Trapped stack related PCI fuel failures

A total of eight ANF fuel rods failed in Barsebeck Unit 1 over the years 1980 to 1982. An extensive program, including hot cell examination, was conducted to determine the cause of these failures and to avoid a recurrence of the problem. The results of the investigation showed that: 1. The fuel pellet characteristics were not a factor. 2. The fuel rod design was characterized by a large pellet cladding gap (10 mil), and a fill gas pressurization of one atmosphere absolute. These features played a major role in the failures through a thermal instability process. 3. The method of operation of Barsebeck Unit 1, characterized by slow control rod withdrawals at full power, can lead to a ''trapped stack'' condition. The slow uncovering of the lower part of the fuel rod provided adequate time for the thermal instability condition and associated fuel swelling effects to propagate down the rod, while stress relaxation of the cladding was prevented by the biaxial stress state created under the trapped stack condition. In addition, volatile fission products were released from the fuel at the tip of the control rod, and were deposited on the inner cladding surface. This occurred on the nearby cold region just below the control rod tip which caused the formation of ''coins'' which enhanced the PCI. 4. Recommendations were issued which avoided a recurrence of failures with this fuel design and this mode of operation. 5. The problem is not likely to occur with current fuel designs which have higher pressurization and smaller gaps. (author). 5 refs, 6 figs, 1 tab