Experimental investigations of the power-ramp failure propensity of light-water reactor fuel

The report reviews experimental investigations, performed at Studsvik, Sweden, of the failure propensity of LWR fuel under power up-ramp conditions. Fuel reliability and reactor production capacity are affected by this performance problem under normal reactor power operation. At present, costly power manoeuvering restrictions are required to minimize the occurrence of failure, in particular during rapid uprating of the core. The power-ramp failure propensity has also some safety implications, namely the release of radioactivity, which must be considered in connection with an accidental rapid increase of core reactivity. Several power-ramp test programmes with different scopes and objectives have been, and are being, executed in parallel using the 50-MW(th) reactor R2. Up to now more than two hundred such fuel ramp tests have been performed. The more comprehensive investigations aim primarily at establishing the power-ramp failure thresholds for normal LWR fuel. Design and operational parameters are being varied. In general the results reveal a strong propensity to ramp failure of fuel rods that are subjected to large and fast ''over-power'' operations. The destructive mechanisms and other criteria relevant to the failure event are discussed. One comprehensive programme of this type, the Studsvik Inter-Ramp Project, is now being executed as a joint international effort. Fourteen organizations representing fuel manufacturers, power reactor utilities and national research organizations in Europe, Japan and the United States of America are participating. Fuel design modifications and special operational modes aiming at suppressing the power-ramp failure probability are also being investigated. Some of the approaches adopted seem quite promising as indicated by the results so far obtained. (author)