves 
V.sub.4 and V.sub.6 are fully open. Valve V.sub.4 serves to depressurize 
the storage tank rapidly, while valve V.sub.6 permits to obtain a rapid 
balance of the pressures in the installation, which ensures a rapid 
drainage of sodium in storage tank 24'. 
(5) sodium-water reaction: the hydrogen bubbles developed in the 
steam-generator tend to repel the sodium on both sides; the generator 
argon pocket is pressurized, in accordance with its function of ram-effect 
preventing pad, and so does the argon pocket of the storage tank. However, 
in view of the large volume of storage tank 24', the pressure therein 
varies very slowly. Within a very short time, the hydrogen bubbles 
becoming bigger and bigger induce the downward drainage of all the 
steam-generator sodium situated lower than the leak. From that moment, the 
steam and hydrogen from the steam-generator penetrate directly into the 
storage tank; actually, ram-effects no longer take place, but there is, 
instead, a gradual rise of the system gas-pressure. With a view to 
restricting such a pressure rise, it is possible to resort to various 
procedures, either successively or simultaneously. 
(a) the specific leakage detectors, or sensors (noise measurement, 
measurement of the hydrogen present in the sodium or in the argon of 
pocket 6a or of tank 24') give the alarm and permit to open valve V.sub.4 
very soon, so as to depressurize the system. In addition, they permit, by 
means of appropriate valves, to depressurize the water-steam circuit and 
isolate the steam-generator at the water inlet and at the steam outlet, 
according to a known procedure; 
(b) pressure, level, and flow rate sensors, by corrolating their readings, 
give the alarm and induce the same operations; 
(c) the pressure within the storage tank reaches a predetermined value 
which causes valve V.sub.4 to open automatically (which, in other words, 
means that valve V.sub.4 acts as a safety valve); 
(d) as an ultimate emergency mean, it is possible to provide a rupturable 
diaphragm M.sub.1 over the argon of storage tank 24'. 
Should all the above devices fail to work, said diaphragm would finally 
rupture. Valve V.sub.5, normally maintained open (for instance by 
locking), could then be closed so as to avoid the introduction of air. 
The argon circuit shown in FIG. 7 is given merely by way of explanation. 
Other arrangements, either more simple or more sophisticated, fulfilling 
the same functions might be resorted to. Said circuit is not shown in its 
entirety; some portions thereof have been omitted, for instance that 
portion corresponding to the device for storing the make-up argon, or that 
portion corresponding to a possible system for recycling argon with a view 
to reducing the consumption thereof, since such devices are no parts of 
the loop according to the invention. 
FIG. 4d represents a concrete application of the secondary loop according 
to the invention, such as shown in FIG. 4a. The reference numerals of FIG. 
4a have been kept in FIG. 4d. The latter, drawn at the same scale as FIG. 
3, clearly indicates what space saving the invention permits with respect 
to the circuit of the prior art. 
Moreover, the drainage system is considerably simplified, since its extra 
ducts comprise only the drains or orifices 25' situated in the upper 
portion of the connection between the intermediate exchanger and the pump 
and capable of being readily connected to argon pocket 6a. The ducts must 
be installed with a given slope (of from about 3 to 5%) and suitably 
directed. It is to be noted, moreover, that, in FIG. 4d, the slope has 
been given a direction permitting to drain the intermediate exchanger 
almost fully by syphon-effect, which was not possible in the prior art. 
FIG. 8 shows a variant of the secondary loop, resorting to an integrated 
pump-exchanger block 90. The exchanger portion 90a of said block comprises 
central stack 90b and the annular bundle of exchange-tubes 90c. As for the 
pump portion 90d, it comprises the free level pump 90e proper with its 
expansion tank 90f. The secondary sodium penetrates into exchanger 90a via 
duct 8 and it leaves pump 90d via duct 10". It is to be noted, however, 
that the same secondary circuit principle is to be found, here again, 
since the bottom of steam-generator 90a is in direct communication, 
through duct 6b, with tank 24', the latter thus constituting at the same 
time the downstream ram-effect preventing tank. FIG. 8 also shows drainage 
duct 92 opening into tank 24'. Said duct, provided with valve V.sub.11 and 
the diameter of which is very small, is used only for draining pipes 8 and 
10", while the drainage of steam-generator and of expansion tank 90f is 
carried out through pipe 6b. 
