ANALYSIS AND MODELING OF FLOW-BLOCKAGE-INDUCED STEAM EXPLOSION EVENTSIN THE HIGH-FLUX ISOTOPE REACTOR

This article provides a perspective overview of the analysis and model ing work done to evaluate the threat from steam explosion loads in the High-Flux Isotope Reactor (HFIR) during flow blockage events. The ove rall work scope included modeling and analysis of core-melt initiation , melt propagation, bounding and best-estimate steam explosion energet ics, vessel failure from fracture, bolts failure from exceedance of el astic limits, and, finally, missile evolution and transport. Aluminum ignition was neglected. Evaluations indicated that a thermally driven steam explosion with more than 65 MJ of energy insertion in the core r egion over several milliseconds would be needed to cause a sufficientl y energetic missile with a capacity to cause early confinement failure . This amounts to about 65% of the HFIR core mass melting and particip ating in a steam explosion. Conservative melt propagation analyses hav e indicated that at most only 24% of the HFIR core mass could melt dur ing flow blockage events under full-power conditions. Therefore it is judged that the HFIR vessel and top head structure will be able to wit hstand loads generated from thermally driven steam explosions initiate d by any credible flow blockage event. A substantial margin to safety was demonstrated.