U.S. patent application number 12/056813 was filed with the patent office on 2009-03-05 for boiling water nuclear reactor and emergency core cooling system of the same.
This patent application is currently assigned to KABUSHIKI KAISHA TOSHIBA. Invention is credited to Yoshihiro KOJIMA, Hirohide Oikawa, Takashi Sato.
Application Number | 20090060112 12/056813 |
Document ID | / |
Family ID | 39974498 |
Filed Date | 2009-03-05 |
United States Patent
Application |
20090060112 |
Kind Code |
A1 |
KOJIMA; Yoshihiro ; et
al. |
March 5, 2009 |
BOILING WATER NUCLEAR REACTOR AND EMERGENCY CORE COOLING SYSTEM OF
THE SAME
Abstract
A boiling water nuclear reactor comprises: a reactor containment
vessel including a dry well and a wet well; a vent pipe connecting
the dry well and the pressure suppression pool; a gravity-driven
water injection pool to hold boric acid aqueous solution; an
emergency core water-injection piping system for causing the boric
acid aqueous solution in the gravity-driven water injection pool to
fall so as to be injected into the reactor pressure vessel in case
of reactor accident; a static containment vessel cooling system
pool; a static containment vessel cooling system heat exchanger; a
dry well connection pipe connecting an upper part of the static
containment vessel cooling system heat exchanger and the dry well;
and a gas vent pipe for discharging noncondensible gas in the
static containment vessel cooling system heat exchanger into the
inside of the pressure suppression pool.
Inventors: |
KOJIMA; Yoshihiro;
(Kanagawa, JP) ; Sato; Takashi; (Kanagawa, JP)
; Oikawa; Hirohide; (Kanagawa, JP) |
Correspondence
Address: |
FOLEY AND LARDNER LLP;SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
|
Family ID: |
39974498 |
Appl. No.: |
12/056813 |
Filed: |
March 27, 2008 |
Current U.S.
Class: |
376/282 ;
376/283 |
Current CPC
Class: |
Y02E 30/30 20130101;
G21C 15/18 20130101; Y02E 30/31 20130101; G21C 9/004 20130101 |
Class at
Publication: |
376/282 ;
376/283 |
International
Class: |
G21C 9/004 20060101
G21C009/004; G21C 9/00 20060101 G21C009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 29, 2007 |
JP |
2007-087617 |
Claims
1. A boiling water nuclear reactor comprising: a core; a reactor
pressure vessel containing the core; a reactor containment vessel
including: a dry well containing the reactor pressure vessel, and a
wet well containing a pressure suppression pool; a vent pipe
connecting the dry well and the pressure suppression pool; a
gravity-driven water injection pool arranged at a position higher
than the core to hold boric acid aqueous solution therein; an
emergency core water-injection piping system for causing the boric
acid aqueous solution in the gravity-driven water injection pool to
fall by gravity so as to be injected into the reactor pressure
vessel in case of reactor accident; a static containment vessel
cooling system pool arranged outside the reactor containment
vessel; a static containment vessel cooling system heat exchanger
arranged in the static containment vessel cooling system pool; a
dry well connection pipe connecting an upper part of the static
containment vessel cooling system heat exchanger and the dry well;
and a gas vent pipe for discharging noncondensible gas in the
static containment vessel cooling system heat exchanger into the
inside of the pressure suppression pool.
2. The reactor according to claim 1, wherein the gravity-driven
water injection pool is arranged in the dry well.
3. The reactor according to claim 2, further comprising a drain
pipe for discharging the condensed water in the static containment
vessel cooling system heat exchanger into the gravity-driven water
injection pool.
4. The reactor according to claim 1, wherein: the gravity-driven
water injection pool is arranged in the reactor containment vessel
and partitioned from the dry well and the wet well; and the nuclear
reactor further comprises a first communication pipe holding the
gas phase section in an upper part of the gravity-driven water
injection pool and the gas phase section above the pressure
suppression pool in the wet well in communication with each
other.
5. The reactor according to claim 4, further comprising a drain
pipe for discharging the condensed water in the static containment
vessel cooling system heat exchanger into the gravity-driven water
injection pool.
6. The reactor according to claim 1, wherein: the gravity-driven
water injection pool is held gas-tight and arranged above the
reactor containment vessel; and the nuclear reactor further
comprises a second communication pipe holding the gas phase section
above the gravity-driven water injection pool and the dry well in
communication with each other.
