Combination containment cooling and residual heat removal condenser system for nuclear reactors

Combination primary containment cooling system and residual heat removal steam condensers (PCCS-CND) operable in a containment cooling mode and in a reactor vessel cooling mode are described. In the containment cooling mode, the PCCS-CND interfaces with the primary containment vessel (PCV) through an isolation valve which can be normally closed or open. In the normally closed valve position, and upon receipt of a high drywell pressure signal, the valve opens allowing the steam in the PCV to flow to the PCCS-CND where it condenses, and the decay heat is transferred to the condenser pool. In the normally open valve position, the PCCS-CND is in the standby containment cooling mode of operation. In the reactor pressure vessel (RPV) cooling mode, the PCCS-CND interfaces with the RPV through isolation valves. Upon receipt of a high temperature signal from the suppression pool or through an operator action, the isolation valves are opened allowing the steam in the RPV to flow to the PCCS-CND where it condenses, and the decay heat is transferred to the condenser pool.

FILED OF THE INVENTION
 This invention relates generally to safety systems for boiling water
 nuclear reactors and, more particularly, to a combination primary
 containment cooling and residual heat removal steam condenser system.
 BACKGROUND OF THE INVENTION
 Boiling water nuclear reactors (BWRs) typically utilize active safety
 systems to control and mitigate accident events. Such safety systems
 transport reactor decay heat to the ultimate heat sink, which is normally
 sea or fresh water. Active safety systems, for example, have both
 high-pressure and low-pressure pumping equipment. Such active systems
 require maintenance and surveillance testing of the safety related
 equipment. In addition, the pumps and other equipments typically require
 AC power to operate.
 An alternative to an active safety system is a passive system. Totally
 passive safety systems have been studied for use in BWRs because of their
 merits in reducing maintenance and surveillance testing of the
 safety-related equipment, and in eliminating the need for AC power,
 thereby improving the reliability of BWR operation and safety. Simplified
 BWRs (SBWRs) have been configured to include totally passive safety
 features that provide more resistance to human error in accident control
 and mitigation.
 There are, however, some tradeoffs when employing totally passive safety
 systems in BWRs. Due to their passive nature, the totally passive system,
 when configured in accordance with nuclear standards of system separation
 and diversity, substantially add to plant size and cost. Therefore,
 passive system applications to BWRs have been limited to small- and
 medium-sized plants having up to about 1000 MWe output.
 A combination active and passive system is described in U.S. Pat. No.
 5,426,681, which is assigned to the present assignee and incorporated
 herein, in its entirety, by reference. The system described in the above
 referenced patent provides many advantages, however, such system has
 separate passive containment cooling systems (PCCS) units and separate
 reactor heat removal steam condenser (RHR-CND) systems. Such separate
 systems are located in separate compartments of a condenser pool and add
 to the plant size and cost.
 It would be desirable to provide a safety system for a nuclear reactor
 which is highly reliable and satisfies safety requirements yet has fewer
 safety components. For example, it would be desirable to provide PCCS and
 RHR-CND systems which perform the same functions as known PCCS and RHR-CND
 systems yet require less space and few components than such known systems.
 SUMMARY OF THE INVENTION
 These and other objects are attained by a combination primary containment
 cooling system and residual heat removal condenser (PCCS-CND) operable in
 a containment cooling mode and in a reactor vessel cooling mode are
 described. In the containment cooling mode, the PCCS-CND interfaces with
 the primary containment vessel (PCV) through an isolation valve, and upon
 receipt of a high drywell pressure signal, the valve opens allowing the
 steam in the PCV to flow to the PCCS-CND where it condenses. The decay
 heat is transferred to the condenser pool.
 In the reactor pressure vessel (RPV) cooling mode, the PCCS-CND interfaces
 with the RPV through isolation valves. Upon receipt of a high temperature
 signal from the suppression pool or through an operator action, the
 isolation valves are opened allowing the steam in the RPV to flow to the
 PCCS-CND where it condenses. As in the containment cooling mode, the decay
 heat is transferred to the condenser pool.
 The combination PCCS-CND performs both functions of containment cooling and
 reactor vessel cooling. The number of components required to satisfy the
 BWR safety requirements of decay heat removal therefore is reduced,
 resulting ultimately in reducing plan size and cost. In addition, the
 subject PCCS-CND can be used to provide backup depressurization of the
 RPV, and provide backup heat removal and inventory control for events such
 as station blackout and reactor isolation. Further, the steam condensate
 from the subject PCCS-CND is allowed to discharge to either the
 suppression pool or to a collection tank located in the PCV. The collected
 condensate can be utilized as a source of water inventory for flooding the
 lower drywell during a postulated severe accident, or back to the reactor
 vessel for long term core cooling.

