Method and device for the extraction of the condenser exhaust gases of a boiling water reactor

In a boiling water reactor having two extraction systems, one in use and the other held as a backup, a method and apparatus for extracting condenser exhaust gases of the reactor include a preheater for heating condenser gases which are then used for heating a recombiner of the extraction system. Auxiliary steam from the turbine is directed to the preheater for heating the condenser gases. A bypass line bypasses a valve in the auxiliary steam pipeline from the turbine to the preheater. The operating extraction system receives heating steam through the auxiliary pipeline, while the backup extraction system has the valve in the auxiliary steam pipeline closed. The recombiner of the backup extraction system is kept heated by guiding steam through the bypass line to the preheater, where the steam expands and is thereby heated, and directing the expanded steam through a vent line to the recombiner.

FIELD OF THE INVENTION 
The invention relates to a method and a device for the extraction of the 
condenser exhaust gases of a boiling water reactor of nuclear power 
stations. 
BACKGROUND 
In the reactor vessels of nuclear power stations, gamma irradiation of the 
water molecules produces, according to the following overall reaction, a 
certain quantity of hydrogen and oxygen in the stoichiometric ratio: 
EQU H.sub.2 O(1).revreaction.H.sub.2 (g)+1/2O.sub.2. 
The resulting gas mixture is called oxyhydrogen gas. When its detonation 
limit (approximately 7 mol % of H.sub.2 (g)) is reached, it explodes as a 
result of the catalytic action of certain metallic substances, and also as 
a result of sparking. 
The steam from the reactor therefore contains a few parts per thousand of 
gases resulting from the breakdown of water, which is called radiolysis. 
In contrast to nuclear power stations of the pressurized water type, there 
is no separation between the water of the primary and secondary circuits 
in nuclear power stations having boiling water reactors, so that the steam 
flows through the turbine. After its expansion work, it enters the 
condenser. Because the pressure in the low-pressure turbine, i.e. before 
the condenser inlet, is already below ambient pressure, there is an 
unavoidable penetration of air into the condenser. The gas mixture 
entering the latter therefore primarily consists of steam, the radiolysis 
gases and ambient air. 
As a result of the stripping of the mixture which takes place during 
condensation, the concentration of the uncondensable component increases 
along the condensation path. The concentration of the oxyhydrogen gas is 
thereby increased. For reasons of safety, systems of this type are 
designed in such a way that the final concentration at the outlet of the 
condenser does not exceed the value 0.04 mol % of H.sub.2 (g). 
A recombiner is arranged between the vacuum pump and the condenser so that 
the concentration of the oxyhydrogen gas, i.e. the danger of explosions, 
does not increase in this vacuum pump as well. This catalyst-filled 
container converts the hydrogen and oxygen into water by catalytic 
combustion, specifically under controlled conditions and below the 
detonation limit. Since this reaction is highly exothermic, the catalyst 
does not require any extraneous heating in normal operation. 
In order to ensure recombination even with very dilute gas mixtures, and to 
accelerate the reaction on start-up, the catalyst is, nevertheless, 
preheated. This is done using a so-called preheater arranged upstream of 
the recombiner. This preheater heats the gases leaving the condenser 
before they are fed into the recombiner, this being achieved by heat 
exchange inside the preheater with heating steam diverted from the 
turbine. 
For reasons of safety, such a device must have a redundant design. For this 
reason, in addition to the first extraction system, there is always a 
second extraction system in stand-by, which can be switched to as 
required. In order for the catalyst to be ready for use when switching 
over, the recombiner of the respective redundant system must also, of 
course, be continuously preheated to a temperature of approximately 
150.degree. C. In the stand-by state, this task is undertaken by an 
electrical heating system. For this purpose, it is necessary either to fit 
electrical windings or to equip the recombiner, the preheater and the 
adjacent components with an electrically-heated jacket. 
However, such an electrical heating system has a large number of 
disadvantages. It must be grounded and externally insulated, which results 
in high costs in addition to a complex structure. In addition, there is 
the latent danger of sparking due to damaged heating elements. Since the 
heat is transferred inward virtually exclusively via lines, this takes a 
relatively long time and also leads to a nonuniform heat distribution. 
Finally, because of the multiplicity of the inputs and outputs of the 
heated components, the electrical heating system must have a separate 
power supply, which makes both assembly and disassembly of the extraction 
system considerably more difficult. 