7. The reactor according to claim 6, further comprising a drain
pipe for discharging the condensed water in the static containment
vessel cooling system heat exchanger into a lower part of the dry
well.
8. The reactor according to claim 1, wherein the gravity-driven
water injection pool is held gas-tight and arranged above the
reactor containment vessel and the nuclear reactor further
comprises a third communication pipe holding the gas phase section
in an upper part of the gravity-driven water injection pool and the
gas phase section above the pressure suppression pool in the wet
well in communication with each other.
9. The reactor according to claim 8, further comprising a drain
pipe for discharging the condensed water in the static containment
vessel cooling system heat exchanger into a lower part of the dry
well.
10. The reactor according to any one of claim 1, further comprising
an emergency dry well water-injection piping system for injecting
the boric acid aqueous solution in the gravity-driven water
injection pool into a lower part of the dry well in case of a
reactor accident.
11. The reactor according to any one of claim 1, further
comprising: an intra dry well heat exchanger arranged in the dry
well; an external heat exchanger arranged outside the reactor
containment vessel; a dry well cooling water pipe connecting the
intra dry well heat exchanger and the external heat exchanger in
order to discharge the heat obtained by the intra dry well heat
exchanger at the external heat exchanger; and boric acid aqueous
solution heating means for heating the boric acid aqueous solution
in the gravity-driven water injection pool by using part of the
heat obtained by the intra dry well heat exchanger.
12. The reactor according to one of claim 1, wherein the pressure
suppression pool can be used to fill the fuel exchange pool with
water when replacing the fuel loaded in the core.
13. The reactor according to any one of claim 1, further
comprising: a boric acid aqueous solution injection system
including a boric acid aqueous solution injection tank arranged
outside the reactor containment vessel to store pressurized boric
acid aqueous solution of a quantity less than quantity of water in
the gravity-driven water injection pool and adapted to inject boric
acid aqueous solution into the core from the boric acid aqueous
solution injection tank.
14. The reactor according to any one of claim 1, further
comprising: a pressure-reducing valve arranged at the reactor
pressure vessel at a position above the core so as to be capable of
discharging steam from the inside of the reactor pressure vessel
into the dry well.
15. An emergency core cooling system of a boiling water nuclear
reactor, the emergency core cooling system comprising: a
gravity-driven water injection pool to hold boric acid aqueous
solution therein, the gravity-driven water injection pool being
arranged at a position higher than the core; an emergency core
water-injection piping system for causing the boric acid aqueous
solution in the gravity-driven water injection pool to fall by
gravity so as to be injected into a reactor pressure vessel
containing the core, in case of reactor accident; a static
containment vessel cooling system pool arranged outside the reactor
containment vessel at a position above a pressure suppression pool;
a static containment vessel cooling system heat exchanger arranged
in the static containment vessel cooling system pool; a dry well
connection pipe connecting an upper part of the static containment
vessel cooling system heat exchanger and the dry well; and a gas
vent pipe for discharging noncondensible gas in the static
containment vessel cooling system heat exchanger into inside of the
pressure suppression pool.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] The present invention contains subject matter related to
Japanese Patent Application No. 2007-87617, filed in the Japanese
Patent Office on Mar. 29, 2007, the entire content of which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] This invention relates to a gravity-driven emergency core
cooling system (ECCS) and also to a boiling water nuclear reactor
(BWR) using the same.
[0003] Studies are being made of introducing equipment that
operates statically (passively), utilizing natural force, without
using dynamic (actively) devices such as pumps mainly in order to
raise the simplicity of safety equipment and improve the
reliability of boiling water nuclear reactors. Among others,
gravity-driven emergency core cooling systems (ECCS) are known for
pouring water into the nuclear reactor in case of an accident as
shown in Japanese Patent Application Laid-Open Publication No.
02-115793 and Japanese Patent No. 2918353, the entire contents of
which are incorporated herein by reference.
[0004] An ordinary boiling water nuclear reactor has a reactor
containment vessel, which includes a dry well and a wet well. The
dry well contains a rector pressure vessel. A pressure suppression
pool is housed in the wet well and the water in the pool is to be
used to fill the fuel exchange pool with water when suspending the
operation of the reactor and replacing the core fuel.
[0005] When a loss-of-coolant accident (LOCA) occurs and the pipe
connected to the reactor pressure vessel is ruptured in the inside
of the dry well, the water level in the reactor pressure vessel
falls. Then, as the signal indicating the water level in the
reactor pressure vessel or the pressure level of the dry well is
detected, the isolation valve arranged at the gravity-driven water
injection pipe connected to a gravity-driven water injection pool
is opened to inject cooling water into the reactor pressure vessel
by means of the gravity-driven ECCS.
