Emergency cooling device for a pressurized water nuclear reactor

An emergency cooling device for a pressurized water nuclear reactor, through the injection of cooling liquid into the primary circuit of the reactor, comprising at least one double injection circuit equipped with two pumps arranged outside the safety enclosure of the reactor and a set of accumulators containing a certain amount of cooling liquid under pressure arranged inside the enclosure. The accumulators are distributed in a first set at a pressure P1 less than the pressure of the primary circuit of the reactor and a second set at the pressure P2 less than the pressure P1. The injection of cooling liquid through one and other set is caused by the automatic opening of valves when the pressure of the primary circuit drops below P1 and below P2, respectively, in the event of a leakage in this circuit. The invention is useful for pressurized water nuclear reactors having any number of primary loops.

FIELD OF THE INVENTION 
The invention relates to an emergency cooling device for a pressurized 
water nuclear reactor operating by injection of a cooling liquid into the 
primary circuit or system of the reactor. 
BACKGROUND OF THE INVENTION 
The primary circuit of a nuclear reactor in the course of operation 
contains water containing boric acid at a pressure in the vicinity of 155 
bars which serves at the same time for the cooling of the core of the 
reactor located in the tank and for controlling the reactivity of this 
core. 
The primary circuit of the reactor in which the steam generators are 
arranged also serves for the transfer of heat from the core of the reactor 
to the steam generators where vaporization of the feed water takes place 
by heat exchange with the primary water. 
If the primary circuit shows a leak, it is necessary to compensate for the 
latter by injection of additional water into this primary circuit. In the 
case of a very large leak and, for example, of a real break in a pipe of 
the primary circuit, it is necessary to send very large amounts of coolant 
liquid constituted by the water supplemented with boric acid, in a very 
short time, to avoid a very considerable temperature rise of the core 
which could lead to melting of the latter. 
In the case of a slight leak accompanied by very slight depressurization of 
the primary circuit, the cooling water is injected at a pressure higher 
than the normal pressure of the primary circuit, by means of a special 
circuit called a volumetric control circuit situated outside the safety 
enclosure in which the vessel of the reactor is located. 
In the case of bigger leakages, it becomes necessary to place in operation 
a device for the emergency cooling of the reactor by the injection of 
cooling liquid, called the safety injection system. 
Such a safety injection system generally includes a group of accumulators 
containing cooling liquid under a pressure in the vicinity of 40 bars 
situated inside the safety enclosure of the reactor and two independent 
injection systems situated outside the safety enclosure. 
The accumulators containing cooling water at 40 bars, generally called 
medium pressure accumulators, are placed in communication with the cold 
branches or legs of each of the loops of the primary circuit with the 
interposition of a valve whose opening occurs at the moment when the 
pressure in the primary circuit drops below the pressure of the 
accumulator. These accumulators are constituted by a water reserve in 
temperature equilibrium with the inside of the safety enclosure and above 
which a gas such as compressed nitrogen maintains a pressure a little 
higher than 40 bars. 
The injection means arranged outside the safety enclosure include at least 
one double circuit equipped with high or medium pressure pumps with at 
least one double circuit equipped with low pressure pumps fed the one and 
the other with cooling water through the boric acid storage tank of the 
pools of the reactor. 
The rated operating pressure of the high pressure or medium pressure pumps 
equiping the one or more injection circuits is close to 100 bars in the 
case of a nuclear reactor with four loops of 1300 MW power, whilst the 
rated operating pressure of the low pressure pumps is less than 20 bars. 
In the case of a serious breach in the primary circuit of the reactor, the 
injection circuits and the medium pressure accumulator come into play very 
rapidly to introduce large amounts of water into the primary circuit to 
avoid the degradation of the fuel assemblies constituting the core of the 
reactor under the effect of too considerable heating and to maintain a 
possibility of subsequent cooling of the core by circulation of a cooling 
liquid. 
The intervention of the accumulators is however limited to a very short 
period following the appearance of the rupture in the primary circuit. 
This period may be less than one minute. 
During this very short time, the primary circuit has passed from the rated 
operating pressure, that is to say 155 bars to a low pressure at the order 
of some bars. 
However, the core of the reactor and the whole of the vessel which contains 
it are still at high temperature, so that it is necessary to maintain the 
low pressure injection circuit in operation for a time which may be rather 
long to produce the cooling of the reactor. 