In the above-described various embodiments, the pumps used were free level 
pumps 12', but, in the secondary loop according to the invention, it is 
just as well possible to use "frozen seal" pumps or electromagnetic pumps. 
As already mentioned, FIG. 5a is a half view in axial section of a pump of 
the "frozen seal" type. Said pump is designated by reference numeral 12". 
It comprises casing 12"b, a wheel 12"c and the driving shaft 12"a 
therefor. There is also provided outer sleeve 12"d, cooled by blades and a 
natural or forced flow of air, generating "frozen seal" 100 of sodium. 
That pump also comprises a tubing 12"e for the inlet of an inert used both 
for preventing said "frozen joint" from being oxidized and for expelling 
said "frozen seal" once melt, so as to allow e.g., the pump to be 
dismounted. 
FIG. 5b shows a possible embodiment of the secondary loop using pumps 12" 
of the "frozen seal" type (or, in some cases, electromagnetic pumps), viz. 
pumps for which it is not necessary to provide an inert gas pressure for 
achieving tightness. 
In FIG. 5b, pump 12" is outside of the tank 24", but it is however in the 
vicinity thereof so as to occupy a low position. Inlet duct 12"a is dipped 
in sodium and passes through the tank wall via an expansion sleeve. It is 
also possible to install the pump directly on the tank according to the 
arrangement shown at 5d. 
FIG. 5c represents a preferred mounting of pump 12". That pump can be 
either of the mechanical type, as shown in FIG. 3', or of the "frozen 
seal" type, as shown in FIG. 5a, or else of the electromagnetic type, well 
known in the field of nuclear-reactors cooled by a liquid metal. According 
to such a mounting mode, the inlet 12"b of pump 12" is directly connected 
to the outlet duct 6b of steam-generator 6. That duct is thus, as well as 
pump 12", outside of tank 24'. The pump outlet 12"c is directly connected, 
by means of the back-flow duct, to duct 10', viz. to the intermediate 
exchanger or exchangers 4. It is possible, of course, to provide several 
back-flow ducts. However, the lower extremity of duct 6b is dipped in tank 
24" under the lowest level of the liquid metal. 
It will be clearly understood that, according to such a preferred 
embodiment, the liquid metal issuing from steam-generator 6 is directly 
introduced into pump 12" via duct 12"b. Actually, in other words, the 
secondary liquid metal circuit does not contain tank 24'. Therefore, in 
normal operation, there is no circulation of liquid metal in tank 24'. 
Such an arrangement is advantageous for the construction and operation of 
the secondary loop. Moreover, since the lower extremity of duct 6b is 
dipped in tank 24' and opens under the lowest level of liquid metal, said 
tank 24' is in a position to act both as an downstream ram-effect 
preventing tank and as an expansion tank. 
It is to be noticed that the embodiment of FIG. 8 ensures the same 
advantage. In view of the fact that the pump is integrated at the upper 
portion of the exchanger, the liquid metal does not flow through tank 24'. 
However, duct 6b provides a communication between said tank 24' and the 
secondary loop. 
FIG. 10 shows a further embodiment of the circuit that distinguishes from 
the others only by the supporting means for the steam-generator. The 
generator envelope is extended by a supporting sleeve 6's, welded to the 
upper wall of tank 24'. One thus dispenses with expansion sleeve 24'a. 
Quite obviously, it is possible, without going beyond the scope of the 
invention, to combine the various variants described concerning the 
various parts of the secondary loop. In particular, it is possible to 
combine the various types of steam-generators associated to their upstream 
ram-effect preventing tank, with the various types of pumps and their 
various modes of installation. 
