CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of International Application No. PCT/DE98/02792, filed Sep. 18, 1998 now WO99/17300 issued Apr. 8, 1999, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     FIELD OF THE INVENTION 
     The invention lies within the field of safety devices for nuclear power plants. The invention relates to a blow-off device for live steam from a nuclear power plant, in which the blow-off device is in communication with at least one live-steam line of the nuclear power plant. The invention also relates to a method of blowing off live steam from a live-steam line of a nuclear power plant. 
     In a nuclear power plant, steam generated in a reactor pressure vessel or in a steam generator is directed through a live-steam line to a turbine. The reactor pressure vessel and the steam generator are accommodated in a reactor building, which has a safety container surrounded by a protective casing. The turbine is located in a turbine building situated outside the reactor building. A line opening into the atmosphere branches off from the live-steam line upstream of a first stop fitting, as seen in the flow direction. Adjoining that line is a line section with a blow-off control valve and a blow-off stop valve, as well as a section with a safety valve, which is connected in parallel with those valves. The line sections that are connected in parallel open out in a common blow-out line. The blow-out line, possibly together with blow-out lines belonging to other steam generators of the nuclear power plant, leads into a muffler or sound absorber, which has an outlet opening to the atmosphere. 
     The live-steam line as well as a feedwater line pass through both the safety container and the protective casing of the reactor building. Barriers between a primary circuit leading through the reactor pressure vessel and the atmosphere are therefore valve cones of the safety valve and of the blow-off stop valve and, in the case of a pressurized water reactor, also heating tubes of the steam generator, which separate the primary circuit from a secondary circuit. 
     In the case of a pressurized water reactor, the steam generated in the steam generator is non-radioactive in normal operation. In the event of a serious incident, damage could occur to the heating tubes in the steam generator, even if that is highly unlikely. As a result, radioactive substances would pass out of the primary circuit of the pressurized water reactor into the live-steam line. In that case, the valve cones of the safety valve and of the blow-off stop valve, together with valve seats, would form a barrier between the primary circuit and the atmosphere. 
     Even though the technology of valve cones and valve seats has been and is being constantly improved, 100% tightness of the seats cannot be guaranteed, since the possibility of foreign bodies or contaminants being located on the valve seat cannot be ruled out, especially if blow-off has been effected beforehand through the affected valves under the conditions of a serious incident. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a blow-off device for live steam from a nuclear power plant which prevents an escape of radioactive substances into the atmosphere even if inadequate tightness of a safety valve or a blow-off stop valve is assumed, and a method of blowing off live steam from a nuclear power plant which enables live steam contaminated by radioactivity to be blown off from a line of the nuclear power plant without radioactive pollution of the atmosphere if an incident occurs in the nuclear power plant, in which the device and the method overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a blow-off device for live steam from at least one live-steam line of a nuclear power plant, comprising a blow-off line for the live steam, the blow-off line in communication with the at least one live-steam line and opening into the atmosphere; a filter disposed in the blow-off line for filtering radioactive substances; and a non-liquid activity holdback agent embedded in the filter. The filter effect may be based on physical or chemical as well as biological principles. 
     With the blow-off device according to the invention, radioactive substances can be removed from the live steam which has passed, for example, a safety valve or a blow-off stop valve that is not closing perfectly. The filtered live steam can then be blown off into the atmosphere without radioactive pollution of the latter. The same applies to such live steam which, for example when a blow-off stop valve is opened, in particular in order to bring about a pressure balance, is to be deliberately blown off into the atmosphere. 
     The blow-off line may be formed, for example, by a line which branches off from a line section through which the live steam passes into the atmosphere in normal operation. The branching line, that is the blow-off line, may be provided, for example, with a stop device and is opened, for example, in the event of an incident. It preferably has a fixed filter, so that very long filter sections can be realized in an advantageous manner. 
     In accordance with another feature of the invention, there is provided a muffler or sound absorber, for example, associated with the filter, the muffler or sound absorber including an inlet opening and a first outlet opening. The combination of the muffler or sound absorber with the filter in one structural unit permits a cost-effective, reliable and simple type of construction of the blow-off device in an advantageous manner. 
     In accordance with a further feature of the invention, the filter is at least partly integrated in a housing of the muffler or sound absorber. As a result, the construction of the blow-off device including a filter and a muffler or sound absorber is further simplified. 
     In accordance with an added feature of the invention, the filter is surrounded by a receptacle having a feed opening which can be closed. This results in it being advantageously possible to wet the filter with live steam only in a demand case (e.g. an incident) and to shield it, with the feed opening closed, in normal operation from the live steam to be blown off. In particular, the blow-off line can be closed by closing the feed opening. 
