Analysis on Jamming Probability of Ball-type SMR Secondary Shutdown System Using Discrete Element Method (DEM)

Abstract

Over the past few years, there have been many attempts and studies on eliminating soluble boric acid in small modular reactors (SMRs). Soluble boron-free concepts can eliminate boric-acid-induced corrosion and simplify the large components related to Chemical and Volume Control System (CVCS). For this reason, some alternative concepts for replacing soluble boron control system have been proposed as the Secondary Shutdown System. Of the concepts, injecting solid neutron absorbers through independent guide tubes has been widely proposed in various studies. In this Ball-type Secondary Shutdown System, jamming can occur during injection of the absorbers, which can lead to a severe accident. However, the studies on jamming of Ball-type Secondary Shutdown System were insufficient. In particular, jamming is a complex phenomenon influenced by various physical parameters, and it is difficult to repeat experimentally. Therefore, the purpose of this study was determined as follows. First, performing the physical modeling of Discrete Element Method (DEM) to be used for jamming analysis, and validating this model through experiments. Second, performing the quantitative analysis of the jamming probability through the validated DEM model.

Discrete Element Method (DEM) is the most commonly used numerical model for describing the mechanical behavior of the granular material. Various force and torque models have been proposed for DEM analysis. Among them, Hertz-Mindlin contact force model is the most commonly used DEM force model. Therefore, it is used in this study to simulate the behavior of the neutron absorbers in the hopper. The jamming probability obtained from experiment and DEM simulation was compared under the same conditions to verify the validity of DEM for jamming analysis. The jamming probability was 3.21 % higher in the simulation than the experiment. Through this, it was confirmed that the Hertz-Mindlin model implemented the jamming phenomena well within the experimental range.

The validated DEM model was used to investigate the effect of various physical parameters (such as particle diameter, hopper outlet diameter, hopper angle, friction coefficient, geometry size, particle density, and the number of particles) on jamming probability. In all simulations, it was confirmed that the jamming probability decreases as the ratio of particle diameter to hopper outlet diameter increases. Especially, the number of particles was found as one of the most important variables in jamming.

Finally, a jamming analysis method that can be applied when the number of particles is not enough was proposed. An analytic solution to calculate the change of jamming probability according to the number of particles was derived. The variation of the particle passing probability according to the number of particles was confirmed through this solution. As a result, it was confirmed that as the number of particles in the hopper increases, the particle passing probability decreases. This study is significant in that it could be a basis for a jamming prediction model in Ball-type SSS.

Keywords

Discrete Element Method (DEM), Granular Material, Jamming, Ball-type Secondary Shutdown System, Soluble boron free reactor, Numerical Simulation, Probability and Statistics