[0006] The injected cooling water gradually raises the water level
in the reactor pressure vessel. As the water level reaches the
ruptured part of the pipe, water spills and falls into the dry well
to submerge a lower part of the dry well which is referred to as a
"dry well lower region" in water. Therefore, a gravity-driven water
injection pool is required to contain a large volume of water of
about 1,500 m.sup.3.
[0007] The gravity-driven water injection pool is held in
communication with the dry well and its performance of injecting
water into the reactor pressure vessel is determined by the water
head of gravity-driven water injection pool. Therefore, the
pressure difference between the inside of the reactor pressure
vessel and that of the dry well needs to be minimized by opening
the pressure-reducing valve before opening the isolation valve
arranged at gravity-driven water injection pipe. While the steam
generated continuously by the decay heat coming from the core is
discharged partly into the dry well by way of the pressure-reducing
valve, it is introduced to and condensed by the heat exchanger of
the static containment vessel cooling system so that the internal
pressure of the reactor containment vessel will not be raised
excessively. In this way, when a LOCA occurs, the core and the
reactor containment vessel are effectively cooled and held safe and
sound.
[0008] On the other hand, when the isolation valve and/or the
pressure-reducing valve of the gravity-driven water injection pipe
become unopenable for some reason or another, there arises a
situation where the core can no longer be cooled. Then, the core
can become damaged and molten. If such is the case, the molten core
melts the reactor pressure vessel and runs through the latter to
fall into and settle in the dry well lower region.
[0009] If the molten core is not cooled and held to a hot
condition, it reacts with the bottom concrete of the dry well lower
region to produce noncondensible gas, which can raise the internal
pressure of the reactor containment vessel. To cope with such a
situation, a piping arrangement may be adapted such that by
detecting the temperature rise of the dry well lower region, the
injection pipe of the gravity-driven water injection pool extending
to the lower part of the dry well is opened to inject cooling water
onto and cool an upper part of the molten core.
[0010] Besides a LOCA, an anticipated transient without scram
(ATWS) can take place to threaten the safety of nuclear reactor.
This is an incident where the attempt to insert the control rods
fails partly or totally although a scram is required for the
nuclear reactor in order to protect the core and the reactor
containment vessel when an accident of total loss of water feed to
the nuclear reactor or an incident of a total closure of the main
steam isolation valve occurs.
[0011] In an ATWS where the trouble proceeds with high internal
pressure of the reactor, accompanying isolation of the reactor, a
large quantity of reactor coolant is discharged in the form of
steam from the main steam safety relief valve into the wet well to
raise the internal pressure of the reactor containment vessel.
Therefore, it is essential to maintain the soundness of the core
and the reactor containment vessel by appropriately maintaining the
water level in the reactor and forcing the reactor to quickly move
into a subcritical state by injecting boric acid aqueous solution
from the boric acid aqueous solution injection tank. The boric acid
aqueous solution injection tank contains only about 20 m.sup.3 of
boric acid aqueous solution that can put the core back into a
subcritical state when mixed with the cooling water in the reactor
pressure vessel and hence is very small if compared with the
gravity-driven water injection pool.
[0012] Assume a situation where an ATWS incident takes place and
the pressure-reducing valve is opened so that water is injected by
the gravity-driven ECCS. In such a situation, there arises a
possibility that the injection of boric acid aqueous solution has
been completed before the injection of water by means of the
gravity-driven ECCS is started to make it difficult to maintain the
subcritical state so that the core may be plunged back into a
critical state because the boric acid is diluted by about 1,500
m.sup.3 cold water injected by the gravity-driven ECCS. On the
other hand, the safety of the reactor can be made more reliable
when the boric acid concentration is held to a certain high level
in such a situation.
[0013] In view of the above-identified circumstances, it is
therefore the object of the present invention to make it possible
to secure the safety of a reactor by reliably maintaining a
subcritical state even when the gravity-driven ECCS is actuated
when an ATWS takes place in a boiling water nuclear reactor.