DETAILED DESCRIPTION OF THE DRAWINGS
 The present invention, in one form, is a combination primary containment
 cooling system (PCCS) and residual heat removal (RHR) system which
 provides containment cooling in the case of a high energy line break in
 the containment, and which provides reactor vessel cooling in the case of
 reactor isolation. Although the system is described herein in specific
 reactor constructions, it should be understood that such system may be
 utilized in other reactor constructions.
 FIG. 1 is a schematic depiction of a nuclear reactor system 10 in
 accordance with one embodiment of the present invention. System 10
 includes a reactor pressure vessel (RPV) 12, including a core 14, a
 primary containment vessel (PCV) 16, and a suppression pool 18. A
 feedwater (FW) line 20 supplies water to RPV 12. Steam turbine 22 is
 coupled to, and drives, a high pressure pump 24. Pump 24 is coupled, by
 line 26, to suppression pool 18, and pumps water from suppression pool 18
 to feedwater (FW) line 20. A steam line 28 carries steam away from RPV 12.
 Also shown in FIG. 1 are two combination primary containment cooling system
 and residual heat removal-condenser systems 30A and 30B, sometimes
 referred to herein as PCCS/RHR-CND units, PCCS-RHR units, or simply as
 PCCS-CND 30A and 30B. PCCS-CND 30A is shown as being coupled to turbine 22
 via cooling line, or flowpath, 32 and valve 34. With such a configuration,
 PCCS-CND 30A may be utilized for cooling exhaust of turbine 22 when valve
 34 is opened. Of course, such exhaust cooling is not necessarily required
 in all applications and is shown in FIG. 1 merely to illustrate a
 contemplated additional use of PCCS-CND 30A.
 PCCS-CND 30A and 30B include condensers, or heat exchangers, 36A and 36B
 that condense steam and transfer heat to water in a large condenser pool
 38 which is vented to atmosphere. Each condenser 36A and 36B is submerged
 in a respective compartment of condenser pool 38 located high in the
 reactor building at approximately the same elevation as the fuel pools.
 Condenser pool 38 is above and outside of PCV 16. PCCS-CND 30A and 30B are
 shown positioned on opposite sides of RPV 12. PCCS-CND 30A and 30B could,
 of course, be positioned at many other locations relative to RPV 12.
 Further, additional PCCS-CND units could be used. For example, a third
 PCCS-CND unit could be located adjacent to, but behind, PCCS-CND 30B.
 Each condenser 36A and 36B is coupled to an upper drum 40 and a lower drum
 42. Steam enters PCCS-CND 30A and 30B through lines, or flowpaths, 44A
 (not shown) and 44B coupled to steam line 28A (not shown) and 28B,
 respectively. A steam-gas mixture may also enter PCCS-CND 30A and 30B
 through lines, or flowpaths, 46A and 46B from PCV 16. In the embodiment
 shown in FIG. 1, a steam-water mixture may also enter PCCS-CND 30A through
 line 32 from turbine 22. The steam is condensed in condensers 36A and 36B
 and falls to lower drum 42. From lower drum 42, the steam condensate and
 the noncondensable gases can be drained and vented through common lines
 48A and 48B having outlets which are submerged in suppression pool 18.
 Heat from PCCS-CND 30A and 30B causes condenser pool 38 temperature to rise
 to a point where the condenser pool water will boil. Condenser pool water
 can heat up to about 101.degree. C. (214.degree. F.). The steam which is
 formed, being nonradioactive and having a slight positive pressure
 relative to station ambient pressure, is vented from the steam space above
 each PCCS-CND 30A and 30B to outside the reactor building via discharge
 vents 50. A moisture separator may be installed at the entrance to
 discharge vents 50 to preclude excessive moisture carryover and loss of
 condenser pool water.
 Condenser pool make-up clean water supply for replenishing the level is
 provided from a so-called "make-up demineralized system" (not shown).
 Level control is accomplished by using an air-operated valve in the
 make-up water supply line. The valve opening/closing is controlled by a
 water level signal sent by a level transmitter sensing water level in the
 condenser pool.
 Cooling/clean-up of the condenser pool water is performed by a fuel and
 auxiliary pools cooling system (not shown). Several suction lines, at
 different locations, draw water from the sides of the condenser pool at an
 elevation above the minimum water level that is required to be maintained
 during normal plant operation. The water is cooled/cleaned and then
 returned to the condenser pool. On the return line for condenser pool
 water recirculation flow, there is also a postLOCA pool water make-up
 connection.