     In accordance with an additional feature of the invention, the first outlet opening can be closed and the inlet opening can be connected through a filter to a second outlet opening of the muffler or sound absorber. As a result, the live steam can advantageously be blown off into the atmosphere in normal operation through the first outlet opening and in the demand case through the blow-off line containing the filter and through the second outlet opening. 
     The blow-out lines of the power plant, for example, which are connected to the safety valve or the blow-off stop valve, open into the inlet opening. 
     In accordance with yet another feature of the invention, there is provided a slide which is movable inside the muffler or sound absorber along an axis and through the use of which the first outlet opening can be closed and/or on which the filter is disposed. 
     For safety reasons, the movement of the slide is preferably exclusively linear (rectilinear) and not curved or angular. The slide generally refers to a movable, preferably guided system part of the blow-off device. The designation “slide”, for example, therefore also stands for a rider, a carriage, a rolling body or a sliding body. In particular the construction in which the first outlet opening can be closed by the slide and in which the filter is disposed on the slide results in the advantage that both the first outlet opening is closed and the filter is positioned at the desired location in the blow-off line, which is cleared, for example after opening of the feed opening, by a single movement of the slide. 
     In accordance with yet a further feature of the invention, the axis of the blow-off device is preferably essentially vertical. As a result, the blow-off device is especially suitable for such muffler or sound absorbers in which the live steam is blown off upward. 
     In accordance with yet an added feature of the invention, the slide is movable under the effect of gravitational force and closes the first outlet opening in an end position of minimum potential energy. In this case, minimum potential energy refers to the absolute minimum within a range in which the slide can be moved. For example, such an end position may be a bottom stop of a guide rail. 
     A particular safety advantage is achieved by the movement under the effect of gravitational force, since gravitational force is a natural phenomena and is effective in a passive manner and does not depend on the presence or functioning of any actuating drives. At least it is not necessary to provide a powerful actuating drive which can move the possibly very large mass of the slide. If at all, an actuating drive which moves, for example, a release latch with a small expenditure of force is necessary. 
     After the movement of such a release latch, the slide, in essentially free fall, can pass, for example, from a top normal position into the end position. A damping device, for example, is provided for the last part of the free fall travel. 
     In accordance with yet an additional feature of the invention, the inlet opening, in the end position of the slide, is connected to the atmosphere through the filter. For example, the feed opening is unlocked for this purpose. 
     The blow-off device may also be constructed in such a way that the movement of the slide in free fall can produce a superimposed impulse, through the use of which a closure of the feed opening can be removed and/or the inlet opening can be connected to the atmosphere through the filter. In an advantageous manner, the changeover of the blow-off device from normal operation to demand case can thereby be largely automated. 
     For example, a drive is also provided with which the slide can be removed from the end position with minimum potential energy and can be moved in particular into the top normal position. 
     With the objects of the invention in view, there is also provided a method of blowing off live steam from a live-steam line of a nuclear power plant, which comprises providing a blow-off line communicating with the live-steam line; and filtering radioactive substances from live steam before the live steam is blown off into the atmosphere through the blow-off line, if an incident occurs in the nuclear power plant. 
     This results in the special safety advantage of providing an additional barrier between the reactor pressure vessel containing radioactive substances and the atmosphere. 
     In a pressurized water reactor, a first barrier is formed of the heating tubes of the steam generator, which separate the radioactive primary circuit from the live-steam line. The valve cones or the valve seats in the stop or safety valves which are assigned to the blow-out lines form a second barrier. In the event of a simultaneous failure of steam generator tubes or a failure or leakage of the stop or safety valves, a third barrier relative to the atmosphere is provided by the filter. This barrier works according to operating principles which are physically and chemically fundamentally different from the two aforesaid barriers and therefore brings about a particular and advantageous increase in safety. 
     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 blow-off device and a method of blowing off live steam, 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary, diagrammatic, elevational view of a blow-off device including a muffler or sound absorber, which is shown in normal operation; and 
     FIG. 2 is a view of the blow-off device similar to FIG. 1, which is shown in a demand case (filter operation). 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1 and 2 of the drawings in detail, there is seen a muffler or sound absorber which is designated overall by reference numeral  1  and has an inlet opening  2  into which a blow-out line  3  of a pressurized water reactor opens. The blow-out line  3  is connected through a blow-off stop valve and/or safety valve, represented by reference numeral  12 , to a live-steam line  8 . 