BRIEF SUMMARY OF THE INVENTION
[0014] In order to attain the object, according to an aspect of the
present invention, there is provided a boiling water nuclear
reactor comprising: a core; a reactor pressure vessel containing
the core; a reactor containment vessel including: a dry well
containing the reactor pressure vessel, and a wet well containing a
pressure suppression pool; a vent pipe connecting the dry well and
the pressure suppression pool; a gravity-driven water injection
pool arranged at a position higher than the core to hold boric acid
aqueous solution therein; an emergency core water-injection piping
system for causing the boric acid aqueous solution in the
gravity-driven water injection pool to fall by gravity so as to be
injected into the reactor pressure vessel in case of reactor
accident; a static containment vessel cooling system pool arranged
outside the reactor containment vessel; a static containment vessel
cooling system heat exchanger arranged in the static containment
vessel cooling system pool; a dry well connection pipe connecting
an upper part of the static containment vessel cooling system heat
exchanger and the dry well; and a gas vent pipe for discharging
noncondensible gas in the static containment vessel cooling system
heat exchanger into the inside of the pressure suppression
pool.
[0015] According to another aspect of the present invention, there
is provided an emergency core cooling system of a boiling water
nuclear reactor, the emergency core cooling system comprising: a
gravity-driven water injection pool to hold boric acid aqueous
solution therein, the gravity-driven water injection pool being
arranged at a position higher than the core; an emergency core
water-injection piping system for causing the boric acid aqueous
solution in the gravity-driven water injection pool to fall by
gravity so as to be injected into a reactor pressure vessel
containing the core, in case of reactor accident; a static
containment vessel cooling system pool arranged outside the reactor
containment vessel at a position above a pressure suppression pool;
a static containment vessel cooling system heat exchanger arranged
in the static containment vessel cooling system pool; a dry well
connection pipe connecting an upper part of the static containment
vessel cooling system heat exchanger and the dry well; and a gas
vent pipe for discharging noncondensible gas in the static
containment vessel cooling system heat exchanger into inside of the
pressure suppression pool.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The above and other features and advantages of the present
invention will become apparent from the discussion hereinbelow of
specific, illustrative embodiments thereof presented in conjunction
with the accompanying drawings, in which:
[0017] FIG. 1 is a schematic elevational cross-sectional view of
the first embodiment of a boiling water nuclear reactor according
to the present invention,
[0018] FIG. 2 is a graph illustrating in comparison the outcome of
analysis of the pressure behavior of the reactor containment vessel
of the first embodiment of the boiling water nuclear reactor
according to the present invention when the core is damaged and the
outcome of analysis of the pressure behavior of the reactor
containment vessel of a conventional boiling water nuclear
reactor;
[0019] FIG. 3 is a schematic elevational cross-sectional view of
the second embodiment of a boiling water nuclear reactor according
to the present invention;
[0020] FIG. 4 is a schematic elevational cross-sectional view of
the third embodiment of a boiling water nuclear reactor according
to the present invention;
[0021] FIG. 5 is a schematic elevational cross-sectional view of
the fourth embodiment of a boiling water nuclear reactor according
to the present invention; and
[0022] FIG. 6 is a schematic elevational cross-sectional view of
the fifth embodiment of a boiling water nuclear reactor according
to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0023] Now, the present invention will be described in greater
detail by referring to the accompanying drawings that schematically
illustrate preferred embodiments of the present invention.
[0024] Throughout the drawings, the same or similar parts are
denoted respectively by the same reference symbols and their
description is omitted.
First Embodiment
[0025] FIG. 1 is a schematic elevational cross-sectional view of
the first embodiment of a boiling water nuclear reactor according
to the present invention. The reactor containment vessel 5 of the
reactor includes a dry well 6 and a wet well 7, and a reactor
pressure vessel 2 of the reactor is contained in the dry well 6. A
pressure suppression pool 20 is arranged in the wet well 7. The
water in the pressure suppression pool 20 is used to fill the fuel
exchange pool with water when the operation of the reactor stopped
to replace the core fuel. The reactor pressure vessel 2 contains
the core 1, the internal structure and the reactor water (not
shown).
[0026] Vent pipes 8 extend from an upper part of the dry well 6
into the pressure suppression pool 20. Main steam pipes 21 are
arranged to feed the main steam generated in the reactor pressure
vessel 2 to the steam turbine (not shown) outside the reactor
containment vessel 5 and main steam safety relief valves 4 are
fitted to the main steam pipes 21 at positions in the dry well 6 so
that steam may be introduced from the inside of the main steam
pipes 21 into the pressure suppression pool 20 by way of the safety
valves 4. A pressure-reducing valve 3 is connected to the reactor
pressure vessel 2 at a position above the core 1.
