A Novel Electromagnetic Technique for Remote Repositioning of Coolant Tube Spacers In CANDU Nuclear Reactors

Abstract

<p>On August 1, 1983, a sudden failure of a coolant tube, known as a pressure tube, was experienced in one of the CANDU reactors at Pickering 'A' Nuclear Generating Station of Ontario Hydro. A contributing factor to the rupture of the tube was a contact for a period of several years between the tube and an outer coaxial tube, called the calandria tube. The contact between the pressure tube and calandria tube occurred because the annular spacers (garter springs), used to maintain the coaxial configuration when the uranium fuel bundles are loaded inside the pressure tube, had shifted out of their design locations during the construction stage and subsequent hot conditioning of the reactor. The displacement of the spacers was attributed to vibration induced by various sources and was determined to have taken place prior to loading the fuel bundles in the reactor.</p> <p>Since the garter springs are not directly accessible by mechanical means, extensive dismantling of the fuel channels would have been necessary to reposition the springs in their designated locations. This Thesis describes a novel electromagnetic method to reposition without the garter springs without dismantling the fuel channels. The method consists of inserting an electromagnetic coil into the pressure tube to a location adjacent to the spacer and passing a time-varying electric current in the coil to induce secondary currents in the pressure tube and the spacer. The interaction between the induced current in the spacer and net magnetic field in the spacer region results in an electro-magnetic force having an axial component large enough to displace the spacer in the required direction. In practice, current impulses generated by discharging a large capacitor bank were used to achieve the required garter: spring displacement.</p> <p>The new method was successfully developed and impIemented to reposition the displaced garter springs in five new CANDU reactors in Ontario. The savings in the reactor repair cost, interest charges and replacement energy costs were in the order of hundreds of millions of dollars. Equally large benefits and savings will be realized if the need to use this technique in commissioned reactors becomes mandatory. The practical aspects of the work including the design of pulse power cables and a coil sufficiently compact to fit in the pressure tube, but strong enough to withstand the stress, were developed by the author at the Ontario Hydro laboratories. The Thesis describes not only this aspect but also develops a model for analysis of the operation and design of the device.</p> <p>There are a number of other applications of the electromagnetic technique which may also benefit from the analysis, especially for electromagnetic metal forming and fusion technologies.</p>