Intrinsically safe emergency cooling device for a pressurized-water nuclear reactor

Intrinsically safe emergency cooling device for a pressurized-water nuclear reactor, comprising an auxiliary circuit which feeds the steam generator and in which is arranged a condenser (10) receiving the steam from the generator and condensing it. The condensate is returned to the steam generator by means of gravity. The device also incorporates a passive adjustment means comprising an adjustable valve (21), a cylinder (23), the chamber of which communicates with the steam pipe (7) and contains a piston (33), and a coupling means (32) between the piston (33) and the shut-off element (30) of the valve (21). The feed rate of the steam generator and the cooling power of the condenser (10) can be regulated in this way.

FIELD OF THE INVENTION 
The invention relates to an intrinsically safe emergency cooling device for 
a pressurized-water nuclear reactor. 
BACKGROUND OF THE INVENTION 
Pressurized-water nuclear reactors incorporate a primary circuit, in which 
the pressurized water for cooling the reactor circulates and transfers the 
heat of the core to the steam generators. The primary circuit usually 
comprises three or four loops, on each of which is arranged a steam 
generator which receives, on the one hand, the pressurized water and, on 
the other hand, the feed water which it heats and vapourizes as a result 
of heat exchange with the pressurized water. 
In the event of an accident in the nuclear reactor, it is immediately 
shut-down by introducing all the control bars of the reactor into the core 
in their position of maximum insertion. The reactor then has to be cooled, 
and this can be achieved by means of the steam generators. However, the 
normal steam utilization and feed water recirculation circuit, which 
incorporates several components such as heating devices and pumps, and 
which therefore has a complex structure, cannot be used for the emergency 
cooling of the reactor after an accident. To ensure this emergency 
cooling, an auxiliary circuit feeding secondary water to the steam 
generator is therefore put into operation for each of the loops of the 
primary circuit incorporating a steam generator, this auxiliary circuit 
then taking the place of the normal steam utilization and secondary-water 
recirculation circuit. 
So that the nuclear reactor can be cooled under all circumstances, in 
particular even if a source of electrical energy is no longer available on 
the reactor site, cooling devices comprising only passive elements have 
been proposed. In particular, an auxiliary feed circuit for the emergency 
cooling of the reactor has been proposed, and this comprises a steam 
condenser located at a higher level than the relatively low level of the 
water of the steam generator, i.e., the equivalent overall level in the 
form of a liquid phase, and pipelines which respectively connect the steam 
outlet of the generator to the inlet of the condenser and the outlet of 
the condenser to the feed water inlet in the steam generator. 
In this auxiliary circuit, the condenser is submerged in a tank filled with 
water and communicating with the atmosphere, and it is cooled as a result 
of the boiling of the water contained in this tank, the steam generated 
being discharged to the atmosphere. 
Such a device is therefore capable, in principle, of cooling the reactor 
without the need for an external energy source, the water recovered at the 
condenser outlet returning to the steam generator under the effect of 
gravity. 
However, during the cooling of the primary circuit of the reactor, the 
operating conditions of the steam generator and the flow of steam 
generated vary greatly, with the result that the condenser has to operate 
under essentially variable conditions, but this greatly complicates the 
design of this condenser which, during certain cooling phases, has in any 
case to operate under conditions very different from its optimum operating 
conditions. 
On the other hand, it is possible to regulate the operating conditions of 
the auxiliary circuit only in so far as this does not require the use of 
an energy source or the involvement of facilities outside the auxiliary 
cooling circuit. 
Finally, the mass of water contained in the cooling tank of the condensers 
already proposed is very large, and this greatly complicates the design of 
the buildings in which these condensers are installed and which in any 
case must be capable of withstanding earthquakes. 
The need to provide an intrinsically safe emergency cooling system to 
prevent the extremely serious consequences of operating a reactor under 
conditions prevailing in the event of an accident has arisen in recent 
years. Such an intrinsically safe system must operate without the need for 
an external energy source, without an operator being involved after it has 
been put into service, and without a regulating working fluid; it must 
also have available a cooling source of virtually unlimited capacity. 
