Patent Number: 061608637
Section: summary

FIELD OF THE INVENTION The present invention relates to a coolant system for a nuclear reactor and, in particular, volume control system having a variable speed pump to regulate the rate of charging flow into a primary coolant circuit. BACKGROUND OF THE INVENTION Nuclear reactors have coolant circulating at high pressures through a coolant circuit that flows past a reactor vessel which heats the coolant producing steam that turns a steam generator to produce energy. Corrosion and wear in a cooling system produces impurities in the coolant. These unwanted impurities must be removed to sustain efficient energy production and transfer of heat. It is known to continuously remove or bleed coolant from a primary coolant circuit to a secondary circuit that removes unwanted impurities. This bleeding is termed letdown flow. Typically, coolant in the secondary circuit is communicated to a letdown cooler before passing to a demineralizer where impurities are removed. Next, the coolant is stored in a makeup surge tank until being pumped back into the primary coolant circuit by a constant speed pump. Flow control valves located downstream of the constant speed pump, are provided to regulate the rate of flow and pressure of the coolant as it is being pumped back into the primary circuit. This replenishing is called charging flow. Loss of coolant is known to occur due to leaks, damaged seals, or during the purification process. Therefore, the makeup surge tank needs to have sufficient storage capacity to enable replacing lost coolant and accommodate for changes in the total coolant volume in the primary circuit. Varying the volume of coolant in the primary circuit also controls the concentration of boron in the primary circuit and is critical for the efficient operation of a nuclear reactor plant. Using control valves to determine the rate of charging flow and letdown flow has many drawbacks. For example, control valves are subject to wear and require maintenance. In addition, maintenance personnel could be exposed to radioactive materials when servicing worn out control valves. Further, a constant speed pump develops constant high pressures which must be reduced by throttling at the control valves to achieve a desired lower pressure. Thus, energy is wasted because pump energy is added to pressurize the coolant to a constant pressure, which energy is in turn dissipated during throttling to a lower pressure before re-introducing coolant into the primary circuit. Charging flow and letdown flow are varied to change the concentration of boron in the coolant for reactivity control or to compensate for volume changes caused by temperature changes in the reactor coolant system. However, power losses are associated with varying the makeup and letdown flows. Therefore, there is an operating incentive to operate at minimum letdown and makeup flows that still provide a proper coolant chemistry. In a conventional system, operating at minimum letdown and makeup flows requires maximum throttling by the control valves. Maximum throttling, in turn, involves maximum energy being wasted. SUMMARY OF THE INVENTION The present invention is directed to a volume control system for a nuclear reactor having a primary coolant circuit for cooling a nuclear reactor and a secondary coolant circuit connected to the primary coolant circuit and including a letdown heat exchanger for receiving a desired amount of coolant being bled from the primary coolant circuit. In addition, a fluid source is provided to receive and store the coolant from the letdown heat exchanger. A variable speed pump is connected to the fluid source and returns coolant into the primary coolant circuit at a variable rate that is selected based in part on a rate of letdown flow. The present invention also includes a reactor coolant system for a nuclear reactor having a primary coolant circuit for cooling a nuclear reactor and a secondary coolant circuit connected to the primary coolant circuit and including a fluid source for receiving coolant which is selectively drained from the primary coolant circuit, the fluid source having a free liquid surface. At least one variable speed pump is in fluid communication with the fluid source and delivers coolant to the primary coolant circuit at variable flow rates and pressures. Moreover, the variable speed pump can be a centrifugal pump having a variable speed drive connected between a power supply and a motor. In one embodiment, an AC variable speed drive is connected between an AC power supply and a synchronous AC motor, however, any suitable type of power supply and motor can be used. The present invention eliminates the need for throttling control valves because a variable speed pump is provided that can deliver coolant into the primary coolant circuit at variable flow rates and pressures. Thus, the present invention is more energy efficient because the variable speed pump only uses a sufficient amount of energy to deliver variable coolant pressure rather than developing constant, high coolant pressure that must be throttled at the control valves to a lower pressure, as in the prior art system. Further, component parts of the coolant system can be redesigned because excess pressures that previously occurred for long time periods, now occur for much shorter time periods. Therefore, the need for very robust component parts is reduced and more cost effective components can be utilized. Still further, throttling control valves are generally high maintenance devices due to the dynamic loading that they encounter. The present invention eliminates the need for throttling control valves and therefore reduces maintenance costs. The AC variable speed drive is used to achieve flow control in the present invention. Thus, maintenance personnel are not at risk of radioactive exposure during repair work since the AC variable speed drive is not part of the coolant circuit.