Exhaust Gas Cleaning System For Handling Radioactive Fission And Activation Gases

Abstract

An exhaust gas cleaning system utilizing the principle of delaying radioactive gases to permit their radioactive decay to a level acceptable for release to the atmosphere, comprising an adsorbent for adsorbing radioactive gas and a container for containing the adsorbent and for constraining gas to flow through the adsorbent, the adsorbent and the container forming simultaneously an adsorptive delay section and a mechanical delay section, by means of a predetermined ratio of volume of voids in the adsorbent to total volume of the container containing the adsorbent, for delaying radioactive gas to permit its radioactive decay to a level acceptable for release to the atmosphere. A method of using an adsorbent for cleaning a radioactive gas containing an isotope which is adsorbed by the adsorbent and containing an isotope whose adsorption by the adsorbent is low as compared to the isotope which is adsorbed and which is short-lived as compared to the isotope which is adsorbed, comprising constraining the gas to flow through the adsorbent with the retention time for the isotope which is adsorbed being at least the minimum for permitting radioactive decay to a level acceptable for release to the atmosphere and with the retention time for the isotope of relatively low adsorption and relatively short life being at least the minimum for permitting radioactive decay to a level acceptable for release to the atmosphere.

Description

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for cleaning radioactive gases to permit safe disposal into the atmosphere.

The radioactive fission and activation gases set free in nuclear reactors must be treated before their release to the atmosphere. The purpose of this treatment is to prevent there occurring an unacceptably high contamination of the environmental air around the nuclear reactors, with the unacceptable radiation load associated with such contamination.

In the treating of such exhaust gases, it is primarily a matter of delaying them to permit their radioactive decay to satisfactory levels. The article, "Reactorabgas und Gebaudeabluftbehandlung im Kernkraftwerk mit Siedewasserreactor (in translation, Reactor Exhaust Gas and Building Exhaust Air Treatment in the Nuclear Power Plant with Boiling Water Reactor)", published in the August 1970, (Volume 16, No. 8) issue of Atom und Strom (in translation, Atom and current), pages 115 to 118, shows a treatment system wherein the gas is first mechanically delayed in a long pipeline circuit, so that short-lived active substances, primarily nitrogen and oxygen isotopes, can decay. That this mechanical delaying is needed has been clear for the known treatment plants, because nitrogen and oxygen can only be delayed adsorptively to a limited extent.

The article gives an introduction to the basics of exhaust gases containing radioactive components. In the case of a boiling water reactor, the gases which arise by activation are primarily isotopes of oxygen, nitrogen, and argon.

Following the mechanical delay section of the treatment system of the article comes a mechanical filter, whose purpose is to collect the solid daughter products arising during the decay of the active gases in the mechanical delay section. Then following the filter comes a purely adsorptive delay section, where the longer-lived radioactive substances are caught on the surface of the adsorbent by free surface forces and consequently delayed. Of concern here are primarily only the noble gases xenon and krypton. The solid fission products resulting in the adsorptive delay section can be active; they are separated at the end of the adsorptive delay section by a mechanical filter. Since the adsorptive delaying action increases with decreasing moisture in the mixture to be delayed, the described system provides a gas drying step before the adsorptive delay section.

The described method has the disadvantage of necssitating a large expense for apparatus, in order to achieve exhaust gas cleaning. This is especially true in the case of the mechanical delay section, which is formed by a long pipeline circuit. Since condensate inherently forms in the pipeline circuit, a complex dewatering system is necessary. In the case of operation at pressures below atmospheric, the problems of condensate removal become greater.

The mechanical delay section is also caused to be large because of the parabolic flow distribution in the pipelines. The flow velocity is thus higher in the center of a pipe than at the wall. This means that the retention time for material to be delayed can take on different values. However, the designing of the delay section has to be carried out on the basis of the flow velocity at the center of the pipes.

The large volumes associated with this prior system requires expensive radiation shielding, both for the pipeline circuit and for the space where the removed condensate is collected.

SUMMARY OF THE INVENTION

An object of the present invention, therefore, is to provide an exhaust gas cleaning system for decreasing the apparatus expense as compared to the above-described system; at the same time, cleaning ability is not to be negatively influenced.

A further object is to avoid problems of condensate removal for the mechanical delay section, to hold the volume of the mechanical delay section as small as possible, and, as a result of such small volume, to hold radiation shielding expenses small.