SUMMARY OF THE INVENTION 
The object of the invention is, therefore, to propose an intrinsically safe 
emergency cooling device for a pressurized-water nuclear reactor 
incorporating a primary circuit, in which the pressurized water cooling 
the reactor circulates and in which at least one steam generator ensures 
the heating and vapourization of feed water, the cooling device comprising 
an auxiliary circuit which feeds the steam generator and which is put into 
operation in the event of an accident in the reactor and which 
incorporates a steam condenser located at a higher level than the 
relatively low level of the water of the steam generator, and pipelines 
which respectively connect the steam outlet of the generator to the inlet 
of the condenser and the outlet of the condenser to the feed-water inlet 
in the steam generator, this cooling device makes it possible for the 
condenser to operate during all the cooling phases of the nuclear reactor, 
up to the transfer to the cooling system at shut-down, without using an 
energy source or facilities outside the auxiliary feed circuit, and 
without the need for a pipe operator to be involved after it has been put 
into service. 
To achieve this object, the cooling device according to the invention also 
incorporates a means of passive adjustment of the feed rate of the 
auxiliary circuit, consisting of: 
an adjustable valve having a shut-off element to make it possible to open 
to a greater or lesser extent the valve arranged on the pipe connecting 
the outlet of the condenser to the inlet of the steam generator in the 
auxiliary circuit, 
a cylinder, the chamber of which contains a movable piston returned in one 
direction by an elastic means, in communication on one side of the piston 
with the pipeline connecting the steam outlet of the generator to the 
inlet of the condenser, and 
a coupling means between the piston and the shut-off element of the 
adjustable valve, to allow this shut-off element to be shifted in the 
opening direction or to be returned in the closing direction as a function 
of the pressure of the steam at the outlet of the steam generator.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows the vessel 1 of a pressurized-water nuclear reactor and part 
of a branch of the primary circuit of this reactor incorporating a steam 
generator 2 and the primary pipelines 4 which allow pressurized water to 
circulate in the steam generator. The entire primary circuit is arranged 
inside a containment shell, one wall 5 of which has been shown in FIG. 1. 
The part of FIG. 1 located on the left of the wall 5 corresponds to inside 
of the containment shell, and the part of FIG. 1 located on the right 
corresponds to the outside of this containment. The steam generator is fed 
with water via a pipeline 6, this feed water coming in contact, inside the 
steam generator 2, with the outer wall of the tubes of a tube bundle, in 
which the primary water circulates. The feed water is thus heated and then 
vapourized, and the steam, after being dried, is discharged via a pipeline 
7 fastened to the upper part of the steam generator. 
The pipelines 7 and 6 form part of the normal steam discharge and 
feed-water recirculation circuit. These pipelines pass through the wall 5 
of the containment shell, in order to convey the steam to the turbines and 
recycle the water recovered to the condenser. 
As can be seen in FIG. 1, the auxiliary circuit of the emergency cooling 
device incorporates part of the pipelines 6 and 7, the condenser 10 of the 
emergency cooling device being branched off between the pipelines 6 and 7 
by means of pipelines 11 and 12. 
Downstream of the junction with the pipeline 11, a shut-off valve 14 is 
arranged on the pipeline 7, and a shut-off valve 15 is likewise arranged 
on the pipeline 6 upstream of the junction with the pipeline 12, if the 
directions of circulation of the steam and the recycled feed water 
respectively are considered. The valves 14 and 15 make it possible to 
interrupt communication between the steam generator and the normal steam 
utilization and feed-water recovery circuit. The operation of closing the 
valves 14 and 15 is carried out immediately after a malfunction or an 
accident in the nuclear reactor, and the auxiliary circuit comprising the 
pipelines 7, 11, 6 and 12 and the condenser 10 then takes the place of the 
normal circuit. Shut-off valves 17 and 18 are likewise arranged on the 
pipelines 11 and 12 respectively. When the auxiliary circuit is in 
service, the valves 17 and 18 are open and the valves 14 and 15 are 
closed. 
The steam leaving the generator 2 is in this case conveyed into the 
condenser 10, the condensate being recovered and returned to the steam 
generator via the pipeline 12. In fact, the building 19 containing the 
condenser 10 is located at a higher level than the relatively low level of 
the water in the steam generator 2, with the result that concentrate is 
recycled into the steam generator as a result of gravitational circulation 
in the pipelines 12 and 6. 