In FIGS. 9a and 9b are represented two preferred embodiments of the 
emergency cooling circuit. As already mentioned, it is often useful to 
provide such a circuit in the secondary loop. 
In FIG. 9a, the emergency exchanger consists, by way of example, of a coil 
E" cooperating with an air stack E', similar to that of FIG. 2. The inlet 
of said coil E" is connected to ducts 8 and 8' through small diameter 
pipes 110 and 110', provided with small diameter valves W.sub.1, W'.sub.1. 
The outlet of coil E" is constituted by tubing 112, provided with small 
diameter valve W.sub.2. The lower extremity of tubing 112 opens into the 
sodium of storage tank 24'. It is to be noted that the upper points of the 
emergency circuit are constituted by the junctions of ducts 110 and 110'. 
Moreover, the various elements of the emergency circuit must meet the 
following requirements as regards their position: 
the upstream junction 110, 110' of exchanger E" is situated on the main 
piping 8, 8' for the introduction of sodium into the steam-generator, at a 
level lower (e.g. by a few meters) than that of the point where main 
piping 8, 8' opens into said steam-generator. Accordingly, it is possible, 
by lowering the sodium free level in the generator, to undrown the points 
where main piping 8, 8' opens into the generator, without undrowning 
junctions 110, 110' of exchanger E". 
recovery junction 112 of the exchanger is itself transferred downstream of 
the generator, to a point the level of which must be lower than, or the 
same as, that of the upstream junction. A specially advantageous 
arrangement, shown in FIG. 9a, consists in transferring the point where 
recovery piping 112 opens into storage tank 24', upstream of the suction 
piping of pump 12', in an area of highly turbulent flow. Therefore, no 
mixer has to be installed on the piping; such a mixer is replaced by the 
storage tank 24' itself. 
emergency exchanger E" is keyed at any level between the thus-determined 
upstream and downstream junctions. However, if it is desired to provide a 
thermosyphon in the emergency exchanger, it will have to be installed at a 
level fairly above that of intermediate exchangers 4. 
The operation of the system is as follows: 
When the generator is in operation, valves W.sub.1 and W.sub.2 are closed; 
the circuit of emergency exchanger E" is constituted by argon, therefore 
stopped, and pre-heated in order that, at any moment, it may be filled 
with sodium without the risk of being choked because of the solidification 
of sodium at any point. Such a safety measure is also necessary in view of 
a possibility of slight leaks in valves W.sub.1 and W.sub.2 : should 
sodium penetrate into the circuit and into emergency exchanger E", it 
would remain in the liquid state. During the downtimes of generator 6 
(steam and water exchanged for an inert gas, e.g. nitrogen), by opening 
valves W.sub.1 and W.sub.2, one fills up the related circuit. 
It can then be used in two various ways: 
(a) in parallel with generator 6: pump 12' of the circuit providing a large 
flow-rate, a portion of the latter passes through generator 6 (without 
being cooled), while the other portion thereof passes through emergency 
exchanger E" in parallel and is cooled therein. At the point of recovery, 
the two sodium streams at different temperatures are fairly mixed, since 
the flow-rate and turbulence are high at that point. 
(b) as a thermosyphon: pump 12' of the circuit is then stopped. With a view 
to forcing the whole thermosyphon flow-rate through the emergency 
exchanger, it is necessary, with the help of e.g. valve V.sub.7 (FIG. 7), 
to introduce argon into the generator pocket until the free level N.sub.5 
of the pocket undrowns the points where the sodium inlet pipes 8 and 8' 
open into the generator (of course, without undrowning the upstream 
junction of the circuit of emergency exchanger E", situated at a lower 
level). Sodium no longer flows through the generator and the whole 
contents of the thermosyphon feeds said emergency exchanger. 
FIG. 9b shows a second embodiment of the emergency circuit. The only 
difference with FIG. 9a lies in the fact that emergency exchanger E" 
constitutes the upper point of the emergency circuit. It is then necessary 
to provide a venting device. The latter can preferably be constituted by a 
small expansion tank 114, the free level N.sub.6 of which can be adjusted 
by introducing argon via duct 116. 
