Electro-Magnetic Forming
: An Emerging Technology for Nuclear Applications R.K.Rajawat, S.V.Desai, K.V.Nagesh, R.C.Sethi |
Today,
Indian Nuclear Industry has taken the shape of a dependable mature
industry. From fuel ore mining to its waste management, it is fully
indigenous. Although as of today, we have mastered most of its critical
and crucial technologies but still we have to have a grip over the
new concepts leading to compact, safer, efficient and hassle free
power delivery systems. To meet these objectives, newer and efficient
technologies are evolving the world over. Electro-Magnetic Forming
(EMF) also called as Pulsed Magnetic Forming (PMF), is one such
technique which is emerging as a viable technique for forming, shaping
and joining metals or even metals to plastics. The principle of operation is based on the application of forces generated as a result of interaction of induced eddy currents with the pulsed magnetic fields. An efficient technique of producing these fields is by discharging the energy stored in a capacitor through an inductive coil placed in the proximity of the job piece. Invariably the job gets repelled from the forming coil.
By suitably placing the coil vis-à-vis the job piece, permanent contraction, expansion, contour formation and joining of mating parts can be achieved. The amount of ‘deformation’ produced depends upon the rate and magnitude of pulsed fields. Good conducting metals such as copper, aluminum, etc., are formed easily because of large magnitudes of eddy currents induced in them. Poor conductors such as stainless steel, zircalloy, etc., require either higher energies or good conducting ‘Drivers’ for their formation. Figure 1 depicts a 2 mm Zircalloy tube expanded to confirm to the shape of a Split die using an Aluminum driver and figure 2 shows free compression of an Aluminum tube to almost a solid rod at an estimated strain rate of 105 per second. In BARC this technique has been established on an experimental basis. Its potential was first realized during the repositioning of ‘Garter Springs’ at nuclear power reactors, Narora Atomic Power Plant (NAPP-II, 1989) and Kakrapar Atomic Power Plant (KAPP-I, 1991). A 20 kV, 80 kJ capacitor bank was employed as a storage device for this job. The technique was extremely successful in repositioning the garter springs. Even today, the channels wherein it was applied are performing remarkably well. This resulted in saving the exchequer a substantial amount of money. Over the course of time, a number of modifications & innovations have been carried out in this technique. A valuable experience has been gained in the design, development & fabriation of capacitor banks and high-pulsed magnetic field coils. During the IX-Plan, fully dedicated EMF equipment; 10 kV, 20 kJ, was developed to demonstrate the welding of Al clad tube to the Al end plug of DHRUVA fuel pin. Feasibility of the existing technique has been established by going through the severest tests. Helium leak rate of these weld joints surpassed the existing requirement of 10-9 std cc/s. Figure 3 shows micrograph of a typical weld achieved by this technique.The signature resembles that of high-energy rate welded joints, as achieved in Explosive Welding Technique.
The complete EMF equipment developed for Atomic Fuel Division (AFD) of BARC during the Plan period is depicted in figure 4.
EMF has also established the feasibility of achieving ‘rolled’ joints for CIRUS Caladria’s Al tube-to Al tube sheet joint. Generally, EMF joints are found to be stronger than the parent metal and same was confirmed during the pull out tests, when the joints failed at the tube rather than at joint. Interestingly, such joints have also been achieved by EMF between Al tube-to SS tube sheet, which are more or less impossible to achieve through the conventional rolling techniques. During the X-Plan, EMF has been selected as a viable option to detach rolled joints of Advanced Heavy Water Reactor. This is a requirement that reactor technologists envisage during re-tubing or de-commissioning stage of the reactor operation. The developed technique, if successful, should provide a neat alternative for decoupling the corroded tubes of existing PHWRs also. Apart from Nuclear Reactors, “Electron and Ion Accelerators Development” is another major DAE programme. EMF technique is being conceived to play a vital role in this area too. Accelerator technology comprises a whole gamut of technologies involving, ultra high vacuum, DC, RF & Microwave accelerating structures, electron and ion beam sources, high voltage power feeds and high power RF & microwave tubes. Each of these technologies makes extensive usage of ‘metal-to-ceramic’ sealing. Forming these seals can very well fall under the umbrella of EMF. This optimism results from the recently established feasibility of achieving vacuum tight joints having less than 10-10 std cc/s helium leak rates. This was attained in typical copper sleeve to Alumina rod/tube joints which in addition, stood a vacuum of 10-6 Torr also. Some of these seals are shown in figure 5.
These joints
were further subjected to a temperatures of 4000C. No deterioration
in the joint performance was observed. These results enthuse the
optimism that the technique is poised to bring in tremendous changes
in the accelerator related technologies too. |