However, immediately after the emergency shut-down of the reactor, when the 
primary circuit is still at a high temperature and the energy released is 
substantial, some of the steam can be discharged to the atmosphere via one 
(or more) valves 20 branched off from the pipeline 7. 
The auxiliary circuit of the emergency cooling device also incorporates a 
means of passive adjustment of the feed rate of the steam generator, 
consisting of an adjustable valve 21 arranged on the pipeline 12 
downstream of the shut-off valve 18 and a device 23 controlling this valve 
and connected by means of a pipe 24 to the steam line 7. This assembly 
will be described in more detail with reference to FIG. 2. 
The condenser 10 is of a new type which forms the subject of a patent 
application filed on the same day as the present application. This 
condenser comprises an assembly of tubes immersed in the cooling water 
contained in the tank 26, the upper part of which is connected to a 
chimney 27. The water in the tank 26 comes to the boil when in contact 
with the tubes of the condenser receiving the steam from the generator 2, 
and the steam formed in the tank 26 is discharged through the chimney 27. 
An assembly 28 which will be described in more detail with reference to 
FIG. 3 makes it possible to supply the tank 26 with further cooling water. 
In FIG. 2, the passive means of adjusting the feed rate in the auxiliary 
circuit have been shown in more detail. 
The shut-off element 30 and the actuating wheel 31 of the valve 21, the 
control device 23 and the coupling means 32 between the device 23 and the 
element 30 have been shown on a very large scale in this diagrammatic 
view. The control device 23 consists of a cylinder, the chamber 23a of 
which contains a piston 33 mounted so as to be sealed and movable between 
two stops 23b machined on the inner surface of the chamber 23a. This 
chamber is put in communication, on one side of the piston 33, with the 
steam line 7 by means of the pipeline 24. The pressure of the steam in the 
pipeline 7 is exerted on the piston 33 which is returned by a spring 34 
bearing on the end of the cylinder 23 and associated with a correcting 
element 34a which makes it possible to ensure the stability of the control 
loop. A means 34b of calibrating the spring 34 is also associated with 
this spring. Such a calibrating means 34b can consist, for example, of a 
screw/nut system which makes it possible to carry out adjustable 
precompression of the spring 34. The steam penetrating into the pipeline 
24 and transmitting the pressure to the piston 33 tends to condense at the 
end of the pipeline 24 and in the chamber 23a of the device 23. The 
pressure is therefore in fact transmitted by means of the condensation 
water 35. 
The coupling means 32 between the piston 33 and the valve wheel 31 integral 
with the shutter 30 consists of an articulated assembly of three rods 36, 
37 and 38, the middle rod 37 being articulated on a fixed shaft 39. The 
displacements of the piston 33, integral with the rod 36, inside the 
chamber 23a are transmitted to the valve wheel 31 by means of the device 
32, so that the shutter 30 of this valve rotates in the valve opening 
direction, when the piston 33 is displaced by the pressurized steam, and 
in the closing direction when the piston 33 is displaced by the restoring 
spring 34 associated with the element 34a, the latter ensuring the damping 
necessary for control stability. 
The valve opens to an extent which is the greater, the higher the steam 
pressure at the outlet of the steam generator 2. In FIG. 2, the device has 
been shown in a position corresponding to an intermediate steam pressure 
below the pressure reached when the piston is put into operation. The flow 
of condensate in the pipeline 12, running off as a result of gravity into 
the pipeline 6 and the steam generator 2, corresponds exactly to the flow 
of steam penetrating into the tubes 40 of the condenser 10. The interface 
between the steam and condensate in the tubes 40 occurs at a certain level 
41. This level varies if the quantity of condensate formed in the 
condenser or the discharge rate of this condensate vary independently of 
one another. For constant values of the thermodynamic characteristics of 
the steam, the condenser cooling heat is substantially proportional to the 
length of the tubes which is not filled with condensate, since most of the 
heat is eliminated as a result of condensation of the steam and only a 
very small proportion as a result of the cooling of the condensate. 
The condenser cooling heat is synchronized, by means of the steam pressure, 
with that generated by the steam generator, and this generated heat 
decreases very sharply from the emergency shut-down of the reactor up to 
the change-over to the refrigeration system at shut-down. 
