58
2 Experiments in Pebble Flows
tivity, as well as not to affect neutron flux. Furthermore, the fixation of the guide
ring is also required not to affect the structure compactness of the reactor core. The
competency of the guide ring in a practical reactor needs more theoretical analysis
and experimental verifications.
The stagnant zone is crucial for the safety of the pebble-bed reactor, and it is not
allowed to exist in a practical reactor core. The possible existence of the stagnant
zone is closely related to the inherent friction angle, the friction developing between
the pebbles and the vessel wall, and the structure of the reactor such as the base
angle and the diameter of the discharge hole, etc. An experimental investigation was
carried out to study the possible existence of the stagnant zone under the experimental conditions. The experimental vessel is first filled with about 70,000 pebbles
to form the initial random pebble packing. Pebbles are all colorless for easy visual
observation. The experimental procedure is the same as that described before. As it is
known empirically that the stagnant zone most likely appears in the regions radially
far from the orifice, black pebbles are inserted from the two side-mounted inlet tubes
to highlight the initially filled pebbles that remain in the vessel. Snapshots are taken
at intervals to record the experimental process, which is partly shown in Fig. 2.5, and
the snapshot at the bottom right shows the final state.
As seen in the snapshot of the final state, the region highlighted with enclosed
lines is a group of pebbles that initially were inserted and remained in the vessel
after about 60 h of recirculation. The discharge rate in the experiment is set to be 150
pebbles per minute so that the number of pebbles for recirculation after 60 h is about
540,000, which is approximately 7–8 times the total number of pebbles initially
inserted into the vessel. Consequently, the highlighted region, which is located at
the joint corner of the cone base section and the rectangular main-body section of
the vessel, is considered as a stagnant zone. It is found that the size of the stagnant
zone decreases as the recirculation continues, which means that it is related to the
running time. Therefore, the limitation of the longest residence time (a fuel pebble
allowed to spend in the reactor core based on reactor physics requirement) mainly
determines the existence and size of the stagnant regions in the rector core. Under
the experimental conditions in this chapter, a stagnant zone is present.
The stagnant zone has a clear boundary, and pebbles off or along the boundary
move out of the vessel in the required time. It is suggested that, instead of a sharp
corner transition from the prime rectangular main-body section to the cone base
section of the vessel, a gradual change along the boundary curve of the stagnant region
obtained from experiments would be favorable for avoiding the stagnant region.
2.3.3 Motion of Pebbles
In the recirculating two-region pebble-bed reactor, the motions of individual pebbles
primarily determine the two-region arrangement and the mixing as well as the stagnant zones’ existence in the pebble bed. If pebbles move in a considerably random
and diffusing way, a large random dispersion will subsequently arise in the pebble
2 Experiments in Pebble Flows
tivity, as well as not to affect neutron flux. Furthermore, the fixation of the guide
ring is also required not to affect the structure compactness of the reactor core. The
competency of the guide ring in a practical reactor needs more theoretical analysis
and experimental verifications.
The stagnant zone is crucial for the safety of the pebble-bed reactor, and it is not
allowed to exist in a practical reactor core. The possible existence of the stagnant
zone is closely related to the inherent friction angle, the friction developing between
the pebbles and the vessel wall, and the structure of the reactor such as the base
angle and the diameter of the discharge hole, etc. An experimental investigation was
carried out to study the possible existence of the stagnant zone under the experimental conditions. The experimental vessel is first filled with about 70,000 pebbles
to form the initial random pebble packing. Pebbles are all colorless for easy visual
observation. The experimental procedure is the same as that described before. As it is
known empirically that the stagnant zone most likely appears in the regions radially
far from the orifice, black pebbles are inserted from the two side-mounted inlet tubes
to highlight the initially filled pebbles that remain in the vessel. Snapshots are taken
at intervals to record the experimental process, which is partly shown in Fig. 2.5, and
the snapshot at the bottom right shows the final state.
As seen in the snapshot of the final state, the region highlighted with enclosed
lines is a group of pebbles that initially were inserted and remained in the vessel
after about 60 h of recirculation. The discharge rate in the experiment is set to be 150
pebbles per minute so that the number of pebbles for recirculation after 60 h is about
540,000, which is approximately 7–8 times the total number of pebbles initially
inserted into the vessel. Consequently, the highlighted region, which is located at
the joint corner of the cone base section and the rectangular main-body section of
the vessel, is considered as a stagnant zone. It is found that the size of the stagnant
zone decreases as the recirculation continues, which means that it is related to the
running time. Therefore, the limitation of the longest residence time (a fuel pebble
allowed to spend in the reactor core based on reactor physics requirement) mainly
determines the existence and size of the stagnant regions in the rector core. Under
the experimental conditions in this chapter, a stagnant zone is present.
The stagnant zone has a clear boundary, and pebbles off or along the boundary
move out of the vessel in the required time. It is suggested that, instead of a sharp
corner transition from the prime rectangular main-body section to the cone base
section of the vessel, a gradual change along the boundary curve of the stagnant region
obtained from experiments would be favorable for avoiding the stagnant region.
2.3.3 Motion of Pebbles
In the recirculating two-region pebble-bed reactor, the motions of individual pebbles
primarily determine the two-region arrangement and the mixing as well as the stagnant zones’ existence in the pebble bed. If pebbles move in a considerably random
and diffusing way, a large random dispersion will subsequently arise in the pebble