Modified Point Kinetics Analysis of Some HTR Transients

Point Kinetics analysis is used as the first approximation to the transient analysis of nuclear reactors. For small cores and for certain transients such an analysis yields reasonable results but, for large cores and for various modes of transients a space dependent transient analysis is needed. Space-Time Kinetics is rather time consuming analysis that needs extensive computer resources. The Pebble Bed High Temperature Reactor (PBHTR) core is very thin and long, with a reflector column placed at the center of it. Presence of graphite and TRISO particles within the core and the absence of metal and phase changing coolant make such a core rather unique. Due to inherent safety characteristics of the design many transients may be tolerated much easily, but a large aspect ratio of the core dimensions may result in difficulties in the analysis of certain transients. Space-Time Kinetics approach for a Pebble Bed High Temperature Reactor may be needed but in order to justify such an elaborate analysis one has to analyze certain transient with Modified Point Kinetics Approach and compare the results with standard point kinetics analysis. The position dependent neutron fluxes, during a transient, obtained from static calculations with the aid of VSOP computer code is used in evaluating the modified kinetics parameters. Standard point kinetics analysis of the transient is performed with the obtained modified point kinetics parameters. Comparison between these methods yields rather profound differences in the behavior of the time dependent flux throughout the transients. It can be concluded that Space – Time Kinetics analysis of some transients should be studied in detail. (author)