The arrangement represented in FIG. 9b can work according to any of the two 
previously described modes. The only differences relate to the way of 
carrying out the filling operation and the operation of the related argon 
circuit. 
(a) filling operation: once the level and pressure regulations of the 
generator argon pocket 6a have been inhibited, they are transferred to the 
small tank 114 of the emergency exchanger circuit (duct 116). By opening 
valves W.sub.1 and W.sub.2, which can be dispensed with, but however act 
as safety devices, the filling of that circuit is automatically obtained 
by counter-pressure effect. The level N.sub.6 of small tank 114 controls 
the pressure of argon in storage tank 24'; the pressure of argon in small 
tank 114 is adjusted to, e.g., 1.1 bar. 
(b) operation when pump 12' of the main circuit is running: the above 
regulation fulfills its function so as to compensate for the variations in 
loss of head likely to result from variations in the pump flow-rate. 
With a view to preventing the undrowning of the points where main pipes 8 
and 8' open into generator 6, it is possible to regulate the sodium level 
by means of argon exhaust or inlet valve V.sub.7, to pocket 6a of the 
generator (where pressure is free). 
(c) operation as a thermosyphon: in the final step of the filling 
operation, argon must be injected into pocket 6a of generator 6, in order 
to lower level N.sub.6 and undrown the opening points of main pipes 8, 8'. 
The operation is then as previously described. As in the case of FIG. 9a, 
it might prove useful to regulate the level of the generator argon pocket 
6a in order to prevent it from rising up again unexpectedly, which would 
lead to by-pass emergency exchanger E". Care must also be taken of 
preventing said level from being lowered to the point of undrowning the 
upstream junction of emergency exchanger E". To that end, a rough 
regulation will be sufficient, either by means of an extra level-sensor in 
the generator pocket, or by using another reference value for regulating 
the pressure of the generator pocket 6a, viz. a reference value equal to 
the pressure of the argon pocket of small tank 114 (e.g. 1.1 bar), 
increased by a pressure equivalent to the height of sodium between the two 
pockets 6a and 114. 
The arrangement represented in FIG. 9b, although more sophisticated than 
that of FIG. 9a, however has the advantage of permitting the operation as 
a thermosyphon, even if the steam-generator is installed lower than the 
level of intermediate exchangers 4. In such a case, when emergency 
exchanger E" is in operation, the static pressure within storage tank 24' 
is higher than when the generator alone is in operation, since the sodium 
must be brought to a higher level than that of the head of generator 6. 
The corresponding pressure increase, e.g. 1 or 2 bars, is perfectly 
admissible, taking into account the margins to be kept when the generator 
is in operation, for withstanding the sodium-water reaction. Indeed, such 
an overpressure of one or two bars, necessary for the operation of 
emergency exchanger E", is then taken in those margins reserved for the 
sodium-water reaction, the latter, in such a case, being no longer to be 
feared, since the steam generator is under a nitrogen atmosphere and, 
therefore, absolutely devoid of water or steam. 
If the emergency circuit is not expected to function as a thermosyphon, 
then the arrangement of FIG. 9a is preferable, though it necessarily 
requires valves W.sub.1 and W.sub.2 between the loop and the storage tank. 
However, even should said valves W.sub.1 and W.sub.2 happen to be slightly 
leaky, the level and pressure regulations would compensate for sodium 
losses. 
In FIG. 11 is shown a first improved embodiment permitting to dispense with 
the rising of argon into the sodium circuit of the secondary loop, and, 
more precisely, into back-flow duct 10'. FIG. 11 shows the lower portion 
of steam-generator 6, with outlet duct 6b opening into the bottom of 
storage tank 24' and, in any case, lower than the lowest level N.sub.2 of 
liquid sodium in said tank. 
There is also represented, in said figure, pump 12' with its inlet tubing 
130' dipped into tank 24', lower than the lowest level N.sub.2 of sodium. 