The steam pressure acts by means of the piston 33 on the opening of the 
valve 21 which regulates the discharge rate of the condensate, thus 
adjusting the level 41 of the interface between the steam and condensate. 
In the event that the heat generated by the steam generator increases, the 
result is a substantially proportional increase in the steam flow produced 
by the generator. But the condensed steam flow depends solely on the 
thermodynamic characteristics of this steam and on the length of the tubes 
40 which is not filled with condensate and not on the steam flow produced 
by the generator. Since the excess steam flow cannot be condensed 
immediately, this results in an increase in the steam pressure. 
The adjustable valve 21 tends to open and the flow rate can increase in the 
auxiliary circuit. The level 41 drops, thus increasing the inner surface 
of the tubes 40 allowing the steam to condense. The condensed steam flow 
increases, thereby making it possible to synchronize the condenser cooling 
heat with the heat generated by the steam generator. 
If the heat discharged by the steam generator decreases, actions in reverse 
result in a reduction in flow in the auxiliary circuit and in a rise of 
the level 41. 
In actual fact, the thermodynamic parameters of the steam (pressure and 
temperature) do not remain constant during the cooling of the reactor 
after an emergency shut-down. The steam, which is at 300.degree. C. and 
86.10.sup.5 Pa at the moment of the emergency shut-down, is at a 
temperature of 157.5.degree. and a pressure of 5.8.10.sup.5 Pa at the 
moment when the reactor shut-down cooling device is put into operation. 
This results in a substantial variation in the exchange conditions in the 
condenser, and this in itself has a regulating effect. In fact, the 
heat-exchange conditions are much more favorable when the steam is at a 
high temperature and a high pressure, i.e., when the power discharged by 
the steam generator is high. 
The residual heat released by the reactor decreases sharply during the 
operation of the auxiliary system with condenser. Since the cooling rate 
of the reactor is substantially constant, the heat generated by the steam 
generator likewise decreases sharply from the emergency shut-down to the 
change-over to the shut-down refrigeration system. This results in a 
continuous and substantial reduction in the flow in the auxiliary circuit. 
However, the pressure and temperature of the steam decrease because of the 
cooling of the reactor. The deterioration in the heat-exchange conditions 
predominates to an increasing extent, and the condenser is progressively 
used as near as possible to its optimum characteristics, i.e., the tubes 
40 are increasingly devoid of condensate, and because of this the 
condenser 10 is designed so that it is capable of discharging the power in 
the final phase of cooling (steam at 157.5.degree. C.) with the tubes 40 
empty of condensate and condensing the steam over their entire length. 
During the cooling of the reactor by means of the auxiliary system, the 
flow rate and the level 41 of the water/steam interface decrease 
simultaneously. The lowering of the level 41 causes a reduction in the 
circulation driving force of the condensed steam flow, and this in itself 
constitutes an additional regulating effect. 
The spring calibration means 34b makes it possible to adjust the cooling 
rate of the reactor. In fact, when, for a given pressure of the steam at 
the outlet of the steam generator, the valve shutter 30 is in such a 
position that the flow of condensation water allows only the residual heat 
of the reactor and the heat released by the primary pump or pumps to be 
eliminated, the reactor cooling rate is zero. By means of this calibration 
of the spring, the spring can be relaxed, thus allowing a greater flow of 
condensation water to pass through, consequently permitting a greater 
extraction of power. The cooling of the reactor is thus achieved. 
Conversely, when the spring is recompressed from the position allowing the 
reactor temperature to be maintained, the reactor can be heated for a flow 
of condensation water below the value which ensures that the residual heat 
and the heat released by the primary pumps are eliminated. 
The device 23 therefore constitutes a control system which makes it 
possible to regulate the cooling rate of the reactor. The nominal value is 
introduced as a result of the calibration of the spring 34. However, the 
relation linking the pressure drop coefficient of the valve 21 to the 
opening angle of the shutter 30 and the relation linking the heat 
eliminated by the condenser and the condensation flow are not absolutely 
linear; as a result, the nominal cooling value introduced by the operator 
by means of the calibration of the spring 34 is only approximate. However, 
it is possible for him to alter the calibration of the spring 34 in order 
to adjust the reactor cooling rate during the operation of the auxiliary 
condenser circuit. 
