Note: Descriptions are shown in the official language in which they were submitted.
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NUCLEAR POWER PLANT
TECHNICAL FIELD OF INVENTION
The present invention relates to nuclear power plants. More specifically, the
present invention relates to pressurized water reactor nuclear power plants
using
a steam cycle.
BACKGROUND OF THE INVENTION AND PRIOR ART
Pressurized water reactor (PWR) nuclear power plants are well known and
employ a steam generation cycle. PWR nuclear power plant comprises a PWR,
a group of boilers, a steam turbine, a moisture separator, a steam reheater,
and
a feedwater heating unit. A reactor coolant is a light water or heavy water
(CANDU reactors). A primary circuit consists of reactor, primary side of
boilers,
and coolant pumps. The reactor coolant is pumped between fuel elements of the
reactor absorbing heat and through the tube side of boilers releasing heat. A
secondary circuit (with light water as a working fluid) includes a secondary
side of
boilers, a steam turbine-generator, a moisture separator, a steam reheater, a
condenser, a group of feedwater heaters, a low-pressure feedwater pump, and a
high-pressure feedwater pump. Saturated steam produced in the boilers is
delivered to the steam turbine-generator. Steam is condensed in the condenser
and returned to the boilers through the group of feed water heaters.
The nuclear power plant has two main consumers of feed water, the boilers and
the boiler blowdown system. The boiler blowdown system is well known in power
generation industry. Usually it comprises a group of throttling valves
connected
to the boilers and to a flash tank having a steam outlet and a drain. In some
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cases, the throttling valves discharge the feed water from the boilers
directly to
the drain.
Nuclear power plants are described in the following patents granted in Canada:
1,223,488 Schluderberg
2,190,855 Tsiklauri
The patent No. 1,223,488 discloses a nuclear power plant having a reactor, a
steam generator, and a steam turbine. The steam generator consists of three
stages to improve a thermal efficiency of the steam cycle.
The patent No. 2,190,855 discloses a nuclear power plant having a reactor, a
steam generator, and a steam turbine. External source of steam is used to
improve the steam quality entering the low-pressure (LP) turbine. This
approach
allows to reduce the steam moisture content in LP turbine, reduce turbine
blade
erosion and improve efficiency of the steam cycle.
The prior art does not address an issue of power uprate of existing PWR
nuclear
power plants. Many of PWR nuclear power plants are presently being uprated by
increasing a reactor thermal output above previous licensing limits. Minor
increase can be accommodated by existing balance-of-plant equipment. Larger
increases (extended power uprates) usually involve a replacement of boilers,
turbines, feedwater heaters, piping, or any combination thereof. A cost of
extended power uprate can exceed 1000 $/kW. In addition, the prior art does
not
address an issue of aging of steam generators when a reactor thermal power is
limited due to deteriorated condition of boilers thus reducing a turbine-
generator
output. Therefore it would be desirable to develop additional options for less
expensive power uprates andlor for refurbishment of nuclear power plant with
limited generation due to aging boilers.
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SUMMARY OF THE INVENTION
The present invention provides a PWR nuclear power plant having a primary
circuit and a secondary circuit. Primary circuit comprises a pressurized water
reactor, a primary side of a group of main boilers, and a reactor coolant
pump.
Secondary circuit comprises a secondary side of the group of main boilers
producing a main pressure steam, a steam turbine, an electrical generator, a
moisture separator, a steam reheater, a condenser, and a feed water supply
unit.
The feed water supply unit comprises lower and higher pressure feed water
heaters, deaerators, lower pressure feed water pump (condensate pump), and
higher pressure feed water pump. The lower pressure feed water heaters use an
extraction steam of the steam turbine (not shown on the drawing). A lower
pressure steam generator is connected to primary circuit downstream of primary
side of main boilers. Steam produced by lower pressure steam generator is used
by steam consumers of secondary circuit. There are many potential consumers
of lower pressure steam in steam cycle power plant. These consumers include
feed water heaters, steam reheater of the moisture separator and reheater unit
(MSR), extraction steam outlets of the steam turbine used to supply additional
steam flow to the turbine, a steam turbine driven feed water pump, a steam
turbine driven condenser cooling water pump, and a steam turbine driven
reactor
coolant pump. Using the lower pressure steam, these consumers increase the
amount of main pressure steam available for main steam turbine or substitute
the
electrical power consumption of the pumps usually driven by electric motors.
