Active Magnetic Bearings for Rotating Machinery in Future Gas Cooled Reactor Plants

In the past, the bearing options available to circulator designers have been essentially limited to gas and oil lubricated systems. In the case of the former, a technological limitation in size has restricted their use to small auxiliary systems. Oil-lubricated bearings are the mainstay of the industry, and many of the circulators operating in the carbon dioxide cooled reactors in the United Kingdom will see service into the second decade of the next century. However this technology (developed in the 1960's) has been superseded by recent bearing advancements, namely the introduction of active magnetic bearings in heavy duty rotating machinery. For the next generation of gas-cooled reactors, namely the modular helium reactor (MHR), the long-term goal of a practical lubricant-free system will be realised with the utilisation of magnetic bearings. This paper covers work done to date on magnetic bearings for helium systems and includes design studies for large machines, experimental testing of vertical rotor systems, and the design and fabrication of a high speed, 10 kWe induction motor drive. The development work done to date is shown to be supportive of vertical rotating machinery needed in future gas cooled reactors, and other potential applications, including the following:- 1) helium turbomachine for the gas turbine plant (GT-MHR) 2) main and secondary system circulators for the future plants involving direct and intermediate helium loops (e.g. high temperature nuclear process heat) 3) shutdown cooling circulator, and 4) rotating machinery in auxiliary systems (e.g. helium purification, gas buffer supply) The magnetic bearing technology base to facilitate near-term deployment of the MHR is emphasised in this paper.