Patent Number: 053316762
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 show an induction furnace 22 for heat treating portions of workpieces, in particular the ends of nuclear fuel rod tubes 24 (one such tube being shown), and a temperature probe 26 that is structured to respond to induction heating in substantially the same way that the tubes 24 respond. The induction heater 22 is adjusted or set up to obtain a flat temperature profile over the portion of the workpiece 24 to be heated, using the probe 26 to determine indirectly the temperature profile that will be obtained when treating the tubes 24. The probe 26 is then removed from the induction furnace 22 and a fuel rod tube 24 is inserted. Whereas the fuel rod tube 24 and the probe 26 have essentially identical temperature responses to the induction heating electromagnetic field, the fuel rod tube 24 is heated to the temperature profile required, which was set up using the temperature probe 26. The fuel rod tubes 24 comprise a zirconium alloy, and are heat treated along an end portion 32 prior to irradiation in a nuclear reactor, to develop a protective cladding 34 of zirconium oxide from two to fifteen microns in thickness, as shown in FIGS. 2 and 3. Preferably, the temperature profile achieved in the induction furnace 22 is flat along the treated length 32, the temperature profile being adjusted or controlled to obtain an even temperature between about 650.degree. and 750.degree. C. along the length 32 of the tube 24. This even profile results in the most efficient heat treatment because the resulting coating 34 of oxide is of even thickness in an amount sufficient to protect the fuel rods 24 from fretting damage, and no time or electrical energy are expended unnecessarily in forming an area in the coating that is unnecessarily thick. Conversely, there is no portion of the end of the tube 24 that is not adequately protected by a too-thin coating. Preferably, the fuel rod tubes 24 are protected in this manner over an endmost four to eight inches, or 10 to 20 cm, namely the length of the fuel rods 24 that extends from the lowermost grid in a fuel assembly (not shown) holding a number of such fuel rods. The induction furnace 22 includes conventional means for generating an alternating electromagnetic field in a cavity 42, and preferably is adjustable to set the required field strength in zones 44 that are adjacent one another axially along the probe 26 and/or the fuel rod 24. The cavity 42 is dimensioned to closely receive at least one tube 24, and also may receive more than one. However, in an arrangement for treating more than one tube 24 at a time, it is preferable to use more than one probe 26 (or to use the probe in one position when there is a tube in each other position), so that the field strength to which the probe 26 is subjected is the same as for a tube 24 in the same position, and the same heating effects are achieved. The temperature probe 26 carries a number of temperature sensors 52 and has conductive tubular structure 54 dimensioned to correspond closely to the nuclear fuel rod tubes 24 at the end 32 to be treated. Accordingly, the probe 26 is at least as long as the cavity 42, and preferably protrudes from the cavity 42 by a distance sufficient that the induced currents from the induction field are affected by the portion of the probe 26 protruding from the cavity 42 in the same way that the currents in a fuel rod tube 24 are affected by the portion that protrudes, even though the fuel rod tube 24 is much longer than the probe 26. The fuel rod tube 24 may be 3 m in length and 0.374" (about 1 cm) in diameter. For a heat treatment cavity, for example, of eight inches (20 cm) in length, the probe 26 is preferably at least fourteen inches (36 cm) in length, thus protruding from the cavity 42 by six inches or 16 cm. In any event, the probe 26 is structured as to diameter, length, wall thickness and shape to respond substantially the same as the fuel rod tube 24 to inducement of current by the electromagnetic field of the induction heater 22. It is possible to make the conductive portion of the probe 26 from zirconium, i.e., the identical same material as used for the tube. However, with heating of the probe over successive measurement cycles, an oxidation layer builds up on a zirconium probe in the same manner that an oxidation layer builds up on the tubes when heated. In addition, numerous cycles of heating can affect the crystalline structure of zirconium alloy, which also would alter the response of the probe over time, and make the response of the probe different than that of a fresh fuel rod tube. Therefore, the conductive portion of the probe 26 according to the invention is an alloy material that is not affected by a buildup of oxide over time, preferably Inconel 600 alloy. The entire conductive tube of the probe can be Inconel 600, or Inconel 600 can be provided at least on the outer surface of a probe that is otherwise made of Zircalloy, to resist oxidation. Inconel 600 alloy is resistant to structural changes due to heating. The alloy is compatible with Zircalloy, and resists galvanic corrosion and oxidation. However, a probe of Inconel 600 alloy has electromagnetic induction and heat transfer properties that are very similar to those of Zircalloy. Accordingly, the alloy probe closely models the performance of an untreated tube, and its response remains stable over a number of heating cycles. The induction furnace 22 can be structured as an eight inch (20 cm) long cylinder or the like, subdivided into zones 44 in which the electromagnetic field strength can be set up or adjusted separately. The furnace 22 can comprise ferromagnetic structures, and preferably is made substantially from Inconel 600 alloy, for the same properties that are advantageous for the probe. The furnace has a bore that closely fits either the tube 24 or the probe 26, having only sufficient clearance to enable the fuel rod tube 24 or the probe 26 to be moved in or out, allowing for temperature expansion. The bore can be 0.375 inches in diameter, slidably accommodating either the probe 26 or the tube 24. At the inner end of the bore 42, the bore can complement the shape of the distal end of the fuel rod tube 24 or the probe 26, both of which have the same shape and dimensions. By making the bore in the ferromagnetic material of the furnace 22 nearly the same size as the probe 26 and tube 24, any air gap and associated dissipation of the electromagnetic field strength is minimized. A plurality of temperature sensors 52 are disposed