This results in the net electrical generator output increase.
The heat used by lower pressure steam generator reduces the temperature of
reactor coolant at the reactor inlet and does not effect the operation of main
steam boilers. The lower pressure steam generator can be represented by lower
pressure steam boiler (evaporator) or by heat exchanger combined with steam
separator and circulating pump. Reactor power and electrical generator output
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can be increased in accordance with consumption of the steam generated by
lower pressure steam generator.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of nuclear power plant according to one
embodiment of the present invention and corresponding to the claims 1, 2, 3,
4,
11, and 13.
FIG. 2 is a schematic representation of nuclear power plant according to
another
embodiment of the present invention and corresponding to the claims 1, 2, 3,
4,
11, and 14.
FIG. 3 is a schematic representation of nuclear power plant according to the
embodiment of the present invention corresponding to the claims 1, 2, 3, 4,
11,
and 15.
FIG. 4 is a schematic representation of nuclear power plant according to the
embodiment of the present invention corresponding to the claims 1, 2, 3, 4,
11,
and 16.
FIG. 5 is a schematic representation of nuclear power plant according to the
embodiment of the present invention corresponding to the claims 1, 2, 3, 4,
12,
and 17.
FIG. 6 is a schematic representation of nuclear power plant according to the
embodiment of the present invention corresponding to the claims 1, 2, 3, 4,
12,
and 17.
FIG. 7 is a schematic representation of nuclear power plant according to the
embodiment of the present invention corresponding to the claims 1, 5, 6, 7, 8,
9,
10, 11, and 13.
FIG. 8 is a schematic representation of nuclear power plant according to the
embodiment of the present invention corresponding to the claims 1, 5, 6, 7, 8,
9,
10, 11, and 14.
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FIG. 9 is a schematic representation of nuclear power plant according to the
embodiment of the present invention corresponding to the claims 1, 5, 6, 7, 8,
9,
10, 11, and 15.
FIG. 10 is a schematic representation of nuclear power plant according to the
embodiment of the present invention corresponding to the claims 1, 5, 6, 7, 8,
9,
10, 11, and 16.
FIG. 11 is a schematic representation of nuclear power plant according to the
embodiment of the present invention corresponding to the claims 1, 5, 6, 7, 8,
9,
10, 12, and 17.
FIG. 12 is a schematic representation of nuclear power plant according to the
embodiment of the present invention corresponding to the claims 1, 5, 6, 7, 8,
9,
10, 12, and 17.
Parts that are not essential to the invention and well known in power
generation
industry, such as feed water heater drains, various valves, control and
instrumentation equipment, etc., are not shown.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1 of the drawings, a nuclear power plant consists of primary
circuit and secondary circuit. Primary circuit comprises a pressurized water
reactor 1, a primary side of a group of main boilers 2, and a reactor coolant
pump
3. Secondary circuit comprises a secondary side of the group of main boilers 2
producing a main pressure steam, a turbine-generator consisting of a higher-
pressure (HP) steam turbine 4 and a lower pressure (LP) steam turbine 5 being
shaft coupled with an electrical generator 6, a moisture separator 7, a two-
stage
steam reheater 8, a condenser, and a feed water supply unit. The feed water
supply unit comprises lower pressure feed water heaters 12, a higher pressure
feed water heater 15, a deaerator 13, lower pressure feed water pump
(condensate pump) 11, and a higher pressure feed water pump 14. The lower
pressure feed water heaters 12 use an extraction steam of the steam turbine
(not
shown on the drawing). A lower pressure steam generator unit (LPSG),
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represented by first heat exchanger 16, steam separator 18 and drain pump 19,
is connected to primary circuit downstream of primary side of main boilers 2.
