The Drop Characteristics Of Control Rod In Lead Bismuth Medium

It is critical to research the dropping of control rod for the safety operation of the reactor.In this paper,the drop characteristics of new accelerator drive subcritical system reactor was analyzed.It was the first time to calculate the rod dropping in lead-bismuth medium.and it has reference value for the safety of ADS reactor.In the whole process of rod dropping of ADS,the forces of the rod is analyzed,which contain gravity,mechanical friction,buoyancy,viscous friction and differential pressure resistance.Fortran language was used to develop a programme,through which the relation between time and displacement and the relation between time and speed curve were got.Under normal operating condition,when the lead-bismuth medium is static,the curve of the rod velocity and time is a straight line from Os to Is.After 1s,the slope of the curve decreases.The displacement of control rod is 900mm at 1.43s,and it means the rod can fall into the bottom of the core in the range of 1.5s.This meet the safety requirements of the nuclear power plant.When the lead bismuth medium is dynamic,the drop time increases with the increase of the flow velocity of lead-bismuth.Under seismic condition,the change trend of rod velocity with time is a straight line from Os to 1.21s,and the range of time is slightly larger than the normal condition.And the slope of the straight line is less than that of the normal condition.After 1.21s,it is a curve and the slope of the curve is decreased.The rod cannot arrive at the bottom of the core in time.It is necessary to adjust the weight of the tungsten block on the driving mechanism.The weight of the tungsten block must be bigger than 39kg.Factorial analysis was used to analyze the influence of the weight of rod,the velocity of the lead-bismuth and the mechanical friction on rod dropping.The results show that the influence of the velocity of lead bismuth is the largest.Compared with the characteristics of rod dropping in different media,the results show that,the displacement of the rod in water is greater than that in the sodium under the same conditions.The displacement in the sodium is greater than that in the lead bismuth.The reason is that the density and kinematic viscosity of the three mediums are different.So in the rod controlling of the lead-bismuth medium,the drive mechanism need to add greater weight in order to meet the safety requirements.