BACKGROUND
Field of the Invention
The present invention relates to a pressure suppression
or release apparatus, and more particularly to a pressure suppression apparatus
used for a reactor container in a nuclear power plant.
Description of the Related Art
Fig. 1 is a sectional view showing a conventional reactor
container. In the reactor container 1, a reactor vessel 3, which includes a reactor
core 2 storing nuclear fuels for electric power generation, is supported by pedestals
4. The reactor container 1 is further equipped with a lower drywell 5 surrounded
by the pedestals 4, an upper drywell 6 surrounding the reactor vessel 3, and suppression
chambers 9 defined by diaphragm floors 7 and having suppression pools 8 therein.
The lower drywell 5 and the upper drywell 6 are both coupled
by connecting tubes 10 and vented tubes 11, and thereby connected underwater to
the suppression pools 8 in the suppression chamber 9. A plurality of drywell coolers
12, which are capable of keeping a cooled condition inside the lower drywell 5 and
the upper drywell 6 in regular operating mode, are disposed as a pressure suppression
apparatus.
Each drywell cooler 12 includes a drywell cooling unit
15 and a fan 16 as a circulator. The drywell-cooling unit 15 includes a casing 14
and cooling coils 13. Cooling water is used to cool the cooling coils 13, and air
inside the lower drywell 5 and the upper drywell 6 is guided to the casing 14. Specifically,
pressure inside the casing 14 is decreased by the fan 16 by realizing pressure difference,
and thereby air circulation is generated through the casing 14. The air circulation
thus generated guides air into the casing 14, and the entering air is cooled by
the cooling coils 13. The cooled air is then circulated to the lower drywell 5 and
the upper drywell 6 through ducts 17 and dumpers 18.
The cooling water in the suppression pool 8 is pumped by
a residual heat removal pump 20 in a residual heat removal line 19, and heat exchange
of the cooling water is made by a residual heat removal heat exchange 21. Spray
heads 22 then spray the cooled water back into the suppression pool 8. This cooling
system cools the reactor container 1 in high-temperature or high-pressure condition.
In the conventional reactor container 1 thus constituted,
a large amount of a fluid mixture including, for example, gases, vapor, and water
at a high temperature is generated in the lower drywell 5 and the upper drywell
6 in case of "loss of coolant accident" (LOCA: an accident in which coolant in the
reactor vessel 3 is leaked). In this case, the mixture is guided to cooling water
in the suppression pool 8 of the suppression chamber 9, and therefore, a pressure
increase inside the reactor container 1 is restrained.
However, if the above-explained LOCA continues and the
cooling system is operated for a long time, gaseous phase including a non-condensable
gas is pressurized by stored water in the reactor container 1, and this may cause
an increase of pressure in the reactor container 1.
In order to avoid this problem, the drywell cooler 12 can
be used to suppress pressure. That is, air in the lower drywell 5 and the upper
drywell 6 can be cooled efficiently, and heat can be exhausted outside of the reactor
vessel 3.
More specifically, by flowing cooling water to the cooling
coils 13 in the drywell cooler 12, vapor in the casing 14 can be condensed. Consequently,
vapor pressure in the reactor container 1 can be reduced by guiding the vapor therein
into the casing 14.
However, a power supply 30, which supplies electric power
to a fan 16 in the dry-well cooler 12, is usually operated only in the regular operating
mode, and is automatically stopped in case of an emergency. Therefore, non-condensable
gas, such as nitrogen gas initially enclosed in the reactor container 1, hydrogen
gas generated by increasing temperature and pressure therein, and the like, gradually
collects around the casing 14. Thereby, the casing 14 holds a large amount of the
non-condensable gas. This deteriorates the heat exchange performance of the drywell
cooler 12, and the rise of vapor pressure in the reactor container 1 cannot be suppressed.
An isolation condenser passive cooling operation in a nuclear
reactor containment is disclosed in the publication EP 0 460 805 A1.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-mentioned
circumstances and is intended to solve the above-mentioned problems. In particular,
the purpose of the present invention is to provide a pressure suppression apparatus
for a reactor container capable of allowing a drywell-cooling unit in case of emergency,
to reduce vapor pressure in the reactor container.
