[en] A principal source of uncertainty in the energetics of FBR core disassembly is the lack of mechanical and thermophysical data on fresh and irradiated fuel under the conditions of interest. The consequences of uncertainties are analysed in two areas: (i) the equation of state (EOS) or irradiated fuel and (ii) the specific heat of molten fuel. The current UK understanding of the role of fission products in the postulated disassembly phase of an HCDA is outlined giving particular emphasis to the possible effects of pre-disassembly heating. The authors draw as far as possible on the rather sparse experimental data and indicate where further work would be most useful. Further they discuss arguments suggesting there exists substantial uncertainty in the currently-accepted values of the specific heat of molten fuel, and show that this lack of knowledge implies that current estimates of accident excursion yield could be exaggerated by more than a factor of two. (author)