Cooling system for cooling a containment chamber constructed for receiving a core melt

A nuclear power plant has a reactor core and a containment chamber for receiving core melt of the reactor core. A cooling system for cooling the containment chamber includes a flooding container to be filled with coolant fluid. A cooling pipe leads from the flooding container to the containment chamber. A passively opening closure element closes the cooling pipe in the flooding container and opens as a function of a level of the coolant fluid.

CROSS-REFERENCE TO RELATED APPLICATION 
This application is a Continuation of International Application Serial No. 
PCT/DE95/01823 published as WO96/20485 Jul. 4, 1996. 
BACKGROUND OF THE INVENTION 
FIELD OF THE INVENTION 
The invention relates to a cooling system having a cooling pipe for cooling 
a containment chamber that serves to receive core melt of a reactor core 
of a nuclear power plant. 
In order to provide safe operation, nuclear power plants have numerous 
diverse and redundant safety systems, including cooling systems, through 
the use of which operating conditions that deviate from normal operating 
conditions can be detected early and counteracted. As a result, such 
safety-critical states as reactor core meltdown are practically precluded. 
In order to control that kind of accident, which is considered 
hypothetical, German Published, Non-Prosecuted Patent Application DE 40 41 
295 A1, corresponding to U.S. Pat. No. 5,343,506, describes a core 
retainer and a method for emergency cooling of a nuclear power plant. The 
core retainer has a catch basin, which is disposed immediately below the 
reactor pressure vessel that encloses the reactor core. Both the catch 
basin and the reactor pressure vessel are disposed inside a reactor 
cavern, which is a concrete structure. Cooling channels extend along the 
floor and the walls of the catch basin between the catch basin and the 
concrete structure and coolant water can be carried through the cooling 
channels. The cooling channels on the floor communicate with a water 
supply and discharge into a cooling pipe that protrudes in siphonlike 
fashion into the water supply. The siphonlike cooling pipe includes one 
part shaped as an inverted U. The apex of the U is located above an 
operative level of the water supply, and although the cooling pipe does 
dip into the water supply, in the vicinity of its apex it protrudes out of 
the water supply. As a result, as long as the level is at the operative 
level, no coolant water enters the cooling channels. It is not until the 
water supply is flooded to a level higher than the apex of the U that 
coolant water enters the cooling channels, resulting in cooling of the 
outside of the catch basin. Cooling of the interior of the catch basin is 
carried out through a flood pipe, which is passed from the water supply 
through the concrete structure into the catch basin. The flood pipe is 
closed in the catch basin by a meltable stopper that does not melt open 
until at a high ambient temperature, thus allowing coolant water to flow 
into the interior of the catch basin. Coolant water is present in the 
flood pipe even during normal operation of the nuclear power plant, and as 
a result the meltable stopper is continuously cooled. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a cooling system 
for cooling a retention or containment chamber constructed for receiving a 
core melt, which overcomes the hereinafore-mentioned disadvantages of the 
heretofore-known devices of this general type and which initiates cooling 
of a catch basin by passive measures and therefore inherently safely. 
With the objects of the invention in view, there is also provided a cooling 
system for cooling a containment chamber for receiving core melt of a 
reactor core of a nuclear power plant, comprising a flooding container to 
be filled with coolant fluid; a cooling pipe leading from the flooding 
container to the containment chamber; and a passively opening closure 
element closing the cooling pipe in the flooding container and opening as 
a function of a level of the coolant fluid. 
Through the use of a closure element that opens as a function of the level 
of the coolant fluid, it is assured that cooling of the containment 
chamber will not ensue by feeding of coolant fluid into the flooding 
container until a safety-critical state exists. The coolant fluid that 
additionally flows into the flooding container is preferably primary 
coolant water, which emerges from the primary coolant loop of the reactor 
core during the safety-critical state. Coolant fluid can optionally be fed 
into the flooding container through a separate coolant fluid reservoir. 
