LWR fuel performance in the United States of America. A user's perspective

The history of commercial LWR fuel is traced over the past two decades to today's highly reliable performance from the perspective of the fuel user, the utility. Commercial power plant operation in the United States of America has resulted in a fuel experience data base of gigantic proportion, over five million fuel rods. This experience has shown that the main aspects of fuel performance, which impact the plant availability and capacity factor, are: (a) reliability and mechanical integrity, and (b) the margin between fuel operating limits and fuel damage limits (i.e. thermal margin), which is established to ensure acceptable fuel conditions under normal operations as well as during postulated accident events. In the 1960s and 1970s fuel failure epidemics due to hydriding and pellet/cladding interaction, respectively, each contributed to significant plant capacity factor losses, as high as 8% for some plants. During this same period other fuel problems, such as densification and clad collapse and fuel rod bow, contributed almost as much through their effect on thermal margin. Concerted industry efforts to provide an understanding of the phenomena and to develop remedies have been largely successful and operation of today's fuel yields much lower capacity losses, typically 2 to 3% for BWRs and approx. 1/2% for PWRs. These capacity losses do not arise from fuel failures, however, but from fuel failure avoidance tactics employed by utilities. It is this less-than-optimal operation that provides the incentive to further improve fuel reliability and to develop a truly forgiving fuel design. US utilities, fuel vendors and the government are co-operating to achieve this goal. (author)