In pump 12', the wheel 132 and the sodium back-flow nozzle 128 are shown 
diagrammatically. According to said first mode of operation, a portion of 
a cranked tube 150 is mounted between the pump outlet duct 10' and the 
pump nozzle 128. That tube is so mounted that its lowest point 150a be at 
a lower level than the lowest level N.sub.2 of the liquid metal in storage 
tank 24'. 
In view of the cranked shape of the pump back-flow tubing in the vicinity 
of the tank bottom, it is unvoidable that, sooner or later, operation 
leaks F.sub.1, F.sub.2 or F.sub.3 will unprime the pump; however, the 
level will be stabilized in the downward leg of the back-flow tubing and 
the loop will remain filled with sodium, without the necessity of 
oversizing the argon regulation. In such a device, the mechanical pump 
cannot be re-started without precaution, since said pump is absolutely 
unprimed. A possible method may consist in inducing a rapid initial 
drainage, permitting to expel towards storage tank 24', the argon trapped 
in the pump, in the downward portion of the back-flow piping and in the 
pump suction piping. One causes then the pump to start at a low speed in 
order to improve the venting of the loop (through the venting means in 
high position); then, the level and pressure regulations being put in 
operation again, the complementary filling of the loop is obtained 
automatically. 
FIG. 12 represents an improved second embodiment, comprising the same 
elements as in FIG. 10, viz. Pump 12' with its long inlet tubing 130' 
extending lower than level N.sub.2, its wheel 132 with its short shaft 12" 
and its liquid metal tank 140', downstream of the hydrostatic bearing. 
According to that embodiment, the extremity 20'a of the associated circuit 
20' (said circuit, if need be, being also used for purification) is 
connected to the pump and, more precisely, to tank 140.degree. provided in 
the supporting sleeve of the pump casing. Of course, said associated or 
dependent circuit 20' comprises a further picking up extremity 20'b in 
storage tank 24' the latter extremity being lower than the lowest level 
N.sub.2. In addition to a possible purification device 50, said circuit 
contains a continuously operating pump 52, preferably of the 
electromagnetic type, so that liquid sodium is permanently picked up in 
the lower portion of storage tank 24' and sodium liquid is permanently 
re-injected into the upper portion of pump 12'. The sodium is thus 
recycled in the pump casing, at a level higher than the upper level of the 
hydrostatic bearing, and more generally higher than the level of any of 
the leaks F.sub.1, F.sub.2, F.sub.3 causing the pump high pressure-portion 
to be in communication with the argon atmosphere of the storage tank. 
When pump 12' is in normal operation, the operational leaks are added to 
the sodium flow provided by associated circuit 20', said circuit, as 
mentioned above, comprising a small special pump 52, usually of the 
electromagnetic type, providing the flow-rate required for purification. 
The upper pump casing is filled with sodium up to the level of the 
overflow windows 148 provided in pump-supporting sleeve 120. 
During the down-times of the pump, a portion of the sodium provided by 
purification will be sucked through the passages for operational leaks. 
The excess of sodium will be expelled as usual by the overflow, provided 
of course that the purification flow-rate be greater than the flow-rate 
sucked by the operational leaks. Such a requirement can be met easily, 
taking into account the values usually adopted for the purification 
flow-rate, e.g. a few scores of liters per second. Indeed, the flow-rate 
sucked through operational clearances generally corresponds to the 
flow-rate generated by the action of gravity through a passage section 
equal to the overall section of the operational leaks under a hydraulic 
head of a few meters (level difference between the operational leaks and 
the free surface of the storage tank. Such a flow-rate remains moderate, 
e.g. from a few liters to a few scores of liters per second. By means of 
the trick disclosed above is obtained a sodium pad over the operational 
leaks. So long as said pad is present, the leaks keep on sucking sodium 
and, therefore, any penetration of argon bubbles into the pump and, from 
the latter, into the circuit, is avoided. Therefore, the unpriming of the 
pump and the gradual drainage of the circuit are avoided. Such incidents 
become quite rare, since their occurrence implies that main pump 12' and 
purification pump 52 must simultaneously come to a stop in an unforeseen 
manner. Besides, such an occurrence would not necessarily entail the 
unpriming and the drainage of the loop. Indeed, if, when main pump 12' is 
stopped, purification pump 52 is also stopped or comes to a stop for a 
limited duration, there occurs an initiation of the loop drainage. Said 
initiation is slow and can be made still slower through the regulation of 
the sodium level of argon pocket 6a of steam-generator 6, according to the 
previously described procedure, even if the argon circuit, not being 
oversized, is insufficient for providing the flow-rate required for 
accurately making up for the argon leaks through the pump. To the extent 
such an operation can be considered as very rare, any disrespect to the 
above mentioned rule of the art is admissible. However, in order that such 
an operation be possible and that the normal conditions be again in force 
when the purification pump is restarted, the secondary circuit and main 
pump 12' must be specially designed; it is necessary that the argon 
bubbles penetrating into the pump be permanently capable of escaping 
upwardly by gravity. To that end, it is only sufficient to give a 
sufficient slope and a suitable orientation to the internal structures of 
the pump and to back-flow pipings 10'. Indeed, if such is not the case, 
and in particular if the circuit is designed according to the principle of 
the above described first variant (FIG. 11), once the pump has been 
unprimed, the operational leaks are no longer sucked; if, under such 
conditions, purification pump 52 is again in operation, and, accordingly, 
reconstitutes the sodium pad above the operational leak orifices, the head 
of said pad will be insufficient for eliminating the argon pocket trapped 
in the pump and under the latter. For causing the pump to start again, it 
will be necessary to proceed as in the first embodiment (FIG. 11). Whether 
one resorts to the first improvement or to the second one or else to a 
combination of both (with, in the latter case, the drawback just 
mentioned), the filling of the initially empty loop by means of the 
initially full storage tank can be easily carried out according to the 
same procedure in all cases. Indeed, as already mentioned, when the tank 
is empty, the pump is fully drowned and, therefore, primed, even in the 
case of the first improvement, since, in that case, the upper point 
constituted by the pump is drained of any amount of argon it may contain, 
towards the argon atmosphere of the storage tank, through the operational 
leaks. In the case of the second improvement, the drainage is continuous, 
since slopes have been provided for the pump internal members and for the 
back-flow piping, said slopes allowing the argon bubbles to come back by 
gravity to the upper point of the loop (viz. The argon pocket 6a of 
steam-generator 6a). 
Under such conditions, the pump can be caused to start in a perfectly safe 
manner. The rotation speed thereof for a substantially zero flow-rate will 
have to be adjusted in such manner that it provides a back-flow head 
slightly greater than the interval between the level of operational leaks 
and the level reached by the free surface of the tank sodium once the loop 
has been filled. Then, according to the method described in the main 
chapter, the storage tank is caused to be gradually pressurized, so as to 
cause the sodium to rise in the loop, by a counter-pressure effect. During 
that operative step, the pump is maintained in rotation and, therefore, it 
provides a slight overpressure upstream of the operational leaks (viz. on 
the inner side with respect to the pump). Therefore, these leaks propell a 
certain amount of sodium towards the argon atmosphere of the storage tank, 
as during a normal operation of the installation; no argon is introducted 
into the pump. 
Quite obviously, it is possible to combine these first two ways of 
protecting the secondary circuit as regards the introduction of inert gas. 
It is to be noted that it is possible to abstain from using such a starting 
procedure, in the case of the second mode of carrying out the improvement, 
either applied alone or in combination with the first mode. In such a 
case, it is necessary that the purification pump be previously started; 
the sodium pad above the operational leaks is fed permanently for all the 
duration of the filling operation and it is partially sucked through the 
operational leaks, which, here again, prevents any penetration of argon. 
FIG. 13 represents a third embodiment of the device adapted to ensure 
protection with respect to any tendency of the gas bubbles to rise in the 
sodium secondary circuit, should pump 12" be stopped. That device is 
applied to the case of FIG. 5c. 