The cooling pipe is closed until the closure element opens and therefore 
is free of water. As a result, during normal operation of the nuclear 
power plant, coolant fluid and particularly coolant water is kept away 
from the containment chamber, thus averting such problematic factors as 
corrosion from coolant water or unintended cooling of a 
temperature-dependent closure element that closes the cooling pipe. 
Moreover, the inherent safety of the nuclear power plant is improved by 
the passively opening closure element, and human error in initiating 
cooling of the containment chamber is precluded. 
In accordance with another feature of the invention, the closure element is 
a float that closes off the cooling pipe. At an operative level of the 
coolant water, this float has such buoyancy that it sealingly closes the 
cooling pipe, for instance through a ball seat. The float is preferably 
movable along a primary axis in a guide, so that unintended slippage of 
the float from its sealing seat is avoided even upon the occurrence of 
jarring of the kind that can be caused by earthquakes, for instance. 
In accordance with a further feature of the invention, the float has an 
interior that can be filled with coolant fluid. A filler pipe passing into 
this interior has an inlet opening for coolant fluid, through which the 
coolant fluid flows in if a flooding level occurs that rises above an 
operative level. The inlet opening may be located geodetically above the 
operative level or geodetically below this operative level. In the latter 
case, the filler pipe is extended from the inlet opening in a U above the 
operative level, so that an apex of the inverted U is located above the 
operative level. In this latter case as well, coolant fluid does not flow 
into the interior of the float until the operative level has been exceeded 
by a predeterminable amount. Coolant fluid flowing into the interior 
lessens the buoyancy of the float, so that beyond a certain fill level of 
the interior, the float leaves its sealing seat, thereby opening the 
cooling pipe. Cooling of the cooling pipe thus ensues in a passive way. 
In accordance with an added feature of the invention, the float has a 
condensed water suction removal device, by which condensed water that 
occurs can be removed by suction during a normal operative state of the 
nuclear power plant. As a result, lowering of the float from the 
occurrence of condensed water and an attendant unintentional initiation of 
cooling of the containment chamber are reliably avoided. 
In accordance with an additional feature of the invention, the containment 
chamber communicates with the flooding container through a return for 
coolant fluid that extends geodetically above the cooling pipe, and in 
particular above the operative level. This return is closed in the 
flooding container by a further closure element that opens as a function 
of the level. Internal cooling of the containment chamber by a coolant 
fluid loop is attained through the use of the return. Coolant fluid 
flowing from the flooding container to the containment chamber flows in 
natural circulation. This assures that during a safety-critical state of 
the nuclear power plant, sufficient coolant fluid is returned to the 
flooding container, and cooling of the containment chamber and in 
particular of the core melt received in the containment chamber occurs. 
In accordance with yet another feature of the invention, the closure 
element that closes the return to the flooding container and which may 
also be a float, has a ball valve. This ball valve may have a floatable 
ball, which is held in a sealing position by a guide path. The ball valve 
protects the closure element from a pressure wave which can occur, for 
instance, from a temperature increase inside the containment chamber. If 
coolant water flows out of the containment chamber into the return, the 
ball of the ball valve floats upward and thereby opens the return to the 
flooding container. 
In accordance with yet a further feature of the invention, the cooling pipe 
is a flood pipe, which discharges into the containment chamber and thereby 
assures direct cooling particularly of the surface of any core melt that 
has flowed into the containment chamber. The flood pipe preferably extends 
horizontally and can be both installed and removed by working from the 
flooding container. Installing the flood pipe in the containment chamber 
from the flooding container has the advantage of permitting the mounting 
to be provided outside the containment chamber, which may be poorly 
accessible and might be affected by radiation. This is especially 
favorable in the case of a containment chamber that surrounds the reactor 
core, since this installation can be carried out after the containment 
chamber is lined in the usual way with a crucible-like guard and 
collection layer. 
