GB2056748A

GB2056748A - A method for eliminating local gas concentrations in stratified atmospheres

Info

Publication number: GB2056748A
Application number: GB7928905A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-08-22
Filing date: 1979-08-20
Publication date: 1981-03-18
Also published as: GB2056748B; FR2463886A1; FR2463886B1

Abstract

Local gas concentrations in stratified atmospheres, in particular in stratified atmospheres present in closed systems, are eliminated by producing a disruption in the stratified atmosphere which impairs the equilibrium of the stratified atmosphere at least locally, so that a gas volume experiences an upwardly or downwardly directed acceleration, thereby causing a convection current. The disruption is caused by producing a local change in density in the respective atmospheric strata by introducing a gas having a suitable density or by supplying or dissipating heat.

Description

SPECIFICATION A method for eliminating local gas concentrations in stratified atmospheres This invention relates to a method for eliminating local gas concentrations in stratified atmospheres in accordance with the preamble of the main claim.

Extensively stable thermal arid/or mixture stratification can form both in the free atmosphere and in particular in atmospheres present in closed systems. The atmospheric strata occur owing to differences in the density of individual components or mixtures of differing temperature and can also be favored additionally depending on the structure of the building. These differences are compensated or equalized by diffusion processes very slowly, the rate of equalization decreasing with a decrease in the concentration differences.

Moreover, such equalization can also be rendered more difficult by locally constricted cross sections in the case of partitioned buildings.

It is known that the occurrence of stratified atmospheres can be prevented or that the elimination of atmospheric stratification can be achieved by providing for a continuous mixing of the atmosphere at least in certain areas. In this way local gas concentrations are prevented from occurring. Fans, ventilators, air circulation plants or similar equipment is employed to mix the atmosphere, for example, but they always require drive aggregates and in most cases must be rather voluminous in design.

Of special importance is the avoidance of local gas concentrations and the elimination of local gas concentrations in the event coolant is lost in the container or reactor. Should such a malfunction occur, molecular hydrogen and oxygen, for example, can form in the coolant by rådiolysis. It is therefore necessary for security reasons to eliminate local concentrations of the released hydrogen.

After a malfunction involving a loss of coolant, a thermally stratified atmosphere forms in a pressurized water reactor, for instance, in which a distinctly warmer atmospheric stratum overlies a relatively cool lower atmospheric stratum.

To eliminate local hydrogen concentration, the hitherto known measures provide for the use of circulation ventilators and/or recombination units. A mixing of the various atmospheric strata is effected with the aid of the circulation ventilators, thereby eliminating local concentrations of hydrogen. The recombination units oxidize the free hydrogen, for example, by catalytic combustion with oxygen, thus reducing the hydrogen content of the atmosphere supplied to these units.

Both measures involve considerable technical outlay, since the equipment must be protected in case of reactor malfunction, dependent on an appropriate power supply and cannot be employed arbitrarily. Furthermore, it is not yet completely clear at which location they can most favorably be disposed within the reactor container. This applies in particular to the spatial arrangement of the intake and resupply ports. If these are disposed in the upper portion of the reactor container, as is the case in a few of the cited facilities, it must be ensured that the hydrogen which is released from a rupture at a low location, for instance, will also reach the intake port.

Another drawback is that it is not entirely impossible that this equipment will become ineffective in the event of a technical defect or, under certain circumstances, will itself act as an undesirable source and ignite the hydrogen-containing atmosphere in the reactor container.

It is significant as far as the technical security considerations are concerned if this equipment is located outside of the reactor container, since every penetration through the container constitutes an increased risk of the container behavior, in particular when the passages and insulating valves in the case of externally installed ventilators or recombination units must be designed with appropriately large dimensions due to the required gas passages.

It must be taken into account in the event that the equipnient is disposed within the reactor, which is not possible in all cases, that after a malfunction with coolant loss, the reactor container cannot be entered due to the fission products released or can only be entered with special protective measures and only for a short time so that repairs cannot be made or can only be made with difficulty.

The known measures are also disadvantageous because reactors already in existence cannot be subsequently equipped with this equipment or can only be equipped with difficulty.

The object of the invention is to provide a method of the type mentioned at the outset which is simple and reliable, which is not prone to malfunction and which is low in cost, it also being possible to execute this method in reactors already in existence.

This object is accomplished in accordance with the invention by means of the characterizing clause of the main claim as well as of claim 11.

According to the inventive method, it is possible in a simple and reliable manner to eliminate local gas concentrations by producing a disruption in the stratified atmosphere which impairs the equilibrium of the stratified atmosphere at least locally so that a gas volume experiences an upwardly or downwardly directed acceleration, thereby causing a convection current.

