Patent Number: 055966136
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the preferred embodiment of the present invention, the pressure inside the containment is reduced by venting the wetwell plenum to an appropriate connection point in the existing offgas system. As shown in FIG. 2, a conventional offgas system comprises two 100% recombiner trains. Generally, the two trains are not operated simultaneously, but rather one train is held in reserve as a backup system. Each train comprises a steam jet air ejector 80 which is connected to the main condenser chamber. The main condenser of a turbine has tubes through which cooling water flows. The tubes provide condensing action for expired steam whereby purified water condensate is recovered for recycling. The main condenser receives a continuous inflow of steam and runs at sub-atmospheric conditions. The main condenser almost produces a high degree of vacuum due to the pressure drop precipitated by condensation. The steam entering the main condenser may have trace quantities of oxygen, hydrogen, argon, nitrogen, krypton, xenon, etc. These trace gases, unless removed, accumulate over time and tend to destroy the vacuum condition as noncondensable gases collect inside the main condenser. The steam jet air ejector takes steam from a turbine extraction stage upstream of the main condenser, or from an auxiliary steam boiler or from a re-boiler that has produced steam of apt pressure, and releases this motive steam through a jet pump nozzle, thereby sucking off noncondensable gas and some steam from suitable collection regions provided in the main condenser. In the event that the nuclear fuel core of the reactor includes leaking fuel rods, radioactive fission products gases, such as krypton and xenon, will leak into the mixture of water and steam flowing out of the core and into the steam separators. Because of the potential for radioactive gases, such as krypton and xenon, to be present in the steam sent to the main condenser and in the offgas diverted therefrom, the offgas must be treated. Therefore, the mixture of noncondensable gas and steam plus added oxygen, if operating under hydrogen water chemistry conditions, is injected into a preheater 82, which heats the mixture to a higher temperature. The preheated mixture is then passed through a recombiner 84 loaded with catalytic material (e.g., a noble metal) which catalyzes the reformation of water molecules from hydrogen and oxygen molecules. This is done to avoid explosive levels of hydrogen and oxygen. The mixture then flows through condensers 86 and 88, which utilize cold water to condense the steam, leaving the noncondensable gas in a gaseous state. The condensed steam, now condensate, drains by gravity to the main condenser hotwell via a loop seal. The noncondensable gas then flows along an outlet line 90, which is monitored by a hydrogen and radiation monitor 92. Depending on the radioactivity of the noncondensable gas as determined by monitor 92, the noncondensable gas is directed either through the bypass valve 100a or the guard bed 93, which is a large bed (typically 8 ft in diameter and 30 ft tall) filled with activated charcoal. If the level of radioactivity of the noncondensable gas is above a predetermined threshold, the bypass valve 100a is closed and inlet valve 100b is opened to allow the noncondensable gas to flow into guard bed 93. As noncondensable gas flows through the guard bed, the fission atomic species, e.g., krypton and xenon, are adsorbed by the charcoal and held up thereby. The krypton takes more than 2 days to pass through the guard bed, whereas the xenon takes more than 4 weeks. While the radioactive fission products are held up, they decay into less harmful products which can then be safely vented to the atmosphere. If the level of radioactivity is below the predetermined threshold, the by-pass valve is opened, the inlet valve 100b is closed and the bypass valve 100c is opened, which allows the noncondensable gas to be vented directly to the atmosphere via the plant vent 98. Depending on the level of radioactivity, the discharge from the guard bed 93 can be directed either to the plant vent 98 via open bypass valve 100c (inlet valve 100d closed) or to the bank of delay beds 94, only two (94a and 94b) of which are depicted in FIG. 2. Each delay bed is a large bed (typically 6 ft in diameter and 20 ft tall), again filled with activated charcoal. Before reaching the plant vent 98, the noncondensable gas is monitored by a radiation monitor 96. If the level of radioactivity of the noncondensable gas is below the predetermined threshold while bypass valve 100c is open, then bypass valve 100c remains open. However, if the level of radioactivity of the noncondensable gas exiting the guard bed 93 is above the predetermined threshold, bypass valve 100c is closed, inlet valve 100d is opened and the flow of noncondensable gas is diverted to the bank 94 of delay beds for further treatment. The delay beds are connected in series, with each successive delay bed holding up the radioactive fission products and thereby providing additional time for radioactive decay. The total holdup times can be extended to any desired duration by passing the radioactive gas through a corresponding number of delay beds. The noncondensable gas is ultimately discharged from the last delay bed and carried to the plant vent for discharge into the atmosphere. Bypass valve 100e, shown closed in FIG. 2 (as are bypass valves 100a and 100c), is only opened during startup to prevent any latent steam in the gas flow from contaminating the charcoal beds. The preferred embodiment of the invention is a piping subsystem, shown in FIG. 3, which releases noncondensable gas from the wetwell plenum 26 and pipes into the delay beds 94 of the offgas system The piping system comprises a main process pipe that communicates the wetwell plenum 26 to a connection point downstream of the guard tank 93 in the offgas system and upstream of the main bank 94 of delay beds. Since the object is to provide extensive holdup to radioactive species in the noncondensable gas, it will be obvious that the connection point could, in the alternative, be upstream of the guard bed or between any two connected delay beds in the main bank 94. The main process pipe 101 is fitted with both an inboard containment isolation valve 102 (preferably featuring a pneumatic-type valve operator) and also an outboard containment isolation valve 104 (preferably featuring a 125-volt DC motor type valve operator). Also incorporated in the main process pipe in accordance with the preferred embodiment is a low-differential-pressure (typically, 10 psid) rupture disk 108 which prevents