Filtered venting system for reactor containment vessel of nuclear power plant

A filtered venting system located in association with a reactor containment vessel installed in a reactor building comprises a filter device disposed in the reactor building and including filter means, a first venting line disposed on an upstream side of the filter device and having one end connected to the reactor containment vessel and another end connected to the filter device, a stand-by gas treatment system including outlet fan means or pump means and connected to the first venting line at the downstream side of the fan or pump means, and a second venting line disposed at a downstream side of the filter device and another end connected to an outlet means/ The filter device being utilized for the stand-by gas treatment system for treating and removing a radioactive substance contained in an atmosphere delivered from the reactor containment vessel.

BACKGROUND OF THE INVENTION 
The present invention relates to a filtered venting system for a reactor 
containment vessel of a nuclear power plant. 
A boiling water type nuclear power plant is well known as one type of 
conventional nuclear power plant, and a typical example of the boiling 
water type nuclear power plant, hereinafter called BWR, is shown in FIG. 
2. Referring to FIG. 2, a stand-by gas treatment system (SGTS) is 
installed in a reactor building 1, and this SGTS 2 operates, when a loss 
of coolant accident (LOCA) as design basis accident (DBA) occurs, to 
maintain a pressure inside the reactor building 1 to be negative by 
releasing an ambient atmosphere in the reactor building 1 into an external 
atmosphere gradually by a small amount and to thereby enclose radioactive 
substance leaking from the reactor containment vessel (RCV) within the 
reactor building 1, thus preventing the radioactive substance from being 
released into the environmental atmosphere. 
Since the SGTS 2 is an equipment for a counter-measure to the LOCA as the 
DBA, the SGTS 2 is considered to be an engineered safety features, and 
accordingly, it is required for the SGTS 2 to apply a single accident 
standard or basis to dynamic equipment or mechanisms and to have an 
anti-earthquake design, which results in the requirement of a highly 
reliable design thereof. 
The SGTS 2 for releasing the ambient atmosphere in the reactor building 1 
is provided with an expensive filter train 4 with high performance for 
removing the radioactive substance in the ambient atmosphere. The filter 
train 4 includes in a row a pre-filter, a high efficiency particulate 
filter and a charcoal filter. The filter train 4 acts to sufficiently 
remove the radioactive substance from the exhausted atmosphere in the 
reactor building 1 fed by the operation of outlet or exhaust fans 5 and 
thereafter to discharge cleaned atmosphere including no radioactive 
substance into the external atmosphere through a stack 6. In the SGTS 2, 
an emergency a.c. power source such as emergency diesel generator (DG) is 
utilized for driving electrically operative valve means 7 for the outlet 
fans 5. In FIG. 2 reference numeral 8 denotes a moisture separator or 
remover, reference numeral 9 denotes a heating coil and reference 
character AO denotes an air operating valve. 
Recently, in nuclear power plants installed in European countries, a 
filtered venting system 10 such as shown in FIG. 3 is provided in the 
reactor building 1 in consideration of an occurrence of a severe accident 
over the DBA. 
The severe accident is an accident which occurs in an assumption of an 
extremely severe state such as in which the function of an external 
electric source is lost and all of the plurality of emergency a.c. power 
sources (DG) are also lost, and concretely, all of the a.c. power sources 
become entirely unusable, called an accident of loss of all a.c. power 
source. At the time of such a severe accident, before the inner pressure 
of the RCV 3 has increased extremely by decay heat generated by a nuclear 
fuel, a rapture disk 11 of the filtered venting system 10 is automatically 
operated to thereby discharge the atmosphere in the RCV 3 into the 
external atmosphere through the stack, thus preventing the RCV 3 from 
being damaged by the increased pressure. 
The radioactive substance in the RCV 3 is removed by a filter vessel 12 
equipped in the filtered venting system 10. The filter vessel 12 as a 
filtering device contains a water filter and a stainless filter, through 
which the radioactive substance contained in the inner atmosphere is fully 
removed and the atmosphere including substantially no radioactive 
substance is then discharged into the external atmosphere. 