 PCCS-CND 30B is shown in detail in FIG. 2, and the following discussion
 regarding PCCS-CND 30B applies also to PCCS-CND 30A. More particularly,
 and referring to FIG. 2, PCCS-CND 30B is operable in a containment cooling
 mode or in a reactor vessel cooling mode. In the containment cooling mode,
 PCCS-CND 30B interfaces with the primary containment vessel (PCV) through
 isolation valve 52. Upon receipt of a high drywell pressure signal, valve
 52 opens automatically allowing the steam in the PCV to flow to PCCS-CND
 30B where it condenses, and the decay heat is transferred to condenser
 pool 38. Alternatively, valve 52 can be configured to be normally open,
 and unit PCCS-CND 30B will be in the standby containment cooling mode.
 In the reactor vessel cooling mode, PCCS-CND 30B interfaces with the RPV
 through isolation valves 54 and 56. Upon receipt of a high temperature
 signal from suppression pool 18 or through an operator action, isolation
 valves 54 and 56 are opened allowing the steam in the RPV to flow to
 PCCS-CND 30B where it condenses, and the decay heat is transferred to
 condenser pool 38. Pressure control valve 58, or as an alternative, an
 appropriately designed flow orifice valve, is located in the piping
 upstream of the isolation valve 54 for the purpose of controlling the
 steam flow to PCCS-CND 30B. A bypass valve 60, located in parallel with
 pressure control valve 58, is allowed to open at a low steam flow to
 maintain continual decay heat removal capability of PCCS-CND 30B at low
 pressures.
 The steam condensate from PCCS-CND 30B is allowed to discharge to either
 suppression pool 18 or as shown in FIG. 3, to a collection tank 62, via
 line, or flowpath, 64, located within containment 16. Collection tank 62
 is a gravity-driven cooling pool of water located at an elevation above
 nuclear fuel core 14. The collected condensate in tank 62 from PCCS-CND
 30B can be utilized as a source of water inventory for flooding the lower
 drywell during a postulated severe accident, via line 66, or fed back to
 reactor vessel 12 for long term core cooling via a line (not shown)
 extending from tank 62 to RPV 12. With respect to the FIG. 3
 configuration, PCCS-CND 30B includes a first output line 68 and a second
 output line 70. First output line 68 is coupled to suppression pool 18,
 via output line, or flowpath, 48B, and transmits gases thereto. Water is
 transmitted via output line, or flowpath, 70 to pool 62.
 An alternative configuration having three PCCS-CND 30A, 30B and 30C is
 shown in FIG. 4. PCCS-CND 30C is configured substantially identically as
 PCCS-CND 30B. Specifically, and referring to FIG. 4, each PCCS-CND 30A,
 30B and 30C is located on one side of RPV 14 in a respective
 subcompartment of condenser pool 38. All pool subcompartments communicate
 to enable full utilization of the collective water inventory, independent
 of the operational status of any given subloop. A valve is provided at the
 bottom of each condenser pool subcompartment that can be closed so the
 respective subcompartment can be emptied of water to allow condenser
 maintenance.
 The combination PCCS-CND described herein performs both functions of
 containment cooling and reactor vessel cooling. The number of components
 required to satisfy the BWR safety requirements of decay heat removal
 therefore is reduced, thereby reducing the number of required safety
 components and resulting ultimately in reducing plan size and cost. In
 addition, the subject PCCS-CND, in conjunction with an active reactor
 inventory supply system such as the reactor core isolation cooling system
 "RCIC", a high-pressure core flooder system "HPCF", or an AC-independent
 water addition system "ACIWA", can be used to provide backup
 depressurization of the RPV, and provide backup heat removal and inventory
 control for events such as station blackout and reactor isolation. This is
 a low to medium pressure system which is unique in operation relative to
 past isolation condenser (high pressure) and passive containment cooling
 (low pressure system).
 Further, the steam condensate from the subject PCCS-CND is allowed to
 discharge to either the suppression pool or to a collection tank located
 in the PCV. The collected condensate can be utilized as a source of water
 inventory for flooding the lower drywell during a postulated severe
 accident, or back to the reactor vessel for long term core cooling.
 Moreover, the subject PCCS-CND can be utilized as a separate system for
 decay heat removal in a completely passive BWR, or in conjunction with
 active RHR systems in a combined active/passive BWR design.
 From the preceding description of the present invention, it is evident that
 the objects of the invention are attained. Although the invention has been
 described and illustrated in detail, it is to be clearly understood that
 the same is intended by way of illustration and example only and is not be
 taken by way of limitation. Accordingly, the spirit and scope of the
 invention are to be limited only by the terms of the appended claims.