50
2 Experiments in Pebble Flows
gravity in the manner of self-organizations, where the collisions between pebbles are
no longer the dominant factor, due to insufficient space to develop.
2.2.3 Side Area Method
The side area method is designed to characterize the stagnant zone located in the
lower corners of the vessel. The stagnant zone plays a vital role in the overall flow
field of the dense pebble flow. It is crucial for the safety of the pebble-bed reactor.
The stagnant zone is defined based on the burn-up level of fuel pebbles and is not
allowed to exist in a real reactor.
Firstly, colorless pebbles were pre-filled in the vessel to form the initial state of
random packing, then colorless pebbles were added in the middle, black pebbles
were added on both sides, with the ratio of 14:122:14. Meanwhile, pebbles were
discharged from the bottom with 150 pebbles per minute. The guide plates were not
installed. As known empirically, the stagnant zone was most likely to appear in both
corners of the vessel. The inserted black pebbles from the two sides were supposed to
highlight the initially filled pebbles that remained in the vessel, as shown in Fig. 2.5.
As seen in the snapshots, two stagnant zones were gradually enclosed up by the
black pebble area and the sides and bases of the vessel, which were initially filled
and remained in the vessel. After about 22 h, the number of recirculated pebbles was
about 2.8 times of the total number, and the stagnant zone still existed. However, the
pebbles in the stagnant zone were not completely at rest. It can be found the size of
the stagnant zone decreased as the recirculation continues. Figure 2.5d shows the size
of the stagnant zone after 30, 40, 50, and 60 h and illustrates that the stagnant zone
exists under the experimental conditions. This is not acceptable in a practical reactor
core. Thus, it could be suggested that a smooth transition from base to sides, a larger
base cone angle, a larger discharge hole, and a narrower vessel will be favorable to
avoid the stagnant region.
2.2.4 Pre-filled Stripes Method
The pre-filled stripes method is designed to display the vertical movement of the
pebble flow. In the beginning, several preloaded stripes of black pebbles horizontally settled in the pebble bed pre-filled with colorless pebbles. Then, colorless glass
pebbles were added from both the middle and sides; meanwhile, pebbles were discharged from the bottom with 150 pebbles per minute. The guide plates were not
installed. Snapshots were taken at intervals to record the experimental process, as
shown in Fig. 2.6.
The change of the preloaded stripes approximately expresses the distribution of
the flow field and displays the propagation of the pebble motion in the pebble packing.
Measuring the distance between the stripes that descended at regular intervals may
2 Experiments in Pebble Flows
gravity in the manner of self-organizations, where the collisions between pebbles are
no longer the dominant factor, due to insufficient space to develop.
2.2.3 Side Area Method
The side area method is designed to characterize the stagnant zone located in the
lower corners of the vessel. The stagnant zone plays a vital role in the overall flow
field of the dense pebble flow. It is crucial for the safety of the pebble-bed reactor.
The stagnant zone is defined based on the burn-up level of fuel pebbles and is not
allowed to exist in a real reactor.
Firstly, colorless pebbles were pre-filled in the vessel to form the initial state of
random packing, then colorless pebbles were added in the middle, black pebbles
were added on both sides, with the ratio of 14:122:14. Meanwhile, pebbles were
discharged from the bottom with 150 pebbles per minute. The guide plates were not
installed. As known empirically, the stagnant zone was most likely to appear in both
corners of the vessel. The inserted black pebbles from the two sides were supposed to
highlight the initially filled pebbles that remained in the vessel, as shown in Fig. 2.5.
As seen in the snapshots, two stagnant zones were gradually enclosed up by the
black pebble area and the sides and bases of the vessel, which were initially filled
and remained in the vessel. After about 22 h, the number of recirculated pebbles was
about 2.8 times of the total number, and the stagnant zone still existed. However, the
pebbles in the stagnant zone were not completely at rest. It can be found the size of
the stagnant zone decreased as the recirculation continues. Figure 2.5d shows the size
of the stagnant zone after 30, 40, 50, and 60 h and illustrates that the stagnant zone
exists under the experimental conditions. This is not acceptable in a practical reactor
core. Thus, it could be suggested that a smooth transition from base to sides, a larger
base cone angle, a larger discharge hole, and a narrower vessel will be favorable to
avoid the stagnant region.
2.2.4 Pre-filled Stripes Method
The pre-filled stripes method is designed to display the vertical movement of the
pebble flow. In the beginning, several preloaded stripes of black pebbles horizontally settled in the pebble bed pre-filled with colorless pebbles. Then, colorless glass
pebbles were added from both the middle and sides; meanwhile, pebbles were discharged from the bottom with 150 pebbles per minute. The guide plates were not
installed. Snapshots were taken at intervals to record the experimental process, as
shown in Fig. 2.6.
The change of the preloaded stripes approximately expresses the distribution of
the flow field and displays the propagation of the pebble motion in the pebble packing.
Measuring the distance between the stripes that descended at regular intervals may