Estimate of total control rod worth based on lower dimensional modelling

Estimation of control rod worths by three dimensional methods is very time consuming. Even with the powerful computers, available today, it is difficult to perform multigroup (25 groups and above) calculations in three dimensions for large fast power reactors. In this paper, a simplified theoretical formula for estimating the total control rod worth from its lower dimensional worth estimations has been framed. The control rod worth estimated in 2-D cylindrical geometry modelling has the cylindricity error and that estimated in 2-D hex/X-Y geometry modelling has the buckling error and the worth estimated in 1-D cylindrical geometry modelling has both the cylindricity and the buckling errors. The simplified formula for estimating the rod worths has been framed by efficiently combining the 2-D and 1-D worths based on the assumption that both, the cylindricity and buckling errors are additive. This formula has been verified by actually performing 3-D calculations for one small fast reactor core (10 MWt) and one medium sized fast reactor core (1200 MWt). The results show that the formula is accurate to within 4% and could be effectively used for fast reactor cores.