In accordance with yet an added feature of the invention, during normal 
operation of the nuclear power plant, the flood pipe is closed in the 
containment chamber with a closure element that opens as a function of 
temperature. During normal operation of the nuclear power plant, it is 
filled with air, and as a result the closure element that opens as a 
function of temperature is thermally insulated from the coolant water of 
the flooding container, and coolant water does not enter the flood pipe 
until during a safety-critical state of the nuclear power plant, so that 
the effects of corrosion are reliably avoided. 
As a result of the thermal insulation of the closure element that opens as 
a function of temperature, reliable opening, and in particular melting 
open, in the event of major heat development inside the containment 
chamber, are assured. The closure element that opens as a function of 
temperature can therefore be constructed in such a way that it opens the 
flood pipe only at high temperatures as compared with a closure element 
that is in direct contact with coolant water. The closure element that 
opens as a function of temperature is preferably resistant to neutron 
radiation, which occurs during normal operation of the nuclear power plant 
in the immediately vicinity of the reactor core and particularly in the 
reactor cavern that receives the reactor pressure vessel. Moreover, it has 
the advantage of using only a single melting element (melting screw, 
melting strip), so that canting and thus belated opening of the closure 
element as would occur if there were a plurality of elements melting at 
different times, is averted. The closure element is furthermore adapted to 
the cross section of the flood pipe, so that installation of the flood 
pipe with the closure element already assembled is assured. 
In accordance with yet an additional feature of the invention, the closure 
element that opens as a function of temperature has a material that melts 
open at a high temperature, for instance above 900.degree. C. This 
material may be corrosion-resistant and radiation-resistant and in 
particular may be silver. The closure element that opens as a function of 
temperature may have a bale closure with a silver tightening screw. The 
bale closure presses a cap sealingly into the flood pipe, so that this 
flood pipe is closed with certainty during normal operation of the nuclear 
power plant. 
In accordance with again another feature of the invention, the closure 
element that opens as a function of temperature can alternatively have a 
closure cap that is sealingly soldered to the flood pipe. Silver can also 
be used as the soldering substance. 
In accordance with again a further feature of the invention, the 
containment chamber has an external cooling device for externally cooling 
at least a floor and/or one wall of the containment chamber. The cooling 
pipe is a supply line connecting the external cooling device to the 
flooding container. During normal operation of the nuclear power plant, 
the supply line is closed by a float. The external cooling device 
preferably has a drain line for the coolant fluid, which returns to the 
flooding container. As a result, coolant fluid, in particular primary 
coolant water that has flowed into the flooding container, returns to the 
flooding container again, so that a coolant loop is provided for the 
external cooling of the containment chamber. 
In accordance with again an added feature of the invention, the containment 
chamber is a crucible-like catch basin disposed below the reactor core. 
Cooling of the catch basin, which ensues passively by a float disposed in 
the flooding container, takes place on the outside of the catch basin by 
the external cooling device and/or in the interior of the catch basin 
through the use of a flood pipe. 
Preferably, a flood pipe is extended thermally elastically from the 
flooding container to the catch basin, discharging into the latter. The 
flood pipe has a compensator outside the catch basin, in particular 
between the wall of the catch basin and a concrete structure that forms a 
reactor cavern. The compensator, which in particular is welded on and has 
a welded-on spherical flange, seals off the catch basin that has an 
interior with a temperature of approximately 300.degree. C., for instance, 
from the external cooling of the catch basin, which has a temperature of 
20.degree. C. to 30.degree. C. The compensator serves to compensate for 
thermal expansions of the catch basin and additionally assures sealing off 
of the flood pipe from a coolant fluid flow for cooling the outer wall of 
the catch basin. 
In accordance with a concomitant feature of the invention, the cooling 
system is also suitable for cooling a propagation chamber located 
laterally below the reactor core. The interior of the propagation chamber 
may be cooled by a flood pipe, which extends from a flooding container 
into the propagation chamber. External cooling of the propagation chamber 
by suitably extended cooling channels, which are flooded with coolant 
fluid through a passively opening closure element, such as a float, inside 
the flooding container, is also possible. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
cooling system for cooling a containment chamber constructed for receiving 
a core melt, it is nevertheless not intended to be limited to the details 
shown, since various modifications and structural changes may be made 
therein without departing from the spirit of the invention and within the 
scope and range of equivalents of the claims. 
