Patent Number: 051805430
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 3, a first aspect of the present invention is to provide a flow path 52 from the reactor coolant system hot leg 54 to the flood up level in the containment (but above the top of the core elevation). This flow path provides for a circulatory flow of water from the containment through the core as indicated by the broken lines and arrows which limits the concentration of boron from in the reactor vessel in the long term cooling mode. The flow path 52 is attached at the bottom of the hot leg 54 so that steam can vent through the upper portion of the pipe. The added dilution flow path is below the flood up water elevation but high enough to achieve natural circulation driven by the lighter (heated) water in and above the core. With the hot leg flow path 52, containment water flows through the core to keep the boron concentration low. Natural circulation of water is shown by the broken lines and arrows and is driven by the differences in water densities. The flow path 52 (one of two shown) works in conjunction with the flow path 56 (one of two shown) which includes a sump screen 13 for communicating water from within the containment into the reactor vessel. A portion of the second flow path is also used to introduce borated water from the passive safety injection system from the various sources, such as the core makeup tanks and the accumulators, as compared with the previous invention shown in FIG. 3 where the normal flow is simply to introduce containment water into the reactor vessel 10 through the second flow path. However, the water is boiled off the core, boron accumulates in the reactor vessel, thereby creating a dangerous situation in which the ability of the reactor core to effect heat transfer is diminished. The flow path 52 thus induces a natural circulatory flow of water from the water flooding the containment through the reactor core based on the differences in water density produced by the heating of water by the reactor core, thereby limiting the concentration on boron in the reactor vessel. Another problem associated with the use of borated water is that in a pressurized water reactor you need to increase the concentration of boron in the reactor when you go from hot to cold conditions while maintaining the core in a subcritical condition. According to the present invention, the core makeup tank elevation relative to the pressurizer elevation and core makeup tank boric acid concentration can be established to ensure that sufficient borated water will be drained by gravity from the core makeup tanks to achieve the required cold shut down boric acid concentration. As shown in FIG. 5, the core makeup tank and pressurizer elevation are established, along with the core makeup tank boric acid concentration, such that when the reactor coolant system water is cooled and shrinks in volume, sufficient core makeup tank boric acid solution will drain into the reactor coolant system to raise the overall concentration to that required to keep the reactor sub-critical. FIG. 4 represents the normal operation with the reactor coolant system water volume at around 6,000 cubic feet. The water in the reactor vessel contains less than 100 parts per million boric acid, while the core makeup tank may contain greater than or equal to 2,000 ppm boric acid. The core makeup tank is located substantially below the pressurizer normal water level. Balance lines 58 and 60 are also illustrated schematically, while flow path 56 connects the core makeup tank to the reactor vessel 10. After cool down, and referring to FIG. 5, the reactor core system water volume shrinks allowing the core makeup tank 40 to drain into the reactor vessel 10 thereby increasing the reactor coolant system boric acid concentration. Thus, high concentration boric acid in the core makeup tanks drain into the reactor coolant system to make up for water shrinkage, and thereby achieves a concentration in the reactor vessel of greater than or equal to about 500 PPM boric acid. An alternative to the partial draining of the core makeup tanks compensating for water shrinking discussed above with respect to FIGS. 4 and 5 is to create a natural circulation flowpath to mix the high concentration boric acid solution in the core makeup tank with the low concentration boric acid solution in the reactor coolant system. Referring to FIGS. 6 and 7, an alternative embodiment is illustrated in which the normal mode of operation would be the same as what is illustrated in FIG. 4. However, in this embodiment, a flowpath 62 is provided between the core makeup tank 40 (which is full of cold borated water) and the pressure balance line 60 which connects the top of the core makeup tank with the cold leg 24. The boration mode illustrated in FIG. 6 can be accomplished prior to the cool down since it is not necessary for the water level to shrink. The