Chapter 6
Applications: Two-Region Pebble Beds
6.1 Experimental Measurements
For a two-region pebble-bed reactor, the first question is whether the two regions can
be established. Consequently, experiments were designed to investigate the establishment process of the two-region arrangement.
The experiment vessel was first filled with about 150,000 colorless pebbles to form
the initial state of the random pebble packing. During the recirculation, black pebbles
were added in the middle, colorless pebbles on both sides, at the ratio of 25:100:25.
Meanwhile, pebbles were discharged from the bottom at 150 pebbles per minute.
After sometime, the black pebbles were expected to replace the pebbles initially
filled in the central region of the vessel, and form the central region of the two-region
arrangement. Likewise, the loaded colorless pebbles were expected to create the side
regions. Different statuses of the development of the two-region arrangement were
recorded by snapshots at intervals, as shown in Fig. 6.1. Because the discharging
rate was set the summation of the three loading rates, the total number of pebbles
remained in the vessel was approximately kept constant at any time. It is shown that a
central region totally filled with black pebbles and side regions completely consisted
of colorless pebbles were shaped after some time of running. Relatively distinct
interfaces separated regions. Mixing zones were developed between the central region
and the side regions. Some black pebbles were found scattering in the side regions
at the spots relatively far from the central region. The maximum size of the mixing
zone was about 6–7 times the pebble diameter, which is too large to accept. At the
same time, the designed size of the central area was expected, namely 440 mm in
width, the 1/5 of the diameter of the central moderator pebbles region designed in
the real pebble-bed reactor.
The reason for pebbles’ scattering is that pebbles bounced far away into the
opposite regions at the top of the pebble packing while they were loaded. So, the
guide plates should be installed at the upper to eliminate the mixings caused by
© Tsinghua University Press 2021
S. Jiang et al., Multiphase Flow and Heat Transfer in Pebble Bed Reactor Core,
https://doi.org/10.1007/978-981-15-9565-3_6
401
Applications: Two-Region Pebble Beds
6.1 Experimental Measurements
For a two-region pebble-bed reactor, the first question is whether the two regions can
be established. Consequently, experiments were designed to investigate the establishment process of the two-region arrangement.
The experiment vessel was first filled with about 150,000 colorless pebbles to form
the initial state of the random pebble packing. During the recirculation, black pebbles
were added in the middle, colorless pebbles on both sides, at the ratio of 25:100:25.
Meanwhile, pebbles were discharged from the bottom at 150 pebbles per minute.
After sometime, the black pebbles were expected to replace the pebbles initially
filled in the central region of the vessel, and form the central region of the two-region
arrangement. Likewise, the loaded colorless pebbles were expected to create the side
regions. Different statuses of the development of the two-region arrangement were
recorded by snapshots at intervals, as shown in Fig. 6.1. Because the discharging
rate was set the summation of the three loading rates, the total number of pebbles
remained in the vessel was approximately kept constant at any time. It is shown that a
central region totally filled with black pebbles and side regions completely consisted
of colorless pebbles were shaped after some time of running. Relatively distinct
interfaces separated regions. Mixing zones were developed between the central region
and the side regions. Some black pebbles were found scattering in the side regions
at the spots relatively far from the central region. The maximum size of the mixing
zone was about 6–7 times the pebble diameter, which is too large to accept. At the
same time, the designed size of the central area was expected, namely 440 mm in
width, the 1/5 of the diameter of the central moderator pebbles region designed in
the real pebble-bed reactor.
The reason for pebbles’ scattering is that pebbles bounced far away into the
opposite regions at the top of the pebble packing while they were loaded. So, the
guide plates should be installed at the upper to eliminate the mixings caused by
© Tsinghua University Press 2021
S. Jiang et al., Multiphase Flow and Heat Transfer in Pebble Bed Reactor Core,
https://doi.org/10.1007/978-981-15-9565-3_6
401