These as well as other objects which will become apparent in the discussion that follows are achieved, according to the present invention, by:

1. an exhaust gas cleaning system utilizing the principle of delaying radioactive gases to permit their radioactive decay to a level acceptable for release to the atmosphere, comprising adsorbent means for adsorbing radioactive gas and container means for containing the adsorbent means and for constraining gas to flow through the adsorbent means, the adsorbent means and the container means forming simultaneously and adsorptive delay section and a mechanical delay section, by means of a predetermined ratio of volume of voids in the adsorbent means to total volume of the container means containing adsorbent means, for delaying radioactive gas to permit its radioactive decay to a level acceptable for release to the atmosphere; and

2. a method of using an adsorbent for cleaning a radioactive gas containing an isotope which is adsorbed by the adsorbent and containing an isotope whose adsorption by the adsorbent is low as compared to the isotope which is adsorbed and which is short-lived as compared to the isotope which is adsorbed, comprising constraining the gas to flow through said adsorbent with the retention time for the isotope which is adsorbed being at least the minimum for permitting radioactive decay to a level acceptable for release to the atmosphere and with the retention time for the isotope of relatively low adsorption and relatively short life being at least the minimum for permitting radioactive decay to a level acceptable for release to the atmosphere.

GENERAL ASPECTS OF THE INVENTION

The objects of the invention are achieved in an exhaust gas cleaning system with a mechanical and an adsorptive delay section for treating radioactive fission and activation gases, wherein the adsorptive delay section simultaneously forms the mechanical delay section for the gas mixture to be treated, this being accomplished by appropriately choosing the ratio of the volume of voids between the particles of the adsorbent making up the adsorptive delay section to the total volume into which adsorbent is filled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of an exhaust gas treatment plant according to the invention.

FIG. 2 is a partially schematic elevational view of a main adsorber of the present invention.

FIG. 3 is a partially schematic elevational view of a preliminary adsorber of the invention.

FIG. 4 is a partially schematic plan view of a plant according to the invention in a vacuum chamber.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As brought out in the BACKGROUND OF THE INVENTION, radioactive gas to be cleaned according to the invention can contain nitrogen and oxygen as isotopes which are relatively unadsorbed and of low half-life, while containing xenon and krypton and isotopes of relatively greater half-life which are adsorbed.

It is preferred to provide the system of the present invention with a preliminary adsorber, with the gas mixture to be treated being first delayed adsorptively there and then being filtered for removal of solid daughter products resulting from active gas decay. Then, in a main adsorber, the remaining, longer-lived isotopes are delayed adsorptively and mechanically, with the resulting solid daughter products being again filtered off.

An example of the invention will now be described with reference to the drawings, firstly with reference to FIG. 1. The radioactive fission and activation gas to be treated is sucked out of a turbine condenser (not illustrated) by evacuation pump 1 and, in mixture with the driving steam, fed to the exhaust gas treating plant. After a slight superheating of the steam/gas mixture in a heater 2, the mixture moves on to recombiner 3, where free hydrogen and oxygen in the mixture are burned to water on the surface of a catalyst, for example of platinum. The heat given off by the exothermic reaction causes a large temperature increase. Temperatures of 400.degree.C in the mixture are comprehended. The thus strongly superheated steam/gas mixture goes next into a condenser 4 and then a cooler 5. The driving steam and the steam from the radiolysis gas being burned to water are largely condensed and withdrawn, so that only an inert gas mixture, composed primarily of air with some remaining steam, comes from cooler 5.

The flow diagram shows a second, parallel line of devices 1 to 5. Such technique is used, for example, in the system described in the article mentioned above.

Following the condenser 4 and cooler 5 is a gas cooling and drying plant 6. The gas leaving the cooler and dryer 6, usually will be at a temperature in the range of 0.degree.-30.degree.C, with a humidity corresponding to a dew point in that range. Then comes preliminary adsorber 7, and a second preliminary adsorber 7', which is used as needed. Following directly on preliminary adsorber 7 is the main adsorber 8. The main adsorber can include one or more adsorption columns. There follows vacuum pumps 9 and 9', by which the treated gas is led to an exhaust air and exhaust gas chimney 10.