circulation flow is illustrated in FIG. 7, which is an enlargement of the broken circle portion of FIG. 6. Basically, hot water rises from the cold leg 24 into the pressure balance line 60 and then through the vent 62 into the core makeup tank 40. The hot water forces the cold core makeup water containing boric acid into the reactor coolant system. An equilibrium boric acid concentration of about 1,000 PPM is achieved as a result of the natural circulation induced by the vent 62. FIG. 8 is another illustration of the embodiment described with reference to FIGS. 6 and 7. The vent 62 has a pipe segment which extends into the core makeup tank 40 and terminates at an elevation below the depressurization system actuator core makeup tank level. The valves 46 are preferably located at or below the depressurization system actuation core makeup tank level. When the reactor coolant pumps are running, the pressurizer is about 60 PSIG less than the cold leg. Thus, check valves will be closed and the pressure difference between the core makeup tank inlet/outlet will be small. Borated water in the core makeup tanks will be mixed simply when the operator opens the core makeup tank inlet/outlet. As the reactor coolant system water shrinks, the pressurizer level will fall and the reactor coolant pumps will trip. Thus, the core makeup tank level and pressurizer level will equilibriate. When the reactor coolant pumps are not running, the natural circulation flow path will allow approximately 100 to 200 gallons per minute from the cold leg to the cold core makeup tank. Another aspect of the present invention incorporates use of two accumulators (one shown in FIG. 1) (tanks partially filled with water with a pressurized covergas such as nitrogen) which provide additional water flow to the reactor in the event of a large pipe break, when the required water addition rate is highest. These tanks deliver water to the reactor when the reactor coolant pressure falls below the nitrogen covergas pressure. This feature permits the high design pressure CMT's and associated piping to be reduced in size since the highest required flow for core cooling after a large pipe break can be initially provided by the accumulators, followed by a lower flow from the CMT's. The check valves provided in the core makeup tank discharge line prevent water from the accumulator from going into the core makeup tank which may be partially drained with accumulator injection. Also since the accumulators can be designed for a lower pressure (corresponding to the covergas pressure of about 100 psig), equipment cost can be reduced. Another aspect of the present invention is illustrated in FIG. 9 in which the core makeup tanks 40 and 41 are provided with fill, drain and sample capability, schematically illustrated by fill and sample lines 40a, 41a, and drain lines 40b and 41b. Similar fill, drain and sample capabilities are provided for the accumulators 42 and 43, as well as the in-containment refueling water storage tank 36. The fill lines 40a, 41a, 42a, 43a and 36a are used to inject water into the respective tanks with an appropriate concentration of boric acid in order to adjust the concentration in the tanks to a desired level. Thus, in a sampling mode, borated water is removed and sampled from the drain and sample lines 40b, 41b, 42b, 43b and 36b to determine the concentration of boric acid. If the concentration is determined to be low, higher concentration boric acid can be injected into the respective tanks through the fill lines while simultaneously removing an equal volume from the drain lines until the desired concentration level is achieved. A refilling supply tank (not shown) can be temporarily or permanently connected to the fill lines. The various tanks containing borated water are isolated normally from the reactor coolant system by means of isolation valves 64 which prevent the contents of the tanks from entering the reactor coolant system. The drain and sample lines 40b, 41b, 42b and 43b include isolation valves 66 to ensure that accidental drain does not occur. The two parallel normally closed valves 50 which are in the pressure balance line from the reactor coolant system cold legs to the top of each core makeup tank are actuated to their open position simultaneously with the parallel core makeup tank discharge isolation valves, on receipt of a core makeup tank actuation signal. Thus, the balance line can be used to permit a large amount of steam to float to the top of the core makeup tank which results in a high flow rate of water to the reactor coolant system from the core makeup tank. It will be recognized by those of skill in the art that numerous modifications and additions may be made to the various structures and the systems disclosed herein and thus it is intended by the appended claims to encompass all such modifications which fall within the true spirit and scope of the invention.