It is hence necessary to use at least two different pump injection systems 
at different pressures arranged outside of the safety enclosure of the 
reactor. 
This equipment used for the safety of the reactor must comply with very 
strict conditions as regards its design and, in particular, each of the 
systems is dual. 
It is obviously possible to use the volumetric control circuit to ensure 
the high pressure injection in the case of rupture in the primary circuit 
but a quite special design of the volumetric control circuit is then 
necessary in order that it may be able to comply with the conditions 
required for a safety system. 
In all cases, the investment necessary for the safety injection circuits 
and the complexity of these circuits are very considerable. 
On the other hand, the low pressure injection circuit contributes to 
ensuring, as soon as the pressure of the primary circuit has fallen to a 
low value, the filling of the vessel with cooling water, and in particular 
the reimmersion of the core, that is to say the re-establishment of 
complete immersion of the latter in the water and its subsequent cooling 
by the flow of water. These extremely important functions must be effected 
by a safety circuit which is to be found outside the enclosure of the 
reactor and in consequence, the design of this circuit must be provided so 
as to obtain extremely good operating safety. 
It has also been proposed to use automatic tripping safety systems arranged 
inside the safety enclosure of the reactor, but these devices are not 
capable of complying with all cases of accidents, in particular in the 
case of breaches of considerable size in a cold branch of the primary 
circuit. It is necessary to join with the automatic tripping devices, 
active equipment such as pumping means possibly integrated into the 
enclosure of the reactor. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide an emergency 
cooling device for a pressurized water nuclear reactor by the injection of 
a cooling liquid into the primary circuit of the reactor, in the case of 
leakage in this primary circuit, including at least one dobule injection 
circuit equipped with pump means and located outside the enclosure of the 
reactor and at least one group of accumulators containing a certain amount 
of cooling liquid under pressure located inside the safety enclosure of 
the reactor and communicating with the cold branches of the primary 
circuit through which the pressurized water arrives at the vessel of the 
reactor, through a valve whose opening causes the injection of cooling 
liquid into the primary circuit, this emergency cooling device having to 
permit a response to all cases of accidents which can occur, even the most 
serious, without the use of a complete pump injection system including 
sub-assemblies enabling injections at different pressures located outside 
of the safety enclosure of the reactor. 
For this purpose, the cooling device according to the invention comprises 
bypassing in each of the cold branches of the primary circuit, with the 
interposition of a valve and independently of one another: 
a first accumulator of refrigerant liquid at a first pressure P1 less than 
the normal pressure of the primary circuit, 
and a second accumulator of refrigerant liquid at a second pressure P2 less 
than the pressure P1, 
the injection of refrigerant liquid through one and other of the 
accumulators of the primary circuit being caused by the automatic opening 
of the corresponding valves, when the pressure in the primary circuit 
falls below P1 and below P2 respectively. 
According to a preferred embodiment of the cooling device according to the 
invention, each of the double injection circuits equiped with pump means 
comprises two pumps arranged in series in a pipe communicating upstream 
with a cooling liquid reserve and downstream with the primary circuits of 
the reactor, the pump located downstream having a rated operating pressure 
very much higher than the rated operating pressure of the pump arranged 
upstream whose delivery is in communication with the suction of the pump 
arranged downstream, a portion of the circuits by-passing with respect to 
the downstream pump provided with a closure valve also enabling the 
placing of the upstream pump in direct communication with the primary 
circuit. 
In order that the invention may be better understood, there will now be 
described purely by way of non-limiting example, an emergency cooling 
device according to the invention with reference to the accompanying 
drawings.

DETAILED DESCRIPTION 
In FIG. 1 is seen, represented diagrammatically, the vessel 1 of a 
pressurized water nuclear reactor enclosing the core 2 of the reactor, 
itself surrounded by the core jacket 3 bounding the circulation of cooling 
water. 
Two loops of the primary circuit have been shown, the loop shown on the 
right FIG. 1 having a rupture in the return pipe for the primary water 
into the vessel, this portion of the loop being called the cold branch. 