[0027] A gravity-driven water injection pool 9 is arranged at a
position higher than the core 1 in an upper part in the inside of
the dry well 6 to constantly store boric acid aqueous solution
therein. In the case of a large scale nuclear power plant, the
gravity-driven water injection pool 9 constantly stores about 1,500
m.sup.3 of boric acid aqueous solution therein. The boric acid
aqueous solution in the gravity-driven water injection pool 9 can
be injected into the reactor pressure vessel 2 by way of the
isolation valve 23 and a gravity-driven water injection pipe 10a.
Additionally, the boric acid aqueous solution in the gravity-driven
water injection pool 9 can be injected into a dry well lower region
6b by gravity by way of an emergency dry well water-injection
piping system 10b.
[0028] A static containment vessel cooling system pool 24 is
arranged above the reactor containment vessel 5 and a static
containment vessel cooling system heat exchanger 12 is immersed in
the static containment vessel cooling system pool 24. The static
containment vessel cooling system heat exchanger 12 includes an
upper header 25, a lower header 26 and a plurality of heat transfer
pipes 27 connecting the upper header 25 and the lower header
26.
[0029] The upper header 25 is held in communication with an upper
part of the dry well 6 by way of a dry well connection pipe 32. A
gas vent pipe 28 is connected to an upper part of the lower header
26 so that the noncondensible gas in the lower header 26 can be
discharged into the inside of the pressure suppression pool 20.
Additionally, a drain pipe 29 is connected to a bottom part of the
lower header 26 so that the condensed water in the lower header 26
can be discharged into the gravity-driven water injection pool
9.
[0030] An accumulator type boric acid aqueous solution injection
tank 11 is arranged outside the reactor containment vessel 5 so
that boric acid aqueous solution can be injected into the core 1 by
way of a boric acid aqueous solution injection pipe 30. In the case
of a large scale nuclear power plant, the boric acid aqueous
solution injection tank 11 typically stores about 20 m.sup.3 of
boric acid aqueous solution, which is by far less than the quantity
of boric acid aqueous solution stored in the gravity-driven water
injection pool 9.
[0031] When an accident (LOCA) occurs and any of the pipes
connected to the reactor pressure vessel 2 is ruptured in the
inside of the dry well 6, a large volume of coolant flows out from
the reactor pressure vessel 2 into the dry well 6 and the steam and
the noncondensible gas flown into the dry well 6 then flow into the
pressure suppression pool 20 in the wet well 7 by way of the vent
pipe 8. The noncondensible gas may be the nitrogen that is
contained in the dry well 6 in advance or the hydrogen that is
produced when the core is damaged.
[0032] Thereafter, the flow rate of the coolant flowing out from
the reactor pressure vessel 2 into the dry well 6 reduces and the
flow of coolant stops when the pressure difference between the dry
well 6 and the wet well 7 becomes sufficiently smaller than the
pressure level that corresponds to the depth by which the vent
pipes 8 are submerged in the water. Then, steam and noncondensible
gas flow toward the static containment vessel cooling system heat
exchanger 12 that shows a smaller pressure difference by way of the
dry well connection pipe 32 and steam is condensed in the static
containment vessel cooling system heat exchanger 12. The depth by
which the gas vent pipe 28 is submerged in the water is made
smaller than the depth by which the vent pipes 8 are submerged in
the water in order to make the latter flow take place
predominantly.
[0033] The steam that is continuously generated by the decay heat
from the core 1 and flows into the dry well 6 is partly led to the
static containment vessel cooling system heat exchanger 12 with
noncondensible gas by way of the dry well connection pipe 32 and
condensed there. Therefore, the internal pressure of the reactor
containment vessel 5 is not raised excessively by the generated
steam. The condensed water produced by the heat transfer pipes 27
of the static containment vessel cooling system heat exchanger 12
accumulates on the bottom of the lower header 26 and falls into the
gravity-driven water injection pool 9 by way of the drain pipe 29.
The noncondensible gas accumulated in an upper part of the lower
header 26 is discharged into the pressure suppression pool 20 by
way of the gas vent pipe 28.
[0034] On the other hand, the water level in the reactor pressure
vessel 2 falls as coolant flows from the reactor pressure vessel 2
to the dry well 6. At this time, in order to prevent damage of the
core 1, after detecting the signal indicating a low water level in
the reactor pressure vessel 2 or a high pressure level in the dry
well 6, the isolation valve 23 arranged at the gravity-driven water
injection pipe 10a, which is connected to the gravity-driven water
injection pool 9, is opened. Then, boric acid aqueous solution is
injected from the inside of the gravity-driven water injection pool
9 into the reactor pressure vessel 2 by means of the gravity-driven
ECCS. The injected boric acid aqueous solution gradually raises the
water level in the inside of the reactor pressure vessel 2. As the
water level reaches the ruptured position of the pipe, boric acid
aqueous solution spills and flows into the dry well 6 to submerge
the dry well lower region 6b in water.