Due to lower temperature of reactor coolant entering the lower pressure steam
generator, the steam pressure produced by LPSG (in the range of 2.5 to 4.5
MPa) is lower than the steam pressure produced by main boilers (in the range
of
4.5 to 6.5 MPa). Steam produced by lower pressure steam generator is used by
lower pressure steam consumers of secondary circuit. These consumers include
feed water heaters, steam reheater 8 of the moisture separator and reheater
unit
(MSR), and one of extraction steam outlets of the HP steam turbine 4. Using
the
lower pressure steam from LPSG, these consumers increase the amount of main
pressure steam available for expansion in the HP and LP steam turbine. The
extraction steam outlet of the steam turbine can be used as an additional
steam
consumer supplying additional steam to the steam turbine by bypassing higher-
pressure steam expansion stages. This results in the net electrical generator
output increase. The heat used by lower pressure steam generator reduces the
temperature of reactor coolant at the reactor inlet and does not efFect the
operation of main boilers 2. The lower pressure steam generator LPSG is
represented by heat exchanger 16 combined with steam separator 18 and drain
pump 19.
The main boilers 2 produce the main pressure steam directed to the HP steam
turbine 4 and higher temperature steam reheater of the MSR 8. After expansion
in HP steam turbine 4, the moisture (in the range of 12 to 15%) from the main
steam flow is removed in moisture separator 7. The steam is reheated in two-
stage steam reheater 8. The lower pressure steam reheater uses the steam
produced by LPSG, the higher-pressure steam reheater uses the main pressure
steam generated by main boilers. The drains of moisture separator 7 and steam
reheater 8 are returned to the steam cycle (not shown on drawing). After MSR,
the main steam continues its expansion in LP steam turbine and condenses in
the condenser 9. Condenser cooling water pump 10 supplies the condenser 9
with sufficient amount of water to remove a latent heat of condensation of the
main steam flow. Lower pressure feed water pump 11 (condensate pump)
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returns the condensate to the steam cycle through lower pressure feed water
heaters 12, deaerator 13, higher pressure feed water pump 14, and higher-
pressure feed water heater 15. The feed water heaters use the turbine
extraction
steam to heat the feed water before it is distributed among the main boilers
and
LPSG.
The LPSG consists of a first heat exchanger 16, a steam separator 18, and a
drain pump 19. The following parameters of the steam cycle, temperatures and
pressures of feed water and reactor coolant are examples and do not represent
actual design values. However, they are close to actual parameters of PWR
nuclear power plant.
After the feed water heater 15, a feed water at 160°C enters the LPSG
through
LPSG feedwater inlet being the first heat exchanger feedwater inlet. Reactor
coolant at 270°C enters the LPSG through LPSG reactor coolant inlet
being the
first heat exchanger reactor coolant inlet. The feedwater, being heated in the
first
heat exchanger 16, reaches the temperature close to the saturation temperature
and, in some modes of operation, partly evaporates. The feed water leaves the
first heat exchanger through first heat exchanger feed water outlet at
220°C to
240°C temperature range, from where it is in part directed to a steam
separator
feed water inlet. The pressure in the steam separator 18 is close or below
saturation pressure of the feed water after first heat exchanger, for example,
being in the range of 2.5 to 3.5 MPa; therefore, the additional amount of the
feed
water can evaporate if the feed water temperature exceeds the saturation
temperature. The steam separated from the water, leaves the steam separator at
saturation temperature through a steam separator steam outlet being a LPSG
steam outlet. The feed water from steam separator 18 returns to the first heat
exchanger 16 feedwater inlet after being mixed with the feed water coming from
the feed water heater 15. The part of the feed water from the first heat
exchanger feed water outlet being a LPSG feed water outlet, is directed to the
boiler 2 feed water inlet. This part of the feed water performs the function
of the
blowdown and is required to reduce the concentration of impurities in this
loop
(first heat exchanger 16, steam separator 18, and drain pump 19). To balance
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the feed water flows and pressures, well known in power generation industry
means can be used, for example, orifices, flow and pressure control valves,
etc.
If the lower pressure steam production in the LPSG is in the range of 10% of
the
main pressure steam generation, and assuming the reactor coolant flow rate
through the first heat exchanger represents 25% of total reactor coolant flow,
the
reactor coolant temperature will be in the range of 255 to 260°C at the
first heat
exchanger reactor coolant outlet being a LPSG reactor coolant outlet.
The lower pressure steam is used by higher pressure feed water heater 15
(through pressure control valve) and by first stage of steam reheater 8 of
MSR.