The construction and method of operation of the invention, however, 
together with additional objects and advantages thereof will be best 
understood from the following description of specific embodiments when 
read in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The figures of the drawings show an exemplary embodiment of a containment 
chamber of the invention that is a crucible-like catch basin but is 
analogously applicable to a containment chamber constructed as a 
propagation chamber. 
Referring now in detail to the figures of the drawings, in which identical 
reference numerals identify identical components, and first, particularly, 
to FIG. 1 thereof, there is seen a fragmentary, longitudinal section 
through a nuclear power plant having a cooling system 1 for cooling a 
containment or retention chamber 2 constructed to receive core melt. A 
reactor pressure vessel 3 which is largely rotationally symmetrical about 
its primary axis 5 is disposed in a reactor cavern 48 formed by a 
supporting structure 36. The reactor pressure vessel 3 contains a reactor 
core 4. The containment chamber 2 is formed in the reactor cavern 48 below 
the reactor pressure vessel 3 through the use of a catch basin 28 for a 
core melt. The catch basin 28 has a floor 24 and a wall 25. A free space 
remains between the support structure 36 on one hand and the wall 25 and 
the floor 24 on the other hand, for an external cooling device 23 of the 
catch basin 28. In the interior of the catch basin 28, the floor 24 and 
the wall 25 are adjoined by a lining 38, for instance of zirconium oxide 
(ZrO.sub.2) tiles. A layer of sacrificial concrete 27, especially for 
lowering the melting point of a core melt, is disposed on the lining 38 
toward the floor 24. A cooling pipe 6 for coolant fluid 7 is constructed 
as a flood pipe 31 which passes from a flooding container 8 through both 
the wall 25 and the adjoining support structure 36, with a slight 
inclination from the horizontal, into the catch basin 28. In the catch 
basin 28, the flood pipe 31 is closed by a closure element 15, in 
particular a closure element that opens as a function of temperature. In 
the flooding container 8, the flood pipe 31 is closed by a closure element 
9 that opens as a function of a fluid level and in particular has a float 
10. A compensator 29 which surrounds the flood pipe 31 between the wall 25 
and the support structure 36, seals off the wall 25 from the external 
cooling device 23 and absorbs thermal expansion of the catch basin 28. The 
float 10 that seals the flood pipe 31 has an interior 11. A filler pipe 
12, which has an inlet opening 13 geodetically above the flood pipe 31, is 
introduced into the interior 11. The inlet opening 13 is likewise located 
above an operative level 14 of the coolant fluid 7, in particular coolant 
water, that is located in the flooding container. The external cooling 
device 23 of the catch basin 28 communicates with the flooding container 8 
through a supply line 26 that extends through the support structure 36 
substantially horizontally below the reactor cavern 48. In the flooding 
container 8, the supply line 26 is likewise closed by a closure element 9 
having a float 10. The closure element 9 of the supply line 26 also has a 
filler pipe 12 which extends into the interior 11 of the float 10, leads 
out of the coolant fluid 7 above the operative level 14 and is bent back 
in a U to enter the coolant fluid 7 again, where it ends in an inlet 
opening 13. A return 20 for internal cooling which is disposed above the 
operative level 14 and thus above the flood pipe 31, extends from the 
reactor cavern 48 into the flooding container 8. Inside the flooding 
container 8, this return 20 is closed by a further closure element 21, 
which has a further float 10 that is immersed approximately half-way into 
the coolant water 7. A ball valve 22 with a float ball is disposed between 
the further closure element 21 and the return 20. Each of the closure 
elements 9, 21 has a respective condensed water suction removal device 19. 
The return 20 extends in the reactor cavern 48 above the catch basin 28 
through both the support structure 36 and an insulation 37 adjoining the 
support structure 36. The return 20 communicates with the interior of the 
catch basin 28. 
During normal operation of the nuclear power plant, the cooling system 1, 
which includes the external cooling device 23, the flood pipe 31, the 
return 20 and the closure elements 9, 21, 15, is closed. In particular, 
both the external cooling device 23 and the flood pipe 31 are filled with 
air. During normal operation of the nuclear power plant, the external 
cooling device 23 serves the purpose of operative air cooling, which 
prevents heating up of the support structure. Cooling air is fed from 
below through airshafts that are located outside the support structure 36, 
into the supply line 26, which is constructed as an annular channel and 
communicates with eight horizontal channels, to the outside of the catch 
basin 28. The cooling air rises on the outside of the catch basin 28 and 
the support structure 36 as it heats up and can escape into a 
non-illustrated reactor building of the nuclear power plant. The annular 
channel likewise communicates through eight pipes with the flooding 
container 8. During an accident involving melting of the reactor core 4, 
the flooding container 8 is flooded with additional coolant fluid, in 
particular coolant water 7, so that the level rises from the operative 
level 14 to an elevated level that is located above the inlet opening 13 
of the float 10. The additional coolant fluid in this case is primary 
coolant water emerging from the primary coolant loop of the reactor core 
4. The additional coolant fluid can optionally be fed from a separate, 
additional coolant fluid supply. The floats 10, which close the flood pipe 
31 and the external cooling device 23, are filled with coolant water 7 and 
sink downward because of the decreasing buoyancy. As a result, both the 
flood pipe 31 and the external cooling device 23 are filled with coolant 
water. When the operative level 14 is exceeded, the external cooling 23 
comes into operation first. A return of coolant water 7 through the 
external cooling device 23 takes place through six horizontally extending, 
non-illustrated channels above the operative level 14 into the flooding 
container 7. The return through the external cooling device 23 and the 
return 20 of the internal cooling are separate from one another. The core 
melt that emerges as the reactor core 4 melts down leads to heat 
development in the catch basin 28, as a result of which the closure 
element 15 of the flood pipe 31 likewise opens, since it opens as a 
function of temperature. As a result, the coolant fluid 7 flows into the 
interior of the catch basin 28 to cool the core melt. The elevated level 
inside the flooding container 8 thereupon drops, for instance by 30 cm to 
60 cm, to a flooding level 32, so that the level of the coolant water 7 is 
at the same height in both the reactor cavern 48 and the flooding 
container 8. The coolant fluid 7 flowing into the catch basin 28 through 
the flood pipe 31 is heated and rises by natural circulation as is 
indicated by flow arrows 30 and flows back through the return 20 into the 
flooding container 8, as is also represented by the flow arrows 30. 
Opening of the closure element 9 of the external cooling device 23 causes 
the coolant water 7 to pass out of the flooding container 8 through the 
supply line 26, as is represented by flow arrows 44, so that it can reach 
the outside of the catch basin 28. The coolant water 7 evaporates there 
and is returned into the flooding container 8 through non-illustrated 
channels. As a result of the evaporation, cooling of the catch basin 28 
from the outside occurs as well. The evaporated coolant water 7 rises 
inside the nuclear power plant, condenses, and passes back into the 
flooding container 8. Effective cooling of any core melt occurring in the 
catch basin 28 is assured over a long period of time through the use of 
the closure elements 9 for both the flood pipe 31 and the external cooling 
device 23, which elements open upon a rise of the level in the flooding 
container 8. 
