Patent Number: 055263860
Section: summary

FIELD OF THE INVENTION The present invention relates generally to improving the efficiency, reliability, safety, steam mixing, and other operational characteristics of nuclear electrical power plants with a light water reactor. And more particularly the instant invention relates to mixing superheated steam from a heat recovery boiler of a gas turbine combined cycle steam system with main steam at the HP stage and the LP stage of the main turbine. BACKGROUND OF THE INVENTION In the operation of nuclear power plants, there are several important problems that significantly increase the cost of operations and maintenance. Some of these significant problems are: inherently low thermal efficiency, erosion-corrosion fuel storage and waste disposal limitations, equipment maintainability and reliability concerns as well as the limited fuel availability and the expensive requirements of storing spent fuel. Various approaches have been tried to solve these problems separately, but few of the existing methods attack more than one of these concerns at a time. Currently, in nuclear power plants (both pressurized water reactors or boiling water reactors), steam at the inlet of the turbine is saturated. This results in lower turbine efficiency and intensive erosion-corrosion problems. There have been several attempts to use single or multiple fossil reheaters to treat the steam of a operating nuclear power plant. However, while higher steam temperatures due to fossil heating between the low pressure turbine stage and the high pressure turbine stage have been achieved, higher efficiency still has not been demonstrated. One such attempt is U.S. Pat. No. 5,361,377 to John A. Miller. The Miller heat exchanger method and apparatus of treating the steam claims an increase in efficiency by use of a fossil fired reheater. A problem with this method is that the fossil reheater still consumes fuel in a process that is inefficient in itself. In other words a big part of the heat from the burning of the fossil fuel goes up the stack. A process is needed that produces electricity, and utilizes the waste heat from that process to treat the steam in an efficient manner. Mixing of the steam going from the LP stage to HP stage of the main turbine with high enthalpy steam is a solution that brings all of these advantages. It is generally recognized that for mixing to be done in non-isothermal fluids, in boundary flow conditions, a pipe length of about 50 diameters is required. In a typical nuclear power plant the typical main steam pipe outside diameter is approximately 0.8 meter. This would require a pipe of 40 to 50 meters in length to achieve through mixing. Therefore, a more efficient means of mixing is needed to mix the steam from a heat recovery boiler with main steam. It is therefore an object of the instant invention to treat the steam prior to entering the HP stage of the main turbine, then again before entering the LP stage by mixing the nuclear steam with high enthalpy steam from an efficient process. SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a means to improve the efficiency, availability, safety, and steam mixing, and economy of nuclear power generation systems. And more particularly an improved means for providing superheated steam to the steam turbine driving the electrical generator. It is a more particular object of the invention to allow for lower hot leg temperatures (T.sub.hot) in the primary cycle of an existing nuclear power plant. The lower T.sub.hot would reduce the rate of erosion-corrosion and extend the lifetime of PWR steam generators. It is a further object of the invention to extend plant life by enabling the delivery of the same amount of electricity for a smaller amount of heat generated by nuclear reaction. In other words, the invention is a combined nuclear-gas turbine cycle that can be used to, inter alia, reduce corrosion damage to the nuclear steam generator by operating the outlet reactor hot leg temperature below the critical corrosion temperature. Corrosion is significantly reduced by operating steam generators at less than 600.degree. F. (315.degree. C.). With the instant invention nuclear power plants can operate below 600.degree. F. (315.degree. C.) without derating the electrical output of the station. The advantage of reduced corrosion is due to reduced steam generator temperature caused by an increased thermal efficiency of the resulting steam cycle. One factor that causes an increase in efficiency is that the additional electrical output from the gas turbines further adds to the electrical grid output of the station. In the instance where the hot leg temperature is reduced below the critical steam generator corrosion temperature, the thermal power of the reactor will be lower, resulting in a potentially longer operating cycle time between reactor refueling cycles. This enables a longer period between refueling which is getting progressively more expensive. The disposal of spent nuclear fuel and waste, which is now a major concern of nuclear industry, will be reduced significantly. In addition, lower reactor power avails opportunities to perform maintenance on many key apparatuses in the secondary system (the steam cycle in PWRs and the coolant cycle in BWRs) while the reactor is at power. By reducing corrosion and erosion the down time is decreased thus enabling the plant to operate with less down time. It is well known in the art that the fuel replacement cost during downtime is from several hundreds of thousands of dollars to one million dollars for each day, depending upon the plant. When the reactor is operating at a lower thermal power, there are, in essence, built-in spare components with the arrangement of currently installed equipment. To underscore the importance of erosion and corrosion in steam equipment it should be noted that maintenance of many secondary-side (i.e. , steam handling) components are often the source of unplanned outages. In the instant invention, the steam turbine has higher efficiency and reduced erosion-corrosion. The reactor steam, prior to entering the high pressure stage of the main turbine, is superheated by a high-pressure-mixer-superheater (HPMS) that utilizes the highly superheated steam from the heat recovery boiler (HRB) of the gas turbine combined cycle. The steam is then conditioned, according to need, by several different components. In the most commonly used application to improve the efficiency of an operating nuclear power plant the steam would next go to the moisture separator reheater (MSR), that is a common component of existing plants. The MSR can be bypassed depending upon the existing steam quality from the high pressure (HP) stage of the main turbine. The steam is next improved for use by the low pressure (LP) stage of the main turbine. When required this is done in a low pressure mixer superheater (LPMS) which mixes steam from the MSR (when used) with HRB steam. The instant invention has a marked increase in the portion of the LP stage of the main turbine that is driven by superheated steam. This enables a reduction in corrosion and erosion. The invention also provides an additional avenue to make nuclear stations safer due to the addition of an operating source of alternative emergency power (i.e., the gas turbines). Additionally, the size of the gas turbine and generator is not limited and the instant invention therefore readily lends itself to operation in conjunction with a large gas turbine and generator that could function as an independent source of power during peak times or alone to supply a grid.