Emergency cooling device for a pressurized water reactor core

The invention relates to a standby or emergency cooling device for the core of a pressurized water reactor. A high pressure reservoir filled with boric acid solution is permanently connected to the reactor vessel via two pipes, preferably arriving at the same level. However, one of these pipes is extended within the vessel by a downwardly directed bend in order to issue below the other pipe. These two pipes have horizontal portions which are intended to prevent any natural circulation under normal operating conditions. Application to the improvement of the safety of pressurized water nuclear reactors.

BACKGROUND OF THE INVENTION 
The present invention relates to an emergency standby cooling device for 
the core of a pressurized water reactor. 
The known cooling devices of this types used in reactors generally comprise 
accumulators filled with pressurized water, which are connected with the 
primary cooling circuit of the reactor via isolating valves, which are 
sensitive to a given lower value of the pressure in the primary circuit. 
The water filling the accumulators is pressurized with nitrogen. 
These known devices have a number of disadvantages. Thus, the injection of 
water from the standby cooling accumulators of the core does not 
necessarily take place at the most favourable moment. Thus, when the 
pressure drop in the primary circuit is due to a relatively small fracture 
of the piping there may be a draining of the primary circuit without an 
adequate reduction in the primary pressure to bring about the draining of 
the accumulators. The primary circuit can then be filled by the "high 
pressure" pumps, however, the latter constitute active systems which may 
be erroneously manipulated by the operators. In addition, the draining of 
the accumulators is linked with a value of the primary circuit pressure 
and not a vessel level, which is the most significant physical magnitude 
of the need for filling the primary circuit with water. 
Moreover, the injection of water into the primary circuit is generally 
terminated by an injection of nitrogen serving to pressurize the water in 
the accumulators. This gas can then be trapped either in the inverted 
U-tubes of the steam generators, or in the upper parts of the hot branches 
of the primary circuit when the steam generators are of the "single pass" 
type, in which the primary fluid enters the generator at its upper end and 
leaves it at its lower end. Thus, in both cases, there is a loss of 
efficiency of the cold source constituted by the steam generators. 
BRIEF SUMMARY OF THE INVENTION 
The object of the invention is to provide an emergency cooling device which 
does not have the disadvantages of the prior art devices and which more 
particularly makes it possible to inject cold water into the core at the 
most opportune moment. 
Therefore, the present invention relates to a device for the emergency or 
standby cooling of the core of a pressurized water reactor in which the 
reactor core is located within a tightly sealed vessel connected by the 
hot and cold branches of a primary cooling circuit with at least one steam 
generator, wherein the device comprises at least one water-filled, high 
pressure reservoir and whose bottom is positioned above the tubes by which 
the hot and cold branches of the primary circuit enter the vessel, said 
reservoir being permanently connected to the reactor vessel by a first 
pipe issuing into the vessel between the said tubes and the high level of 
the core and by a second pipe issuing into the vessel below the first 
pipe. 
As a result of the device according to the invention, the injection of the 
emergency water is no longer controlled by isolating valves sensitive to a 
given lower pressure within the primary circuit and is instead controlled 
by a reduction of the primary fluid level in the reactor vessel. The 
starting level is determined by the first pipe connecting the reservoir to 
the reactor vessel, the steam formed in the case of a reduction in the 
level entering the said pipe to start the draining of the reservoir. 
According to a first variant of the invention, the two pipes enter the 
vessel at substantially the same level. Preferably, the two pipes then 
have horizontal portions which are either not or only slightly thermally 
insulated of an adequate length to prevent any initiation of the flow by 
natural convection between the vessel and the reservoir under normal 
operating conditions. None of the pipes connecting the vessel and the 
reservoir is equipped with a valve. 
According to a second variant of the invention, when it is desired to 
create a flow by natural convection between the vessel and the reservoir, 
the two pipes enter the vessel at different levels. 
According to another feature of the invention, the second pipe may have 
within the vessel a downwardly directed bend or elbow and entering a 
funnel extended by a tube making it possible for the injection water to on 
the one hand partly mix with the hot water of the vessel and on the other 
to arrive directly at the bottom of the vessel. 
