Patent Number: 044118597
Section: summary

Gamma sensors, such as are used to measure the heat rate of nuclear reactors, depend upon the heating of a thermally isolated heater body by gamma rays. The heat so generated is proportional to the specific power output of the nearby fuel rods. To measure the heat generated in the heater body, it is permitted to escape to a sink through a controlled heat path of closely held dimensions. The temperature drop along such heat path is directly proportional to heat rate (watts/gm) in the heater body and therefore proportional to power in adjoining fuel rods. A thermocouple, or thermocouples, arranged to measure the temperature drop along the controlled heat path produces a signal proportional to this power. This temperature drop and the signal are not greatly affected by the temperature of the heat sink. However, when the temperature of the heat sink changes by a considerable amount, say 50.degree. C., then there is a corresponding and noticeable second order change in the thermal conductivity of the controlled heat path, which is reflected as a change in the temperature drop and as an error in the signal output; even though the gamma ray flux is constant. The gamma sensor disclosed in the aforementioned application gives very accurate measurements when the bore into which it is inserted is that of an unfueled guide tube of a nuclear reactor through which coolant circulates rapidly. In such an installation, it follows that the outer sheath of the sensor will have a fairly well defined temperature. The readings of the gamma sensor will therefore be accurate enough, or cn accurately be corrected to take care of the aforementioned second order change in thermal conductivity of the heat path with changes in the temperature of the outer sheath of the gamma sensor, which outer sheath acts as a heat sink. However, when such gamma sensor is inserted into a dry bore of a nuclear reactor fuel core, the outer sheath of the sensor may assume a non-uniform temperature, depending upon whether the portion of the outer sheath in question touches or does not touch the bore. Those portions of the outer sheath which touch the bore will be kept cool by the contact, while those portions which bridge the bore will heat up from the gamma ray flux. SUMMARY OF THE INVENTION In the present invention a centering device spans the annular gap between the gamma sensor and the bore into which it is inserted. The centering device has the dual function of providing a thermal bridge at the correct spot on the gamma sensor for a heatsink and of centering the gamma sensor in the bore of the fuel core or guide tube into which it is inserted. In accordance with certain embodiments of the invention, the centering device is a separate elastically deformable element mounted externally on the outer sheath of the sensor and in thermal contact therewith by means of an annular groove, for example, to also engage the internal bore surface of the fuel core or guide tube. The centering device is axially located on the sensor within the cold region of the heater body close to the boundary of an adjacent hot region which is coextensive with the thermal resistance gap portion of the heater body having a reduced diameter. Thus, the centering device may be aligned with the cold junction of the differential thermocouple associated with the sensor at an axial location coinciding with a flat portion of the axial temperature gradient. Development of any asymmetrical temperature gradient is thereby avoided by preventing any unintended thermal conduction between the outer sheath and the bore surface. In accordance with another embodiment of the invention, the outer sheath of the gamma sensor is radially deformed from a circular cross-section to establish a thermally conductive path by thermal contact with the heater body and the bore surface of the guide tube along the cold regions as well as to function as a centering means.