Following an HCDA the roof of a CT fast reactor may be threatened by coolant impact. Large computer codes are used to analyse model experiments in which roof impact situations occur, but discrepancies between prediction and measurement have given rise to some misgivings over the usefulness of the numerical models. In most instances experimental roof assemblies are made over-strong in order to simulate a fixed roof configuration to simplify the analysis. Recent trends in pool-type LMFBR roof designs however, do not preclude the possibility of structural response during the impact process. Such roof motions could have a feedback effect on the hydrodynamic loading due to fluid-structure interactions.This article aims to develop at a fundamental level understanding of the impact process and assess the relevance and magnitude of fluid-structure interaction effects. Reference is made to four 1/30th scale experiments, set up to verify the ideas developed in this work, and to provide quality data for code validation purposes.The impact of a one-dimensional liquid slug on a solid slab is investigated using a simplified form of the Rankine-Hugoniot shock equations derived under the joint assumptions of slight compressibility and small Mach number; both assumptions are well justified for the applications in mind. In the first instance the roof slab is considered to be freely supported and of finite thickness. A detailed picture of the shock and expansion wave propagations is built up from the basic equations including the effects of wave reflections at boundaries and wave-wave interactions. Particular attention is paid to the impulse transfer mechanism from the slug as this controls the roof slab acceleration. Bulk fluid cavitation effects are noted.Roof flexural response is then taken into account, together with the effects of the hold-down constraints. It is seen that even very minor structural responses can result in significant mitigation of the impulse loading.Guidelines for the application of the work to HCDA analysis in pool reactor geometries is presented.