In typical steam cycle, total steam consumption would represent about 7% of
main steam flow rate. By substitution of the turbine extraction steam by lower
pressure steam from LPSG, the amount of main pressure steam available for
expansion in the main steam turbine would increase. The turbine-generator
output would increase by approximately 6% with reactor thermal output increase
of approximately 7.5%.
The extraction steam outlet of the HP steam turbine 4 can be used as an
additional lower pressure steam consumer supplying additional steam to the
steam turbine by bypassing the turbine governor and higher-pressure steam
expansion stages. This results in additional net electrical generator output
increase.
Referring to FIG. 7 of the drawings, other lower pressure steam consumers are
shown. The reactor coolant pump 3, the higher-pressure feed water pump 14,
and the condenser cooling water pump 10 are shown being driven by a steam
turbine drive using lower pressure steam from LPSG. These steam turbine
drives are preferably of backpressure type, thus reducing the required size of
steam piping and the turbines. Steam turbine drive steam outlets can be
connected to one of the lower pressure feed water heater 12 steam inlets or to
the deaerator 13 steam inlet (not shown on the drawing). These pumps usually
have an electric drive. Therefore, electrical consumption by these pump drives
is
substituted by mechanical energy generated by steam turbine drives increasing
net electrical output of nuclear power plant. Typically, the reactor coolant
pump 3
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(or group of pumps) consumes about 4% of turbine-generator output, the higher-
pressure feed water pump 14 (or group of pumps) consumes about 1 % of
turbine-generator output, and the condenser cooling water pump (or group of
pumps) consumes about 0.6% of turbine-generator output. As a result, the net
nuclear power plant electrical output can be increased by 5.6% by increasing
the
reactor thermal output by approximately 6.5% without any changes to existing
secondary circuit equipment (turbine-generator, feed water heaters, main
boilers,
etc.).
Nuclear power plant can utilize any combination of lower pressure steam
consumers shown on FIG.1 and FIG.7 with appropriate electrical generation
increase.
Referring to FIG. 2 of the drawings, the nuclear power plant differs from the
one
shown on FIG. 1 by having a second heat exchanger 17 being connected in
parallel with the first heat exchanger 16 by reactor coolant and by feed water
flow. The first heat exchanger 16 is used to preheat the feed water before it
enters the main boiler 2. The second heat exchanger 17 is used to heat the
feed
water before it enters the steam separator 18 of LPSG. The nuclear power plant
of FIG.8 differs from the one shown on FIG 2 by shown lower pressure steam
consumers. The reactor coolant pump 3, the higher-pressure feed water pump
14, and the condenser cooling water pump 10 are shown being driven by a
steam turbine drive using the lower pressure steam from LPSG.
Referring to FIG. 3 of the drawings, the nuclear power plant differs from the
one
shown on FIG. 1 by the steam separator of LPSG being replaced by lower
pressure steam boiler 20. The part of feed water flow after the first heat
exchanger 16 of LPSG moves through the tubes of the lower pressure steam
boiler 20, generates a lower pressure steam on secondary side of lower
pressure
steam boiler 20, and returns by the drain pump 19 to the first heat exchanger
16
feed water inlet. The remaining part of the feed water after first heat
exchanger
16 of LPSG is directed to main boiler 2 thus increasing the efficiency of main
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cycle by supplying the main boiler with the feed water of higher temperature
and
increasing the main steam pressure and steam generation flow rate. The nuclear
power plant of FIG.9 differs from the one shown on FIG 3 by shown lower
pressure steam consumers. The reactor coolant pump 3, the higher-pressure
feed water pump 14, and the condenser cooling water pump 10 are shown being
driven by a steam turbine drive using the lower pressure steam from LPSG.
Referring to FIG. 4 of the drawings, the nuclear power plant differs from the
one
shown on FIG. 3 by having a second heat exchanger 17 being connected in
parallel with the first heat exchanger 16 by reactor coolant and by feed water
flow. The first heat exchanger 16 is used to preheat the feed water before it
enters the main boiler 2 consequently increasing the efficiency of main steam
cycle by supplying the main boiler with the feed water of higher temperature
and
increasing the main steam pressure and steam generation flow rate. The second
heat exchanger 17 is used to heat the feed water before it enters the lower
pressure steam boiler 20 of LPSG. The nuclear power plant of FIG.10 differs
from the one shown on FIG 4 by shown lower pressure steam consumers. The
reactor coolant pump 3, the higher-pressure feed water pump 14, and the
condenser cooling water pump 10 are shown being driven by a steam turbine
drive using the lower pressure steam from LPSG.