The essential components of the exhaust gas cleaning system are the preliminary adsorber 7, and perhaps 7', and the main adsorber 8. The preliminary adsorber, which usually represents the smaller part of the adsorption plant, serves essentially for the decay of the short-lived fission products, present as the vastly greater portion of the gaseous nuclide mixture. In conformance with the high proportion at which the short-lived fission products are present, there is a correspondingly large heat and daughter product production. The forming solid daughter products are simultaneously filtered off. The preliminary adsorber is constructed in such a manner that the adsorbent, and the solid daughter products, which deposit in the adsorbent and are thus filtered off simultaneously, can be exchanged easily and without danger. Further details will become apparent with the description of FIG. 3 below.

The main adsorber, which usually represents the larger part of the system, serves both for mechanically and adsorptively delaying the remaining, longer-lived isotopes and for the filtering of the resulting solid daughter products. The container of the main adsorber 8 is, therefore, of larger volume than the preliminary adsorber and is provided with simple installations for an improbable, yet conceivable, exchange of the adsorbent.

By choosing a suitable ratio of volume of voids in the adsorbent to the total volume of container containing adsorbent, which ratio may be controlled for example by the particular shape or size of adsorbent particles used, it is possible to provide in main adsorber 8 for the further decay of poorly adsorbable gas components in the voids between the adsorbent packing bodies.

The main adsorber 8 can be provided in the form of a cylindrical vessel having a floor on which the adsorbent lies. Gas flow-through can be either from below upwards, or from above downwards. An especially advantageous embodiment of the main adsorber is illustrated in FIG. 2. A central tube 12 is arranged in standing container 11; tube 12 divides the container 11 into two spaces 13 and 13' for receiving adsorbent 50, for example activated carbon. These spaces are flowed through, one after the other. Depending on particular circumstances, more central tubes 12 of other diameters can be arranged concentrically in the standing container 11, so that a larger number of separate spaces is provided for receiving adsorbent. The locations of gas entrance and exit connections, and filling and emptying connections for adsorbent, can be seen likewise in FIG. 2.

In order to catch solid particles which might be carried out of the main adsorber, no additional filter is needed. Rather, this catching is done by using liquid-piston type rotary blowers to forward the gas. The liquid needed for blower operation then catches any low-activity solid impurities present in the gas. The separation of the largely decayed and cleaned gas from the liquid proceeds in a gas-water separator. A suitable liquid-piston type rotary blower is shown in FIGS. 6 -47 on page 6-24 of PERRY'S CHEMICAL ENGINEER'S HANDBOOK, McGraw-Hill, New York, 1963.

A preferred embodiment of the preliminary adsorber 7 (or 7') is shown in FIG. 3. Included are a cylindrical upper part 14 and a conical lower part 15. Pipe 16 extends from above down into the interior of the chamber formed by parts 14 and 15; gas enters from this pipe into adsorbent 51, for example activated carbon. Lid 20 closes the top of the chamber. A gas distributor 17 on the end of pipe 16 provides for the emission of the gas into the adsorbent while at the same time preventing an encroachment of the adsorbent from the area between the outside of pipe 16 and the inner walls of parts 14 and 15, into pipe 16. For reasons of radioactivity and easy filling and emptying, the preliminary adsorber 7 is arranged between an upper shielding floor 18 and a lower shielding floor 19. A filling pipe 21, for the filling of packing bodies carrying adsorbent, is accessible from above the upper shielding floor 18 and opens into the adsorbent chamber through lid 20. For the emptying of adsorbent, a discharge tube 22 extends downwardly from the lower end of conical part 15, through lower shielding floor 19, into the space beneath. Either a part, or all, of the adsorbent can be extracted from the adsorbent chamber through tube 22 and into one or more transport vessels. Control of the adsorbent emptying is accomplished by a push gate 23 in the discharge tube 22. Because the pipe 16 extends down into the conical lower part 15, it is possible to empty first always the most strongly loaded adsorbent through discharge tube 22. Thus, that portion of the adsorbent in the vicinity of the gas distributor 17 provides the greatest filtering and retention of solid daughter products. Fresh adsorbent can then be added for make-up purposes through filling pipe 21.

A transport car 24 with vessel 25 can serve for receiving and carrying away the emptied adsorbent. Sealing between the vessel 25 and discharge tube 22 is done using a plastic sack according to conventional welding techniques.

As indicated by the arrows in FIGS. 3 and 2, a radioactive gas to be cleaned first flows from the cooling and drying plant 6 in FIG. 1 into pipe 16 in FIG. 3, out into the adsorbent 51, upwards in part 14, and out through pipe 52 in lid 20. Pipe 52 is connected to pipe 53 in FIG. 2. Gas flowing into pipe 53 flows upwards through space 13' and then turns down through the outer, annular space 13, finally to leave, in cleaned state, through pipe 54. Adsorbent is filled into space 13' through connection 55a; into space 13 through connection 55b. Adsorbent may be removed from space 13 through connection 56b and from space 13' through connection 56a.