The loop shown on the left of FIG. 1 does not have a rupture. This loop 
includes a steam generator 5 whose lower portion beneath the tube plate 
bearing the bundle 6 is divided into two parts 7 and 8 respectively in 
communication with the hot branch 9 and with the cold branch 10 of the 
loop of the primary circuit. 
The hot branch 9 is in communication on the inside of the tank with the 
inside of the jacket 3 containing the core 2 of the reactor. 
The cold branch in which a primary pump 11 is located is in communication 
with the peripheral inner portion of the vessel arranged around the jacket 
3. 
In normal operation of the reactor, the circulation of the primary water is 
effected along the arrows 12 constituted by a single line. The primary 
water becoming heated in contact with the core rises inside the inner 
portion of the jacket 3 passes into the hot branch 9, into the entrance 
compartment 7 of the steam generator, into the bundle 6, into the exit 
compartment 8 from the generator and then into the cold branch 10 to be 
introduced into the peripheral annular space of the vessel between the 
jacket 3 and the wall of the latter. This water which is cooled to produce 
steam in contact with the secondary water in the steam generator 6 
descends to the lower portion of the vessel before reascending into the 
central portion of the latter inside the jacket 3 where it is heated in 
contact with the fuel assemblies constituting the core. 
To maintain the primary circuit at a pressure and at a temperature which 
are completely defined, a pressurizer 14 is arranged on one of the loops 
of the reactor (in FIG. 1, for example, the pressurizer is located in the 
hot branch of the right hand loop). 
The elements of the right hand loop of the primary circuit corresponding to 
the elements of the loop shown on the right in FIG. 1 bear the same 
reference numerals, but with the sign '(prime). 
It is seen that the cold branch 10' of the right hand loop has undergone a 
rupture with complete displacement of the piping, so that the leakage 
space is equal to the double cross-section of the branch of the primary 
circuit. 
This accident corresponds to the largest size of breach that it is possible 
to imagine, and in particular, as well be apparent below, this accident is 
more serious than a rupture of the same size of a hot branch of one of the 
loops of the primary circuit. 
There has also been shown in FIG. 1, very diagrammatically, a medium 
pressure accumulator in each of the loops (references 16 and 17), each 
constituted by a cooling water reserve containing boric acid surmounted by 
nitrogen at 42 bars. 
These accumulators are connected to the cold branches of the loops of the 
primary circuit through a valve whose opening occurs when the pressure in 
the primary circuit drops below 42 bars. 
From the appearance of the rupture in the cold branch 10', the water of the 
primary circuit escapes through the broken pipe and the pressure in the 
primary circuit drops rapidly to abnormally low values. 
Control and warning signals formed from the pressure recorded at the level 
of the pressurizer first cause the emergency shut-down of the reactor, 
i.e., the dropping of the control rods, and then the safety injection. 
In the case of ruptures of small size, it is possible to introduce 
sufficient water, by using only the pump system, the pressure in the 
primary circuit not dropping sufficiently to cause the tripping of the 
medium pressure accumulators. 
On the other hand, for a real rupture of a pipe of the primary circuit, as 
shown in FIG. 1, the pressure drop causes the placing of the accumulators 
16 and 17 in operation injecting boric water into the cold branches of the 
primary circuit. 
The circulation of the water in the primary circuit in the case of rupture 
has been shown by the arrows 20 constituted by a double line. 
In the case of a real rupture located in a cold branch, there appears in 
the course of the decompression stage a reversal of the core flow, the 
steam flow then rising again in the annular collector (between the jacket 
of the core and tank) in a first stage prevents the water from the medium 
pressure accumulators from descending into the tank, this water rejoining 
the breach directly by flowing around the top of the jacket of the core. 
This by-pass phenomenon ceases during the end of the decompression and the 
water coming from the three subsystems of the injection circuit then 
participates in the actual filling of the vessel. 
In the course of this accident the role of the medium pressure accumulators 
is to inject a very large flow for a relatively short time corresponding 
to the filling of the lower part of the vessel and of the part comprised 
between the jacket of the core and the vessel. As soon as the water level 
starts to rise in the core, steam production is generated in contact with 
the fuel rods. The removal of this steam retarded by the considerable 
resistance constituted by the tubes of the steam generators and the pumps 
in the case of a cold branch rupture, very considerably limits the speed 
of rise of the level of the water in the core. 