[0035] The gravity-driven water injection pool 9 holds boric acid
aqueous solution by an amount that is sufficient to completely
submerge the dry well lower region 6b in water. The gravity-driven
water injection pool 9 is held in communication with the dry well 6
and its performance of injecting water into the reactor pressure
vessel 2 is determined by the water head of gravity-driven water
injection pool 9. Therefore, the pressure difference between the
inside of the reactor pressure vessel 2 and that of the dry well 6
needs to be minimized by opening the pressure-reducing valve 3
before opening the isolation valve 23 arranged at gravity-driven
water injection pipe 10a. Additionally, the steam generated
continuously by the decay heat coming from the core 1 is discharged
partly into the dry well 6 by way of the pressure-reducing valve
3.
[0036] In this way, the core 1 and the reactor containment vessel 5
can be cooled and the soundness thereof can be maintained when a
LOCA occurs.
[0037] On the other hand, when the isolation valve 23 and/or the
pressure-reducing valve 3 of the gravity-driven water injection
pipe 10a becomes unopenable for some reason or another, there may
arise a situation where the core 1 can no longer be cooled. Then,
the core 1 can become damaged and molten. If such is the case, the
molten core melts the reactor pressure vessel 2 and moves through
the latter to fall into and settle in the dry well lower region 6b.
If the molten core is not cooled and held to a hot condition, it
reacts with the bottom concrete of the dry well lower region 6b to
produce noncondensible gas, which can raise the internal pressure
of the reactor containment vessel 5. In such a situation, the
embodiment detects the temperature rise of the dry well lower
region 6b and opens the emergency dry well water-injection piping
system 10b from the gravity-driven water injection pool 9 to inject
boric acid aqueous solution into and cool the molten upper part of
the core.
[0038] In an anticipated transient without scram (ATWS) where a
high pressure situation of the reactor continues while the reactor
is isolated, a large volume of reactor coolant is discharged into
the wet well 7 through the main steam safety relief valve 4 in the
form of steam to raise the pressure in the reactor containment
vessel 5. Therefore, the reactor is quickly moved into a
subcritical state by injecting boric acid aqueous solution from the
boric acid aqueous solution injection tank 11 into the core 1 by
way of the boric acid aqueous solution injection pipe 30, while
maintaining the water level in the reactor to an appropriate level.
However, the boric acid aqueous solution injection tank 11 contains
only about 20 m.sup.3 of boric acid that can put the core 1 back
into a subcritical state when mixed with the cooling water in the
reactor pressure vessel 2 and hence is very small if compared with
the gravity-driven water injection pool 9.
[0039] Assume now a situation where an ATWS takes place and the
pressure-reducing valve 3 is opened so that water is injected by
the gravity-driven ECCS. In such a situation, even if the injection
of boric acid aqueous solution has been completed before the
injection of water by means of the gravity-driven ECCS is started,
the boric acid is not diluted by the gravity-driven ECCS because
the boric acid aqueous solution is contained in the gravity-driven
water injection pool 9. Thus, the subcritical state of the core can
be maintained.
[0040] Furthermore, this embodiment can maintain the reactor in a
subcritical state not only in an ATWS that accompanies isolation of
the reactor but also in a situation where the attempt to insert the
control rods fails partly or totally when a LOCA occurs. Thus, the
reactor can be maintained in a subcritical state even if a gigantic
earthquake of a magnitude that exceeds the design magnitude takes
place, although such a situation can hardly arise in reality, and
even if a LOCA occurs consequently to make it impossible to insert
control rods because the fuel channels are bent.
[0041] Furthermore, in a situation where a LOCA occurs and the
isolation valve 23 and/or the pressure-reducing valve 3 of the
gravity-driven water injection pipe 11a becomes unopenable for some
reason or another so that the core 1 is damaged and molten to
damage the reactor pressure vessel 2 and the molten substances are
accumulated in the dry well lower region 6b before the isolation
valve of the emergency dry well water-injection piping system 10b
is opened to cool the molten substances, the molten radioactive
substances can be highly reliably prevented from falling back into
a critical state by the injected boric acid aqueous solution.