If a first atmospheric stratum with a first specific density overlies a second atmospheric stratum having a second specific density, the first specific density being smaller than said second specific density, the equilibrium can be disrupted either in the first or second atmospheric stratum or even simultaneously in both atmospheric strata.

In the following, the atmospheric stratum having the lower specific density will be designated as the light atmospheric stratum and that having the higher specific density as the heavy atmospheric stratum.

Now, if a heavy gas is introduced into one region of the light atmosphere so that there is such a change in the density of the light atmosphere that it becomes heavier at least locally than the heavy atmosphere, a downwardly directed acceleration will occur in this local region so that a downwardly directed current will form. This downward current will cause an upwardly directed current to occur in another region of the atmosphere so that the result will be convection in the entire atmosphere. Consequently, the gas of the first atmospheric stratum will mix with the gas of the second atmospheric stratum.

The formation of a convection current can be promoted in this case by introducing a light gas into the heavy atmosphere at substantially the same time the heavy gas is introduced into the light atmosphere. The light gas is introduced into the heavy atmosphere at a location which is horizontally spaced from the introduction site of the light atmosphere. The specific density of the heavy atmosphere is reduced by introducing light gas to such an extent that the resultant new specific density is lower than that of the light atmosphere so that an upwardly directed acceleration occurs in the region which results in an upward directed gas current.

If a heavy atmosphere overlies a light atmosphere, a convection current can be provoked by supplying to the heavy atmosphere a gas having such a specific density that at least locally in the region of the heavy atmosphere such a specific density is formed that either the interface between the two atmospheric strata is disrupted and heavy gas flows into the light atmospheric stratum or that, while still retaining the atmospheric stratification, the entire atmosphere is set in rotation, the region to which the heavy gas is supplied then sinking downwardly.

The formation of a convection current can be promoted in this case as well by feeding a light gas into another region of the light atmosphere so that the heavy atmospheric stratum will be moved upwardly there due to the buoyancy of this light gas.

In accordance with the invention the changes in density in the respective atmospheric strata required to produce a disequilibrium can also be provoked instead of supplying a gas by extracting or supplying an amount of heat. Lower density in a region is then obtained by heating and greater density is achieved by cooling.

Depending on the respective gas making up the atmospheric strata, different gases can be supplied in succession. For instance, if a convection current has already formed, it can be maintained by supplying a gas either continuously or at various intervals. The gas must then have such a density that it experiences an acceleration in the total atmosphere which does not oppose the direction of the convection current, since the convection current would otherwise slowed down, if not eliminated completely.

Inert gases are especially well suited to execute the inventive method. One advantage is that no chemical reaction occurs between such a gas and the other gases in the atmosphere. In addition, equipment or machines located in the room in which the atmosphere is enclosed will not be impaired or attacked chemically by an inert gas.

If the inventive method is provided for the container of a reactor, for example, in whose coolant system molecular hydrogen can be formed which will then escape into the reactor container should a rupture occur in a coolant line, the gas to be supplied can then advanta geously be fed directly into the coolant line. It is advantageous that the gas be supplied directly to that location at which the hydrogen escapes into the reactor container.

Let us assume that the inventive method is to be employed in a reactor. A gas can be supplied to the region of an atmospheric stratum for outside the reactor container without any difficulty. In so doing, only apertures or pipes with a small cross section have to be provided in the wall of the container. Since no mechanical equipment is required, the inventive method is extremely reliable.

In particular, no elements are required which under certain circumstances could cause the hydrogen to be ignited as would be the case, for example, with drive aggregates for circulation ventilators. Yet another advantage of the inventive method is that it can also be employed in reactors already in existence.

However, even if the required density change in the atmospheric strata is effected by supplying and extracting heat, the inventive method is still reliable and safe. In such a case, it is sufficient to provide heat exchangers. These heat exchangers can be supplied from outside the reactor container with a medium with the desired temperature.

If desired, the inventive methods can also be employed together with recombination units mounted inside or outside the reactor. In this case, the hydrogen-containing atmosphere is transported in accordance with the inventive method from the site of hydrogen release, i.e.

the rupture in the primary coolant line, for example, to the recombination unit.

The two inventive methods can also be used simultaneously or in succession.

The inventive method in which a gas is supplied to change the specific density of an atmospheric stratum will now be described in the following by way of example in conjunction with a malfunction involving a loss of coolant in the container or a reactor with reference to the figure in which a pressurized water reactor is schematically illustrated in a sectional view.

In the figure, number 1 3 designates the container of a reactor. The interior of said container is partitioned, thereby forming spaces 2 to 10. Each of these spaces communicates with at least one neighboring space by overflow apertures.

Steam generators DE are located in spaces 3, 5, 8 and 4, 7, 10 and are connected by primary coolant lines 14 to a reactor pressure container RDB located in space 9. Number 1 designates the site at which a rupture in the primary coolant line is supposed to occur.

