Patent Number: 043476236
Section: summary

BACKGROUND OF THE INVENTION Pressurized-water and boiling-water nuclear reactors utilize a vessel to contain the fuel rods with their associated fissionable material. Coolant water is circulated through the vessel and is heated (PWR) or partially vaporized (BWR) by heat transfer from the fuel rods. The heated coolant of a PWR is utilized to vaporize a secondary through heat exchangers. In the power generation application of nuclear reactors, the vapor powers a steam turbine rotating a generator. In the event that a malfunction of the system takes place so that the normal coolant circulation is interrupted, the reactor is shutdown by inserting absorber rods into the core, thereby interrupting the nuclear reaction. However, even after shutdown of the reactor, the fission products in the fuel rods continue to produce heat generally referred to as "decay heat". Without the normal coolant flow, this decay heat could melt the fuel rod cladding, the fuel, and the vessel itself, releasing radioactive fission products into the secondary containment building and thereby increasing the risk that radioactive material would be introduced into the atmosphere with the associated potential danger to the public. The potential for release of radioactive material into the environment has led to the development of emergency core-cooling systems. All nuclear reactors must now have provision for maintaining sufficiently low temperatures after a malfunction that the integrity of the fuel rods will be insured. The primary malfunction with which the emergency core-cooling systems are concerned is a loss-of-coolant accident. In such an accident, the primary coolant system develops a leak or rupture resulting in some of the primary coolant water being lost from the system. When the leak is a relatively minor one, the primary coolant system can continue to function to cool the core after the shutdown so long as the small quantity of coolant being lost is replenished. The replenishment of coolant through a small leak is accomplished by a high-pressure injection system. However, in the event of a large rupture developing in the primary coolant system, a different emergency core-cooling system becomes effective. According to conventional design, such an emergency core-cooling system operates in two distinct phases. Initially, accumulator tanks are discharged and/or pumps operated to rapidly refill the vessel with borated coolant water. Subsequently, the new coolant is circulated through the pressure vessel. Steam leaking into the reactor containment is condensed, removing the reactor heat from the system. Power for the pumps is obtained from an independent power source such as a Diesel engine. Typically, a complete emergency core-cooling system injects water into each of the primary coolant loops of the reactor so that the break in a single coolant loop will not defeat the operation of the emergency core-cooling system, pumping water into the other primary coolant loops. It will be apparent that the provision of such systems sufficient to maintain a safe temperature within the vessel is an expensive and critical component of the overall power generating system. Such conventional emergency core-cooling systems are rendered more expensive and less reliable because each of the possible contingencies for such a system's operation adds further to the design capacity requirements. For example, Diesel generator sets have a high startup failure rate, requiring a backup electric system to increase the reliability. A critical time lag may develop before the cooling water from the pumps is injected. Water coming into contact with the hot core is then partially vaporized by the fuel elements. The steam in the vessel collects in the plenum of the vessel producing a back pressure to the entry of new coolant water. The steam problem, generally referred to as "steam binding", limits the design of the reactor plant so that the initial temperature rise caused by the steam binding effect does not endanger the integrity of the reactor core. Therefore, it is desirable to have a system for removing heat from the vessel that increases the reliability of decay heat rejection from the reactor after an accident and has a short startup time. Such a system is particularly desirable if it is capable of operating independently of an electrical power source. SUMMARY OF THE INVENTION An exemplary embodiment of the invention will be described in association with a pressurized-water reactor. However, it is to be understood that the system has also application in other reactor systems incorporating a circulating coolant and that the system in particular has application to a boiling-water reactor. In the exemplary embodiment, the dependence on electric power systems of prior art emergency core-cooling systems is overcome by utilizing the energy from the reactor itself to power the emergency system. The heat energy is utilized by a unique jet pump providing operating characteristics closely corresponding to the requirements for emergency core-cooling over a wide range of failure