2.4 Pebble Flow Mechanism Analysis
67
pebbles flow out from the orifice one by one, while voids spread upwards through the
pebble packing; in this way, the various arrangements of the pebbles are gradually
formed. The propagation of the voids can be regarded as a kind of disturbance.
Furthermore, pebble motion is not simultaneously to be activated as soon as
the voids emerge. Firstly the equilibrium arches are created with several pebbles
inside the pebble packing, when certain conditions are met. When the equilibrium
conditions are broken down with the pebbles around the arches moving away, a fresh
local flow starts at the place where the arch is formed, during which the above pebbles
are motionless, because there is no new disturbance propagated over the equilibrium
arches. This means that the equilibrium arches cut off the propagation of the void
disturbance from underneath. Furthermore, the size of the voids below the arches is
not large but only with the volume of less than one pebble to several pebbles because
the size of the pebbles is much less than that of the vessel.
Part A and Part B of Fig. 2.14 display the arrangement of the pebbles inside
the pebble packing, which is only the two-dimensional arrangement of the pebbles
for the sake of graphic expression. Part A shows a tight arrangement of pebbles, in
which the pebble of No. 9 is restricted and locked by several surrounding neighbors,
undergoes long-lasting frictional contacts with each other, gathers in crowds, and
groups to move down together under gravity, when the supporting pebbles, e.g., No.
7 and No. 8, move away. However, it is not always the case of being closely restricted
or locked by their neighbors. It is possible for the pebbles to travel through the voids
inside the pebble packing and temporarily become free travelers. During flowing, the
assembly of the pebbles is squeezed, stretched, and rotated. Then the new assembly
of pebbles is consequently formed in a self-organization manner. This is a so-called
self-organization mechanism of the quasi-static pebble flow, which is related to the
distribution of the inside voids, geometric parameters, and material parameters. It is
the self-organization that causes the mixing of the different pebbles during quasistatic pebble flow, in which the competition mechanism of pebbles exists.
2.4.4 Short Summary
The present section draws the following conclusions from the results of the experiments performed on the two-dimensional scaled model of a practical pebble-bed
reactor core. Under the experimental conditions, a stable two-region arrangement in
the core model is found to be formed and maintained. The mixing zone between the
regions is constrained to a reasonable size. Guide plates are effective in reducing the
mixing zone with no noticeable effect on the shape and the size of the two-region
arrangement. However, the application of the guide ring in a practical reactor core
needs further theoretical and experimental verifications. A stagnant zone is present
at the corner of the core model under the experimental conditions. The existence
and size of the stagnant zone are related to running time and are determined by the
requirement of the physical design of the reactor core. The motion of individual
pebbles in the pebble bed is random to some extent, which results in the existence
67
pebbles flow out from the orifice one by one, while voids spread upwards through the
pebble packing; in this way, the various arrangements of the pebbles are gradually
formed. The propagation of the voids can be regarded as a kind of disturbance.
Furthermore, pebble motion is not simultaneously to be activated as soon as
the voids emerge. Firstly the equilibrium arches are created with several pebbles
inside the pebble packing, when certain conditions are met. When the equilibrium
conditions are broken down with the pebbles around the arches moving away, a fresh
local flow starts at the place where the arch is formed, during which the above pebbles
are motionless, because there is no new disturbance propagated over the equilibrium
arches. This means that the equilibrium arches cut off the propagation of the void
disturbance from underneath. Furthermore, the size of the voids below the arches is
not large but only with the volume of less than one pebble to several pebbles because
the size of the pebbles is much less than that of the vessel.
Part A and Part B of Fig. 2.14 display the arrangement of the pebbles inside
the pebble packing, which is only the two-dimensional arrangement of the pebbles
for the sake of graphic expression. Part A shows a tight arrangement of pebbles, in
which the pebble of No. 9 is restricted and locked by several surrounding neighbors,
undergoes long-lasting frictional contacts with each other, gathers in crowds, and
groups to move down together under gravity, when the supporting pebbles, e.g., No.
7 and No. 8, move away. However, it is not always the case of being closely restricted
or locked by their neighbors. It is possible for the pebbles to travel through the voids
inside the pebble packing and temporarily become free travelers. During flowing, the
assembly of the pebbles is squeezed, stretched, and rotated. Then the new assembly
of pebbles is consequently formed in a self-organization manner. This is a so-called
self-organization mechanism of the quasi-static pebble flow, which is related to the
distribution of the inside voids, geometric parameters, and material parameters. It is
the self-organization that causes the mixing of the different pebbles during quasistatic pebble flow, in which the competition mechanism of pebbles exists.
2.4.4 Short Summary
The present section draws the following conclusions from the results of the experiments performed on the two-dimensional scaled model of a practical pebble-bed
reactor core. Under the experimental conditions, a stable two-region arrangement in
the core model is found to be formed and maintained. The mixing zone between the
regions is constrained to a reasonable size. Guide plates are effective in reducing the
mixing zone with no noticeable effect on the shape and the size of the two-region
arrangement. However, the application of the guide ring in a practical reactor core
needs further theoretical and experimental verifications. A stagnant zone is present
at the corner of the core model under the experimental conditions. The existence
and size of the stagnant zone are related to running time and are determined by the
requirement of the physical design of the reactor core. The motion of individual
pebbles in the pebble bed is random to some extent, which results in the existence