Circulator performance during THTR trial and post-commissioning operation

Since the first trial runs the circulators have scored more than two years of trouble-free operation during the trial operation and commissioning phases. At no time the circulators gave reason for plant shutdown. The circulators which had been ordered in 1972 were delivered already ten years ago. Only two out of the six circulators were tested on a test stand in the workshop. In 1987 another identical standby circulator was supplied in support of an initiative to increase the basic conditions of plant availability. In view of the excellent result of the test period it can be assumed that the standby circulator will never be required. The 6 circulators equipped with oil bearings are each provided with a separate oil system whose pressure corresponds to that of the primary system. Safety aspects resulting from licensing requirements led to extensive efforts regarding the oil systems. These efforts involve also that part of the buffer helium flow which is released from the circulators with the oil draining from the bearings. During the commissioning phase it has been verified that the internal barrier helium system of the circulators is capable to retain the lubrication oil inventory within the circulator range, even if the external make-up barrier helium supply from the helium purification system is shut off. Thus it is ensured that leakage of oil into the primary helium system is ruled out by the shaft seal system. Further development work on the circulators is currently being continued for the only reason that active magnetic bearings permit vertical arrangement of the circulators with the impeller at the lower end without requiring the operation of an extremely complicated and expensive oil system. Apart from that, the experience gained with the THTR operation did not result in any changes in the design characteristics which had already been used in the AVR circulators. 7 refs, 13 figs