Patent Number: 044180357
Section: summary

BACKGROUND OF THE INVENTION This invention relates in general to monitoring of the cooling action and condition of coolant in a nuclear power reactor. A required safety measure for operation of nuclear power plants involves detection of coolant level in the reactor fuel core, to provide warnings of dangerous coolant loss. The installation of measuring and monitoring systems for such purposes is regarded as very costly because of equipment costs and modification of reactor design to accommodate such equipment. Further, providing only coolant level information is sometimes insufficient for recognition of inadequate fuel core cooling caused by significant reactor malfunction such as high void fraction-pumped flow and stagnant boil off. On the other hand, non-significant coolant loss detection must be avoided to prevent unnecessary and wasteful power plant shutdowns. It is, therefore, an important object of the present invention to monitor coolant conditions in an economically feasible and meaningful manner. An additional object is to provide advanced warnings of inadequate fuel core cooling by a monitoring system covering a full range of operation. SUMMARY OF THE INVENTION In accordance with one embodiment of the present invention, loss of coolant in the fuel core of a nuclear reactor may be detected through a simple rearrangement of local power rate sensors of the type disclosed in prior application Ser. No. 888,881 filed Mar. 21, 1979, now U.S. Pat. No. 4,298,430 owned in common with the present application by the same assignee. In this type of sensor, the gamma radiation heated sensor body is designed to exhibit a varying temperature distribution because of heat transfer to the coolant within the fuel core. Such sensors include double junction thermocouples at each of a plurality of vertically spaced local measurement zones. The tip portion of the thermocouple at which the faster acting junction is located, is vertically positioned above the other junction so that the faster acting junction normally acts as the cold junction spaced above the hot region of the sensor core as long as reactor coolant is in thermal contact with the sensor throughout. When a drop in coolant level occurs below the faster acting junction, however, the faster acting junction then acts as the hot junction. Thus, depletion of the coolant manifested by a drop in coolant level below the faster acting junction will produce a reversal in polarity of the differential signal voltage across the junctions. Such a reversal in polarity may be used to trigger an alarm or initiate an action sequence in the event of a reactor accident resulting in coolant loss. In accordance with another embodiment of the invention, an electrical heating device is embedded within the sensor body amongst the double junction thermocouples to increase the level of internal heating above that produced by gamma radiation. The level detection operation is thereby enhanced and data obtained by the signal output of the sensors for determining the heat transfer characteristics of the external surface of the sensors at critical levels. An important aspect of the invention resides in the aforementioned modification of gamma sensors so as to render them operative for multiple function monitoring purposes, including the monitoring of coolant level, determination of heat transfer coefficients, and temperature surveillance as well as to monitor local power distribution. The advantages and benefits of the invention are further enlarged by extension of the sensors into the upper dome of a nuclear reactor vessel in order to monitor coolant conditions within the dome. An economical system may thereby be designed for providing safety monitoring facilities based on existing sensors for monitoring local power distribution.