Patent Number: 044118597
Section: description

DETAILED DESCRIPTION Referring now to the drawings in detail, FIG. 1 illustrates a gamma sensor 10 of the type disclosed in the aforementioned prior U.S. Pat. No. 4,298,430. The sensor has an outer sheath 11 enclosing an elongated metallic heater body 12 that is generally cylindrical in shape and of a constant outer diameter except along reduced diameter portions 14 at axially spaced measurement zones of the sensor, one of which is shown. Under the influence of an ambient gamma ray flux, the cylindrical body 12 is heated. The heat so generated along the larger diameter portion of the body can escape radially outward, to sheath 11 and then to the ambient surround, such as flowing reactor coolant or the dry bore 16 of an instrument tube 18. However, the heat generated in portion 14 can not escape radially outward, since the axial gap space 17 forms a thermal resistance. Heat must therefore escape by flowing longitudinally along portion 14 and then radially on either axial side thereof. Accordingly, portion 14 forms a hot region as compared to cold regions on either axial side thereof. A temperature plot or gradient taken along the axis of bore 20 of the body 12 when the sheath 11 is uniformly cooled to a constant temperature, would have the characteristic indicated by curve 22 in the graph of FIG. 2. The temperatures of the hot spot 24 and a cooler spot 26 as shown on curve 22, are sensed by a differential thermocouple 32 with hot junction 28 and cold junction 30 as shown in FIG. 1. The electrical output of the thermocouple is directly dependent on the thermal gradient and is a measure of the gamma ray flux. The sheath 11 would be uniformly cooled to a known temperature if surrounded on all sides with flowing reactor coolant. If the gamma sensor is inserted into a dry instrument tube, some parts of the sheath may bridge the small annular gap 33 between the sheath 11 and the bore 16 of the instrument tube and make poor thermal contact with the instrument well and thereby disturb the symmetrical heat distribution depicted by curve 22 in FIG. 2. For example, if a portion of the sheath 11 were in poor thermally conductive relation to the tube 18, the temperature plot, taken along the axis of the gamma sensor for the same gamma ray flux as before is shown by dotted line 22' in FIG. 2. It will be seen from curve 22' that its peak 24' is not aligned with the hot junction 28 so that the differential temperature signal output of the thermocouple will be in error. To avoid such signal error, establishment of poor thermally conductive thermal condition between sheath and instrument tube is precluded by providing a centering device 34 for each measurement zone as shown in FIG. 1. The centering device 34 as more clearly seen in FIGS. 3 and 4 includes a pair of split ring sections 36 seated within an annular mounting groove 38 on sheath 11. The fact that two ring sections are separable permits them to be inserted into the groove in touching contact with each other. A spring 40 made of a helix with the two ends hooked together, is stretched over the circumferential groove about the exterior edge of the ring sections. The bias of spring 40 holds the ring sections in place. The springiness of the individual turns of spring 40 permits the turns thereof to conform to the shape and diameter of bore 16 over any foreseeable variation in bore size. Since the turns of spring 40 press tightly against the ring sections and bore 16, the thermal resistance between sheath 11 and bore is considerably decreased, compared to that between a bore and a sheath loosely lying within. Furthermore, if the bore is a wet bore, flow of reactor coolant therealong is not significantly hindered, since the fluid can flow along the bore by passing between successive turns of the helix spring 40. Each centering device 34 is located by means of the mounting groove 38 as close as possible to the hot thermal resistance gap 18 but sufficiently spaced therefrom to correspond to a flat portion of the resulting axial temperature gradient on curve 22 shown in FIG. 2 at point 26, which may also be aligned with the cold junction 30 of the thermocouple. Such location of the centering device will insure that there is no disturbance of the symmetrical characteristic of curve 22 because of poor thermal contact between sheath 11 and tube 18. FIGS. 5 and 6 illustrate an embodiment of the invention in which a centering device in the form of a ribbon spring 42 is snapped into groove 38 in sheath 11. The ribbon spring is provided with a split 41 so that it can be spread over the larger diameter of sheath 11. In other respects the gamma sensor of FIG. 5 is similar to that of FIG. 1. While the thermal resistance of the thermal bridge of spring 42 is somewhat higher than that of centering device 34 hereinbefore described, it is adequate for many applications and simpler in construction. The ribbon spring 42 also will not obstruct the longitudinal flow of reactor coolant. FIG. 7 illustrates another embodiment wherein the thermal bridge is made of a springy mass 44 of tubular knitted fabric of very fine stainless steel wire. The knitted tube is collapsed to 30% density, thereby forming a sponge-like sleeve, similar to those used to demist compressed gases. The sponge-like sleeve is slipped over the sheath 11' of a gamma sensor and welded into place. The springy mass 44 will reduce the thermal resistance between sheath 11' and bore 16 and will also permit longitudinal flow of reactor coolant. The embodiment of FIG. 4 has the obvious advantage that the thermal bridge is fixed to the gamma sensor. FIGS. 8, 9, and 10 illustrate another embodiment which provides for good thermal contact between a sheath generally referred to by reference numeral 46 and bore 16 while providing for good centering of the sheath within the bore and permitting longitudinal flow of reactor coolant. As in other embodiments, the sheath 46 is in contact with the cold regions of the heater body for cooling thereof. At portions 48 of the sheath 46 enclosing the gaps 17, the sheath is cylindrical while along portions 50 and 52 it is deformed into an ellipsoid, the major exterior dimension of which contacts the body 12. Along alternate axially spaced portions 50 and 52, the sheath 46 is deformed in perpendicular directions. Because of the sliding fit, there is good thermal contact between the sheath 46 and bore 16. At the same time, the thermal resistance gaps 17 are enclosed by the sheath 46 to preserve its isolation. Good contact is established at major and minor axis points 54 and 56 as shown in FIG. 9 so that 90.degree. spaced portions of the bore and heater body are engaged to provide a better centering action. Except for the enlarged portions 50 and 52, the sheath 46 is cylindrical and in thermal contact with the cold regions. It will be apparent that the thermal bridge afforded by this form of construction is very rugged. The thermal bridges and centering means described herein are useful not only in combination with gamma sensors, which are generally used while fixed in place, but are also useful with the traveling type of gamma thermometers, which are used for scanning operations.