Patent Number: 051788217
Section: description

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1 of the drawing, a water cooled nuclear fission reactor plant 10 includes a reactor pressure vessel 12, containing a core of fissionable fuel material 14, such as enriched uranium oxide pellets contained within sealed metal tubes grouped into conveniently sized bundles. During reactor power generation operations, the neutron incited fission reaction of the fuel material is controlled by neutron-absorbent control rods or blades positioned into appropriate amounts and patterns of control rod withdrawal to produce prodigious amounts of thermal energy. The core of fuel assemblies is positioned centrally in a lower region of the reactor pressure vessel 12 spaced inward therefrom and substantially submerged in coolant water 16 which circulates through the fuel core 14 to carry away heat and form steam for work, such as driving a turbine for generating electrical power. Control rods 18 containing a neutron absorbing material such boron, are reciprocally moveable into and out from the fuel core 14 to control or govern the rate of the neutron incited fission reaction of the fuel, or to terminate the reaction. This in turn regulates the quantities of heat produced by the fuel core 14 for generating steam to perform work. Typically such nuclear reactor plants 10 are provided with an auxiliary cooling water system, including a supplementary cooling water reservoir containing a standby supply of supplementary water coolant. Cooling water auxiliary systems heretofore were commonly activated and driven with a propellant gas under pressure, such as nitrogen, as a means for propelling supplementary water coolant from a reservoir tank into the reactor vessel 12. A propellant gas source for a supplementary coolant reservoir normally consisted of high pressure gas supply tank connected with the reservoir through fluid conduit. Such means are susceptible to leakage and in turn malfunction. The reactor pressure vessel 12 of the plant 10 contains a fuel core shroud 20 encircling the inward spaced fuel core 14 as shown in FIGS. 1 and 2. The fuel core shroud 20 extends a distance both above and below the surrounded fuel core 14 to form a fuel core lower plenum area 22 comprising the region within the shroud beneath the fuel core and a fuel core upper plenum area 24 comprising the region within the shroud above the fuel core. The fuel core shroud 20 is typically an open ended cylinder in configuration, and is of smaller diameter than the pressure vessel 12 being positioned inward away from the side wall of the reactor pressure vessel 12 to provide an annular area 26 between the outside wall of the fuel core shroud 20 and the inside wall of the reactor pressure vessel 12. The annular area 26 forms a downward flow path for the circulation of coolant water 16 through its cooling cycle comprising condensed coolant feedwater 16 supplied by the vessel inlet 28 along with reactor vessel recirculating liquid water coolant 16 flowing downward through the annular area 26. The coolant water 16 then continues down and around the lower edge of the fuel core shroud 20 and upon reversing flow direction, the coolant water 16 passes upward within the fuel core shroud 20. Thus the circulating coolant water 16 flows in sequence up through the core lower plenum area 22, the fuel core 14 and the core upper plenum area 24. On passing through the heat producing fuel core 14 the coolant water 16 absorbed heat therefrom converting a portion of the coolant water to steam. The steam admixed with the remaining liquid water coolant on passing through the core upper plenum area 24, and beyond, are substantially separated by apt means whereupon the steam exits from the pressure vessel 12 to perform work. The remaining liquid water coolant 16 again reverses its flow path above the top of the fuel core shroud 20 and passes down within the annular area 26 along with some returned feedwater usually originating as condensed steam and purified make up water usually originating from power station condensate water storage tanks or evaporators to endless repeat this cooling cycle. This circulating coolant water system maintains the heat producing core of fissionable fuel submerged within the coolant water flowing thereover as a means of governing the reactor's temperature and in turn the reactor vessel pressure through continuous heat transfers from the fuel core to the circulating coolant water and the evaporation of a portion thereof into steam. However, in the unlikely event of a significant breach of a coolant water containing component of the reactor, such as a main pipe, with a resultant reactor depressurization and with a resultant substantial loss of coolant water from about the heat producing fuel core uncovering same, and/or interrupting the circulation of the coolant