In FIG. 2, the further closure element 21 of FIG. 1, having a float 10 and 
a ball valve 22 with a floatable ball, is shown on a larger scale. At the 
operative level 14, the float 10 is immersed approximately halfway in the 
coolant water 7. The floatable ball of the ball valve 22 rests on a ball 
position holder 33 that extends downward from the return 20 to the float 
10. Even in the event of a pressure wave arising in the reactor cavern 48 
and propagating through the return 20, the ball valve 22 seals off the 
float 10, so that the float remains protected. The float 10 is guided in 
guides 35, and it is thus displaceable along an axis 49. The ball valve 22 
has a vent 34. During a normal operating state of the nuclear power plant, 
the return 20 is dry and in particular is filled with air. If the level 
inside the flooding container 8 rises from the operative level 14 to a 
flooding level 32, which is located geodetically above the further closure 
element 21, then the coolant water 7 reaches the ball valve 22 through the 
return 20. After the entry of the coolant water 7 into the ball valve 22, 
the floatable ball rises and uncovers an opening 50, through which the 
coolant water 7 can flow out of the return 20 into the float 10. As a 
result of the inflowing coolant water 7, the buoyancy of the float 10 
decreases, and it sinks along the axis 49 in the flooding container 8, and 
therefore the coolant water 7 can flow out of the return 20 into the 
flooding container 8 in natural circulation. 
The flood pipe 31 of FIG. 1 is shown on a larger scale in FIG. 3. Inside 
the catch basin 28, the flood pipe 31 is closed by the closure element 15 
that opens as a function of temperature and has a bale closure 16. The 
flood pipe 31 is surrounded between the support structure 36 and the catch 
basin 28 by the compensator 29, which rests sealingly on the catch basin 
28 in a ball sealing seat 38. 
On a larger scale, FIG. 4 shows the closure element 15 of FIG. 3 that opens 
as a function of temperature. The bale or hoop closure 16 acts through a 
bale or hoop 42 to press a cap 40 firmly into a ball sealing seat 39 of 
the flood pipe 31. The bale 42 is firmly connected to the flood pipe 31 
through a tightening screw 17, which has a melting bolt 43. The melting 
bolt 43 is formed of silver with a melting temperature of about 
960.degree. C. A splash protector 41 between the melting bolt 43 and the 
cap 40 is disposed parallel to the flood pipe 31, to protect the melting 
bolt 43 against escaping coolant water 7. As a result, it is assured that 
melt-through of the melting bolt 43 is not delayed by evaporating coolant 
water 7, even if the ball sealing seat 39 should leak. 
FIG. 5 shows an alternative embodiment of a closure element 15, which opens 
as a function of temperature, for the flood pipe 31. The closure element 
15 has a closure cap 18, which is soldered to the flood pipe 31 at two 
solder strips 45 through a silver strip 46 that surrounds the flood pipe 
31. An insulator 47 having an air cushion is introduced between the silver 
strip 46 and abutting portions of the flood pipe 31 and the closure cap 
18. If high heat develops in the containment chamber 2, the solder strips 
45 and if applicable the silver strip 46 melt open, so that the closure 
cap 18 falls off and the flood pipe 31 opens. The closure elements 15 
shown in FIG. 4 and FIG. 5 each have only one melting element 43, 46. As a 
result, the danger of unequal melting open of two melting elements that 
close the closure element, with the possibility of belated opening of the 
closure element, is averted. 
The invention is distinguished by a cooling system with a cooling pipe for 
cooling a containment chamber constructed to receive a core melt. The 
cooling is tripped through the use of a passive closure element. The 
closure element opens as a function of the level of coolant water in a 
flooding container, so that coolant water flows into the containment 
chamber or along its outside surfaces. The closure element preferably has 
a float which due to its buoyancy closes off the cooling pipe. The float 
is constructed in such a way that when a level of cooling water that is 
above an operative level is reached, the float is filled with coolant 
water through a filler pipe, and the cooling pipe sinks downward into the 
flooding container, thereby opening. The cooling system has a return that 
is extended above the fluid pipe that feeds coolant water into the 
containment chamber. Through the use of the return and the fluid pipe, a 
natural circulation of the coolant water develops, thereby assuring 
effective cooling of the containment chamber and the core melt caught 
therein.