RETRAN modeling of the PWR rod ejection transient

The rod ejection transient is a postulated Condition IV event initiated by the mechanical failure of a control rod mechanism pressure housing. Such a failure results in the rapid ejection of a road cluster control assembly from the core followed by a fast reactivity insertion. A sever asymmetric core power distribution may result, possibly leading to fuel rod damage. Reactor protection for the transient is provided by negative reactivity feedback effects and by reactor trips on high neutron flux levels. This transient has been modeled for Virginia's Power's Surry and North Anna Nuclear Power Stations using RETRAN-02. The analysis is performed in two parts. First, the core average power history is calculated using a single-loop model with point kinetics and three axially stacked core control volumes. The ejected rod's reactivity is inserted linearly over 0.1 seconds. The negative reactivity feedback effects due to Doppler and moderator temperature changes, and the reactor trip are also modeled. The effect of the locally peaked core flux shape is approximated by applying a conservative power weighting factor to the Doppler reactivity feedback. The core average power history is adjusted to represent peak core power conditions and input to the hot spot thermal-hydraulics analysis model. The hot spot model predicts maximum fuel enthalpy and temperature transients. This model has two control volumes, one for the hot spot location and the second for a sink for flow from the hot channel. From these results, the amount of fuel damage and the radiological consequences can be assessed