[0042] FIG. 2 is a graph illustrating how the internal pressure of
the reactor containment vessel 5 is expected to change when the
molten substances accumulated in the dry well lower region 6b do
not fall back into a critical state and when they fall back into a
critical state. It is assumed for FIG. 2 that the reactor pressure
vessel 2 is damaged and the molten substances of the core 1 are
accumulated in the dry well lower region 6b and that the operation
of injecting boric acid aqueous solution from the gravity-driven
water injection pool 9 into the dry well lower region 6b by way of
the emergency dry well water-injection piping system 10b at time
"t0".
[0043] With this embodiment, boric acid aqueous solution is poured
onto the molten substances from above to prevent them from falling
back into a critical state so that the internal pressure of the
reactor containment vessel 5 does not rise excessively and hence
the reactor containment vessel 5 is not damaged as indicated by the
solid line 60 in FIG. 2. The net results will be that no
radioactive substances are discharged into the surrounding
environment on a large scale.
[0044] If, on the other hand, the gravity-driven water injection
pool 9 simply holds water according to the conventional art, the
molten substances accumulated in the dry well lower region 6b can
fall back into a critical state. Then, steam would be generated to
an extent that exceeds the level corresponding to decay heat and
would be far above the steam condensing capacity of the static
containment vessel cooling system heat exchanger 12, so that the
internal pressure of the reactor containment vessel 5 might rise as
indicated by the dotted line 62 in FIG. 2 to consequently damage
the reactor containment vessel 5.
[0045] The boric acid aqueous solution injection system including
the boric acid aqueous solution injection tank 11 and the boric
acid aqueous solution injection pipe 30 shown in FIG. 1 may be
omitted from the above-described embodiment.
Second Embodiment
[0046] FIG. 3 is a schematic elevational cross-sectional view of
the second embodiment of a boiling water nuclear reactor according
to the present invention. In this embodiment, the gravity-driven
water injection pool 9 holding boric acid aqueous solution is
isolated from the dry well 6 by means of a partition wall 14. The
gas phase section above the liquid surface of the gravity-driven
water injection pool 9 is held in communication with the upper gas
phase section in the wet well 7 by way of a first communication
pipe 13.
[0047] The boric acid aqueous solution injection system including a
boric acid aqueous solution injection tank 11 and a boric acid
aqueous solution injection pipe 30 similar to its counterpart
illustrated FIG. 1 are omitted in the embodiment shown in FIG. 3.
However, the boric acid aqueous injection system may be arranged in
this embodiment as in FIG. 1. This statement also applies to all
the succeeding embodiments.
[0048] This embodiment operates like the first embodiment in the
sense that it can maintain the core in a subcritical state by
holding boric acid aqueous solution in gravity-driven water
injection pool 9. Furthermore, in this embodiment, after the boric
acid aqueous solution in the gravity-driven water injection pool 9
is discharged, the gas phase section of the gravity-driven water
injection pool 9 operates as part of the gas phase section of the
wet well 7. When a severe accident that involves damage to the core
occurs in a boiling water nuclear reactor, the hydrogen that is
generated as a result of the damage to the core is apt to be
accumulated in the wet well 7, so that the rise of the internal
pressure of the containment vessel 5 can be effectively suppressed
by increasing the volume of the gas phase section of the wet well
7. Therefore, this embodiment can effectively suppress the rise of
the internal pressure of the containment vessel when hydrogen is
generated.
Third Embodiment
[0049] FIG. 4 is a schematic elevational cross-sectional view of
the third embodiment of a boiling water nuclear reactor according
to the present invention. In this embodiment, the gravity-driven
water injection pool 9 holding boric acid aqueous solution is
arranged above the ceiling of the reactor containment vessel 5. The
gas phase section above the liquid surface of the gravity-driven
water injection pool 9 is held in communication with an upper part
of the dry well 6 by way of a second communication pipe 13a.
Additionally, the drain pipe 29 connected to the bottom of the
lower header 26 of the static containment vessel cooling system
heat exchanger 12 extends down into the dry well lower region 6b in
this embodiment.
[0050] This embodiment operates like the first embodiment in the
sense that it can maintain the core in a subcritical state by
holding boric acid aqueous solution in gravity-driven water
injection pool 9. Additionally, the design limitations of the
inside of the reactor containment vessel 5 are alleviated so that
the reactor containment vessel 5 can be made compact particularly
in terms of diameter.
