Iterative Method of Flow Computation in Guide Thimble during Rod Dropping

In the previous rod dropping computation, it was assumed that the flow in the discharge orifices of the guide thimble are all of outflow throughout the rod dropping course, while, in fact, the inflow occurs. To handle this problem, it is assumed that inflow and outflow exist in each discharge orifice. The energy dissipation is analyzed on every flow section, having changes in shape or area. By introducing the flow indexes to represent the flow status of each orifice, a set of equations of the Bernoulli and flow conservation is subsequently developed by accumulating all flow status with these indexes. The flow status, pressure and velocity in the guide thimble are iteratively solved and used to compute the drag force on the control rod assembly. The dropping time is finally solved by substituting the drag force into the equation of dropping motion. The dropping time is obtained as 2.133 s while it is 2.036 s if the flow reversed is neglected and the outflow is assumed. It is indicated that the flow reverse has a significant effect on the dropping course and dropping time. Hence, this iterative method is a preferable choice of the flow computation for the guide thimble.