Back-flow duct 12"c of pump 12" comprises an upper point 160. Duct 12'c 
between the pump outlet and said upper point 160 has a sufficient slope 
and a suitable orientation allowing the gas bubbles to escape and to reach 
upper point 160 by gravity. A vent 162 permits the exhaust of that gas. 
However, in order to maintain the requested pressure in tank 24', an 
equivalent amount of gas is re-injected into tank 24', e.g. as indicated 
in FIG. 7. 
Quite obviously, said device may also be used if pump 12" is dipped in the 
tank, in which case back-flow duct 12"c passes through the upper wall of 
tank 24'. Again, that protective device may be combined with those 
previously described. 
In short, the improvements suggested aim at still more improving the 
possibility of reducing the cost of a cooling secondary circuit, and of 
improving its realiability and its safety and also of rendering the 
working thereof easier, by avoiding that, should the main pump, assumed to 
be provided with the shortest possible shaft, be stopped in an unforeseen 
manner, said pump be unprimed and that the loop be gradually drained. To 
that end, the improvements suggested consist either in a particular design 
of the back-flow pipes of the pump or in an appropriate installation of 
the piping for repelling the sodium provided by purification and sucked by 
a pump (e.g. an electromagnetic pump), independent of the main pump. It 
must be added that, in each of said three modes of operation, it is 
endeavoured to reduce the operational leaks of liquid metal in the pump, 
to the largest possible extent. 
The main advantage of the circuit for evacuating the residual power, or 
emergency circuit, according to the improvements suggested, is definitely 
as follows: with respect to the usual devices, the devices according to 
the invention permit to dispense with large diameter valves in the main 
pipings and also with mixers which, in usual systems, are rendered 
necessary by the confluence, in the main piping, of two sodium streams at 
different temperatures. 
It follows from the above description that the secondary loop forming the 
object fo the invention has several advantageous features, some of which 
are likely to provide a final remedy to some of the unfavorable features 
of normal secondary loops. In other cases, the loop according to the 
invention provides a substantial improvement. To sum up, the various 
advantages provided by the secondary loop according to the invention are 
as follows: 
that loop permits, in a perfectly safe manner, to place an ordinary free 
surface pump in low position, therefore with a good NPSH coefficient, thus 
allowing a higher speed of rotation, a wheel of smaller diameter, a less 
heavy driving mode and, finally, less expensive a motor-pump unit; 
it permits to reduce the number of operational tanks in the loop: if the 
steam-generator is provided with an argon pocket, a single tank is 
sufficient, said tank performing several functions: storage, expansion, 
ram-effect preventing tank and recovery of contaminated sodium; 
it permits to place the circuit heavy elements in a low position (the above 
storage tank and the pump), which promotes their support, in particular to 
withstand seismic stresses; 
it promotes the reduction of the length of large diameter pipings (main 
pipings); 
it permits to simplify some auxiliary devices and even to dispense with 
some of them, e.g. drainage valves and pipings, level balancing circuit, 
overflow circuit, etc.; in addition, it renders the filling and draining 
operations easier; 
it permits to reduce the importance of the rupturable diaphragms 
considerably or even to dispense with same, said diaphragms being 
expensive and entailing obligations as regards exploitation (periodical 
maintainance) with the risk of incidents likely to have important sequels 
as regards safety (sodium leaks and fires, loss of the normal circuits for 
the evacuation of power); 
it permits to withstand sodium-water reactions readily; 
for all the above reasons, it permits to design a system that is less 
bulky, less high and, therefore, less cumbersome and less costly to 
install; 
in view of the above reasons, it leads to a loop containing a smaller 
amount of sodium with, accordingly, a favorable effect on the size of the 
storage tank and, more generally, on the importance of various devices: 
pre-heating devices, heat-insulating devices, supporting devices, etc.; 
finally, for all the above reasons considered together, the secondary loop 
according to the invention lessens the importance of the control to be 
associated to thes systems.