Space-dependent analysis of BWR core nuclear thermal hydraulic instability and thermal margin

Nuclear thermal hydraulic oscillations in BWR cores were analyzed by the space-dependent BWR core transient program STANDY. In a simulation of instability in the Lasalle-2 unit, the oscillations that caused a scram were successfully reproduced. The maximum thermal margin decrement was far smaller than the initial margin, and significant margin to thermal limits existed at the time of scram. An analysis of hypothetical control rod insertion suggested that the oscillations could have been suppressed by only a few control rods. Analyses of a core destabilized by various parameters were also carried out to examine thermal margin sensitivity during the oscillations. The results showed that, regardless of which parameters were assumed to make the core unstable, thermal margin changes were substantially smaller than the initial margin expected under operation conditions to cause an instability.