The medium pressure accumulators ensure the operation of rapid filling of 
the lower part of the vessel, of the annular collector (between the core 
jacket and vessel) and the bottom of the core. 
It is therefore necessary to have an injection means enabling the water 
injection to be maintained for a sufficient time to ensure the reimmersion 
of the core, and only during this stage, if it is desired to contend with 
all accidents which can occur in the primary circuit and in particular 
with a rupture of a cold branch. 
For all types of breach, from complete reimmersion of the core, the 
injection pumps enable the removal of the residual power to be ensured and 
the cooling of the core to be ensured in the long term, either by means of 
the remainder of the capacity of the reservoir of the pools (direct 
injection phase), or when this reservoir reaches a low level, by means of 
the water or cooling liquid contained in the sump of the confinement 
enclosure or containment structure (recirculation phase). 
We will now describe with reference to FIG. 2, an emergency injection 
system according to the invention which enables such a rupture to be 
contended with and the production of reimmersion of the core after rupture 
without bringing into play two additional pumping systems located outside 
the confinement enclosure of the reactor. 
In FIG. 2 is shown diagrammatically the emergency injection system of which 
a portion is arranged inside the confinement enclosure of the reactor and 
of which the other portion is situated outside. 
In FIG. 2 the part of the device situated inside the sealed enclosure 21 is 
shown to the right of the latter while the part arranged inside the 
enclosure 21 is situated to the left of the latter. 
The emergency cooling device shown in FIG. 2 is associated with a nuclear 
reactor with four cooling loops. 
There are shown in FIG. 2, the four pipes 22a, 22b, 22d, which permit the 
medium pressure accumulators 24 and the low pressure accumulators 25 to be 
connected to each of the cold branches of the reactor. 
There is also shown two pipes 23a and 23b which permit the pump injection 
circuits to be connected to the hot branches of the reactor. 
In each of the pipes 22 is shunted as a by-pass a medium pressure 
accumulator 24 and a low pressure accumulator 25. 
The medium pressure accumulators 24 each contain a reserve of about 30 m3 
of water with 2000 ppm of boron under nitrogen at a pressure P1 of 25 to 
30 bars. 
The low pressure accumulators 25 each contain 20 m3 of water with 2000 ppm 
of boron under a nitrogen pressure P2 of about 15 bars. 
The medium pressure accumulators 24 are connected to the pipes 22 through a 
valve 26 which is always open when the reactor is in operation and a valve 
27 associated with a valve 28 placed in the pipe 22, these two valves 27 
and 28 enabling injection of borated water contained in the accumulator 
24, into the cold branch when the pressure of the primary circuit drops 
below the pressure of the accumulator (25 to 30 bars). 
The low pressure accumulators 25 are connected to the pipes 22 through a 
valve 29 always open when the reactor is in operation and a valve 31 which 
enables injection of water into the pipe 22 as soon as the pressure in 
this pipe drops below the pressure in the accumulator 25 (about 15 bars). 
The pipes 22 are also connected through valves 32 and valves 33 and 34 to 
injection lines through pumps of which the active portions are arranged 
outside of the confinement enclosure 21. 
Each of the injection lines include a medium pressure pump 36 mounted in 
series with a boost pump 37 which is a low pressure pump from which the 
delivery permits the supply of suction of the pump 36. 
The pump 36 is arranged downstream of the pump 37 with respect to the 
supply reserve of these pumps constituted by the supply reservoir of the 
pools of the reactor 40. 
This reservoir contains 3000 m.sup.3 of water with 3000 PPM of boron. 
Branched to the pump 36 is placed a by-pass circuit 41 which can be closed 
by a valve 42. This by-pass circuit 41 permits direct injection of the 
borated water into the primary circuit through the single low pressure 
pump 37, when the valve 42 is opened. 
A junction is also possible between the suction of the pump 37 and the sump 
of the confinement enclosure 21 which ensures the collection of the 
injected cooling fluids and of the condensed steam in this container 21, 
the cooling of this water by another circuit enabling in the long term its 
recycling at low pressure and at lower delivery rate to the primary 
circuit. 
There exists, in fact, in addition to the cooling systems by extraction of 
heat from the core and from the primary circuit, which have been 
described, a sprinkler system of the reactor enclosure which transfers and 
exchanges the extracted energy to an outer cold source. 