According to another feature of the invention, the device also comprises a 
cooling circuit having a heat exchanger located within the reservoir. This 
cooling circuit can be the low pressure emergency injection circuit which 
is used for ensuring the pressure drop of the primary circuit in the case 
when a crack or opening occurs thereon. As soon as the primary pressure 
has dropped sufficiently, this cooling circuit ensures the filling of the 
primary circuit via a three-way valve. 
The device according to the invention can also have two pipes for cooling 
the reactor on shutdown respectively connecting the upper part of the 
reservoir with at least one hot branch via at least one electrovalve and 
the lower part of the reservoir with at least one cold branch via an 
electrovalve and a circulating pump. As a result of this characteristic, 
the device according to the invention can also be used as a high pressure 
device for cooling the reactor on shutdown. For the latter function, the 
water reservoir of the cooling circuit must itself be cooled. 
The device according to the invention may also comprise a heating circuit 
for the water contained in the high pressure reservoir, said circuit 
having a coil located in the reservoir and whose ends are respectively 
connected to at least one hot branch and at least one cold branch by means 
of at least one electrovalve. 
According to yet another feature of the invention, the water contained in 
the reservoirs can be a boric acid solution, whose boric acid 
concentration is controlled and adjusted by means of a connecting circuit 
by which the reservoirs are connected to an external chemical and 
volumetric control circuit.

The drawing very diagrammatically shows the vessel 10 for a pressurized 
water nuclear reactor, whose core 12 is located within the vessel 10 and 
is supported by a ferrule or collar 14. The reactor core is traversed by a 
cooling fluid 16 which flows in the direction of the arrows. The cooling 
fluid 16, generally constituted by pressurized water in this type of 
reactor, extracts the heat given off by the nuclear fission in the reactor 
core and the transfer thereof to the not shown steam generators by a 
primary cooling circuit. The primary circuit is constituted by a number of 
loops, but only a hot branch 18 and a cold branch 20 are partly shown in 
the drawing. 
According to the invention, at least one high pressure reservoir 22 filled 
with boric acid solution 24 is placed in the reactor confinement enclosure 
26. In practice, for safety reasons, there are at least two reservoirs 
like reservoir 22. Each reservoir 22 is placed at a level such that the 
bottom thereof is positioned above the hot branch 18 and cold branch 20 of 
the primary circuit. A first pipe 28 permanently connects the top of 
reservoir 22 to reactor vessel 10 between hot branch 18 and cold branch 20 
of the primary circuit and the high level of reactor core 12. A second 
pipe 30 connects the bottom of the reservoir 22 to vessel 10 which it 
preferably enters at the same level as pipe 28 in order to prevent the 
natural flow of water between vessel 10 and reservoir 22. In a not shown 
variant, pipes 28 and 30 can enter the vessel 10 at two different levels 
when it is desired for the boric acid solution to flow by natural 
convection between the vessel and the reservoir. 
As illustrated in the drawing, pipe 30 is extended downwards within the 
vessel 10 by an elbow or bend 29, whose end enters a funnel 31 
constituting the upper end of a tube 33 issuing in the vicinity of the 
bottom of the vessel. The end of bend 29 is at a level close to the high 
level of core 12 in such a way that if the level in vessel 10 drops the 
steam formed firstly enters the pipe 28 to start the drainage of reservoir 
22. The free space left between the end of bend 29 and funnel 31 makes it 
possible for the injection water from reservoir 22 to partly mix with the 
hot water of the vessel, the remainder of the injection water passing 
directly to the bottom of the vessel via tube 33. 
As illustrated in the drawing, each of the pipes 28 and 30 has a 
substantially horizontal part forming an obstacle to the natural 
convection of boric acid solution 24 between the reactor vessel and the 
reservoir. As neither of the pipes 28 and 30 is equipped with a valve, 
reservoir 22 is directly and permanently connected by said two pipes with 
reactor vessel 10. 
In the represented embodiment, the water 24 in reservoir 22 is cooled by 
means of a cooling circuit 34 having a coil-shaped heat exchanger 36 
located within reservoir 22, a circulating pump 35 and a valve 37. The 
cooling fluid flowing in circuit 34 comes from a reservoir 42 and is 
permanently cooled, for example, by means of a second exchanger 38 within 
reservoir 42. 
