2.2 Phenomenological Methods
55
horizontal motion is higher than that in the upper section. This is entirely different
from that of the drainage pebble operation mode. Secondly, experimental results
indicate the maximum horizontal motion does not emerge on the slope of the base
cone, but deviates from it for a distance. This suggests that there exists a slow flow
zone near the base cone. Thirdly, the similar propagation process of the pebble motion
is shown through the pre-filled central core method, although it is not as clear as that
in the pre-filled stripes method. It can be expected that the smaller friction of the
particle-wall, smaller vessel width, and larger cone angle, and larger drainage hole
will produce a smaller slow flow zone.
2.3 Pebble Flow in Two-Region Beds
2.3.1 Formation of Two-Region Arrangements
Experiments were designed to investigate the establishment of the two-region
arrangement and the mixing zone between the regions. The vessel was first filled
with about 70,000 colorless pebbles, thus forming the initial state of random pebble
packing. In the recirculation of the pebble bed, black pebbles were loaded from the
central inlet tube with colorless pebbles from the two side-mounted inlet tubes. Meanwhile, pebbles that moved out of the vessel were discharged from a single outlet tube.
After some time, black pebbles were expected to replace pebbles initially filled in
the central region of the vessel, forming the central region of the two-region arrangement. Likewise, loaded colorless pebbles were expected to create the side regions.
Snapshots at intervals recorded different statuses of the development of a two-region
arrangement. The process of the two-region arrangement has already been shown
in Fig. 2.3. Figure 2.9 here shows the final state of the two-region-arrangement. A
central region entirely filled with black pebbles and side regions wholly consisting
of colorless pebbles were shaped after some time of running. Relatively distinct surfaces separated regions. Mixing zones were developed between the central region
and the side regions. The maximum size of the mixing zone was about 4–5 times of
the pebble diameter. Also, a tiny quantity of black pebbles was found to be scattered
in the side regions at the spots relatively far from the central region, as well as a
few colorless pebbles scattered in the central region. The reason for the pebbles’
scattering is that pebbles bounced far away into the opposite regions at the top of
the pebble bed during the pebble loading. In the physical calculations for the tworegion designed PBMR reactor, the mixing zone was statistically estimated to be 25
cm, but for the hot shutdown margin with the control system, 33 cm was used [9],
which is about 5.5 times the diameter of the fuel pebbles. Comparatively, the mixing
zone obtained from the experiment is in reasonable agreement with that practically
utilized, but the safety margin is not large enough.
55
horizontal motion is higher than that in the upper section. This is entirely different
from that of the drainage pebble operation mode. Secondly, experimental results
indicate the maximum horizontal motion does not emerge on the slope of the base
cone, but deviates from it for a distance. This suggests that there exists a slow flow
zone near the base cone. Thirdly, the similar propagation process of the pebble motion
is shown through the pre-filled central core method, although it is not as clear as that
in the pre-filled stripes method. It can be expected that the smaller friction of the
particle-wall, smaller vessel width, and larger cone angle, and larger drainage hole
will produce a smaller slow flow zone.
2.3 Pebble Flow in Two-Region Beds
2.3.1 Formation of Two-Region Arrangements
Experiments were designed to investigate the establishment of the two-region
arrangement and the mixing zone between the regions. The vessel was first filled
with about 70,000 colorless pebbles, thus forming the initial state of random pebble
packing. In the recirculation of the pebble bed, black pebbles were loaded from the
central inlet tube with colorless pebbles from the two side-mounted inlet tubes. Meanwhile, pebbles that moved out of the vessel were discharged from a single outlet tube.
After some time, black pebbles were expected to replace pebbles initially filled in
the central region of the vessel, forming the central region of the two-region arrangement. Likewise, loaded colorless pebbles were expected to create the side regions.
Snapshots at intervals recorded different statuses of the development of a two-region
arrangement. The process of the two-region arrangement has already been shown
in Fig. 2.3. Figure 2.9 here shows the final state of the two-region-arrangement. A
central region entirely filled with black pebbles and side regions wholly consisting
of colorless pebbles were shaped after some time of running. Relatively distinct surfaces separated regions. Mixing zones were developed between the central region
and the side regions. The maximum size of the mixing zone was about 4–5 times of
the pebble diameter. Also, a tiny quantity of black pebbles was found to be scattered
in the side regions at the spots relatively far from the central region, as well as a
few colorless pebbles scattered in the central region. The reason for the pebbles’
scattering is that pebbles bounced far away into the opposite regions at the top of
the pebble bed during the pebble loading. In the physical calculations for the tworegion designed PBMR reactor, the mixing zone was statistically estimated to be 25
cm, but for the hot shutdown margin with the control system, 33 cm was used [9],
which is about 5.5 times the diameter of the fuel pebbles. Comparatively, the mixing
zone obtained from the experiment is in reasonable agreement with that practically
utilized, but the safety margin is not large enough.