The calibration of the spring has to be modified periodically during the 
normal operation of the reactor, to take into account the fact that the 
depletion of the fuel causes an increase in the residual heat, especially 
during the first cycle of the fuel (a new core) when this increase is 
substantial. 
The adjustment device operating without any external energy source 
therefore makes it possible to achieve self-regulated and stable operation 
of the auxiliary cooling circuit. 
When the operating schedules for the reactor prevent the reactor from being 
cooled immediately after an accident, during the operations of treating 
the primary cooling fluid with boric acid, the cooling device according to 
the invention can be adapted to these operating conditions. For this 
purpose, the control device 23 can be uncoupled from the valve 21 during 
the borication operations and recoupled at the end of these operations. It 
is also possible, by means of the calibration of the spring, to enter a 
zero cooling value during the entire period of the borication operations 
and then to introduce a nominal value for effective cooling after these 
operations. 
Referring to FIG. 3, it will be seen that the condensation heat in the 
condenser 10 is absorbed as a result of the heating and boiling of the 
water filling the tank 26. The steam generated escapes through the chimney 
27, and make-up water is supplied to the tank 26 via a feed line 42. A 
constant level of cooling water in the tank 26 is maintained by means of 
an adjustment device consisting of a cylinder 43, the chamber 43a of which 
contains a piston 44 mounted so as to be sealed and movable between stops 
43b. The chamber 43a is connected, on one side of the piston 44, to the 
lower part of the tank 26 by means of a pipeline 45. The piston 44 is 
returned to its upper position counter to the action of the water pressure 
in the tank 26 by means of a spring 47 bearing on the bottom of the 
cylinder 43. A correcting element 47a associated with the spring 47 makes 
it possible to ensure the stability of the adjustment device. A 
calibration means 47b is likewise associated with the spring 47. The 
piston 44 is perforated completely through by a channel 44a which, as 
shown in FIG. 3, can be an extension of an inlet channel 48a and an outlet 
channel 48b passing through the wall of the cylinder 43. The channel 48a 
is connected by means of a pipeline 49 to a tank containing high-pressure 
compressed gas 50. The channel 48b is connected by means of a pipeline 51 
to the upper part of a make-up water tank 52 containing blanket gas. When 
the level of the cooling water in the tank 26 of the condenser 10 is above 
a nominal value, the pressure of water in the lower part of this tank, 
transmitted to the adjustment device 43 via the pipe 45, keeps the piston 
44 in the low position counter to the action of the spring 47. In this 
position, the channel 44a is no longer an extension of the channels 48a 
and 48b. The piston 44 ensures that the pipelines 49 and 51 are shut off. 
When the level in the tank 26 drops below a nominal value as a result of 
the formation of steam eliminated via the chimney 27, the pressure 
decreases in the chamber 43a of the cylinder 43 above the piston 44, and 
as a result of the action of the spring 47 the piston rises into the 
position shown in FIG. 3. Under the effect of the pressure difference, 
compressed gas from the tank 50 flows into the upper part of the make-up 
water tank 52, thus making it possible to supply make-up water to the tank 
26 via the pipeline 42, to the moment when the level in the tank 26 has 
returned to above the minimum level selected as the nominal value. The 
pipelines 49 and 51 and the channel 44a passing through the piston 44 have 
a diameter which is suitable for allowing a sufficient flow to pass 
through, to compensate for the vapourization in the tank 26 of the 
condenser in all the operating phases. This device for automatically 
supplying further cooling water to the condenser 10 makes it possible to 
limit the mass of water contained in the tank 26 located at a level higher 
than the relatively low level of the water of the steam generator. 
The calibration of the spring 47 fixed by the means 47b makes it possible 
to maintain the level in the tank 26 as constant as possible during the 
operation of the condenser 10. When the condenser 10 is in operation the 
tank 26 contains a two-phase water/steam mixture, of which the mass per 
unit volume is less than the mass per unit volume of the cold water 
filling the tank 26 before the condenser is put into operation. 
Consequently, the adjustment which is made by means of the device 43 and 
which is a function of pressure in the lower part of the tank 26, makes it 
possible to maintain the mass of water in the tank 26 constant during all 
the operating phases of the condenser, until it is shut off. The level of 
water in the tank 26 changes from a high level during the operation of the 
condenser to a lower level when this condenser is shut off. 
