A low-power testing program was completed in January 1966, and operation cycles at 20, 50, 75, 90, and 100 MW began. From the time it attained its design power of 100 MW in September 1966, a little over five years from the beginning of its construction, until it was temporarily shut down in late 1986, HFIR achieved a record of operation time unsurpassed by any other reactor in the United States. By December 1973, it had completed its 100th fuel cycle, each lasting approximately 23 days.