Neutronic modeling for a modular high temperature pebble bed reactor during a reactivity accident

Neutronic modeling for the modular high temperature pebble bed reactor during a reactivity accident was investigated for safety analysis purposes. Three existing dynamic models, i.e., the point reactor, adiabatic, and improved quasi-static models were compared each other, and their accuracy was discussed. A one dimensional numerical experiment was performed for simulating the severest reactivity accident involving withdrawal of all absorber rods in the reflector region. The results showed that the adiabatic model gave the upper bound of the power excursion since it over-estimated the input reactivity worth. Although the point reactor model required the minimum computation time, it under-estimated the input reactivity worth and gave less accuracy in predicting the power excursion. The improved quasi-static model gave the highest accuracy and its computation time was comparable to the adiabatic model. (author)