After the boiling water reactor has been switched over to the second 
extraction system, it is an essential requirement for it to be possible to 
preheat the catalyst of the first extraction system. An electrical heating 
system is therefore obviously required for this extraction system as well. 
However, the outlay and the concomitant costs are therefore further 
increased. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the invention is to avoid all these 
disadvantages by providing a novel, simple, cost-effective and reliable 
device for the extraction of condenser exhaust gases of a boiling water 
reactor. A further intention is to specify a method for operating the 
device. 
According to the invention, this is achieved in that, in a device according 
to the invention, a bypass bypassing the shutoff element of the auxiliary 
steam pipeline is arranged at this shutoff element. A throttle element is 
arranged, downstream of the vent orifices, in the vent tube. The vent tube 
is functionally connected to the recombiner. 
The advantages of the invention are based on the fact that the catalyst of 
the second extraction system, which is in the standby state, can be heated 
using a less expensive heat source, specifically the heating steam which 
is in any case present at the auxiliary steam pipeline. The electrical 
heating system can therefore be omitted, which leads to a simple, 
cost-effective and reliable device for the extraction of condenser exhaust 
gases. In addition, the heat transfer within the preheater takes place 
exclusively by forced convection and in direct contact, so that the 
process runs substantially faster and more uniformly. 
It is particularly expedient for the throttle element to be designed as a 
multiple throttle, consisting of a series circuit of orifices arranged 
off-center. With such a multiple throttle, the heating steam is fed 
through the orifices in meander fashion, and therefore undergoes 
particularly strong deceleration. In this way, the pressure of the heating 
steam located in the preheater can be better reduced to the pressure 
prevailing in the recombiner. 
When the boiling water reactor is operated using the first extraction 
system, a small quantity of heating steam is continuously fed past the 
shutoff element arranged in the auxiliary steam pipeline of the second 
extraction system and fed into the preheater. This heating steam is 
strongly heated in the preheater by expansion and then throttled to the 
pressure prevailing in the recombiner. The strongly heated heating steam 
is fed into the recombiner and therefore preheats the latter. The heating 
steam is then condensed in the downstream-connected exhaust gas condenser 
and fed back from there into the main condenser. 
The heating steam expands in the preheater by approximately two orders of 
magnitude, which causes it to be very strongly superheated. Even 
temperatures of approximately 170.degree. C. can thereby be reached. For 
this reason, the entire extraction system which is in the stand-by state 
remains dry and can advantageously be flushed with hot steam. When the 
boiling water reactor is switched over to the second extraction system, 
the high temperature of the preheated catalyst also ensures immediate 
recombination of the radiolysis gases and therefore prevents concentration 
of the oxyhydrogen gas.

Only those elements which are essential for understanding the invention are 
shown. The flow direction of the working medium is indicated with arrows. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, a turbine 2 
and a main condenser 3 are connected to the boiling water reactor 1 of a 
nuclear power station and are connected to each other by an exhaust steam 
line 4. In order to extract the condenser exhaust gases 5, the main 
condenser 3 is connected to a first extraction system 6 and to a second 
extraction system 7, one of which is continuously in operation and the 
other of which is in stand-by as a backup. Both extraction systems 6, 7 
have the same design. 
FIG. 1 represents the first extraction system 6, which is in operation. It 
consists of a vacuum pump 8 which is connected via a H.sub.2 --O.sub.2 
recombiner 9, which contains a platinum-palladium catalyst 10, to the main 
condenser 3. A preheater 11 is connected upstream of the recombiner 9 and 
an exhaust gas condenser 12 is connected downstream of the recombiner 9. 
The preheater 11 is connected via an extraction line 13 to the main 
condenser 3 and, via an auxiliary steam pipeline 14, to the turbine 2. A 
shutoff element 15, 16, designed as a control valve, is respectively 
arranged both in the auxiliary steam pipeline 14 and in the extraction 
line 13. The control valve 15 located in the auxiliary steam pipeline 14 
has a bypass 17. One condensate line 18, 19 runs in each case from the 
preheater 11 and from the exhaust gas condenser 12 to the main condenser 
3. The extraction line 13 leads, starting from the main condenser 3, 
through the preheater 11, the recombiner 9, the exhaust gas condenser 12 
and the vacuum pump 8, to an exhaust gas outlet 20. 
