440
6 Applications: Two-Region Pebble Beds
of the mixing interface in the relatively steady state increase with the rise of size
ratio for R D >1.0, which is hardly influenced by region ratio RL. On the other
hand, when RL increases, the width of the central region is reduced and the mixing
interface becomes closer to the centerline (axis) of the bed.
• The effects of R D and R L on the mixing degree reflect the different influencing
mechanisms. The former is caused by the size and the number of particles that
reflect the spatial distribution effects of particles. In contrast, the latter is caused
by dynamical discharge features that are related to temporal movement effects.
• The effect of region ratio on PSMI of two-region pebble bed is also remarkable.
There is an increasing tendency of PSMI when the region ratio or size ratio rises.
When the size ratio varies from 0.5 to 2.0, the diffusion of small particles into
the 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. Also, 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.
In addition, the two-region pebble flow of a three-dimensional model based on
real reactor HTR-10 is simulated, aiming to verify the feasibility of two-region core
design and to investigate the effect of loading ratios and density difference. First of
all, the feasibility of the two-region core design is verified. Issues concerning the
feasibility of the two-region design, including the establishment and maintenance of
the two-region arrangement, the mixing zone on the interface, and the stagnant zone
existence, are analyzed.
A method to determine the boundary of the central region has been proposed,
and it is found that after a period of circulation flow, the pebble bed can reach the
stable state with invariable central boundary and a stable discharging ratio, which
is consistent with loading ratio of middle and side pebbles, and the mixing zone
between the regions is constrained to a reasonable and acceptable range. The mixing
extent at different heights and dispersion of pebbles are investigated, indicating a
higher level of mixing at the bottom.
Then, to study the effects of density difference and loading ratio, issues about the
size of the central region, stagnation condition, and the retention of initial loading
pebbles are analyzed. Results show that the loading ratio does not influence the
retention rate, but it would significantly affect the two-region configuration. Both
the shape of the central region and the stagnant condition would be affected by
pebble density. Compared to single-density conditions, increasing the middle pebble
density (L-H-L condition) can accelerate the pebble flow and reduce the size of the
central region, leading to a larger stagnant area and longer total retention time. This
is not beneficial for the core safety. When increasing the side pebble density (H-LH condition), although the flow of middle pebbles is much slower and the central
6 Applications: Two-Region Pebble Beds
of the mixing interface in the relatively steady state increase with the rise of size
ratio for R D >1.0, which is hardly influenced by region ratio RL. On the other
hand, when RL increases, the width of the central region is reduced and the mixing
interface becomes closer to the centerline (axis) of the bed.
• The effects of R D and R L on the mixing degree reflect the different influencing
mechanisms. The former is caused by the size and the number of particles that
reflect the spatial distribution effects of particles. In contrast, the latter is caused
by dynamical discharge features that are related to temporal movement effects.
• The effect of region ratio on PSMI of two-region pebble bed is also remarkable.
There is an increasing tendency of PSMI when the region ratio or size ratio rises.
When the size ratio varies from 0.5 to 2.0, the diffusion of small particles into
the 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. Also, 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.
In addition, the two-region pebble flow of a three-dimensional model based on
real reactor HTR-10 is simulated, aiming to verify the feasibility of two-region core
design and to investigate the effect of loading ratios and density difference. First of
all, the feasibility of the two-region core design is verified. Issues concerning the
feasibility of the two-region design, including the establishment and maintenance of
the two-region arrangement, the mixing zone on the interface, and the stagnant zone
existence, are analyzed.
A method to determine the boundary of the central region has been proposed,
and it is found that after a period of circulation flow, the pebble bed can reach the
stable state with invariable central boundary and a stable discharging ratio, which
is consistent with loading ratio of middle and side pebbles, and the mixing zone
between the regions is constrained to a reasonable and acceptable range. The mixing
extent at different heights and dispersion of pebbles are investigated, indicating a
higher level of mixing at the bottom.
Then, to study the effects of density difference and loading ratio, issues about the
size of the central region, stagnation condition, and the retention of initial loading
pebbles are analyzed. Results show that the loading ratio does not influence the
retention rate, but it would significantly affect the two-region configuration. Both
the shape of the central region and the stagnant condition would be affected by
pebble density. Compared to single-density conditions, increasing the middle pebble
density (L-H-L condition) can accelerate the pebble flow and reduce the size of the
central region, leading to a larger stagnant area and longer total retention time. This
is not beneficial for the core safety. When increasing the side pebble density (H-LH condition), although the flow of middle pebbles is much slower and the central