However, such a filtered venting system 10 has been designed to 
countermeasure a severe accident, which is a rare accident beyond the DBA, 
and therefore, it will be said that the design has not been made by 
specifically paying attention to the application of a single accident 
standard to a general dynamic equipment and to the assurance of the 
reliability of the system such as anti-earthquake design. Nevertheless, it 
is assumed that the accident of loss of all power source resulting in the 
severe accident is mainly caused with high possibility due to the 
earthquake, and in the prior art any redundant design or suitable 
countermeasure is not made in this view point. Accordingly, the described 
prior art system is not satisfactory in the reliability in an assumption 
of a case of a severe accident. 
Furthermore, the filtered venting system for the severe accident such as 
shown in FIG. 3 is not designed to be automatically operated at the time 
of an occurrence of the severe accident. Thus, during steady operation of 
the reactor, it is necessary for an operator to open isolation valves 13, 
now being closed, in accordance with a personal judgement of the operator, 
and this involves much load to the operator. In this point of view, too, 
it will be said that the system shown in FIG. 3 is also not designed with 
high reliability. In FIG. 3, reference character L denotes a level meter. 
On the contrary, as described hereinbefore, to the SGTS 2 for the design 
standard accidents designed for the DBA, the redundant design and the 
anti-earthquake design have been made to thereby automatically operate at 
an occurrence of the LOCA. In this view point, the SGTS may be said to 
have a design ensuring the sufficient reliability. However, the SGTS 2 
requires the location of the a.c. power source for operating dynamic 
equipments such as outlet fans 5 and the electrically actuating valves 7, 
and accordingly, such system 2 cannot be operated at all at the severe 
accident caused by the accident of loss of all a.c. power source. For this 
reason, in spite of the provision of the expensive SGTS 2 for the DBA, in 
the European countries, an independent filtered venting system such as 
system 10 is additionally provided for the specific countermeasure to the 
severe accident, resulting in additional working and involving much cost. 
Moreover, since the filtered venting system is a system for the specific 
severe accidents, when it is intended to incorporate such filtered venting 
system 10 in the existing nuclear power plant, there is a fear of giving 
an erroneous understanding to the public such that the existing power 
plant is a type liable to cause such severe accident. 
In the actual technology, the SGTS 2 is only used for the DBA and the 
filtered venting system 2 is only used for the severe accident, so that 
the additional application of the filtered venting system 10 to the SGTS 2 
not only requires much additional cost but also gives the wrong 
understanding to the people. 
Furthermore, since the filtered venting system 10 is a system for the 
specific severe accident and the reliability thereof is less than that of 
the SGTS 2, in actual, the introduction of the filtered venting system 10 
applies much load to the operator, thus providing a problem. 
SUMMARY OF THE INVENTION 
The present invention conceived in consideration of the prior art of the 
characters described above and an object of the present invention is to 
provide a filtered venting system in association with a reactor 
containment vessel installed in a reactor building of a nuclear power 
plant, having the combined function of a stand-by gas treatment system and 
a filtered venting system for countermeasure to an occurrence of a design 
basis accident and a severe accident with highly improved reliability. 
This and other objects can be achieved according to the present invention 
by providing a filtered venting system located in association with a 
reactor containment vessel installed in a reactor building comprising a 
filter device disposed in the reactor building and including filter means, 
a first venting line disposed on an upstream side of the filter device and 
having one end connected to the reactor containment vessel and another end 
connected to the filter device, a stand-by gas treatment system connected 
to the first venting line, and a second venting line disposed at a 
downstream side of the filter device and another end connected to an 
outlet means, wherein the filter device is utilized as filtering means for 
the stand-by gas treatment system for treating and removing a radioactive 
substance contained in an atmosphere delivered from the reactor 
containment vessel. 
According to the filtered venting system of the structure described above, 
a filtered venting system for the severe accident is positively utilized. 
The first venting line is connected to the upstream side of the filter 
device and the downstream side of the outlet fans or pump means of the 
stand-by gas treatment system is connected to the first venting line to 
thereby utilize the filter device as filter means for removing the 
radioactive substance in the stand-by gas treatment system. Accordingly, 
it is made possible to eliminate a location of an expensive filter train 
utilized in a conventional system, thus remarkably decreasing cost. 
Furthermore, the filtered venting system according to the present invention 
can be constructed as a single system having a combined function of the 
venting function of a filter venting system for countermeasure to a severe 
accident and a function of the stand-by gas treatment system, thus being 
effective.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
One preferred embodiment of the filtered venting system located in 
association with a reactor containment vessel according to the present 
invention will be described hereunder with reference to FIG. 1. 
Referring to FIG. 1, a filtered venting system 20 to countermeasure severe 
accidents is installed in a reactor building 21 of a light water reactor. 
In the reactor building 21, there is also installed a reactor containment 
vessel (RCV) 22, in which a reactor pressure vessel 23 is incorporated. A 
suppression pool 24 is arranged at a lower portion of the RCV 22 and the 
suppression pool 24 includes an gas chamber 24a to which a venting line 25 
of the filtered venting system 20 is connected. 
To the venting line 25 are incorporated in order isolation valves 26, 26, a 
rupture disk 27 and a check 30 constructed as a filtering device. In the 
filter vessel 30 there are disposed a water filter 31 and a stainless 
fiber filter 32. The filter vessel 30 has a top portion from which a 
venting line 33 of downstream side extends, and to the venting line 33 are 
incorporated in order a check valve 34, a pressure control throttle 35 and 
a rapture disk 36 as an assembly to be connected to a stack, not shown, 
through which the venting line 33 opens to the external atmosphere. 
As described before, the venting line 25 disposed upstream side of the 
filter vessel 30 has one end communicated with the gas chamber 24a in the 
suppression pool 24 and the other end connected to the filter vessel 30. A 
line 39 of a stand-by gas treatment system (SGTS) 38, called hereinlater 
SGTS line 39, is connected at its one end to a portion of the venting line 
25 disposed at the upstream side of the filter vessel 30. The SGTS line 39 
has another end opened to an inner ambient atmosphere in the reactor 
building 21 and is equipped with at its intermediate portions in order, 
inlet or intake valves 40, outlet fans 41, isolation valves 42 and a check 
valve 43 all of the SGTS 38. Outlet valves 45 for the SGTS bypassing the 
rapture disk 36 are incorporated to the downstream side venting line 33. 
As described above, the stand-by gas treatment system (SGTS) 38 is 
integrated with the filtered venting system 20, thereby constituting an 
integrated filtered venting system as a single system. 
A bypass circuit 46 bypassing the isolation valves 26 and the rupture disk 
27 is incorporated to the venting line 25 disposed upstream side of the 
filter vessel 30, and isolation valves 47 are assembled with this bypass 
circuit 46 for the venting operation of an operator. Concretely, the 
isolation valves 47 are disposed for the purpose such that the operator 
carries out the venting operation before the inner pressure of the RCV 22 
reaches an actuating pressure of the rupture disk 27 or the operator 
carries out a back-up operation in case of failure of the rupture disk 
operation. Redundancy or multiplicativeness is applied to the dynamic 
equipments, except for the check valves 28, 34 and 43 and the rupture 
disks 27 and 36, which have to be operated after the accident. 
The filter vessel 30 includes a gas chamber 30a above the water filter 31, 
and an inert gas, preferably N.sub.2 gas, supply line 48 is communicated 
with the gas chamber 30a to supply the inert gas thereinto. The interior 
of the filter vessel 30 is filled up with the inert gas such as N.sub.2 
during a reactor steady operation period by the supply of the N.sub.2 gas 
from the feed line 48 for preventing burnable gas such as H.sub.2 or CO 
gas contained in the atmosphere in the RCV 22 from burning in the filter 
vessel 30 after the accident. 
Further, a filter means of a kind other than that mentioned above, such as 
a sand filter, may be disposed in the filter vessel 30 in substitution for 
the stainless fiber filter 32, but it is not necessarily required to 
always fill the interior of the filter vessel 30 with N.sub.2 gas. In this 
case, it will be unnecessary to dispose the rapture disk 36 and the outlet 
valves 45 for the SGTS to the downstream side venting line 25. 
Furthermore, it may be possible to make redundant the check valve and the 
rapture disk for the improvement of the reliability of the system. 
A water feed line 50 is connected to the filter vessel 30, and in FIG. 1, 
reference numerals 51 and 52 denote a drain line and a deaerator line, 
respectively. 
The filtered venting system for the reactor containment vessel of the 
structure described above operates in the following manner. 
If the DBA occurs in the light water reactor of a nuclear power plant, at 
least one series of DGs are provided to be operative, and accordingly, the 
outlet fans 41, the inlet valves 40, the isolation valves 42 and the 
outlet valves 45 of the SGTS 38 can be made operative by the operation of 
the DG. 
Accordingly, the equipments such as outlet fans 41 start to operate 
automatically in response to a signal informing the detection of the 
occurrence of the DBA and the outlet fans 41 start to suck the ambient 
atmosphere in the reactor building 21. In this operation, since the outlet 
or exhaust line including the outlet fan or pump of the SGTS 38 according 
to this filtered venting system is connected to the venting line 25 
disposed at the upstream side of the filter vessel 30 of the system 20, 
the sucked ambient atmosphere in the reactor building 21 is introduced 
into the filter vessel 30, in which the radioactive substance contained in 
the sucked atmosphere is then removed. The atmosphere cleaned by the 
filtering function of the filter vessel 30 is discharged externally into 
atmosphere through the stack, not shown. During this operation, the 
ambient atmosphere in the reactor building 21 is sucked from the SGTS 38 
by the outlet fans 41 and then treated, so that the radioactive substance 
leaking at the DBA from the RCV 22 into the reactor building 21 can be 
prevented from further releasing into the external atmosphere, whereby the 
safeness to the public environment and people can be ensured. 
On the contrary, when the severe accident occurs, it is considered that the 
all DGs become unusable. In such a case, all the dynamic equipments 
including the outlet fans 41 and the inlet valves 40 of the SGTS 38 will 
become inoperative. Further, since the dynamic systems including such as a 
core cooling system become also inoperative, the core is damaged and the 
radioactive substance is released from the damaged nuclear fuel, and 
hence, there causes a fear of releasing the radioactive substance into the 
RCV 22 and the inner pressure in the RCV 22 becomes high pressure due to 
the decay heat released by the nuclear fuel. 
However, when the inner pressure reaches to a constant value, the rupture 
disk 27 operates to thereby deliver the atmosphere in the RCV 22 into the 
filter vessel 30 through the venting line 25. On the way of this flow of 
the atmosphere, the radioactive substance contained in the atmosphere of 
the RCV 22 can be fully removed in and by the filter vessel 30 and the 
cleaned atmosphere is then discharged into the environmental atmosphere 
through the stack. As described above, at the occurrence of the severe 
accident, the atmosphere in the RCV 22 can be automatically released into 
the environmental atmosphere in accordance with the increasing of the 
inner pressure in the RCV 22, so that any driving source such as a.c. 
power source for this purpose, whereby the pressure in the RCV 22 can be 
maintained to a value approximately of an atmospheric pressure and the 
soundness of the RCV 22 can thus be maintained. In the assumption of an 
occurrence of the severe accident, such a condition as that the 
radioactive substance is infinitively released into the environment can be 
preferably prevented, thus ensuring the safeness to the public. 
Moreover, as described hereinbefore, according to the integrated filtered 
venting system 20, the radioactive substance can be removed by utilizing 
the same filter vessel 30 in an occurrence of the DBA as well as the 
severe accident for ensuring and maintaining the safeness to the public. 
Furthermore, since the filtered venting system according to the present 
embodiment is provided with the safety function as the stand-by gas 
treating system essential to the occurrence of the DBA, the filtered 
venting system and, hence, the total power plant can be designed and 
installed as an engineered safety features in dependency on the safeness 
standard prescribed by a national standard with the high reliability and 
performance being maintained, whereby the reliability, such as redundant 
design or anti-earthquake design, of the venting function can be ensured 
at the occurrence of the severe accident. 
In the described preferred embodiment, the venting line 25 of the filtered 
venting system 20 is connected to the gas chamber 24a in the suppression 
pool 24, the venting line 25 may be communicated with a drywell 54 defined 
in the RCV 22. Furthermore, many other changes or modifications for the 
arrangements of the outlet fans 41, the inlet valves 40, the isolation 
valves 42 and the line 39 for the SGTS 38 may be made according to the 
present invention, and for one example, the outlet fans 41 may be 
substituted with outlet pump means.