FIGS. 4 and 5 show a second embodiment of the condenser which, where a 
nuclear reactor having three loops and therefore incorporating three steam 
generators 2 is concerned, consists of three aero-condensation assemblies 
arranged in casings 65a, 65b and 65c distributed round the containment 
shell 5 of the reactor. The elements of the reactor and of the cooling 
circuit shown in FIG. 1 and in FIGS. 4 and 5 and corresponding to one 
another bear the same reference symbols. Each of the steam generators 2 is 
connected, by means of pipes 11 and 12 branching off between the pipes 6 
and 7 of the feed-water and steam circuit, to the cooling elements 60 of 
an aero-condenser incorporating a casing 65 in the form of a portion of a 
cylinder, which is coaxial relative to the containment shell 5 of the 
reactor and which is fastened to the cylindrical surface of the latter by 
means of vertical webs making it possible to absorb the forces exerted by 
the casing 65 and the cooling elements 60 via the containment shell 5. The 
casing of the aerocondenser is open in its lower part 67 and constitutes a 
chimney having an upper part 66 provided with guide elements, so that the 
atmospheric air passes through the casing from the bottom upwards in 
natural circulation. Each casing 65 contains cooling elements 60. As 
before (although not shown), a device for adjusting the flow of feed water 
is associated with each of the aero-condensers. This adjustment device is 
actuated by the pressurized steam at the outlet of the steam generator and 
acts by means of a valve arranged on the pipe 12 returning the feed water 
to the steam generator. 
This arrangement of the air-condensers round the containment shell of the 
reactor makes it possible to design and produce the condenser so that it 
is as efficient as possible. On the other hand, the advantage of an 
aero-condenser is that it operates with a source of cooling fluid of 
infinite capacity. 
In FIGS. 6 and 7, it can be seen that the cooling elements 60 are made in 
the form of finned tubes 68 which are inclined relative to the horizontal 
plane and which are connected at one of their ends to steam collectors 61 
and at their other end to condensate collectors 62. The tubes 68 can be 
connected to the collectors so as to constitute successive assemblies of 
parallel tubes of any number. These assemblies of tubes 60 are arranged in 
the aero-condenser casing 65 which constitutes a cooling-air circulation 
chimney ensuring condensation inside the tubes 68. 
FIG. 7 shows a modular assembly of three tubes 68 communicating with one 
another by means of steam collectors 61 and condensate collectors 62 
located at their ends. It emerges that, in the same way as in a condenser 
with submerged vertical tubes, such as that shown in FIG. 2, the 
separating meniscus between the steam and condensate forms at a certain 
level 69 for a certain extraction of power in the condenser. If the 
condenser 10 shown in FIG. 2 is replaced by an aero-condenser, such as 
that shown in FIGS. 4 to 7, the device adjusting the flow of condensate 23 
makes it possible to maintain the level 69 in a fixed position for a 
constant power extracted by the steam generator. When the power increases, 
the flow of condensate increases, and as before the level 69 drops, thus 
increasing the length of the tubes ensuring condensation and therefore 
extraction of power by means of the condenser. 
Where both a condenser with tubes submerged in the cooling water and an 
aero condenser are concerned, self-regulation of the operation of the 
reactor cooling device is therefore achieved. This self-regulation is 
obtained in a very simple way by adjusting the condensate draw-off rate at 
the condenser 10 as a function of the steam pressure at the outlet of the 
steam generator. 
The invention is not limited to the embodiment which has been described. 
Thus, the condenser can be of a different type from those envisaged, from 
the moment when the regulation of the flow of condensate as a function of 
the steam pressure at the outlet of the steam generator makes it possible 
to adjust the cooling power of the condenser. 
It is possible to utilize other embodiments of the passive means of 
adjusting the auxiliary feed rate, i.e., the flow of condensate, and in 
particular other coupling means between the piston and the shut-off 
element of the regulating valve. It is also possible to use other elastic 
piston-return means, for example a volume of gas trapped between the face 
of the piston opposite the face exposed to the steam and the bottom of the 
cylinder. 
The cooling device according to the invention can be used in any nuclear 
reactor, whatever the number of loops in the primary circuit, an auxiliary 
steam-generator supply circuit and a condenser being associated with each 
of the loops on which there is a steam generator.