Patent Number: 055419690
Section: summary

BACKGROUND OF THE INVENTION The present invention is directed to a water level indicator for use in a nuclear power plant. During maintenance periods at a nuclear power plant, it is important for safety reasons to know the level of water in the hot leg of the water line travelling from the reactor vessel to the heat exchanger to ensure proper reactor core cooling. Present methods of measuring the water in this particular pipe have proven inadequate because of inaccuracies in the measurement of the water level in the pipe or the inability to make the water level reading from the control room of the power plant. During nuclear plant operation, water is heated under pressure in the reactor vessel. The water then travels through the hot leg pipe of the water circulation system to the heat exchanger/steam generator where the water is cooled. The cooled water then travels through a circulation pump to be returned to the reactor vessel for reheating. The water serves as the medium to cool the reactor core and transfer heat to the heat exchanger steam generator. During maintenance periods, water needs to be drained from the heat exchanger/steam generators to facilitate examination of the generator's internal equipment. While the steam generator does not serve its heat exchanging function during these maintenance periods, the water is cooled by another heat exchanger as the water continues to function as a coolant for the core. Since the hot leg pipe lies in essentially a horizontal plane, the precise level of water in the pipe is important. Should the water in the pipe be too high, or full, the inside of the generator, where work crews are performing examinations, could be flooded. Should the water level in the hot leg pipe be too low, or empty, this could indicate a water level too low to maintain proper cooling of the nuclear core. Because the hot leg has a direct fluid connection to the reactor vessel, the hot leg is in a convenient position for measuring the water level to ensure proper water level in the core. Therefore, being able to measure the precise water level in the hot leg pipe is of crucial importance for safety. It is of additional safety consideration that the level may be measured from the nuclear plant's control room where other safety equipment is monitored. SUMMARY OF THE INVENTION The primary object of the present invention is to provide apparatus to accurately measure the level of water in the hotleg of a pressurized water reactor system. It is a further object of the invention that the measurement of the level of the water can be monitored from a remote location such as the control room. It is another objective of the invention that the monitoring can be performed during all periods of plant operation and maintenance. It is still another objective of the invention that a measurement system can be permanently installed on the hot water leg pipe. It is yet another object of the invention to have a water level measurement system that can selectively be isolated from the hot leg water pipe. In fulfillment of these and other objectives, the midloop water level monitor of the present invention comprises a remote chamber fluidly connected to the pipe for measurement of the level of water in the pipe. In particular, a remote tank forming a chamber is located at an elevation substantially the same as that of the hot water pipe. A lower connecting pipe extends from the bottom of the hot water pipe to the bottom of the chamber and an upper connecting pipe extends from the top of the hot water pipe to the top of the chamber. The result is that the level of water in the chamber is substantially the same as the level of water in the pipe. Inside the chamber are means such as heated junction thermocouples for measuring the level of water in the chamber. It is also possible to use mechanical measurement systems, such as a float, connected to a rheostat or other electrical means of generating an electrical signal commensurate with the water levels. Isolation valves are located on each of the connecting pipes to allow isolation of the chamber from the main pipe such as during power generating operation of the power plant. This allows maintenance of the water level monitor without disrupting plant operation. In addition, because the water level monitor need only be at the same elevation as the hot leg pipe, the monitor can be located at any remote distance from the hot leg pipe. The isolation valves allow maintenance at this remote location during plant operation. These isolation valves can also be used to protect the detection equipment in the chamber from the pressure and heat of the water traveling through the pipe when the plant is in full operation. Since the hot water pipe lies in a substantially horizontal plane, it is important to be able to measure the water in this pipe with accuracy because a change of even a fraction of an inch within this pipe can result in an excessive level of water, which could flood the steam generator chamber during an examination by plant personnel, or an insufficient level of water, which could result in degradation of cooling of the reactor core .