Patent Number: 044252978
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawings in detail, FIG. 1 illustrates a typical boiling water reactor installation, generally referred to by reference numeral 10, for a gamma radiation type of sensor, generally referred to by reference numeral 12. The sensor 12 is inserted into the reactor from the top by means of a gripping cone 14, in a manner already known in the art. When inserted, the lower end of the sensor projects from the bottom wall 16 of the reactor vessel through a high pressure gland 18 and is connected by means of a pin socket coupling 20 to a signal cable 22 extending to the instrumentation site for the reactor. The sensor 12 is vertically positioned as shown between adjacent fuel assemblies 24 having vertically enlongated channels 26 enclosing bundles of fuel rods 28. These fuel assemblies extend vertically within the reactor vessel between a top fuel guide structure 30 and a lower grid 32 below which the sensor projects. Some of the fission products from the fuel rods during power generation, in the form of gamma radiation is detected by the sensor 12 at a plurality of vertically spaced measurement zones. In practice, there are between four and ten of such measurement zones from which local power generation measurements are obtained through each sensor. The outer diameter of the sensor is made as large as practically possible for the available space between the adjacent fuel assemblies, and is externally exposed to a body of coolant within the reactor vessel in order to establish a uniform heat sink temperature therefor. As more clearly seen in FIGS. 2 and 3, showing an enlarged portion of the gamma sensor 12, an elongated monolithic gamma radiation absorbing body 34 is provided made of a material such as stainless steel No. 316 or zircaloy which generates heat when exposed to gamma radiation, without any change in its properties. The heat so generated flows radially outward to the external surface 36 of the body 24 which is maintained at a heat sink temperature by the coolant, such as water, in direct contact therewith throughout. A plurality of double junction thermocouple cables 38 is mounted internally within the body 34 and is connected to the instrument cable 22, in order to measure differential temperatures at the axially spaced measuring zones, one of which is shown in FIGS. 2 and 3. The sensor, therefore, has associated therewith differential thermocouple junctions 40 and 42 for each measuring zone as shown in FIG. 3. The thermocouple cables are mounted within a central bore 44 formed in the elongated body 34 through which its longitudinal axis extends. The body is generally cylindrical and of a constant diameter along a major portion thereof interrupted at reduced cross-sectional area portions 46 located within each measuring zone. The axial interruptions in the otherwise constant diameter of the body at the reduced diameter portions 46 form cold regions within which the thermocouple junctions 42 are located. As a result of such configuration, the temperature gradient in the measurement zone follows the curve 48 shown in FIG. 4. As shown in FIGS. 3 and 4, the cold junction 44 is located approximately midway within the cold region surrounding the reduced diameter portion 46. The hot junction 40 adjacent the tip of the thermocouple cable 38 is aligned with the constant diameter portion of the body 34 adjacent to the reduced diameter portion 46 in axially spaced relation to cold junction 42. Thus, temperature measurements made through the thermocouple cable will be influenced by one-dimensional, radial heat flow through the body 34 during reactor power operation at the axial locations of the thermocouple junctions 40 and 42. The temperature differential (.DELTA.T) between the interior of body 34 and the heat sink surface for such a radial heat flow arrangement is approximately .DELTA.T=(9r.sup.2 /4K), as compared to the equation applicable to an axial heat flow arrangement; .DELTA.T=(qL.sup.2 /8K). In the foregoing equations, (q) is the rate of heat generated by gamma ray absorption, (r) is the radius of the major portion of the body, (k) is its thermal conductivity and (L) is the axial length of the reduced diameter portion of the body. Thus, for both pressurized and boiling water types of reactors, the temperature differential signal obtained from an axial flow type sensor is a function of axial length of the reduced diameter portion of the heater body of the sensor. When utilizing a radial heat flow type of sensor in accordance with the present invention, the differential temperature signal obtained is a function of the outer diameter or radius (r) of the sensor body. Accordingly, by use of a larger diameter sensor for the larger space available in a boiling water reactor as compared to a pressurized water reactor, a larger differential signal output is theoretically possible. In practice, the differential temperature measurement through the raidal heat flow sensor is up to 4.degree. C. in a pressurized water reactor and is up to 22.degree. C. in a boiling water reactor. The foregoing equations are approximate in that they omit a negligible factor depending on the radius of the reduced diameter portion of the sensor body and disregard those heat losses which are mimimal only for the radial heat flow type of sensor. Such heat losses are avoided because the entire external surface of the body 34 is in direct thermal contact within the coolant establishing a uniform heat sink temperature throughout. Thus, more accurate and reliable differential signal measurement of local power generation is achieved. The only drawback may reside in the limitation on the magnitude of the temperature differential imposed by the more restricted space for the sensor in a pressurized water reactor. Whether such a drawback is significant will depend on the signal noise level to be encountered. In the design and construction of the sensor 12, the dimensions of the reduced diameter portions 46 are not significant, as hereinbefore demonstrated, in so far as signal level is concerned, but do affect the structural strength of the sensor. In order to strengthen the sensor and offset the weakening effect of the reduced diameter portions, radial fins 50 are provided as more clearly seen in FIG. 2. These fins are made of material having a high structural strength and a high thermal conductivity so as to have a negligible affect on heat sink temperature and the radial flow of heat. The accuracy of the measurements obtained through the sensor 12 will also depend on its calibration before installation. Calibration is effected by passing an electric current longitudinally through the sensor body 34 by connection to an electrical power source 52, as diagrammed in FIG. 5, causing internal electrical heating of the body. A non-uniform volt drop ordinarily occurs along the length of the sensor body as indicated by curve 54 in FIG. 6, because of the higher electrical resistance of the reduced diameter portions 46. The volt drop is therefore modified to render it uniform as indicated by curve 56, by establishing current paths in parallel with the reduced diameter portions 46 of the body during electrical heating for calibration purposes. Toward that end an annular filler ring 58 is fitted about the reduced diameter portions of the body as shown in FIG. 5. The filler is made of a material having high thermal and electrical conductivity as well as a low melting point temperature. Thus, silver solder, etc., may be suitable. The quantity of the filler utilized in such as to equalize the volt drop per unit length of the reduced diameter portion 46 with the major diameter portions to obtain the uniform volt drop curve 56 shown in FIG. 6. The high conductivity of the filler will avoid any additional heating affect on the body 34. The low melting point for the filler will enable it to be readily removed by melting after calibration is completed. Once the body 34 is electrically heated after being prepared as herein described, a differential temperature signal (.DELTA.T) is obtained across the thermocouple junctions 40 and 42 as shown in FIG. 5 and the heating current varied in order to plot the signal (.DELTA.T) versus the heating effect of the current as indicated by the calibration curves 60 in FIG. 7. The slopes of these curves plotted for each sensor individually, represent sensitivity factors in terms of .degree.C. per watt, per gram. In practice, the sensitivity factors for the radial heat flow sensors is between 4 and 40.degree. C./watt/gram.