The cooling circuit 34 is connected to the primary circuit via a standby or 
emergency injection circuit 32 controlled by a valve 39, which opens when 
it receives an emergency injection signal and by a three-way valve 49 
which, as soon as the primary pressure has dropped sufficiently, permits 
the filling of the primary circuit by blocking the recirculation of the 
emergency injection water towards reservoir 42. 
Preferably, the emergency cooling device according to the invention also 
has pipes 40 and 60 for cooling the reactor on shutdown, which 
respectively connect the upper part and bottom of reservoir 22 with one or 
more of the hot and cold branches 18, 20 respectively of the primary 
circuit. These pipes 40 and 60, by means of a pump 42 and after opening 
the electrovalves 44 and 64, make it possible to remove the sensible heat 
of the primary circuit and the residual power of the reactor core during 
the cold shutdown of the latter, no matter what the temperature and 
pressure of the primary circuit water. 
A heating circuit 70 for the water in reservoir 22 is connected with pipe 
40 upstream of electrovalve 44. Circuit 70 has an electrovalve 69 and a 
coil 71, preferably located at the bottom of reservoir 22 and issuing into 
the nozzle of pipe 60. The ends of coil 71 are thus indirectly connected 
by pipes 40 and 60 to the hot and cold branches 18, 20 respectively of the 
primary circuit. Thus, this circuit is arranged in such a way that it 
causes no movement of the water of reservoir 22 towards the primary 
circuit, thus making it possible to keep the boron concentration of the 
reservoir at a constant level. 
Obviously, the water in reservoir 22 can be heated by any means (e.g. 
electricity). 
In the represented embodiment, the boric acid concentration of the water 24 
contained in reservoir 22 is controlled and adjusted by means of a circuit 
50 comprising two electrovalves 52. Circuit 50 connects reservoir 22 to a 
conventional chemical and volumetric control circuit outside the 
confinement enclosure 26 of the reactor. 
Finally, a purging or draining circuit 54, controlled by an electrovalve 
56, issues into the upper part of reservoir 22. 
The operation of the pressurized water reactor partly described with 
reference to the drawing is identical to that of known reactors of this 
type and will not be further described here. 
During an accidental decompression of the primary circuit resulting from a 
loss of primary fluid through an opening or crack formed in a pipe, the 
optionally cold boric acid solution contained in reservoir 22 is 
automatically discharged by pipe 30 into reactor vessel 10 as soon as the 
water level in said vessel drops below the entry point of pipe 28. Thus, 
the putting into operation of the device according to the invention, is 
automatically controlled by the drop in the water level in the reactor 
vessel, without it being necessary to use one or more electrovalves. In 
addition, this is in no way linked with the opening of valves. Thus, the 
cooling of reactor core 12 is always ensured as soon as the latter is 
exposed to the risk of being drained, i.e. at the most opportune moment, 
no matter what the size of the crack or opening formed in the primary 
circuit. Thus, the device according to the invention is much more 
efficient than the prior art devices in which the putting into operation 
is controlled by isolating valves sensitive to a given pressure in the 
primary circuit. 
The device described relative to the drawing also makes it possible to 
ensure the cooling of the reactor on shutdown by opening electrovalves 44 
and 64 and by making the primary fluid flow between vessel 10 and 
reservoir 22 by means of pump 42 located in pipe 60. The primary fluid is 
then cooled in reservoir 22 by means of cooling circuit 34 and in 
particular exchanger 36. The present device thus makes it possible to 
remove the sensible heat from the primary circuit and the residual power 
from the reactor core 12 during the cold shutdown of the reactor, without 
it being necessary to wait for the temperature and pressure of the primary 
circuit water to drop significantly compared with their normal operating 
value. 
Obviously, the invention is not limited to the embodiment described 
hereinbefore. In particular, the pipes 40 and 60 for cooling the reactor 
on shutdown can optionally be eliminated and in this case the device will 
only carry out the standby cooling of the reactor core in the case of an 
accident through loss of primary fluid. Circuit 50 which makes it possible 
to control and adjust the boric acid concentration in the water 24 
contained in reservoir 22 can also be eliminated or replaced by any 
equivalent device.