The present invention provides a pressure suppression apparatus,
including: a reactor vessel that stores nuclear fuel; a reactor container that stores
the reactor vessel and defines a drywell room; a drywell cooler that cools a fluid,
such as gases, liquids, vapor, water, or mixtures in the drywell room so as to suppress
pressure in the drywell room; a circulator that guides fluid in the drywell room
into the drywell cooler; and an exhauster that exhausts a non-condensable fluid
from the drywell cooler.
Here, the exhauster may include a fan. The fan may be constituted
such that the fan functions when the circulator stops. Further, the circulator may
function by a regular power supply, and the fan may function by an emergency power
supply.
The exhauster may include a door that links the inside
to the outside of the drywell cooler. The door may be closed when the circulator
operates, and the door may be opened when the circulator does not operate. The door
may be operated mechanically, or the door may be operated by a pressure difference
between the inside and the outside of the drywell cooler.
Further, the exhauster may include a material that melts
at a certain temperature and then links the inside and the outside of the drywell
cooler. The material may melt at a temperature that is higher than that in the reactor
container in regular operating mode.
The circulator may include a first fan, and the exhauster
may include a second fan. Here, the first fan may be larger than the second fan.
BRIEF DESCRIPTION OF DRAWINGS
The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate several preferred embodiments
of the present invention and, together with the description, serve to explain the
principles of the present invention.
- Fig. 1 is a sectional view showing a conventional reactor container.
- Fig. 2 is a sectional view showing a reactor container of the present invention.
- Fig. 3 is a sectional view showing the pressure suppression apparatus in the
reactor container according to a first embodiment of the present invention.
- Fig. 4 is a sectional view showing the pressure suppression apparatus in the
reactor container according to a second embodiment of the present invention.
- Fig. 5 is a sectional view showing the pressure suppression apparatus in the
reactor container according to a third embodiment of the present invention.
- Figs. 6A and 6B are sectional views showing the pressure suppression apparatus
in the reactor container according to a fourth embodiment of the present invention;
Fig. 6A indicates when a fan operates, and Fig. 6B indicates when the fan stops.
DESCRIPTION OF PREFERRED EMBODIMENTS
Preferred embodiments of a pressure suppression or release
apparatus for a reactor container of the present invention will now be specifically
described in more detail with reference to the accompanying drawings. Wherever possible,
the same reference numbers will be used throughout the drawings to refer to the
same or like parts.
First, a pressure suppression apparatus arranged in a reactor
container is explained by referring to Fig. 2, which illustrates a sectional view
of the reactor container.
The feature of the present invention is that a non-condensable
fluid exhauster such as a second fan 23 is disposed in the reactor container 1 together
with the circulator such as the fan 16, and the second fan 23 is operated by an
emergency power supply 31.
In case of an emergency, such as the LOCA for example,
the cooling coils 13 in the drywell cooler 12, which is a pressure suppression apparatus,
can continue to condense vapor in the casing 14. However, as time progresses, non-condensable
gas, such as nitrogen gas initially enclosed in the reactor container 1, hydrogen
gas generated by increasing temperature and pressure therein, and the like, gradually
collects around the casing 14.
In this situation, the regular power supply 30 is already
disconnected due to safety, and many devices including the fan 16 stop.
However, the second fan 23 may be connected to the emergency
power supply 31 in the present invention. Therefore, as soon as the regular power
supply 30 is disconnected, the emergency power supply 31 is connected automatically,
and thereby the second fan 23 is operated accordingly. By using the second fan 23,
the non-condensable gas is exhausted to outside of the drywell cooling unit 15 or
to inside of the suppression chamber 9 through an exhaust duct 24.
Partial pressure of the non-condensable gas around the
cooling coils 13 decreases; partial pressure of the non-condensable gas in the casing
14 of the drywell cooling unit 15 also decreases, and heat exchange performance
of the apparatus can be maintained. Consequently, increases in the internal pressure
of the reactor container 1 can be suppressed for a long time.
Fig. 3 is a sectional view showing the drywell cooler 12
in the reactor container in Fig. 2. As shown in Fig. 3, the second fan 23 is disposed
in the drywell cooling unit 15 of the drywell cooler 12, which acts as a pressure
suppression apparatus of the reactor container 1. One end of the exhaust duct 24
is connected to the exit of the second fan 23, and another end thereof is connected
to a check valve 25. The check valve 25 is equipped so as to avoid air or vapor
around the drywell cooling unit 15 flowing into the casing 14, and closes when the
second fan 23 stops.
