Method of avoiding or inhibiting intermixing of the atmosphere which exists in an enclosed space with a gaseous substance present in the same space

In a method of inhibiting or avoiding intermixing of the atmosphere which exists in a closed nuclear reactor containment space with a gaseous substance present in the same space a change in density of the atmosphere is induced in either lower or upper area of the space such that a stable atmosphere stratified into superimposed layers is formed in that space due to the change in density.

BACKGROUND OF THE INVENTION 
In the present context an enclosed space is to refer, quite generally, to a 
zone which are sealed, largely or entirely, by a structural enclosure. 
Such a zone may be, for instance, a closed building, a closed container, 
or an area which is substantially enclosed within a building. 
In industry, technical equipment often is housed in sealed spaces. When an 
operational disturbance or an accident occurs it may happen that a 
substance escapes which is contained, for example, in a pipeline system of 
a plant or in vessels associated with such plant or equipment. This may 
happen because a rupture or leak has formed. If the substance leaking out 
is such as to react chemically or physically with the atmosphere 
surrounding the plant, this is a potential risk for the plant. If the 
substance is an inflammable, volatile liquid, like benzene, alcohol, or 
the like, or an inflammable gas, like methane, hydrogen, or the like, the 
danger that a normal or explosion-type combustion will occur grows with 
the length of time such substance issues from the plant and has time to 
intermix, for instance, with the oxygen of the air. 
If the substance so leaking is a chemically aggressive gas, there is a risk 
that even elements or parts of the plant at a great distance from the leak 
will be attacked or endangered if the gas spreads at sufficiently high 
concentration. 
If the gas or gas mixture released by the chemical engineering plant or 
having formed subsequently, is a poisonous substance an essential gain in 
safety may be achieved if the latter can be confined to a limited area of 
the plant, thereby avoiding or at least restricting or delaying its 
release through such means as ventilation ducts, skylights, doors, and 
hallways, or the like, in order that protective measures devised for 
disaster control may be prepared and carried through successfully. 
As an example of a conceivable accident in the chemical industry, reference 
may be made to the release of chlorine gas from a container or process 
circuit. This poisonous gas is especially dangerous when released close to 
the ground. If no more than minor heating occurs, such as by a fire 
breaking out at the same time, this gas may escape readily in an upward or 
lateral direction from a building and then cool down rapidly to such an 
extent that it will spread disastrously in the surroundings near to the 
ground. This may pose a threat to entire towns, particularly if 
meteorological conditions of inversion and unfavorable wind conditions 
prevail so that protective measures devised for disaster control, such as 
the evacuation of the population, cannot be carried out effectively. On 
the other hand, it is conceivable that the gas in question, which is 
relatively heavy at normal temperatures, will flow out in a downward or 
lateral direction into lower zones, for example through passages and gates 
although it was initially released in an upper region. It is possible as 
well that this gas will pass through ventilation ducts, even if the air 
circulating equipment is shut off, thus reaching zones which should remain 
accessible to afford an opportunity of controlling the accident. In any 
such case even reduced or delayed release would amount to an effective 
protective measure. 
Similar, yet specific dangers threaten with nuclear power plants featuring 
light water reactors. Light water reactors comprise a primary coolant 
system in which "ordinary" water is used as the coolant. 
In the case of light water reactors, allowance is made for the occurrence 
of serious accidents by way of technical safety measures providing for 
automatic shut-down of the light water reactor and switching on additional 
specific auxiliary means. 
A rupture, or at least a leakage, occurring in a primary coolant line is 
called a loss of coolant accident and classified as a serious accident. 
When such loss of coolant accidents occur, for instance, molecular 
hydrogen may be formed within a short period of time by a metal-water 
reaction in the core area. 
The explosiveness of an air/hydrogen mixture highly depends on the 
concentration of the reaction partners present. If, in such event, the 
proportion of air and of the oxygen available in the air reach a 
sufficiently high concentration, the high proportion of hydrogen will 
cause the formation of a gas mixture which may very easily be ignited. 
However, in the case of a combustion or explosion the effect may be of 
limited intensity if the amount of air or of the oxygen it contains which 
have become mixed with the hydrogen is insufficient or only partly 
sufficient to react with all of the hydrogen present. If an intermixing 
under such circumstances can be limited or avoided from the very beginning 
by not supplying further air, i.e. additional oxygen, to the place of the 
high hydrogen concentration, this will clearly reduce the hazardousness of 
the accident in question. Gaining but a few hours of time will play an 
essential role, all the more so as the short-lived fission products then 
will already have become decomposed to a large extent by natural decay. 