The method of the present invention provides longer delay times if pressures above atmospheric are used and also if cooling is provided. But then, the use of a below-atmospheric pressure chamber can improve the exhaust gas cleaning system still further. Such is illustrated in FIG. 4. Thus, usually, radioactive exhaust gas cleaning systems are operated at pressures below atmospheric for reasons of safety.

The below-atmospheric or vacuum chamber includes a concrete housing 30 for the purpose of providing shielding from radiation. The inner walls of this housing are lined with a heat insulation 31. The vacuum chamber surrounds the main adsorber 8, which in this embodiment includes three adsorption chambers 32, 32', and 32". The vacuum chamber is accessible through an air lock 33, which is closed during normal operation. Ventilation of the vacuum chamber is done using an air introduction line 34 and an air extraction line 35.

The air sucked out of the vacuum chamber, as well as the air which is introduced, is passed through an ultrafine filter 36 and an activated carbon filter 37 by means of a blower 38, whose suction side faces filter 37, in conformance with the arrows indicating air flow in the vacuum chamber. On the pressure side of blower 38, throttle valves 39 and 40 are adjusted with respect to one another to recycle a portion of the air while sending the remainder to the chimney 42. The air recycled to the vacuum chamber can be sent through a cooler 41, as shown, with the gas in line 34 being in the range -20.degree.C to +20.degree.C. The throttle valve 57 controls the admission of fresh air to the system.

FIG. 4 further shows that the gas mixture to be treated in the main adsorber 8 is first passed through a compressor 43, whose pressure side faces chamber 32 in the drawing. Upon leaving the main absorber, the gas moves through an expansion valve 44 and thence out of the vacuum chamber and into chimney 42. In this system the pressure within chamber 30 is in the range 0.95-1 atm. and the pressure downstream of compressor 43 is in the range 0.8-20 atm.

A significant advantage of the exhaust gas cleaning system of the present invention resides in its elimination of a separate mechanical delay section, so that the total system becomes cheaper and less demanding of space. Since the mechanical delay takes place after the cooling, condensing, and drying provided by apparatus components 4, 5, and 6 described above, condensate does not occur during the mechanical delay. By filling the adsorbers with adsorbent, a substantially constant gas flow velocity is obtained across the adsorber cross sections; this enables an excellent utilization of the volume available for mechanical delay.

The space saving aspect of the present invention means that lesser amounts of radiation shielding are needed.

Because the preliminary adsorber is arranged between two shielding floors, it is possible to fill and empty adsorbent with safety.

The dividing of the main adsorber up into a plurality of concentric adsorbent-receiving chambers also saves space, as compared with a practice of providing the adsorbent in a plurality of separate columns.

The preferred activated carbon, which is used as adsorbent, has the following specification:

particle size 1-3 mm internal surface 1000 m.sup.2 /g bulk density 0.5 g/m.sup.3

This carbon is manufactured by Bergwerksverband, Essen-Kray, Frillendorferstra.beta.e 351. The trademark name is "Aktivkohle Typ VRG 1-3".

The release rates at the stack are based on the permissible exposure rates at the location of the power plant. The permissible rates both in the United States of America and in Germany are in the range of 5-50 mrm/a (millirem per annum). This means-depending on the meteorological conditions at the site-values at the stack-outlet of the power plant in the range of 1-100 Curie/h (see the article, mentioned on page 3, lines 13 to 19.)

To design the exhaust gas cleaning system for handling the radioactive fission and activation gases there must be known the kind, the quantity and the activity of the radioactive gases to be treated in connection with the maximum radiation level in gases to be released to the atmosphere as mentioned above.

Examples of the kind, the quantity and the activity of the radioactive gases in connection with a boiling water reactor are given in the above-mentioned article and also in the article: "The off-gas system of the power plant Gundremmingen" published in the May 1971 (Volume 13, No. 5) issue of Kerntechnik, pages 205-213, especially Table 1 and 2. According to these publications, the usual flow rates for the mixture of air, fission gases and gases formed from water by activation, are in the range of 10-80 m.sup.3 /h at 1 atm. In the system according to the invention the quantity of activated carbon is calculated on the basis of the fission gases, that is, the isotopes of the noble gases, xenon and krypton, to provide sufficient delay time in the absorptive delay section. Taking the absorptive coefficient for xenon at normal conditions at 600-1000 cm.sup.3 /g, the amount of carbon required for this purpose will ordinarily be in the range of 10 to 100 tons.