     Live steam M entering the muffler or sound absorber  1  through the blow-out line  3  is blown off into the surrounding atmosphere  4  through the use of the muffler or sound absorber  1 . The muffler or sound absorber  1  has a housing  5  with a bottom part which is provided with thermal insulation  6 . An upright  7  serves to fasten the muffler or sound absorber  1 . 
     FIGS. 1 and 2 respectively show the blow-off device in normal operation and in filter operation (demand case), e.g. in the event of an incident. Respective main flow directions in the different operating modes are indicated by respective arrows  9  and  10 . 
     In normal operation shown in FIG. 1, the live steam M, after passing the inlet opening  2 , flows through a first perforated-plate configuration  11  and a first steel-wool configuration  13  into a first outlet opening  14 . From there, the live steam M flows through a second perforated-plate configuration  15  and a second steel-wool configuration  17  into a plurality of segmental chests  19 . The segmental chests  19  have protective flaps  21 , which can be swung open under the pressure of the outflowing live steam M, so that the live steam M can escape into the surrounding atmosphere  4 . 
     A filter  31  for radioactive substances is located centrally between the segmental chests  19 . The filter  31  is enclosed by a receptacle  33 . The filter has a close-meshed screen  35  in order to provide a homogenous deposit of an activity holdback agent. A top end wall  33 A of the receptacle  33  is permeable to gas or steam in sections, so that the live steam M, after flowing through the filter  31 , reaches a second outlet opening  36 , from which it flows out into the atmosphere  4  through a perforated-plate tube  37 . The perforated-plate tube  37  carries a canopy  39  for protecting the filter  31 . 
     A bottom end wall  33 B of the receptacle  33  is permeable to gas and steam in a certain section, a so-called feed opening  41 . The feed opening  41  can be closed by two flaps  43 . In the demand case shown in FIG. 2, the flaps  43  are opened, so that the live steam M passes in the direction of the arrows  10  through the filter  31  and the second outlet opening  36  into the atmosphere  4 . Radioactive substances are then retained in the filter  31  containing the activity holdback agent. 
     Furthermore, a slide  51 , which is formed from the receptacle  33  and a sliding tube  52  extending the receptacle  33 , is shown in FIGS. 1 and 2. The slide  51  therefore also carries the filter  31  and the flaps  43 . In FIG. 1, the slide  51  is shown in its top end position (normal position). The slide  51  is movable in the direction of a vertical axis  53 . In order to provide guidance, a guide shroud  55  is provided in a top part of the blow-off device and a guide collar  57  is provided in a bottom part. The slide  51  is held in the depicted top end position by a non-illustrated release device, for example a latch. 
     At the start of the demand case, i.e. if the filter operation is desired, the release latch is actuated and the slide  51  drops in free fall into a bottom end position of minimum potential energy. This end position is shown in FIG.  2 . The slide then lies on a truncated cone  61  having a cone opening  63 . In this end position, the first outlet opening  14  of the muffler or sound absorber  1  is covered by the slide  51 , in particular by the sliding tube  52 , and is thus closed. Since the flaps  43 , which are released by the free fall, are opened, the live steam M entering the muffler or sound absorber  1  through the inlet opening  2  can only pass into the atmosphere  4  after passing the filter  31 , as is shown by arrows indicating the flow direction  10 . 
     An unlocking system which opens the flaps  43  automatically during free fall of the slide  51  is not explicitly depicted in FIGS. 1 and 2. 
     A first annular groove  65  and a first seal  67  are provided in order to seal off the slide  51  from the housing  5  of the muffler or sound absorber  1  in the top end position illustrated in FIG. 1. A second annular groove  69  made in the truncated cone  61  and having a second seal  71  serves to seal off the slide  51  from the truncated cone  61  in the bottom end position illustrated in FIG.  2 . 
     In the demand case shown in FIG. 2, a blow-off line which is designated overall by reference numeral  73  and leads to the surrounding atmosphere  4  is formed by the first perforated-plate configuration  11 , the first steel-wool configuration  13 , the bottom part of the housing  5 , the truncated cone  61  and the slide  51 . This blow-off line has the filter  31  for radioactive substances.
A blow-off device for live steam includes a filter for radioactive substances in order to increase operational safety in a nuclear power plant. The filter is preferably integrated in a housing of a muffler or sound absorber and is disposed on a slide which is movable along an axis under the effect of gravitational force and in particular also serves as a closure for an outlet opening of the muffler or sound absorber. A method of blowing off live steam is also provided.