It was suggested to fill the safety containments of a reactor with an inert 
gas during operation so as to prevent any inflammation or explosion of a 
substance reacting with the oxygen of air. However, this involves 
considerable disadvantages in normal operation and obviously cannot be 
realized with all plants. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of this invention to provide a method of 
avoiding or inhibiting intermixing of the atmosphere which exists in an 
enclosed space with a gaseous substance present in the same space. 
This and other objects are met, in accordance with the invention, in that a 
change in density of the atmosphere is induced in one area of said space 
such that an essentially stable atmosphere stratified into superimposed 
layers is formed because of the change in density. 
The method of the invention permits the formation of a substantially stable 
thermal stratification and/or mixture stratification in an enclosed space. 
The method according to the invention causes changes in density such that 
the atmosphere becomes stratified. As soon as a stratified atmosphere is 
established, the differences in density between the strata of the 
atmosphere balance only slowly by diffusion processes, as analyses and 
experiments have shown. 
The method according to the invention can be realized in an extremely 
simple manner. 
Fundamentally, the invention provides two possibilities of effecting a 
change in density, either by introducing a gas or a liquid or by heat 
input and/or heat removal. It is also feasible to realize both 
possibilities at the same time or in timed sequence. A particularly simple 
way of realizing the method according to the invention by gas supply 
resides in disposing a heating means in the ceiling area of the space 
and/or a cooling means in the bottom area of the space. These means may be 
provided in the upper area, for instance, at the ceiling of the space and 
in the lower area, for instance, at the bottom of the space. It is also 
possible to provide for heating or cooling from outside, for instance by 
steam or water. 
The subject matter of the invention will be described further, by way of 
example, with reference to the accompanying panying drawings:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The method according to the invention will be described, by way of example, 
with reference to a loss of coolant accident in the safety containment of 
a reactor, referring to FIG. 1, the reactor being a pressurized water 
reactor. 
In FIG. 1 reference numeral 13 designates the safety containment of a 
reactor. The interior of the safety containment is subdivided by 
partitions into areas which are in fluid communication, such as spaces 2 
to 10. Each of the spaces communicates with no less than one adjacent 
space through overflow apertures. 
The spaces 3, 5, 8, and 4, 7, 10 house steam generators SG which are 
connected by way of primary coolant lines 14 to a reactor pressure 
container RQC arranged in space 9. Reference numeral 1 marks the place at 
which a rupture of the primary coolant line is assumed to have taken 
place. 
Gas feed lines 11 and 12 lead into the interior of the safety container 13. 
The gas feed lines 11 open into lower areas of the safety container, 
namely into spaces 8 and 10. The feed lines 12 lead through the cupula of 
the safety container 13, terminating in the upper area of the safety 
container in space 2. 
In general, the gas feed lines 11 and 12 are laid so as to be protected at 
the inside by concrete walls and are passed to the outside of the safety 
container at a certain location. If desired, a manifold may be used. 
Control of the lines may be effected by remote controlled double 
instruments, as is the case with the sampling metering system. 
If a loss of coolant accident has occurred, releasing coolant as well as 
hydrogen at the place of rupture 1 of the primary coolant line 14, the 
hydrogen escaping will spread in space 10 from which it may first pass on 
to the other spaces 5 to 9 communicating with space 10. 
The method according to the invention makes it possible to prevent or at 
least inhibit the distribution of the hydrogen in the entire containment. 
In the case of the embodiment shown this is achieved by using the gas feed 
line 12 which is provided in the upper area of the safety container 13 to 
introduce a gas whose density is less than the density of the atmosphere 
in the safety container 13. It is assumed that the gas supplied is helium. 
The helium supplied through the feed lines 12 spreads in the area of the 
cupula of the safety container 13 where it mixes with the existing 
atmosphere. In this way the specific density of the atmosphere in the 
upper area of the safety container 13 is reduced, adopting a value which 
is lower than that of the specific density of the atmosphere in the lower 
area of the safety container 13. 
The pressure drop reduced by the supply of, for instance, helium would 
cause a pressure balancing flow between the volumes of space 2, on the one 
hand, and spaces 3 to 10 which, in general, are called equipment spaces. 