The cooling water used by this sprinkler system is collected through the 
sump of the confinement enclosure at the same time as the water escaping 
from the primary circuit in the case of rupture of the latter. This sump 
and its cooling system enable recirculation of the injected borated 
cooling water into the safety enclosure. 
The valves 32 as well as the valve 43 located in the pipe 23 permit the 
injection of cooling water either into the cold legs through pipes 23 or 
at the same time into the cold legs and into the hot legs through pipes 22 
and 23. 
The pumps 36 have a rated operating pressure of the order of 100 bars for a 
flow rate higher than about 500 m.sup.3 per hour. 
The pumps 37 have a rated operating pressure of the order of 15 bars for a 
flow rate higher than 500 m.sup.3 /h. 
The placing of the pumps of the injection systems outside the enclosure in 
operation is actuated as soon as a break has been detected in the primary 
system. 
Referring to FIG. 3, the operation of the emergency cooling device shown in 
FIG. 2 will now be described in the case of a real break in one of the 
cold legs of the primary system, that is to say in the case of the most 
penilizing accident which has been described with reference to FIG. 1. 
In FIG. 3 as plotted the delivery rates injected by each of the components 
of the emergency cooling device and the total delivery rate injected as a 
function of time taking as the origin the appearance in the primary 
circuit. 
To demonstrate the effectiveness of the injection device described above 
the following unfavorable assumptions have been made: 
only one pumping unit 36-37 has been able to be placed in operation and 
this pumping unit which delivers simultaneously into the four cold legs of 
the reactor has its useful delivery, that is to say useable for cooling 
the core of the reactor, reduced by a quarter by the injected water 
flowing through the broken cold leg of the primary circuit, 
a quarter of the delivery rate from all of the medium pressure accumulators 
flows through the breach of the primary circuit and is not useable for 
cooling the reactor, 
a quarter of the delivery rate of the low pressure accumulators is also 
lost through the breach of the primary circuit of the reactor. 
There have been shown the delivery rate curve from the medium pressure 
accumulators 50 in solid lines, the delivery rate curve from the low 
pressure accumulators 51 in mixed lines, the delivery rate curve from the 
pumping circuits 52 in hatched solid lines and lastly the injected total 
delivery rate curve 53 in dashed lines. 
It is seen that towards 15 seconds after the break of a cold leg of the 
primary circuit, the medium pressure accumulators start to inject their 
water reserve into the cold legs of the primary circuit, the pressure 
having dropped in this circuit below their triggering pressure. 
The maximum injection delivery rate is reached 20 to 30 seconds after the 
appearance of the breach in the primary circuit. 
The pressure in the circuit has then fallen back to some bars and the low 
pressure accumulators which have commenced the injection of their water 
reserves as soon as the pressure has fallen towards 15 bars in the primary 
circuit have then reached their maximum delivery rate. The total delivery 
rate injected is then at its maximum. 
This injection at very high delivery rate enables, as in the case of the 
use of devices according to the prior art, the filling of the lower part 
of the vessel and at the same time the cooling of the vessel and a part of 
the primary circuit and the lower part of the core. 
The delivery rate injected by the medium pressure accumulators is cancelled 
practically 50 seconds after the break in the cold leg. 
The delivery rate injected through the low pressure accumulators decreases 
progressively after passage through its maximum. This is due to the lower 
pressure difference between the accumulator and the primary circuit, which 
is practically at the pressure of the environment of the reactor building. 
The flow rate of these low pressure accumulators is only practically 
cancelled towards 400 to 500 seconds after the appearance of the break. 
Before the end of injection, from the medium pressure accumulators, the 
pumps of the injection circuit have been placed in operation supplying an 
additional delivery rate constant in the course of time which is added to 
the delivery rate from the low pressure accumulators. 
After 50 seconds the delivery rate injected is no longer constituted by the 
delivery rate from the low pressure accumulators and the pumping circuit. 
The delivery contribution from 50 seconds to 450/500 seconds by the low 
pressure accumulators completes the delivery rate contribution of the 
pumping system to ensure the more rapid reascent of the water level in the 
core of the reactor, preserving the driving hydraulic load height between 
the jacket of the core and the vessel. 
