Patent Number: 046702121
Section: summary

FIELD OF THE INVENTION Our present invention relates to a temperature sensor and, more particularly, to a rapid-response temperature sensor and especially a fast-response temperature sensor for measuring the temperature in a coolant loop of a pressurized water nuclear reactor. BACKGROUND OF THE INVENTION It is important to provide temperature sensors with high response times in nuclear reactor applications and, in particular, for measuring the temperature at upstream and downstream sides of the nuclear reactor core in a coolant loop of a pressurized water nuclear reactor. A pressurized water nuclear reactor (PWR) generally has a coolant loop or circulation path which includes a pump for displacing the water coolant under pressure through the nuclear reactor core, an expansion chamber or accumulator for taking up pressure surges, and as a heat exchanger, a boiler or vapor generator which is heated by the coolant at elevated temperatures derived from the PWR. In a system of that type, a temperature sensor can be provided to effect three principal functions: 1. Measurement of the power generated. The measurement of the power generated can be effected by application of a law of the type: EQU P=Q.sub.1 -Q.sub.2 with EQU Q.sub.(1,2) =MCT.sub.(1,2) in which: Q is the quantity of heat supplied per unit of time and Q.sub.1 is the heat per unit of time at the inlet of the exchanger while the heat flow at the outlet of the heat exchanger is represented at Q.sub.2 ; PA0 M is the mass flow of the heat carrying fluid per unit of time; PA0 C is the specific heat of the fluid at the temperature under consideration; and PA0 T is the absolute temperature of the fluid. If the quantities of the fluid entering and leaving the exchanger are equal: EQU P=MC.DELTA.T where .DELTA.T=T.sub.1 -T.sub.2. The precision with which the power can be measured thus required precise knowledge of the quantity of water circulated in the loop, the absolute temperature in order to determine with precision the specific heat, and the difference between the inlet and outlet temperature of the exchanger or the vapor or steam generator. The energy produced by the nuclear reactor can only be withdrawn in a useable manner under thermodynamicokinetic conditions in the core of the reactor which can be generalized as requiring a high volumetric flow rate and a low thermal gradient. Present instruments permit determination of flow rates with the precision of about 1% and determination of temperature with a precision of about 0.1.degree. C. 2. The safety of a PWR reactor is assured utilizing various sensors disposed throughout the plant perhaps the most important of them are temperature sensors which provide an indication of the instantaneous temperature prevailing at critical points in the cooling loop. As a consequence, the speed with which the temperature sensor responds to a change in temperature is basic to avoidance of incidents or accidents and to permit personnel to take immediate action to safeguard the reactor and its environment. Present standards require that the thermometric sensor be capable within a maximum of four seconds, of providing a temperature which represents 63% the true value of a changed temperature to which the sensor is subject, i.e. 63% of the true temperature or temperature variation. 3. It is essential, considering the lowered exposure levels now deemed to be permissible for nuclear power plant operating personnel to provide the sensors so that they may be readily replaced in a minimum of time and with a minimum of complexity. In the past, as the discussion below will show in greater detail, it has been deemed to be advantageous to provide the temperature sensors in bypasses of the pressurized water loop, i.e. in additional piping which is separate from the traversed by the main pressurized water flow. This system had the disadvantage of complicated mounting and dismounting, thereby requiring prolonged presence of the plant personnel at the high radiation sites. Because the temperature was taken in a bypass, moreover, the response time to the actual temperature in the cooling loop was low. This system could, however, utilize a sensor which made use of the change in resistivity of a platinum wire. Another arrangement which was used provided a tubular finger or closed-end sheath which could project into the path of the liquid and into which a sensor was inserted. The adjustment of the sensor within the sheath was complex, irregular clearances were defined and the response time was poor because of the clearance required between the sensing end of the sensor and the sheath. The system here was also susceptible to significant vibration which caused rapid deterioration of the sensor. OBJECTS OF THE INVENTION It is the principal object of our present invention to provide a temperature measuring instrument which overcomes the drawbacks of the prior art temperature measuring instruments described above and which, in particular, represents a significant advance in the use of platinum filaments as variable resistivity sensors. Another object of this invention is to provide an instrument which has a greater precision of temperature