The preheater 11 consists of a cylindrical housing 21 which is connected at 
both ends to the extraction line 13 (FIG. 2). A tube bundle 23, consisting 
of a plurality of coaxially arranged heat exchange pipes 22, is arranged 
inside the housing 21. The heat exchange pipes 22 extend as far as both 
ends of the housing 21 and receive a central vent tube 24. A steam 
distribution space 25, which is connected via a heating steam inlet 26 to 
the auxiliary steam pipeline 14 and, via a condensate outlet 27, to the 
condensate line 18, is constructed inside the housing 21 and annularly 
around the tube bundle 23. A plurality of vent orifices 28, which connect 
the vent tube 24 to the steam distribution space 25, are constructed in 
this vent tube 24 (FIG. 3, FIG. 2). A throttle element 29, designed as a 
multiple throttle, is arranged downstream of the vent orifices 28 in the 
vent tube 24. This multiple throttle 29 consists of a series circuit of 
orifices 30 arranged off-center (FIG. 4). The vent tube 24 is closed in 
the direction of the main condenser 3, but has an opening 31 toward the 
recombiner 9 (FIG. 2). 
During operation of the boiling water reactor 1, both the main condenser 3 
and the turbine 2 are continuously functionally connected to one of the 
two extraction systems 6, 7. If the first extraction system 6, as already 
described above, is active, its control valves 15, 16 are open (FIG. 1). 
The control valves 15, 16 of the second extraction system 7 then remain 
closed (FIG. 5). 
As a result, heating steam 32 enters the preheater 11 of the first 
extraction system 6 via the auxiliary steam pipeline 14, and condenser 
exhaust gases 5, which entrain radiolysis gases 33, amongst other things, 
with them, enter the preheater 11 of the first extraction system 6 via the 
extraction line 13. In this preheater, the condenser exhaust gases 5 
cooled in the main condenser 3 to approximately 50.degree. C. are reheated 
by heat exchange with the heating steam 32 diverted from the turbine 2, 
and thus reach a temperature of above 150.degree. C. This being the case, 
the heating steam 32 condenses and a condensate 34, which is fed back via 
the condensate line 18 to the main condenser 3, is formed. Subsequently, 
both the condenser exhaust gases 5, including the radiolysis gases 33, 
which are now heated, and the heating steam 32 not condensed in the 
preheater, enter the recombiner 9 via the extraction line 13. Because of 
the high temperatures prevailing in this recombiner, catalytic combustion 
of most of the radiolysis gases 33 contained in the condenser exhaust 
gases 5 to give water takes place in spite of the short residence time. 
The resulting steam 35 is, together with the residual condenser exhaust 
gases 5, fed into the exhaust gas condenser 12 via the extraction line 13. 
Further heat exchange with the water used as coolant 36 takes place in 
this condenser. This being the case, a further condensate 37 results, 
which is likewise fed back to the main condenser 3 via the condensate line 
19. The relatively low proportion of exhaust gases 38 which then remains 
is extracted by the vacuum pump 8 and discharged to the ambient air via 
the exhaust gas outlet 20. 
During this, the second extraction system 7, which is in the stand-by 
state, continuously receives a small quantity of heating steam 32 via its 
bypass 17 from the auxiliary steam pipeline 14 (FIG. 5). For this purpose, 
the bypass 17 is designed in such a way that it allows approximately 5 
grams of heating steam 32 per second to pass through. To this end, a 
control valve 39 may also be arranged in the bypass 17. The pressure of 
the heating steam 32 present at the auxiliary steam pipeline 14 is equal 
to approximately 10 to 15 bar. After it has entered the preheater 11, the 
heating steam 32 expands through the vent orifices 28 and is throttled by 
the multiple throttle 29 to the pressure prevailing in the recombiner 9. 
Because of the orifices 30 multiply arranged one after another and 
off-center, the heating steam 32 is fed in meander fashion through the 
throttle element 29 and therefore undergoes particularly strong 
deceleration. When it expands by two orders of magnitude, the heating 
steam 32 is superheated very strongly and reaches a temperature of above 
150.degree. C. With this temperature, the catalyst 10 of the second 
extraction system 7 is continuously preheated, so that recombination of 
the radiolysis gases 33 is immediately ensured when switching over to this 
extraction system 7. 
The strongly heated heating steam 32 is condensed in the 
downstream-arranged exhaust gas condenser 12 and the condensate 37 is fed 
back into the main condenser 3. 
If it is intended to switch the boiling water reactor 1 over to the second 
extraction system 7, it is merely necessary to close the control valve 15, 
16 of the first extraction system 6 and simultaneously to open the control 
valves 15, 16 of the second extraction system 7. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.