This would mean introducing helium under pressure together with atmosphere 
from space 2 into spaces 3 and 4, while the air in space 2 would mix with 
the remaining helium by free convection. By virtue of this process the 
spaces 2, 3, and 4 have a lower density than spaces 5 to 10 disposed at a 
lower level so that a stable stratification is obtained. If the difference 
between the specific density of the atmosphere enriched with helium gas 
and the specific density of the atmosphere prevailing in spaces 5 to 10 is 
sufficiently great, a stable superimposed barrier layer will form to 
prevent intermixing by convection between the hydrogen in spaces 5 to 10 
and the oxygen of the great volume contained in space 2, and vice versa. 
The only oxygen available for reaction with the hydrogen issuing from the 
leak 1 in the primary coolant line 14 is the oxygen contained in the 
atmosphere in spaces 5 to 10. Under the conditions prevailing during an 
accident the proportion of oxygen in this very area should be clearly less 
than during normal operation. 
It should further be noted that when fixing the quantity of helium gas to 
be supplied it must be kept in mind that, because of the leaking hydrogen, 
a change in specific density to be compensated and depending on the 
composition of the atmosphere will take place in spaces 5 to 10, too. This 
circumstance induced by the accident itself can be influenced 
advantageously by the method of the invention in that a heavy gas, 
preferably an inert gas, is introduced in addition through lines 11 so as 
to achieve at least partial balance of the change in density caused by the 
hydrogen. 
It should be noted that when employing the method of the invention in the 
manner described above the simultaneous feeding through lines 11 and 12 
may influence the above-mentioned overflow processes to such an extent 
that the pressure increases in spaces 2 and 5 to 10, respectively, are 
equalized or that controlled overflow is obtained. 
The use of the method according to the invention in a chemical plant will 
be described with reference to FIG. 2 which shows such a plant 
diagrammatically. A vertically disposed boiler 201 is connected to a 
reservoir 202. Furthermore, the upper and lower ends of the boiler 201 are 
connected to a chemical reactor 203. The chemical plant consisting of the 
boiler 201, the reservoir 202, and the chemical reactor 203 is installed 
in a building 205. A cooling means 215 is provided in the lower area of 
the building 205, and a heating means 214 is provided in the upper area 
thereof. A gas feed line 213' opens into the interior of the building 205 
in the lower area, approximately at one third of the height of the 
building 205. This gas feed line 213' is connected to a gas supply means 
213 by way of valves not designated specifically. A gas feed line 212' 
opens into the upper area of the building, approximately at two thirds of 
the height of the building 205, this line being connected to a gas supply 
means 212 by way of valves not designated specifically. A building 207 
housing auxiliary equipment and comprising a window 208 is located next to 
the building 205. A control stand 211 is arranged in the auxiliary 
equipment building 207 which is connected to the reactor building 205 by a 
walkable connecting passage 209. 
Between the building 205 housing the chemical plant and the building 207 
housing the auxiliary equipment there is an air conditioning unit 210, 
reference numeral 204 designating an air circulation fan which is 
indicated diagrammatically and may also comprise, for example, a heating 
or a cooling means. Connecting lines lead from the air circulation fan 204 
to the buildings 205 and 207 where they end. Part of these connecting 
lines is connected to the suction end of the air circulation fan 204, and 
another part is connected to the output end of the air circulation fan 
204. Those connecting lines connected to the inlet end of the air 
circulation fan 204 withdraw atmosphere from the upper areas of the 
buildings 205 and 207, and this atmosphere is introduced into the lower 
area of the buildings 205 and 207 through the other part of the connecting 
lines. 
The connecting lines which are in fluid flow communication with the upper 
areas of the buildings 205 and 207 and lead to the suction end of the air 
circulation fan 204 are in fluid flow communication with the gas supply 
means 212 through a gas feed line 217'. 
A gas feed line 216' establishes fluid flow communication between the gas 
supply means 213 and those connecting lines which lead into the lower 
areas of the buildings 205 and 207 and are connected to the outlet end of 
the air circulation fan 204. 
Controllable valves are provided in the gas feed lines 216' ahd 217' to 
control the rate of flow passing through these lines 216' and 217'. These 
valves are not designated specifically. 