measurement than prior art instruments, which has a more rapid response to temperature variation and indeed a response which is in excess of the standard now applied for nuclear reactor application, and which allows the sensor to be replaced conveniently and simply. It is also an object of our invention to provide an improved temperature-measuring instrument whose static and dynamic characteristics are such that the measurements obtained are more reproducible than has hitherto been the case. SUMMARY OF THE INVENTION According to the invention a thermally conductive tubular element or sheath, closed at one end, is mounted so that at least this end projects into the path of the circulated water of a pressurized water reactor in the form of a glove-like finger, the finger having a cylindrical bore terminating at its end within the path of the water in a blind bore defined by a uniform thickness bore. The temperature sensor is provided with a platinum filament whose change in electrically conductivity or resistivity provides a measurement of the temperature, and which extends through this bore and has a sensing tip which is likewise tapered frustoconically to fit snugly within the frustoconical taper of the blind bore so that over the length of the tip and at least the greater part of the length of the blind bore there is snug, firm, all-around surface contact between the wall of the bore and the surface of the tip. We have found that this eliminates the vibration effect entirely and provides highly reproducible dynamic and contact effects and further ensures a rapid transfer of heat through the wall of the sheath so that the response at the sensor far exceeds the acceptable standards applied heretofore in nuclear reactor applications. More generally the configuration of the instrument allows it to be adapted to various liquid containing structures, for example, enabling it to be inserted and sealed in place in the pipelines or other elements of the cooling loop. Both the response time and the insensitivity to vibration are greatly increased over prior art systems. Since the instrument can be inserted directly into pipelines, it is not necessary to provide bypass paths. Advantageously, the sensor is pressed by elastic means, e.g. a coil spring, in the direction of taper into the blind bore and thus is spring biased to maintain the firm contact of the tip in the sheath. According to a feature of the invention, the temperature sensor is fixed in the sheath by a sleeve connected by a screwthread to the open end of the sheath which lies outside the pipe and forms an overhang against which the coil spring is seated, the opposite end of the coil spring being braced against a shoulder of the sensor in a direction biasing the latter in the direction of the taper of the tip of the sensor and the closed end of the sheath. The taper angle has been found to be important and specifically we have determined that the angle between the generatrices of the tapered surface and the axis should be three degrees with a tolerance of 1 minute. Advantageously, the parts of the sensor and the sheath which are in surface contact with one another in the tapered region should have a roughness with an arithmetic mean value Ra less than 0.2. Preferably and by way of a best mode example, the sheath can be formed from a forged or rolled austenitic stainless steel such as that represented by the standard designation Z2 CND 17-12 with a wall thickness of 3.5 millimeters while the temperature sensor has an outer member formed from pure silver which has been subjected to surface treatment and contains a platinum wire resistor having a resistance of 100 ohms at 0.degree. C. for use in the primary cooling loop of a pressurized water nuclear reactor. Under these conditions the response time is 2.56 seconds to 63% of the temperature increase at a temperature of about 285.degree. C., a water pressure of about 155 bars and a water velocity of 6 meters per second. According to a feature of the invention, the sheath need not be drawn or machined from a single piece but can be realized in two parts, one of these parts forming the shank while the other part forms the blind bore. The first part, therefore, can be cylindrical and open at both ends while the second part is frustoconical and forms a closure for the end of the first part to be received in the pipe. From the practical point of view it is preferred to start with a central cylindrical bore in a block of material and to drill from the tail end with traction to form the frustoconical zone with high precision. The surfaces are then polished and ground. Of course, the instrument can be fabricated by other means. A preferred alternative mode of fabricating the sheath is by electrodischarge machining or electrochemical machining. The tip of the sensor can be turned, swaged or otherwise shaped to form the taper and can be polished and otherwise finished. Advantageously, the interior of the sensor which receives the platinum wire of given resistivity at a particular temperature can maintain the thermal contact between the wire and surrounding shell of the sensor by compaction of high purity boron nitride in a powdered state between shell and wire.