Integrated modeling of postulated rod drop incident in a PHWR

Reactivity devices of standard 220 MWe Indian PHWRs include adjuster rods (AR) and regulating rods (RR) which are meant for maintenance of steady power level, adjustment of global power level and distribution and provision of additional reactivity whenever required. There is an upper and a lower absorbing element at each AR/RR location. In a postulated event of mechanical failure of lower absorbing element, it drops out of the core and inserts positive reactivity in the core. Analysis of such an incident is presented in this paper. Time required for the rod to fall through the surrounding medium is estimated by integrating the equation of motion of the rod. The reactivity insertion rate so obtained is used to simulate the associated power transient. It is observed that due to high-reactivity insertion rate, reactor period decreased rapidly, leading to actuation of reactor shutdown system based on low reactor period. This prevents any significant rise in the fuel and clad temperatures. It is found that peak fuel and clad temperatures attained during such a transient are well below the safety limits. (author)