Referring to FIG. 5 of the drawings, the nuclear power plant differs from the
one
shown on FIG. 3 by the lower pressure steam boiler 20 being connected
downstream of main boiler 2 primary outlet and heated directly by reactor
coolant. In this case, the steam separator and the drain pump are not
required.
The nuclear power plant of FIG.11 differs from the one shown on FIG 5 by shown
lower pressure steam consumers. The reactor coolant pump 3, the higher-
pressure feed water pump 14, and the condenser cooling water pump 10 are
shown being driven by a steam turbine drive using the lower pressure steam
from LPSG.
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Referring to FIG. 6 of the drawings, the nuclear power plant differs from the
one
shown on FIG. 5 by the lower pressure steam boiler 20 being connected to
reactor coolant header downstream of main boiler primary outlets and heated
directly by reactor coolant. The lower pressure steam boiler uses a part of
total
reactor coolant flow; the other part of the flow goes through the bypass
(orifice
21 ). The nuclear power plant of FIG.12 differs from the one shown on FIG 6 by
shown lower pressure steam consumers. The reactor coolant pump 3, the
higher-pressure feed water pump 14, and the condenser cooling water pump 10
are shown being driven by a steam turbine drive using the lower pressure steam
from LPSG.
The nuclear power plant has two main consumers of feed water, the boilers and
the boiler blowdown system. The boiler blowdown system is well known in power
generation industry. Usually it comprises a group of throttling valves
connected
to the boilers and to a flash tank having a steam outlet and a drain. In some
cases, the throttling valves discharge the feed water from the boilers
directly to
the drain. It is required to remove impurities from the feed water in boilers.
Main boilers 2 have blowdown outlets connected to boiler blowdown system (not
shown on drawings). The lower pressure steam boiler 20 of LPSG has a
blowdown outlet connected to boiler blowdown system (not shown on drawings).
This is required to reduce the concentration of impurities in the lower
pressure
steam boiler 20.
The same numbers on the drawings were assigned to similar pieces of
equipment, for example, to the main boilers 2 and to the lower pressure feed
water heaters 12. The steam reheater 8 of moisture separator and reheater unit
MSR is shown having two reheat stages (no numbers assigned on the drawings).
However, the nuclear power plant implementing the present invention can have
only one reheat stage. It will result in slightly lower thermal efficiency of
the
steam cycle.
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Terminology used in this application has the following meanings. The drive of
a
pump is a source of mechanical energy (electrical motor, diesel, steam engine,
gas turbine, steam turbine) used by the pump. The drive can be directly shaft
coupled with the pump, or through a transmission, gearbox, etc.
Flow communication means that the same fluid moves from first piece of
equipment (component, element) to second piece of equipment, or the same fluid
moves to the first piece of equipment (component, element) and to the second
piece of equipment. It does not imply a direct connection by pipes without
additional equipment being installed between those two pieces of equipment.
Other pieces of equipment (elements, components) can be installed between the
first and the second piece of equipment (for example, isolating valves,
pressure
control valves, pumps, etc.).
Boiler is a heat exchanger having a primary and a secondary side. A heating
fluid (reactor coolant) moves through the primary side of the boiler (usually
through the tube side) and transfers the heat to the secondary side (usually
the
shell side). The secondary side liquid (feed water) boils producing a steam.
Part
of feed water is being removed from the boiler as a blowdown to reduce a
concentration of impurities in the boiler. Reactor coolant is a light or heavy
water
used to remove the heat from the reactor fuel and transfer it to the boiler.
After LP steam turbine, the steam condenses in the condenser transforming into
the water, which is often called a condensate or, downstream of lower pressure
feed water pump (condensate pump), a tower pressure feed water. The same
condensate is traditionally called a feed water (higher pressure feed water)
downstream of higher pressure feed water pump.
Several preferred embodiments of the present invention have been shown and
described. However, it is apparent to those skilled in the art that many
changes
and modifications may be made without departing from the invention as it is
defined in the appended claims.
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