A Mechanism Explaining the Instability of Ebr-I, Mark Iii

A US perspective on fast reactor fuel fabrication technology and experience part I: metal fuels and assembly design

“…One at.% of burnup corresponds to roughly 9.4 GWd/MTHM. Important result Loss of reactivity with time Partial melting of the core [14] Improved stability of the fuel [15] More compact core [16] Impurity level amounts (200-500 ppm) of Si were alloyed with the Mark-I metal fuel to minimize the fission gas release, thereby minimizing the cladding stresses that occurred as the fuel swelled radially against the cladding and axially against engineered fuel lift-off restraints. Although minimized by techniques such as Si additions, these swelling-induced stresses led to limitations of the useful lifetime of Mark-IA metal fuels [9,12].…”

Terminal Report for the Mark Iv (Plutonium) Loading in Ebr-I

“…For example, it seems unlikely that even a highly-skilled analyst would have anticipated the difficulties experienced w^ith SRE''^"/ or with the earlier loadings of EBR-I. (13) For extremely small, highly-concentrated cores, such as the Mark IV loading of EBR-I, factors beyond the control and the ability of the analyst combine to limit the usefulness of the direct approach. As the results of many measurements,^ ' including those discussed in this report, it is clear that the feedback is strongly dependent on intangibles such as clearances between fuel rods, between fuel rods and hexes, and even between hexes.…”

“…Total Feedback = Prompt Feedback + Delayed Feedback, (13) in which the first term defines the feedback associated primarily with pow^er-density changes in the fuel, and the second term describes the feedback associated with expansion effects in the cladding, coolant, and structure. On a relative basis, the first term is prompt and the second term is delayed.…”

“…(10) As the result of a comprehensive experimental program, concerned mainly with transfer-function and power-coefficient measurements, it was concluded that those features responsible for the instability noted in the earlier loadings could be eliminated by rather elementary changes in mechanical design. (11)(12)(13) The operation of the Mark IV core, fueled entirely with plutonium, introduced a variety of problems which required careful scrutiny before loading and eventual operation. (14) Although many problems were of a more or les s conventional nature, others were complicated by physical, chemical, and neutronic properties peculiar to plutonium.…”