[en] In a loss-of-flow (LOF) accident in an LMFBR, the mode and timescale of disruption of fuel can establish the probability of a subsequent energetic excursion. To investigate these phenomena, in-pile disruption of internal-fission-heated fuel pellets, both fresh UO₂ and preirradiated mixed oxide, was recorded by high speed cinematography. Neither fuel frothing nor dust-cloud breakup occurred under the simulated LOF conditions. Instead massive and very rapid fuel swelling, not predicted by current transient fuel-swelling and gas-release models, occurred. This may be the initial and dominant mode of fuel disruption in a LOF. A new transient fuel-swelling model, FISGAS, has been developed at Sandia