Patent Number: 052271277
Section: description

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.