any gas outflow in this piping whatsoever unless rupture occurs by virtue of pressure inside this main process pipe on the wetwell plenum 26 side of disk 108 exceeding the design opening (rupture) pressure differential. Also, one or more restricting orifices 106 of apt size are provided at apt locations along the main process pipe 101 to limit the release rate of noncondensable gas from the wetwell plenum 26 as may be found desirable, depending on various considerations of the system design. This pressure suppression subsystem also includes apt stored-energy type auxiliary power services (not shown) that will both passively allow closure of the isolation valves 102 and 104 if so signaled--e.g., via provision of a precharged pneumatic accumulator to "passively" power-close the inboard containment isolation valve 102, and connections to safety-grade DC battery power to "passively" power-close the outboard containment isolation valve 104 when signaled--as well as industry common auxiliary services to provide elective opening/closing of these valves for test purposes, or to allow isolation of the rupture disk 108 from excess wetwell pressure which might develop, for example, during containment leaktight integrity pressure testing. The subsystem is further provided with apt controls and sensors, and the like, to meet the appropriate needs for proper responses during LOCAs, as well as routine plant operations including testing, maintenance, etc. A part of this complement of instrumentation includes one or more process radiation monitors 110, located at one or more apt points in the subsystem main process piping 101, which upon detection of radiation levels in excess of predetermined limits, will issue a signal which actuates the containment isolation valves 102 and 104, thereby preventing the release of such radioactivity to the environs. The holdup action in the charcoal beds 94 is known to be a strong, and favorable, function of at least four parameters: (1) the total mass of the charcoal, (2) the atomic weight of the gas, (3) whether the gas is monatomic or diatomic, and (4) the ambient gas pressure which the charcoal is experiencing. For example, gases of light atomic weight and low boiling point (nitrogen, oxygen) pass through the charcoal rapidly. But the radioactive gases krypton and xenon, which have high atomic weight and are monatomic, are retained in the activated charcoal for vastly longer holdup times. This is a key (though not essential) benefit derived from choosing to discharge the gases released from the wetwell plenum through the relatively enormous mass of the charcoal beds. The harmless gases are readily exhausted, while any potentially hazardous gases are retained for very long periods in the charcoal--a condition which is highly desirable. Water vapor present in the released wetwell gas stream does have a tendency to "wet"--and render less effective--activated charcoal. For this reason, it is common practice in nuclear power station offgas systems based on charcoal for holdup action, to incorporate some form of water vapor removal of the process gas prior to its entry into the charcoal beds. The impact of this phenomenon on the expected resultant reduction in holdup times that would be experienced by the proposed configuration and operating mode have been studied, and it was found that the reduction in holdup times for krypton and xenon will be less than 10%--a wholly inconsequential amount--over the time period during which the subsystem of the present invention would be allowing release of significant quantities of the noncondensable gases (again, principally nitrogen, with a trace of oxygen) which are such important contributors to the post-LOCA containment pressure. Note should also be made that the design pressure of the steel vessels comprising the charcoal tanks of the offgas system is typically 350 psig. This is sufficient to withstand any prospective elevated pressure loading caused during the LOCA and manifested down the main process pipe to the offgas system, for the peak wetwell pressure during a design basis accident in these containments is well below 55 psig. The subsystem could be configured with multiple pipes leading from the wetwell airspace to the offgas system, to secure some benefits of redundancy and/or increased process flow capacity (although at perhaps higher capital cost). The inboard containment isolation valve could be moved to a location on the main process piping outboard of the wetwell to improve accessibility for maintenance. An alternative discharge for the effluent passing down the main process pipe of the subsystem could be directly to the environs instead of to the offgas system charcoal beds, if the speed of closure of isolation valves were made rapid enough to isolate flow to limit radioactivity actually released to acceptably small quantities. Another alternative connection point for the downstream end of the main process pipe could be to the main condenser, since this component already provides means for moving noncondensables from the condenser into the offgas system and would be at an absolute pressure even below that in the offgas system. Various alternatives for location, types, control responses, etc., of the subsystem instrumentation and controls can be used, as would be evident to one practiced in the (system engineering) art. Water vapor removal, available in many alternative methods, could be applied to the process gas stream prior to its entry to the charcoal tanks. The action upon detection of high or potentially high radioactivity releases to the environs could be to close a discharge valve just ahead of where the offgas process stream is released to the plant stack, as an alternative to closing the containment isolation valves. The prospective benefit in this alternative is that the charcoal can continue to uptake noncondensables from the wetwell due to its not-yet-fully-saturated state. Suitable isolation, and timing thereof following a LOCA, for portions of the offgas system which normally lie upstream of the charcoal guard bed could be provided via controls and valves, which may be new or may already be present in any given offgas system, depending on design optimization and simple licensability considerations. This could prevent, for example, possibly unwanted migration of the wetwell gas into equipment in the turbine building of these plants. The foregoing preferred embodiments have been disclosed for the purpose of illustration. Other variations and modifications will be apparent to persons skilled in the design of passive pressure suppression systems. All such variations and modifications are intended to be encompassed by the claims hereinafter.