Fuel pin behavior under the slow power ramp transients in the CABRI-2 experiments

Slow ramp-type transient-overpower tests were performed within the framewor k of the international CABRI-2 experimental program. The implemented power transients of similar to 1% nominal power/s correspond to a control rod wit hdrawal-type accident in a liquid-metal-cooled fast breeder reactor (FBR). The analysis of the tests includes the information elements derived from th e hodoscope signals, which were assessed quantitatively and supported by de structive and nondestructive posttest examinations. These tests, performed with fuels of various geometries, demonstrated the high margin to failure o f such FBR fuel pins within the expected power level before the emergency r eactor shutdown. At the same time, these tests performed with high- and low -smear-density industrial pins led to clarification of the influence of pel let design on fuel pin behavior under high overpower condition. With the hi gh-smear-density solid fuel pellet pin of high burnup level, the retained g aseous fission products played an important role in the solid fuel swelling , leading to clad deformation and failure at a maximum heating rate of 81 k W.m(-1), which is much greater than the end-of-life (EOL) linear rating of the pin. With the low smear-density annular pellet pin, an important fuel s welling takes place, leading to degradation of the fuel thermal conductivit y. This effect was detected at the power level around 73 kW.m(-1), which is also much higher than the EOL value of the pin. Furthermore, the absence o f clad deformation, and consequently of failure even at the power level goi ng up to 134.7 kW.m(-1), confirmed the very high margin to failure. In cons equence, it was clarified that gaseous fission products have significant ef fects on failure threshold as well as on thermal performance during over-po wer condition, and such effects are significantly dependent on fuel design and power operation conditions.