at spaced points along the probe 26, preferably corresponding with the heating zones 44. The zones can be placed, for example, at one inch (2.5 cm) centers, with the temperature sensors 52 centered in their respective zones. The temperature sensors 52 are mounted in heat transfer relationship with the conductive casing structure 54 of the probe, for example being attached to the inner wall of the tubular metallic casing of the probe 26, thereby sensing the probe temperature at a discrete location along the probe. Each of the probe temperature sensors 52 is operable to produce a signal representing the temperature of the probe 26 at the respective zone 44. The sensors 52 are wired to the proximal end 56 of the probe 26 by leads 58 passing internally through the probe 26. A connector 62 or similar means can be provided for coupling the temperature sensors 52 to means 72 for decoding the signal, such as a meter or readout. Alternatively, the leads 58 can extend from the proximal end 56 of the probe 26 for suitable connection to meters, amplifiers, digitizers, feedback controls or other similar means. After monitoring the temperatures obtained at each zone 44 on the probe 26 with induction heating, the electromagnetic field in the cavity 42 can be adjusted or controlled for each zone 44 to obtain predetermined operating conditions. The induction furnace 22 produces an alternating electromagnetic field that induces a current in the conductive structure 54 of the probe 26 or tube 24. The currents produced are eddy currents, and their energy is released substantially in resistive heating of the conductive structure 54. Individual adjustment of the zones 44 is preferable because the localized induced currents are produced as a function of the coupling of the field to the conductive structure 54. Adjacent the distal end 74 of the probe or tube, which is closed, the conductive material 54 extends transversely across the axis of the probe or tube, whereas at intermediate zones the conductive structure 54 is simply a section of hollow cylinder. In order to obtain the same flux density in the more extensive conductive material at the end 74, as is obtained in the less extensive conductive material at intermediate points (thereby to obtain a comparable temperature rise), the field intensity is adjusted accordingly. The temperature sensors 52 can be special type K thermocouples, disposed in the probe 26 at regularly spaced points and arranged to sense the temperature of the thermally and electrically conductive metal portion 54 of the probe 26. These points can be linearly spaced along the probe 26 as in FIG. 2, or arranged at different angles around the probe 26 as suggested by FIG. 1. Also, as shown in FIG. 4, the temperature sensors 52 can be placed on opposite sides of the tubular casing 54 in an alternating manner. Any of these mountings is effective to measure the probe temperature in the respective zone. FIG. 4 shows an internal structure of the probe 26, the same reference numbers being used throughout the drawings to identify the same elements. In a preferred embodiment, eight special type K thermocouples are placed in the probe 26 at one inch intervals, beginning at the insert or distal end 74 of the probe so as to also monitor the temperature at or adjacent the chamfer 76 or similar shape at the end 74 of the fuel rod tube 24. Wire leads 58 from each thermocouple pass along the body of the probe 26 through a potting material 78 such as magnesium oxide, to the proximal end 56 of the probe 26, protruding from the furnace 22. The proximal end 56 can be sealed, and provided with connector means 62 for coupling the thermocouples to the meters, digitizers or similar means for converting the thermocouple signals to temperature information. The temperature information as thus obtained can be used initially when setting up the furnace 22 by individual adjustment of the power output in each respective zone 44 of the induction furnace 22, to obtain a flat temperature profile. After initial set up, the probe 26 can be used as necessary to monitor the profile being maintained, allowing regular recalibration, calibration following maintenance procedures, etc. The probe 26 is also useful in the event of a change of operating conditions, for example if the furnace 22 is to be set up to process fuel rod tubes for a shorter time and higher temperature or a longer time at lower temperature. These adjustments can be made via manual controls, as shown schematically in FIG. 2, or the probe 26 can be coupled to a control means (not shown) for automatically determining the power levels needed for a predetermined (e.g., flat) temperature profile, and setting the furnace 22 to maintain such levels. Accordingly, the method of heat treating a workpiece 24 such as a nuclear fuel rod tube, or other workpiece having an at least partly conductive structure, involves providing a temperature probe 26 with a conductive portion 54 substantially corresponding in response to electromagnetic induction heating to the conductive structure of the workpiece 24, and a plurality of temperature sensors 52, spaced on the probe 26. The probe 26 is inserted into the induction furnace 22, and heated by electromagnetic induction of current in the conductive portion 54 of the probe 26. Temperatures of the probe 26 are detected at different points by the temperature sensors 52, for verifying or setting the required temperature profile. After such verifying or setting of the temperatures as generated in the probe 26 by induction and detected in the probe by the temperature sensors 52, the probe 26 is removed. The workpiece 24 is inserted into the induction furnace 22 in place of the probe 26, and heat treated. By first adjusting at least one of a power level and a duration or duty cycle of the electromagnetic induction as a function of the temperatures of the probe 26 as detected by the temperature sensors 52, using a probe 26 that responds to the induction energy in a manner similar to the fuel rod tube or other workpiece 24, the workpiece is heat treated at the required temperature profile. The invention having been disclosed in connection with the foregoing variations and examples, additional variations will now be apparent to persons skilled in the art. The invention is not intended to be limited to the variations specifically mentioned, and accordingly reference should be made to the appended claims rather than the foregoing discussion of preferred examples, to assess the scope of the invention in which exclusive rights are claimed.