Patent Number: 061608637
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 shows a schematic view of a nuclear reactor cooling system 20 having a primary coolant circuit 22 connected to a secondary coolant circuit 24. The primary coolant circuit 22 includes a reactor 26 connected to a steam generator 28 by a coolant line 30. A fluid source 32 having a free liquid surface 34 also communicates with the reactor 26 and ensures sufficient coolant volumes in the primary coolant circuit 22. The secondary coolant circuit 24 has a drain line 36 connected to the coolant line 30 for bleeding a predetermined amount of coolant to a letdown heat exchanger 38. A makeup tank 40 is coupled to the letdown heat exchanger 38 and includes a free liquid surface 42. The makeup tank 40 receives coolant from the heat exchanger 38 and stores the coolant until it is needed for controlling volume levels in primary coolant circuit 22. A charging pump 50 is connected to the makeup tank 40 and pumps coolant from the secondary coolant circuit 24 back into the primary coolant circuit 22. Preferably, the charging pump 50 is a variable speed pump that can deliver coolant at variable pressures and flow rates, as desired to change coolant volumes and/or match letdown flow rates with charging flow rates. Charging and letdown flow rates can be varied to effect changes in the boron concentration in the coolant to control reactivity. Further, coolant volume changes occur due to temperature changes in the primary coolant circuit 22. By utilizing variable speed charging pumps 50, the present invention eliminates the need for throttling valves to reduce constant coolant pressure developed by a constant speed pump, as in conventional reactor coolant systems. FIG. 2 shows a portion of a charging pump system 60 having a makeup tank 62 connected to first and second variable speed charging pumps 64, 66. Each variable speed pump 64, 66 preferably has an AC synchronous motor 68 connected to an AC variable speed drive 70. However, other suitable variable speed arrangements and motors can be used without departing from the spirit of the present invention. For example, the present invention can have any suitable type of motor, including but not limited to, an asynchronous induction motor. Moreover, the present invention is not limited only to AC. Other suitable power sources can be used including, for example, DC. More specifically, AC variable speed drive 70 is located between an AC power supply bus (not shown) and a synchronous AC motor 68 of each charging pump 64, 66. Coolant flow is varied by modifying the frequency of a power supply (not shown) using the AC variable speed drive 70, which results in varying the speed of the respective charging pump 64, 66. Pump speed reduction reduces the pressure developed by the pump and reduces the rate of coolant flow. In addition, a pressurizer level control system 80 is optionally, but preferably, connected to each variable speed drive 70 and includes a pressurizer level control processor 82 to monitor and control the level of pressure and flow rates in the reactor coolant system 20. Pressurizer level is maintained as a function of desired plant power by adjusting charging pump speed. Thus, pressurizer level control system 80 independently controls the speed of the first and second variable speed charging pumps 64, 66. Charging flow will match letdown flow when pressurizer level is maintained constant. Preferred embodiments of the present invention have been disclosed. A person of ordinary skill in the art would realize, however, that certain modifications would come within the teachings of this invention. Therefore, the following claims should be studied to determine the true scope and content of the invention.