Gas feed lines 11 and 1 2 are provided and open into the interior of the reactor container 1 3. One gas feed line 11 terminates in spaces 8 and 10 in each case. The feed lines 1 2 are located in the upper portion of the container 1 3 and serve to introduce gas into space 2.

It is already known from experiments on the reactor container that thermal atmospheric stratification can be expected in the container in the long term, i.e. hours or days, after a malfunction with an attendant loss of coolant has taken place. The atmosphere in the upper portion of the reactor has a distinctly higher temperature than the atmosphere in the lower section of the reactor. This is termed a thermal inversion. It is also conceivable that in extreme cases a warmer layer will form between the lower and the upper atmospheric strata. This warmer stratum would act as a thermal barrier layer.

After the atmosphere has stratified, the atmospheres in the individual spaces interconnected by overflow apertures will not show any appreciable differences in pressure. Due to the lower temperature of the atmospheric stratum present in the lower section of the reactor container, it has a greater specific density than the atmosphere in the overlying warmer stratum. This means that, according to physical laws, the atmosphere with the lower temperature would experience an opposite "barrier acceleration" if this atmosphere attempted to ascend contrary to the inverse temperature stratification.

This thermal barrier effect can be overcome if the specific density of the lower atmospheric stratum is smaller than that of the higher atmospheric stratum. Such a state could be achieved by adequately enriching the atmosphere using the hydrogen which escapes during a reactor malfunction. Under the expected conditions with respect to the differing temperatures of the atmospheric strata, there would have to be a hydrogen concentration in the lower stratum which should be avoided for safety reasons. The limit which is decisive for safety considerations amounts to 4% by volume of hydrogen, i.e. at the lower combustion or explosion limit.

In accordance with the invention, the density of the lower atmospheric stratum is reduced in the present example at least locally to such an extent that the thermal barrier effect can be overcome so that the hydrogen is distributed throughout a larger volume before the hydrogen concentration in the lower section of the container or of the lower atmospheric stratum has reached an undesirably high value.

This is accomplished in accordance with the invention in the present exemplary case by introducing a light, preferably an inert gas such as helium, for example, into the lower container spaces 8 and/or 10. This reduces the density of these lower atmospheric strata, i.e. compared to the total system increasing buoyancy is imparted to this atmosphere as the concentration increases. Finally, this buoyancy overcomes the thermal barrier acceleration and can thus initiate the corresponding convection. The convection which then occurs would form in the shape of a roller which flows preferably through spaces 3, 5, 8, 9, 10, 7, 4 and 2.

As long as there is a stratified atmosphere, the lower atmospheric stratum, for example, filling spaces 5 to 10 and the upper one filling spaces 2 to 4 of the reactor container, convection through spaces 4, 3 and 2 is impossible without auxiliary measures for a long time, since the thermal stratification is stable. In the lower atmospheric stratum itself, however, a convection current can occur through spaces 7, 6, 5, 8, 9, 10. This convection current, however, would only mix the gases within the lower atmospheric stratum in which the hydrogen in particular is released in space 1 0. Hence, local hydrogen concentrations can consequently occur in the lower atmospheric stratum and the magnitude of such hydrogen concentrations exceeds the permissible safety value of 4% by volume.

Instead of the introduction of a light inert gas such as helium, for example, the introduction of steam can also be taken into consideration. If steam is used as the light gas, it would be possible to eliminate or at least reduce the temperature inversion at one specific location similar to the introduction of an inert gas which is also warmed up, so that the local difference in density due to the hydrogen, the steam and/or the inert gas present would result in the formation of a corrresponding convection current, thereby building up a convection roller flowing through spaces 3, 5, 8, 9, 10, 7,4 and 2.

The technical implementation of the inven tive method is not associated with any appreciable difficulties. It is only important that means be provided which are capable of supplying the desired gas to the atmosphere inside the reactor container at predetermined locations. Such means can be pipes 11 and 1 2 which are connected to a source of gas located outside of the container. Such a gas source could be gas cylinders (gas bottles) in which the highly pressurized gas is normally in the liquid state. The storage of such gas cylinders is not problematical, since there is no safety risk involved, especially if the gas is an inert gas such as helium. The requisite regulatory means such as valves, siide gates, etc. can be provided outside the reaction container so that no special safety measures have to be taken.

It can also be taken into consideration to add an inert gas such as helium to the coolant itself, e.g. in the cooler to dissipate after-heat or in the vicinity of the residual heat cooling pump, etc., since in the event of a reactor malfunction involving the coolant, the gas will be transported by the coolant to that location at which a rupture has occurred in the coolant line.

The time and duration of a singular or repeated introduction of a gas can be determined by referring to the instruments for monitoring the temperature and hydrogen concentration. Such instruments will be provided in any case.

One can also consider storing the gas source itself, i.e. the helium cylinders, for example, in the interior of the reactor container. Measures, however, should be taken to prevent damage to the gas cylinders and the associated valves and lines in the event of a reactor malfunction involving a loss of coolant. The advantage in this case would be that the wall of the reactor container would not have to be penetrated. In many cases, however, lines are provided anyhow for the purpose of taking samples (e.g. hydrogen measurements) which pass through the reactorcontainer and which could be utilized to introduce the gas.

Claims

1. A method for eliminating local gas concentrations in stratified atmospheres, in parti-cular in closed systems such as the containers of reactors, characterized in that a first atmospheric stratum having a first specific density and overlying a second atmospheric stratum, the-specific density of which is greater than said first specific density, is supplied with such an amount of a gas with such a specific density that, at least locally in the region proximate to said second atmospheric layer, the specific density of said first atmospheric stratum assumes a value greater than that ofsaid second specific density, or said second atmospheric stratum is supplied with such an amount of a gas with such a specific density that, at least- locally in the region proximate to said first atmospheric stratum, the specific density of said second atmospheric stratum assumes a value smaller than that of said first specific density, or that a first atmospheric stratum having a first specific density and overlying a second atmospheric stratum whose specific density is smaller-than said first specific density is supplied with such an amount of a first gas with a specific density greater than said first specific density that, in a region proximate to said first atmospheric stratum, the specific- density of said second atmospheric stratum assumes a value greater than- that of the specific density of said second atmospheric stratum, or the second atmospheric stratum is supplied with such an amount of a second gas with a specific density smaller than the specific density- of said second atmospheric stratum that, in a region proximate to said second atmospheric stratum, the specific density of said first atmospheric stratum assumes a value smaller than that of said first specific density, whereby, due to the change in density of the region respectively proximate to an atmospheric stratum, an upwardly or downwardly directed acceleration occurs which causes convection.

2. A method according to claim 1, wherein said first atmospheric stratum is supplied with a first gas in a first region and said second atmospheric stratum is supplied with a second gas in a second region which is spaced from the first region of the first atmospheric stratum in a horizontal direction.

3. A method according-to claim 1- or 2, wherein various gases are supplied to at least one of said atmospheric strata either simultaneously or in succession.

4. - A method according to one of claims 1 to 3, wherein a mixture of different gases is employed as the gas to be supplied.

5. A method according to one of claims- 1 to 4, wherein an inert gas is employed as the gas to be- supplied.

6. A method according to claim 5, wherein helium is used as said inert gas.

7-. A method according to one of claims 1 to 4, wherein steam is used as the gas to be supplied.

8. A method according to one of the preceding claims, wherein the gas is supplied to the coolant.

9. A method according to-one-- one- of the pre ceding claims, wherein the temperature of the gas supplied to an atmospheric stratum is higher than the temperature of said atmospheric stratum.

10. A method according to one of claims 1 to 8, wherein the temperature of the gas supplied to an atmospheric stratum is lower than the temperature of said atmospheric stratum-.

1 1. An apparatus for eliminating local gas concentrations in stratified atmospheres, in particular in closed systems such as the containers of reactors, characterized in that such an amount of heat is extracted from a first atmospheric stratum having a first specific density and overlying a second atmospheric stratum, the specific density of which is greater than said first specific density, that, at least locally in the region proximate to said second atmospheric stratum, the specific density of said first atmospheric stratum assumes a value greater than that of said second specific density, or such an amount of heat is supplied to said second atmospheric stratum that, at least locally in the region proximate to said first atmospheric stratum, the specific density of said second atmospheric layer assumes a value smaller than that of said first specific density, or that such an amount of heat is extracted from a first atmospheric stratum having a first specific density and overlying a second atmospheric stratum, the specific density of which is smaller than said first specific density, that, in a region proximate to said first atmospheric stratum, the specific density of said second atmospheric stratum assumes a value greater than that of the specific density of said second atmospheric stratum, or such an amount of heat is supplied to said second atmospheric stratum that, in a region proximate to said first atmospheric stratum, the specific density of said first atmospheric stratum assumes a value smaller than that of said first specific density, whereby, due to the change in density of the region respectively proximate to an atmospheric stratum, an upwardly or downwardly directed acceleration occurs which causes convection.

1 2. An apparatus according to claim 11, wherein an amount of heat is extracted from a first region of said first atmospheric stratum and an amount of heat is supplied to a second region of said second atmospheric stratum which is spaced from said first region of said first atmospheric stratum in a horizontal direction.

1 3. A method as claimed in Claim 1, substantially as described herein.

14. An apparatus as claimed in Claim 11, substantially as described herein with reference to the accompany drawing.

1 5. The features as herein described, or their equivalents, in any novel selection.