Fourth Embodiment
[0051] FIG. 5 is a schematic elevational cross-sectional view of
the fourth embodiment of a boiling water nuclear reactor according
to the present invention. As in the case of the third embodiment
(FIG. 4), the gravity-driven water injection pool 9 holding boric
acid aqueous solution is arranged above the ceiling of the reactor
containment vessel 5 in this embodiment. Then, the gas phase
section above the liquid surface of the gravity-driven water
injection pool 9 and the gas phase section above the pressure
suppression pool 20 in the wet well 7 are held in communication
with each other by means of a third communication pipe 13b.
[0052] This embodiment provides advantages similar to those of the
third embodiment. Additionally, this embodiment also provides
advantages similar to those of the second embodiment because the
gas phase section of the gravity-driven water injection pool 9
operates as part of the gas phase section of the wet well 7 after
the boric acid aqueous solution in the gravity-driven water
injection pool 9 is discharged.
Fifth Embodiment
[0053] Now, the fifth embodiment of a boiling water nuclear reactor
according to the present invention will be described below by
referring to FIGS. 1 and 6. FIG. 6 is a schematic elevational
cross-sectional view of the fifth embodiment of a boiling water
nuclear reactor according to the present invention. This embodiment
is a modification of the first embodiment. This embodiment has the
arrangement shown in FIG. 1 commonly with the first embodiment.
[0054] Like the first embodiment, the sixth embodiment is adapted
to use boric acid aqueous solution as water held in the
gravity-driven water injection pool 9. As the temperature of the
gravity-driven water injection pool 9 decreases, boric acid becomes
deposited. This embodiment utilizes waste heat of the dry well
cooling system to suppress the deposition of boric acid in the
gravity-driven water injection pool 9 and maintain the state of the
boric acid dissolved in water. The dry well cooling system is same
as the one arranged in conventional boiling water nuclear reactors
in order to suppress the internal temperature of the dry well 6
below a predetermined level during normal operation of the nuclear
reactor as shown in Japanese Patent Application Laid-Open
Publication No. 2001-215191 (the entire content of which is
incorporated herein by reference).
[0055] As shown in FIG. 6, an external heat exchanger 40 is
arranged outside the reactor containment vessel. It is typically
part of a sea water system 42 including a sea water pump 41. An
intra dry well heat exchanger 43 is arranged in the dry well 6,
while dry well cooling water pipes 44 and a dry well cooling water
pump 45 are arranged to supply cooling water cooled by the external
heat exchanger 40 to the intra dry well heat exchanger 43. A fan 46
is additionally arranged in order to forcibly circulate gas in the
dry well 6 so that the inside of the dry well 6 is efficiently
cooled by the intra dry well heat exchanger 43.
[0056] An intra injection pool heat exchanger 47 is arranged in the
gravity-driven water injection pool 9 and the water warmed by the
intra dry well heat exchanger 43 is supplied to the intra injection
pool heat exchanger 47. The cooling water that is used to warm the
boric acid aqueous solution in the gravity-driven water injection
pool 9 by means of the intra injection pool heat exchanger 47 and
whose temperature is lowered is fed to the external heat exchanger
40 by way of the dry well cooling water pipes 44 and the dry well
cooling water pump 45. Thus, the cooling water is cooled further.
Isolation valves 50 are fitted to the dry well cooling water pipes
44 at positions where the cooling water pipes 44 penetrate the
reactor containment vessel 5.
[0057] This embodiment provides advantages similar to those of the
first embodiment. Additionally, this embodiment can suppress the
deposition of boric acid in the gravity-driven water injection pool
9 because it warms the boric acid aqueous solution in the
gravity-driven water injection pool 9. Additionally, this
embodiment is economically advantageous since waste heat from the
existing dry well cooling system can be utilized.
Other Embodiments
[0058] While several embodiments are described above, they are only
some of the conceivable embodiments of the present invention and
the present invention is by no means limited thereto.
[0059] For example, the boric acid aqueous solution injection tank
11 and the boric acid aqueous solution injection pipe 30 of each of
the above-described embodiments may be arranged in a manner as
shown in FIG. 1 or omitted as shown in FIGS. 3 through 5.
Additionally, the emergency dry well water-injection piping system
10b can be appropriately omitted.
[0060] The fifth embodiment is a modification of the first
embodiment that is formed by adding the functional feature of
warming the boric acid aqueous solution in the gravity-driven water
injection pool 9, utilizing waste heat from the dry well cooling
system. The same additional functional feature may be added to the
second through fourth embodiments.
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