The role of the low pressure accumulators is then to ensure the complement 
of the delivery rate from the medium pressure pumps in the function of 
reimmersion of the core, by a relatively low delivery rate during a 
sufficient time to ensure the complete immersion of the core. 
Then, the pump circuit acts alone to effect the cooling of the reactor over 
a long period. 
If the pressure is low (large size breach), the medium pressure injection 
pumps are by-passed and the recirculation of the water is only ensured by 
the booster pumps. 
It is hence seen that is is possible to carry out the various functions : 
rapid cooling and partial filling of the vessel, complete reimmersion of 
the core and long-term cooling of the vessel with a group of pressure 
accumulators and a single pumping unit, due to the delivery rate spread 
over time of the low pressure accumulators, during the phase of 
reimmersion of the core of the reactor. 
In the case of the device according to the invention, the association of a 
medium pressure pump 36 with a booster pump 37 permits a sufficient 
delivery rate to be ensured to compensate for the primary water loss in 
the case of a small or medium breach and to avoid the passage of the core 
to an unimmersed state, i.e. to a state where the core is no longer 
immersed in cooling water. 
In the case of a small or medium breach, the device according to the 
invention precludes the passage of the core to an unimmersed state, solely 
by bringing into play the pumping system to the exclusion of the average 
and low pressure accumulators. 
The improved performance of the pumping system of the invention enables the 
triggering pressure of the medium pressure accumulators to be lowered by 
40 bars to about 25 to 30 bars, which represents an economy in the design 
of the accumulators. 
A relatively high triggering pressure of the medium pressure accumulators 
is necessary in the device of the prior art, in the case of an accident of 
less seriousness than a real break in a cold leg of the primary circuit, 
since the injection delivery rate of the pumping system did not permit the 
phase of emergence of the core to be limited sufficiently. 
In the case of a real break in a cold leg of the primary circuit, with the 
pressure falling rapidly in this circuit, the injection at relatively high 
pressure through the medium pressure accumulators does not have any 
advantage. 
In the case of a small or medium breach in the primary circuit or a rupture 
of the steam piping, the supplement through the injection of water by 
means of medium pressure injection pumps supplied by the booster pumps, 
themselves supplied by cooling water from the reservoir of the pools of 
the reactor, is ensured throughout the decompression phase of the primary 
circuit. 
Finally, for a longer duration, the cooling takes place by circulation with 
injection at the same time into the cold branches and into the hot 
branches through the low pressure pumps with recirculation of the water 
recovered in the sump of the safety enclosure. 
Finally, in the case of a considerable breach in the primary circuit, such 
as a real break in a cold leg after the reimmersion of the core has been 
effected, the longer duration cooling also takes place solely by water 
circulation due to the pumping circuit with recovery of the water in the 
sump of the confinement enclosure. 
This circulation can be assured solely by the low pressure booster pump, 
the valves 42 enabling the short-circuiting of the medium pressure pumps 
being open. 
Injection can be effected by circulation solely in the cold legs or by 
circulation at the same time in the hot legs and in the cold legs, by 
opening the valves 43. 
It is hence seen that the safety cooling device according to the invention 
enables the realisation of all the necessary functions whatever the 
accident occuring in the primary circuit of the reactor, by using a single 
injection system including active elements located outside of the safety 
enclosure and two accumulator units whose triggering pressures are 
different. 
The triggering of the whole of the low pressure accumulators enables the 
reimmersion of the core to be ensured after decompression of the primary 
circuit. 
The invention is not limited to the embodiments which have just been 
described; it comprises on the contrary all modifications thereof. 
Thus it is possible to conceive the use of accumulators of any type, from 
the moment when their automatic triggering by pressure drop in the primary 
circuit is possible. 
It is also possible to conceive the use of two accumulator assemblies at 
different triggering pressures with an injection circuit having pumping 
means arranged outside of the enclosure, of any type. 
The two-pump arrangement in series in the pumping circuit has however the 
advantage of enabling functioning of the installation at high delivery 
rate and at medium pressure at the same time as operation at high delivery 
rate and low pressure by using only one of the pumps which is a low 
pressure pump and by short-circuiting the other pump. 
Finally the emergency cooling device according to the invention is 
applicable to all types of pressurized water nuclear reactors whatever the 
number of loops of the primary circuit.