Patent Number: 046702121
Section: description

SPECIFIC DESCRIPTION In FIG. 1 we have shown a prior art primary coolant loop for a pressurized water nuclear reactor. The primary pump 5 feeds the water to the reactor at an elevated pressure and a pressurizer or accumulator 3 is provided upstream of the steam or vapor generator represented as a boiler 4. The temperature sensors are here represented at 6 and 7 and are disposed in respective circuits 8 and 9 corresponding to hot and cold loops, respectively forming bypasses of the main coolant circulation path. Obviously the presence of such additional bypass loops can create, in a nuclear reactor installation, problems which range from increased capital cost, because of the need to ensure that all of the auxiliary or bypass paths conform to the high safety standards for nuclear reactors, to the difficulty of replacing the temperature sensors. Large numbers of welded joints, valves and the like must be fabricated with the exceptional care required for nuclear reactor components and the bypass system effectively constitutes a thermal storage which effects the speed with which the sensors can respond to a change in conditions in the primary coolant. Indeed, the need to remove the sensors in the course of use of the reactor makes it necessary to provide the isolating valves as well as valves 12 for voiding the radioactive liquid from the apparatus and a reservoir or other means shown at 13 to recover the voided radioactive liquid. FIG. 2 shows a portion of a sensor 6 drawn to a larger scale. In this system a cable 14 has its two leads connected at 15 to the opposite ends of a platinum wire 16 shown disposed in a coil and whose resistivity is measured to indicate the instantaneous temperature. Early systems of this type wire compelled to put up with the comparatively short life of the sensors and the fact that the piping in which the sensors are installed have a fragile characteristic made necessary by the need for means enabling rapid change of the sensors to minimize the presence in the contaminated or radioactive zones by the personnel for changing them. In a second arrangement also belonging to the prior art and represented in FIG. 3, the tubular sheath 17, which is closed at its end, has a cylindrical bore 18 in which the cylindrical sensor 19 with its platinum wire is received with clearance. Even the most precision manufacture of this assembly can not always ensure precise arrangements between the sensor 19 and the sheath 17 so that the response time of this unit is poor. The inability to ensure precise positioning of the two components contributes to the sensitivity of the instrument to vibration, such vibration causing rapid deterioration of the sensor. The sensor shown in FIGS. 4-6 and 8 can be utilized by threading it directly into a pipe 20 of the circulation and indeed two sensors are shown in the primary coolant circulation of FIG. 7 at 22, 23. In particular, each sensor comprises a tubular sheath 22 which is closed at its end projecting into the path of the liquid in the pipe, this sheath being constituted like a rigid finger of a glove whose open end is accessible from the exterior of the pipe 20. The temperature sensor 23 is removably received in this sheath. Approximately midway along its length, the sheath 22 is provided with a male thread which is engaged in a female thread of the pipe 20, the threaded connection being represented at 24 and being sealed by a weld deposit 25 all around the instrument. Over close to its entire length, the sheath 22 is provided with a cylindrical bore 26 extended at the closed end of the sheath by a blind bore 27 of frustoconical form. In the embodiment of FIG. 4 beyond the cylindrical bore, the frustoconical bore 27 is formed in a cap fitted by a joint 30 sealingly welded onto the sheath 22. The latter thus is composed of two parts, namely, the principal part 29 provided with a cylindrical bore and the cap part 28. A butt weld may seal these two parts together at the joint 30. Alternatively, the frustoconical part may be formed in the piece 29 (FIG. 6) and the frustoconical end can be closed by a plug 32 welded to the main part of the sheath at 33. In the embodiment of FIG. 5, the cap 28 is slightly shorter than that of FIG. 4. FIG. 4 also shows the sensor 23 which is of generally cylindrical form and has an end hereinafter referred to as a tip, which is likewise frustoconical and which, as can be seen from FIG. 8, can be defined by a frustoconical shell 34a receiving a plug 34b and a coil holder 34c of electrically nonconductive material defining a space 34d receiving the turns 34e of a platinum wire coil whose ends 34f and 34g are connected to the conductor cable 34h and are led thereby out of the sensor. Compacted boron nitride can be provided around turns and between the turns and the shell 34a for effective heat conduction between this silver shell and the platinum wire. The temperature measuring instruments 22, 23 can be disposed directly in the primary cooling path (FIG. 7) advantageously immediately downstream of the pump 5 and immediately upstream of the boiler 4 although other arrangements may be used. As can be seen from FIG. 4 as well the tip 34 is connected to a cylindrical shank 23a formed at its upper and with an annular shield 36 against which a coil spring 35 is seated. A boss 23b at the upper end of the shank 23a is externally threaded and can be engaged by a threaded sleeve 38 having an inwardly extending overhang 37 against which the spring 7 bears. By simply loosening the sleeve 38 and removing it, the pressure of the coil spring sealing the tip 34 in the frustoconical bore 27 will be eliminated and the sensor easily withdrawn from the sheath. It will be apparent, therefore, that the sensor of the invention can be readily replaced and removed and tests have shown that the frustoconical fit of the tip 34 in the bore prevents detrimental vibration and provides greatly increased response rates. Naturally the need for bypass piping is also avoided.