The voids in the activated carbon fill constitute a mechanical delay section for the activation gases, such as the isotopes of oxygen and nitrogen. In the case where the active carbon has properties similar to those of the preferred form referred to above, the void volume of the bed usually is in the range of 30 to 50% which provides sufficient delay time for the activation gases.

The filters 36 and 37 in FIG. 4 are conventional high efficiency filters for aerosols which are commonly used in nuclear and other industrial ventilating systems. Examples of such devices are those manufactured by the firm of Delbag in Berlin, Germany.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

Claims

It is claimed:

1. Apparatus for processing radioactive exhaust gases including fission gases and activation gases, said apparatus comprising preliminary adsorber means including a container, main adsorber means including a container, and adsorber material filling each of said adsorber containers, said main and preliminary adsorber means being connected to form a series delay path between a source of such exhaust gases and an exhaust gas chimney to constrain such exhaust gases to flow through said absorber material and to be delayed by said delay path in order to undergo radioactive decay, said delay path providing essentially the only gas delay between the source and the chimney, said preliminary adsorber means serving to permit radioactive decay of those exhaust gas components which constitute short-lived fission products and to filter out the resulting daughter products, said preliminary adsorber means being provided with shielding means and comprising means permitting easy replacement of said adsorber material.

2. A system as claimed in claim 1 wherein said main adsorber means include at least one concentrically arranged tube means for dividing the interior of said main adsorber means container into a plurality of space means for receiving said adsorber material, and means for causing radioactive gas to flow through said space means one after the other.

3. Apparatus as defined in claim 2, wherein there are a plurality of centrally disposed pipes arranged concentrically.

4. Apparatus as defined in claim 1 wherein said container of said preliminary adsorber means is composed of a cylindrical upper part and a conical lower part, said replacement permitting means include a discharge opening in said lower portion for the easy and safe discharge of adsorber material, and said preliminary adsorber means further comprise a gas inlet pipe extending into the interior of said conical lower part for introducing radioactive exhaust gas into said adsorber material.

5. A system as claimed in claim 4 wherein said preliminary adsorber means further include an upper shielding floor means above said upper part for shielding radiation emitted in said upper and lower parts, and a lower shielding floor means below said lower part for shielding radiation emitted in said upper and lower parts, and said replacement permitting means further include pipe means for delivering adsorber material down through said upper floor means and into said upper and lower parts, and a discharge tube means for extracting adsorber material from said discharge opening down through said lower floor means.

6. Apparatus as defined in claim 5 further comprising a vacuum chamber in which said main adsorber means is situated, and means connected for controlling the temperature of said chamber.

7. Apparatus as defined in claim 1 for processing exhaust gases whose fission gases include krypton and xenon and whose activation gases include nitrogen isotopes and oxygen isotopes.

8. Apparatus as defined in claim 1 wherein said main adsorber means filled with adsorber material forms simultaneously an adsorptive and a mechanical delay path by a predetermined ratio of the void volume in said adsorber material to the volume of said container for said adsorber material.

9. An exhaust gas cleaning system utilizing the principle of delaying radioactive gases to permit their radioactive decay to a level acceptable for release to the atmosphere, comprising adsorbent means for adsorbing radioactive gas, said adsorbent means being divided into two portions, a preliminary adsorber containing one of said portions, for delaying radioactive gas and for filtering out solid daughter products, and a main adsorber containing the other of said portions, for further delaying radioactive gas which has passed through said preliminary adsorber means and for further filtering out of solid daughter products, said preliminary adsorber including a cylindrical upper part, a coaxial conical lower part connected to and tapering narrower downwardly from the cylindrical upper part, said conical part having means for adsorbent removal, and a pipe opening into the interior of said conical lower part for the introduction of radioactive gas into said adsorbent means, an upper shielding floor above said upper part for shielding radiation emitted in said upper and lower parts, a lower shielding floor below said lower part for shielding radiation emitted in said upper and lower parts, means for filling the portion of said adsorbent means associated with said preliminary adsorber extending downwardly through said upper floor into said upper and lower parts, and a discharge means extending downwardly from said extraction opening means through said lower floor.