water, the fuel core and associated components within the reactor pressure vessel soon overheat. In accordance with this invention a standby supply of coolant water for more completely submerging the heat producing core of fissionable fuel of the reactor is provided for any instances involving inadvertent loss of coolant water. Coolant water of the standby supply in this invention is retained within the reactor pressure vessel, and the activation and the impelling or driving means are inherently passive responding to any overheating within the reactor pressure vessel. Referring to the drawings, at least one elongated chamber 32 is provided within the annular area 26. Elongated chamber(s) 32 is provided with an open lower end, and a closed upper end having a small orifice bleed vent 34. Small orifice bleed vent 34 in the closed upper end of chamber 32 is preferably vented through a small diameter tube such as a capillary tube 36 passing through the shroud 20 into the core upper plenum area 24. With the foregoing means and arrangement of this invention, coolant water 16 flowing through the reactor pressure vessel 12 during normal operating conditions fills the elongated chamber 32 through its open lower end replacing the initial gaseous contents as the gaseous contents are vented out therefrom over a prolonged period of time through bleed vent 34. Thereafter, upon the occurrence of a loss of coolant water 16 due to an accident or the like, the inevitable resultant depressurization of the hot reactor coolant will everywhere throughout the body of liquid comprising coolant water 16 produce steam formation in the form of small bubbles in a process well known as steam flashing. The flashing formation of steam within the coolant water contents of the water filled elongated chamber 32 will similarly occur because of the comparably hot conditions of this water. This inherently produced steam rising to the bleed vented closed top and rapidly expanding within the chamber 32 as the reactor depressurization transient proceeds will drive the remaining coolant water contents of the chamber 32 out through the open lower end into the pressure vessel 12 supplementing the leaking volume of coolant water 16 for submerging and cooling the heat producing fuel core. Thus, supplementary coolant water is supplied from a source within the reactor pressure vessel 12 and is applied by inherently passive means. Referring to FIGS. 1 and 3 of the drawings, a preferred embodiment of this invention comprises a plurality of spaced apart elongated chambers 32 arranged in an encircling configuration around and within the annular area 26 to provide for an enhanced supply of supplementary coolant water. The elongated chambers 32 desirably are of tubular-like construction of suitable length and diameter to reside in annular area 26 without effectively obstructing coolant water flow therethrough. An alternative or additional measure of this invention of either replacing or modifying the bleed vent 34/capillary tube 36 means and/or the passive steam driving phenomenon, comprises providing a duct 38 communicating from the vent 34 in the closed top of chamber(s) 32 to a vent and source of auxiliary pressure external of the reactor pressure vessel 12. Namely, the duct 38 extends out from the reactor pressure vessel 12 and is in communication with a valve for venting gases from the chamber(s) 32, and/or with a source of fluid pressure 40 to either provide the driving force to expel supplementary coolant water from chamber(s) 32, or augment inherently produced steam in chamber(s) 32 in expelling the supplementary coolant water out into the reactor vessel. Another embodiment of this invention is illustrated in FIGS. 2 and 4 of the drawings. In this version of the invention a supplementary coolant water chamber 42 of annular configuration is provided. Chamber 42 induces an open lower end and closed upper end having at least one small orifice bleed vent 44 communicating with a capillary tube 46 for bleeding gases from the chamber. Alternatively annular chamber 42 can be provided with a duct 38 extending outside of the reactor pressure vessel 12 for venting and/or connecting to source of auxiliary gas pressure. Annular chamber 42 extending around annular area 26 can be formed by providing an annular skirt 48 having a top wall section 50 extending generally inward from the inside of the reactor pressure vessel 12 into a limited portion of the annular area 26, with a side wall section 52 extending therefrom downward within annular area 26, generally concentric to the reactor pressure vessel 12 and the fuel core shroud 20. Thus, the outer portion of annular area 26 is closed off except at its bottom to provide a supplemental coolant water chamber 42 while the inner portion of annular area 26 remains open and available for the normal circulation of coolant water.