Chemical apparatus and process disinfecting – deodorizing – preser – Control element responsive to a sensed operating condition
Reexamination Certificate
1999-07-22
2003-02-25
Knode, Marian C. (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Control element responsive to a sensed operating condition
C422S171000, C422S177000, C376S277000, C376S301000
Reexamination Certificate
active
06524534
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention pertains to an apparatus for removing flammable gas, which is generated by some reaction, especially hydrogen or oxygen.
An apparatus for removing flammable gas is used, as one of the useful examples, in a primary containment vessel (PCV) in an atomic power plant, and removes the flammable gas generated in PCV at the time of an accident. A general type of PCV and an apparatus for removing flammable gas contained in it are explained in reference to figures using the case of a boiling water reactor (BWR) as an example.
FIG. 31
is an outlined systemcross section showing a conventional primary containment vessel. A primary containment vessel (PCV)
102
houses a reactor pressure vessel (RPV)
101
, in which a reactor core
107
is built, and comprises of an upper dry well
103
and a lower dry well
104
for enclosing the RPV
101
and a wet well
105
equipped with a suppression pool
105
a
connected via a vent pipe
106
to the upper dry well
103
. And the RPV
101
is enclosed by a biological shield wall
108
.
If a primary cooling system pipe of the atomic reactor, such as main vapor pipe
109
connected to the RPV
101
, is broken, a high-temperature and high-pressure primary coolant of the atomic reactor is discharged into the upper dry well
103
in PCV
102
, rapidly raising the pressure and temperature in the upper dry well
103
. The high-temperature and high-pressure coolant discharged into the upper dry well
103
is mixed with a gas in the upper dry well
103
, discharged into the water of the suppression pool
105
a
through the vent pipe
106
, and cooled. Most of the thermal energy discharged from the RPV
101
is absorbed in the suppression pool
105
a.
The water of the suppression pool
105
a
is poured into the RPV
101
by an emergency core cooling system, but the coolant absorbs decay heat from the reactor core in the long term and is discharged to the dry well from an opening in the broken pipe. So, at that time, the pressure and temperature in the upper dry well
103
are always higher than those of the wet well
105
. Under such a long-term phenomenon, water, which is a coolant, is decomposed by radiation in a light water reactor of the atomic power plant, generating hydrogen gas and oxygen gas.
Furthermore, when the temperature of a fuel cladding is raised, a reaction is caused between the vapor and zirconium of the fuel cladding material (called a Metal-Water reaction), so that hydrogen gas is generated in a short time. The hydrogen gas generated in this manner is discharged into PCV
102
from the opening in the broken pipe, and the concentration of the hydrogen gas in PCV
102
is gradually raised. Also, since the hydrogen gas is noncondensable, the pressure in PCV
102
is also raised.
When a certain effective countermeasure to such a state cannot be taken and the concentration of the hydrogen gas and that of the oxygen gas are raised to 4 vol % and 5 vol % , respectively, that is, when the concentration of the flammable gas exceeds a certain flammability limit, the gases reach the flammable state. If the concentration of the hydrogen gas is further raised, there is a possibility that an explosive reaction will occur.
As an effective countermeasure to such a situation, in a conventional nuclear power plant of BWR, the inside of PCV is strictly prevented from becoming a flammable atmosphere due to a large amount of hydrogen gas generated in a short time by the Metal-Water reaction, by means of substituting the inside of PCV by nitrogen gas, so that the concentration of the oxygen gas is kept at a low level and therefore a special safety level is achieved.
Also, the gas in PCV is absorbed to the outside of PCV by a flammable gas concentration controller installed outside PCV to remove the hydrogen gas; it has a recombining apparatus and a blower, with the hydrogen gas and oxygen gas being recycled to water by recombination occurring by raising the temperature. The residual gas is cooled and recycled to PCV, so that an increase in the concentration of the flammable gas is suppressed.
Also, as a method that statically controls the flammable gas concentration without the above-mentioned external power source, a method that arranges several catalytic recombining apparatuses for promoting a recombination reaction using an oxidizing catalyst of hydrogen set in PCV has been developed. The constitution of such an apparatus for removing a flammable gas is described in U.S. Pat. Nos. 5,301,217, 5,473,646, 5,167,908, for instance.
FIG. 32
is an oblique view showing an outline of the conventional catalytic recombining apparatus
110
installed in PCV
102
. A case
112
for housing an oxidizing catalyst of hydrogen
111
is equipped with two openings
113
a
and
113
b
that are always opened. Also, the gas in PCV is introduced into a catalyst layer from the opening
113
b
installed at the lower side of the case
112
. If the concentration of the flammable gas in PCV is raised, the recombination reaction of hydrogen and oxygen is caused in the case
112
by the catalyst
111
. The gas is warmed by the heat of the reaction and discharged from the opening
113
a
at the upper side of the case
112
. A naturally circulating flow passing through the catalytic, recombining apparatus is then formed by the heat by the reaction due to the catalyst
111
.
Compared with the conventional flammable gas concentration control system that absorbs the gas in PCV to the outside of PCV and recycles the hydrogen gas and the oxygen gas to water due to its recombination by raising the temperature, the above-mentioned catalytic recombining apparatus is economically excellent, and easily repaired without special operations at the time of an accident. However, the catalyst housed in the catalytic recombining apparatus may be poisoned by a substance represented by an iodine compound present in PCV at the time of an accident, so that there is a possibility that the performance of removing flammable gas is lowered.
In order to prevent the decrease in the catalyst function due to the catalyst poison, a method that separates the catalyst from the gas atmosphere in usual condition, releases the separation of the catalyst using the increase of the temperature in PCV as a trigger, and removes the catalyst poison by interposing a filter before passing a gas through the catalyst, is proposed. However, although the filter proposed by this method is suitable for the removal of grease particles or aerosol particles, it is not suitable for the removal of gaseous iodine, which has a large catalyst poison effect.
Also, when the temperature in PCV is used as a trigger for the release of the catalyst separation, there is a great possibility that the separation will be released although the oxygen concentration in PCV is still low at the initial stage of an accident, so it is not optimum as a timing of the separation release.
When the flammable gas concentration control system or the apparatus for removing flammable gas is not operated, several tens of hours pass until the atmosphere in PCV reaches a flammability limit at the time of an accident. In other words, even if the apparatus for removing a flammable gas is not operated for several tens of hours, it does not reach the flammability limit. So, separating the catalyst from the gas atmosphere during such a period is an effective way to prevent the decrease of the function of the catalyst.
However, in the above-mentioned catalytic recombining apparatus in which the openings are always opened, or in the above-mentioned catalytic recombining apparatus in which the temperature in PCV is used as a trigger to release the catalyst separation, the time exposed to a high-concentration catalyst poison present in the initial stage of the accident occurrence is so long that the function of the catalyst decreases. As a result, it can also be considered that the catalyst does not function sufficiently for the case wherein the promotion of the recombination reaction is much expected.
SUMMARY OF THE INVENTION
The
Arai Kenji
Harada Makoto
Osawa Yasuo
Tahara Mika
Yamamoto Yuji
Doroshenk Alexa A.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Knode Marian C.
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