types. A subcooler for increasing the differential between the saturation temperature of the coolant water for the then existing pressure and coolant water temperature further enhances the characteristics of the jet pump design. The subcooler and jet pump together are designed to accommodate the coolant in such a manner that substantially all the flashing of the coolant into steam will occur in the divergent section of the nozzle. Since flashing does not take place in the throat of the nozzle, the flow does not reach sonic velocity (become choked) in the nozzle throat and the pump produces an almost constant mass flow rate over a wide range of temperature/pressure relationships. Because flashing in hot water in the nozzle is the driving force that distinguishes the jet pump according to the invention, the pump is referred to hereinafter as the flash jet pump. The flash jet pump is further distinguished from conventional designs in its use of an extremely high expansion area ratio. Expansion area ratio refers to the ratio between the size of the nozzle outlet area to the nozzle throat area. For conventional jet pumps, this ratio is over a maximum range of 1:1 to 8:1. In the flash jet nozzle, the area ratio is in the range of 10:1 to 50:1. The high area ratio is caused by the high density of liquid water compared to that of steam and the fact that flashing is suppressed in the convergent nozzle section. The supersonic two-phase flow exiting the divergent nozzle impinges upon coolant water drawn into the suction side of the flash jet pump. The mixing of the supersonic two-phase jet with the coolant water produces a combined high-velocity flow which is converted into a pressure rise for pumping. The pressure rise is increased by a divergent section in the conduit so that substantially all of the remaining kinetic energy of the water is converted into pressure for forcing the combined coolant flow through the inlet connection into the reactor vessel. The coolant water is drawn from a supply of additional coolant. The source of supply of the additional coolant is a storage tank or the reactor building sump. The reactor building sump collects the water lost from the primary cooling loop due to the break. Thus, the system becomes self-sustaining with the water being lost from the reactor vessel being picked up by the flash jet pump and recirculated. For maximum advantage, the subcooler is in the form of a downcomer pipe connected to the outlet connection of the vessel and having a vertical extent of 20 to 40 feet. The effect of the downcomer pipe is to apply a static pressure head on top of the circulating pressure of the coolant thereby raising the saturation temperature and increasing the differential between the saturation temperature and coolant water temperature. For the capability of the system to deal with a wide range of loss-of-coolant accidents and to minimize fuel rod damage under all circumstances, operation of the system in its transfer mode for reflooding the vessel with an initial charge of coolant water is necessary. The operation of such a transfer system assumes that the rupture is sufficiently large that a substantial coolant fraction is lost from the vessel. The transfer system is utilized to transfer borated water into the vessel in the minimum possible time. The pump for the transfer system incorporates the same principles previously described in association with the recirculation system. That is, the pump is a flash jet pump powered by hot water generated by the reactor heat. THe hot water can be taken from the secondary side of the steam generator or from a hot water storage tank. The steam generators contain a substantial quantity of heated water at the moment of a loss-of-coolant accident. Since the transfer system must operate only for an initial period, a finite quantity of heated water is sufficient to refill the reactor vessel. A subcooler is connected to the flash nozzle. The subcooler is a downcomer pipe with a vertical elevation difference between the hot water tank and flash jet pump. Cold water is drawn from a storage tank containing borated water by the flash jet pump and discharged into the reactor vessel. It is therefore an object of the invention to provide a new and improved flash jet coolant pumping system. It is another object of the invention to provide a new and improved flash jet coolant pumping system that has few moving parts. It is another object of the invention to provide a new and improved flash jet coolant pumping system with a highly reliable cooling action. It is another object of the invention to provide a new and improved flash jet coolant pumping system which operates over a wide pressure range. It is another object of the invention to provide a new and improved flash jet coolant pumping system that reliably transfers an initial charge of water into a reactor vessel. Other objects and many attendant advantages of the invention will become more apparent upon a reading of the following detailed description together with the drawings in which like reference numerals refer to like parts throughout and in which: