Patent Number: 044252978
Section: summary

BACKGROUND OF THE INVENTION This invention relates generally to in-core, gamma ray heat sensing means for determining local power generation in a nuclear reactor. According to prior copending application, now U.S. Pat. No. 4,298,430, owned in common with the present application by the same assignee, several gamma sensor assemblies for local power measurement are disclosed wherein an elongated monolithic body of gamma ray heated material mounts thermocouples for measuring temperature differentials produced internally in the elongated body at a plurality of axially spaced measurement zones. The temperature differential is a direct function of the average volumetric heat flow rate for the measurement zone which is in turn directly related to local power generation. The disclosure in U.S. Pat. No. 4,298,430 is hereby incorporated herein by reference. Other related, prior copending applications, now U.S. Patents, owned in common herewith are U.S. Pat. Nos. 4,313,792 and 4,356,061. In U.S. Pat. No. 4,298,430, aforementioned, the claims are directed to gamma ray sensors within which axial heat flow patterns are established to produce the temperature differential measurements for each measurement zone in a pressurized water type of nuclear reactor where space for insertion of the sensor is restricted to 7.5 mm. for example, to limit the outer diameter of the elongated sensor body. Such temperature differentials are produced at each measurement zone by a thermally resistive region or gap in the body enclosed by an outer heat sink tube in thermally conductive relation to the constant outer diameter portion of the elongated sensor body. In such an arrangement, the resulting axial heat flow pattern is such as to render the thermocouple junction in the high thermal resistance region hot and the junction in spaced adjacency thereto, cold. The high thermal resistance gap has an axial length which is directly related to the differential temperature signal output of the sensor, disregarding gap heat losses and assuming a uniform heat sink temperature for the sensor. Further, the outer diameter of the sensor body has no direct affect on the level of the signal output. It was therefore, believed that only with a sensor of the type having the axial heat flow pattern could a suitable signal output level be obtained despite installation space limitations, by appropriate selection of the axial gap length. However, the axial heat flow type of sensor does have signal reliability problems because of axial gap heat losses resulting from deterioration of the high thermal resistance in the axial gap and deviations in heat sink temperature as a result of poor contact developed between the sensor body and the outer heat sink tube. It is therefore, an important object of the present invention to provide a gamma sensor for measuring local power generation in a nuclear reactor, that is more reliable with respect to signal error and yet provide an adequate output signal level as well as to exhibit the necessary physical strength under installational conditions. An additional object is to provide such an improved gamma sensor which retains the attribute of direct electrical calibration. SUMMARY OF THE INVENTION In accordance with the present invention, a gamma sensor is provided wherein controlled radial heat flow paths are established within the elongated gamma ray heated body by direct exposure of its external surface throughout to coolant, producing a uniform heat sink temperature at the external surface both along the outer diameter portions and along the reduced cross sectional portions. Radial heat flow within each measurement zone occurs so that the reduced diameter portions are colder than the portions of the body adjacent thereto, producing a differential temperature signal in the thermocouple device that is a direct function of only the outer diameter of the sensor body. Thus, because of the larger available space for the sensor within a boiling water type reactor, temperature differentials of up to 22.degree. C. may be obtained by use of larger diameter sensor bodies, as compared to 4.degree. C. for sensors of the radial heat flow type fitted to the smaller available space in a pressurized water type reactor. The radial heat flow type of sensor of the present invention is not only similar in construction, but avoids the disadvantages of the axial heat flow type aforementioned, with respect to deterioration of the thermal resistance of the enclosed axial gap and development of poor contact between the heater body and outer heat sink tube. To insure structural integrity for the radial heat flow type of sensor body, having no outer heat sink tube, thin metal fins are provided axially across the annular space at the reduced diameter portions of the sensor body. The material and dimensions of such fins is such as to have a negligible affect on the differential temperature signal produced. One of the advantages of the gamma ray type sensors emphasized in the prior copending application aforementioned, is the ability to effect electrical calibration by conducting electrical current longitudinally through the elongated monolithic body of the sensor to internally generate heat simulating the heat internally generated during sensor use by gamma radiation. However, since the reduced diameter portions of the sensor body have higher electrical resistance and therefore produce heat at a higher rate than the larger diameter portions of the body, the electrical heating effect would be the reverse of the gamma heating effect in the axial heat flow type of sensor. In order to preserve the direct electrical calibration attribute for the radial heat flow sensor, certain calibration measures are taken in accordance with another aspect of the present invention to substantially render the rate of electrical heat generation constant throughout the sensor body. During calibration, the annular spaces formed at the reduced diameter portions of the sensor body are partially filled with a predetermined quantity of low melting point fillers of high thermal and electrical conductivity. The quantity of filler in such as to equalize the voltage drop per length across the reduced and major diameter portions of the sensor body. The high thermal conductivity of the filler is necessary to render any additional temperature drop along radial paths therethrough to be negligible during calibrating heat generation. After calibration, the filler is melted off.