The positions shown in FIG. 2 of the heating means 214 and the cooling 
means 215 are given as an example only. The cooling means 215 also could 
be provided in the form of a cooling coil placed, for instance, in the 
floor of the building 205. 
If a disturbance or an accident occurs during the operation of the chemical 
plant, for example releasing a gaseous substance whose specific gravity is 
higher than that of air from the upper end of the boiler 201, this gaseous 
substance will sink inside the building 205. As soon as the accident has 
been determined, a gas will be introduced from the gas supply means 213 
through the gas feed line 213'. 
The type of gas provided in the gas supply means 213 depends on the kind of 
chemical process employed and the gaseous substances it produces. The 
positions of the gas feed lines which open into the building 205 and of 
which only gas feed line 213' is shown in FIG. 2 depend, moreover, on the 
structure of the chemical plant and on its positional relationship within 
the building 205. 
If, as is the case with the example chosen, a dangerous gaseous substance 
must be expected whose specific density is greater than that of air at the 
temperatures prevailing, preferably the gas supply means 213 will contain 
a gas whose specific density is higher than that of the dangerous gaseous 
substance at the temperature conditions prevailing in the building 205. If 
a leak has occurred at the upper end of the boiler 201, setting free the 
dangerous gaseous substance, gas from the gas supply means 213 will be 
introduced through the gas feed line 213' into the interior of the 
building 205 by suitably controlling the valves provided in the gas feed 
line 213. This gas, fed into the interior, will sink to the bottom of the 
building 205. As time passes, the gas flowing into the building 205 
through the gas feed line 213' will fill or enrich the lower area of the 
building. The dangerous gaseous substance flowing out of the upper end of 
the boiler 201 and sinking down will reach the area of the building 205 in 
which the gas introduced has become distributed. The dangerous gaseous 
substance will float on the gas introduced because, as assumed, it has a 
lower specific density than the gas introduced under the existing 
temperature conditions. Further supply of gas makes it possible to shift 
the dangerous gaseous substance into the upper area of the building 205 so 
that it will be located in this upper area of the building 205, remaining 
remote from the lower area of the building 205. 
It should further be noted that not only the gas supplied through gas feed 
line 213' but also the dangerous gaseous substance exiting out of the 
upper end of the boiler 201 will mix at least partially with the air 
contained in the building 205 so that the relationships will change 
somewhat as regards the different specific densities. In the present case, 
the mean density of the mixture of gas supplied and air available in the 
lower area of the building 205 will be lower than the specific density of 
the gas supplied but higher than that of the atmosphere in the upper area 
mixed with the poisonous substance. The latter atmosphere, moreover, may 
be reduced by additional introduction of a light gas or by heating in the 
upper area. 
It should also be considered that the introduction of gas as well as the 
emergence of the dangerous gaseous substance cause a pressure rise within 
the building 205. If connecting lines to an air conditioning unit are 
provided, as is the case with the present example, a hazardous gaseous 
substance may issue through these connecting lines. In the case at issue, 
for instance, this may be prevented substantially by introducing a low 
specific density gas through gas feed line 217' into the connecting lines 
of the air conditioning unit and introducing the same gas already fed 
through gas feed line 213' also through gas feed line 216'. This means 
that essentially a gas having a high specific density would flow into the 
lower area of the building 205, while a gas of low specific density would 
reach the upper area of the building 205. In this manner the 
stratification of densities prevailing inside the building 205 may be 
maintained or even enhanced. 
The connecting lines establishing flow communication between the building 
207 which houses the auxiliary equipment and the air conditioning unit 210 
contain blocking means adapted to be closed, mainly for reasons of safety, 
gas tight so that no person working in the building 207 will be 
endangered. If, on the other hand, nobody is left in the building 207 
because of an actual danger, the building 207 can be filled through the 
connecting lines leading to the air conditioning unit 210 with at least 
one of the gases available in the gas supply means 212 and 213 so that it 
is guaranteed that no dangerous gaseous substance will enter the building 
207 housing the auxiliary equipment. 
As an additional measure, the cooling means 215 arranged in the lower area 
of the building 205 may be switched on to produce an additional rise of 
the mean density of the gas mixture available in this area. Switching on 
of the heating means 214 so as to reduce the density of the gas mixture in 
the upper area of the building 205 may be advisable, depending on the 
given circumstances and taking into consideration the pressure conditions 
prevailing within the building 205.