Here, the second fan 23 directs air towards the lower surface
of the drywell cooling unit 15, and the exit thereof faces the direction of gravity,
as shown in Fig. 2. This is because nitrogen, which is a non-condensable gas initially
enclosed in the reactor container 1, is heavier than air. The nitrogen gas collects
downward of the casing 14, and can be exhausted easily.
However, the position of the second fan 23 is not limited
to this example, and can be disposed on a side surface or an upper surface of the
drywell cooling unit 15. Similarly, the number of second fans 23 is not limited
to one; a plurality of second fans can be disposed in each drywell cooling unit
15.
Further, heat exchange performance of the drywell cooling
unit 15 can be maintained as long as the second fan 23 exhausts non-condensable
gas out from the casing 14. For this reason, the size of the second fan 23 can be
smaller than that of the fan 16.
Fig. 4 is a sectional view showing the drywell cooler 12
according to a second embodiment of the present invention. The feature of the second
embodiment is that doors 26a, 26b and 26c, which open when temperature and pressure
in the drywell cooling unit 15 increase, are disposed on the casing 14 of the drywell
cooling unit 15. The doors, vents, panels, shutters, or openings provide a pressure
release or suppression mechanism. The doors 26a, 26b and 26c are disposed in an
upper surface, side surface, and lower surface, respectively.
When the dumper 18 on the duct 17 opens or closes, the
doors 26a, 26b and 26c are operated simultaneously. That is, the doors 26a, 26b
and 26c are connected to the exit of the dumper 18 mechanically, by means of springs,
for example, and are opened by using force generated when the dumper 18 closes.
When the fan 16 operates, the inside of the casing 14 is
depressurized due to exhaustion thereof. Therefore, it is possible to operate the
doors 26a, 26b, and 26c such that they are closed when the fan 16 operates, and
they are opened automatically by gravity when the fan 16 stops.
In this embodiment, the second fan 23 shown in Fig. 3 is
not required. Because of the specific gravity against air, the upper door 26a and
the side door 26b can exhaust mainly hydrogen, and the lower door 26c can exhaust
mainly nitrogen, out from the casing 14.
According to this embodiment, nitrogen, which is heavier
than vapor among non-condensable gas, can be exhausted mainly from the lower door
26c, and hydrogen, which is lighter than vapor among non-condensable gas, can be
exhausted mainly from the upper door 26a and the side door 26b. Thereby, heat exchange
performance of the drywell cooling unit 15 can be improved.
In addition, the second fan is not required for this embodiment;
additional equipment such as the emergency power supply are also not needed. Therefore,
the structure of the apparatus can be simplified.
Fig. 5 is a sectional view showing the drywell cooler 12
according to a third embodiment of the present invention. The feature of this embodiment
is that materials 27 having a low melting point comprise a portion of the casing
14, and the materials 27 and the casing 14 form the enclosure of the drywell cooling
unit 15.
These materials 27 melt under certain atmospheric condition
that are chosen in designing a nuclear reactor; the materials 27 melt when the atmosphere
exceeds an environmental temperature in the reactor container 1 for example. If
the temperature inside of the reactor container 1 becomes higher than a predetermined
temperature, the materials 27 melt and then openings appear in the wall of the casing
14. Thereby, non-condensable gas in the casing 14 can be exhausted outward.
Figs. 6A and 6B are sectional views showing the drywell
cooler 12 according to a fourth embodiment of the present invention. Here, Fig.
6A indicates when the fan 16 operates, and Fig. 6B indicates when the fan 16 stops.
In this embodiment, a second door 28 is disposed in the
lower surface of the casing 14. The second door 28 is closed by absorptivity of
the fan 16 in a normal operation mode as shown in Fig. 6A, and is opened by gravity
when the fan 16 stops. This also makes it possible to exhaust non-condensable gas
in the casing 14 outward.
The foregoing discussion discloses and describes several
exemplary embodiments of the present invention. As will be understood by those skilled
in the art, the present invention may be embodied in other specific forms without
departing from the scope thereof. Accordingly, the disclosure of the present invention
is intended to be illustrative, but not limiting, of the scope of the invention,
which is set forth in the following claims. Thus, the present invention may be embodied
in various ways within the scope of the spirit of the invention.
For example, the explanation above is made when the present
invention is applied to a BWR (Boiled Water Reactor) nuclear power plant, however,
the present invention is also applied to other types of plants such as a PWR (Pressurized
Water Reactor) nuclear power plant.