424
6 Applications: Two-Region Pebble Beds
voids among big particles should be taken into consideration. The PSMI in the mixing
region only is investigated. Due to the diffusion between multi-sized particles, the
mixing degree in the mixing region will be enhanced by the rising size ratios.
In general, increasing particle size ratio will have a more evident influence on
the mixing and dispersion of pebbles than increasing the region ratio of the tworegion bed. In addition, when large-sized particles of large inertia (e.g., for R D =2.0)
flow downward, the stagnant region will decrease significantly. On the contrary, the
stagnant region will be rather large for R D <1.0.
6.3 Density Difference and Loading Ratio Effects
It is necessary to verify the feasibility of two-region core design, to study the flow
characteristic of pebbles, and to investigate the effect of loading ratios and density
difference. Therefore, the two-region pebble flow of a three-dimensional model based
on real reactor HTR-10 is simulated. This will benefit the design and arrangement
of a two-region pebble bed and provide a better understanding of the flow pattern
inside the pebble-bed reactor.
6.3.1 Simulation Setup
For verifying the feasibility of the two-region core design and exploring the flow
pattern in real pebble beds, a three-dimensional model based on the HTR-10 reactor is applied, which is composed of a cylinder as the main body and a tapered
hopper as the bottom. There are five inlets at the top of the bed for inserting fuel
particles with one set in the top center, and the other four symmetrically arranged
around. Besides, a discharging hole is located at the center of the hopper bottom
to form a dynamic reactor core. By setting the loading and unloading speeds, the
number of pebbles remained in the core can be controlled. The simulated cylinder
is 1,800 mm × 2,500 mm in diameter, and more details about the parameters used in
the simulation are listed in Table 6.3.
The effect of density difference on the two-region-designed dynamic core was
investigated here. In the referential case (i.e., the single-density case), all pebbles in
the middle and side regions have the same density of 1846 kg/m
3 . By changing the
density of pebbles within the middle region and the side region, two other conditions
were conducted. One has light pebbles inserted in the middle region while heavy
pebbles in the side region (named as the “H − L − H " cases). The other one inserts
heavy pebbles in the middle region while light pebbles in the side region (named as
the “L − H − L" case). The densities of the light pebbles and the heavy pebbles are
1846 kg/m
3 and 2399.8 kg/m
3 , respectively.
The simulation process can be divided into two stages: the initial stacking stage and
the circulation flow stage. During the initial stacking period, a cylindrical baffle with
6 Applications: Two-Region Pebble Beds
voids among big particles should be taken into consideration. The PSMI in the mixing
region only is investigated. Due to the diffusion between multi-sized particles, the
mixing degree in the mixing region will be enhanced by the rising size ratios.
In general, increasing particle size ratio will have a more evident influence on
the mixing and dispersion of pebbles than increasing the region ratio of the tworegion bed. In addition, when large-sized particles of large inertia (e.g., for R D =2.0)
flow downward, the stagnant region will decrease significantly. On the contrary, the
stagnant region will be rather large for R D <1.0.
6.3 Density Difference and Loading Ratio Effects
It is necessary to verify the feasibility of two-region core design, to study the flow
characteristic of pebbles, and to investigate the effect of loading ratios and density
difference. Therefore, the two-region pebble flow of a three-dimensional model based
on real reactor HTR-10 is simulated. This will benefit the design and arrangement
of a two-region pebble bed and provide a better understanding of the flow pattern
inside the pebble-bed reactor.
6.3.1 Simulation Setup
For verifying the feasibility of the two-region core design and exploring the flow
pattern in real pebble beds, a three-dimensional model based on the HTR-10 reactor is applied, which is composed of a cylinder as the main body and a tapered
hopper as the bottom. There are five inlets at the top of the bed for inserting fuel
particles with one set in the top center, and the other four symmetrically arranged
around. Besides, a discharging hole is located at the center of the hopper bottom
to form a dynamic reactor core. By setting the loading and unloading speeds, the
number of pebbles remained in the core can be controlled. The simulated cylinder
is 1,800 mm × 2,500 mm in diameter, and more details about the parameters used in
the simulation are listed in Table 6.3.
The effect of density difference on the two-region-designed dynamic core was
investigated here. In the referential case (i.e., the single-density case), all pebbles in
the middle and side regions have the same density of 1846 kg/m
3 . By changing the
density of pebbles within the middle region and the side region, two other conditions
were conducted. One has light pebbles inserted in the middle region while heavy
pebbles in the side region (named as the “H − L − H " cases). The other one inserts
heavy pebbles in the middle region while light pebbles in the side region (named as
the “L − H − L" case). The densities of the light pebbles and the heavy pebbles are
1846 kg/m
3 and 2399.8 kg/m
3 , respectively.
The simulation process can be divided into two stages: the initial stacking stage and
the circulation flow stage. During the initial stacking period, a cylindrical baffle with