402
6 Applications: Two-Region Pebble Beds
Fig. 6.1 Establishment of the two-region arrangement
the pebble’s bouncing on the free surface. As a result, the maximum width of the
mixing zone was improved and was smaller on a scale of about 3–4 times the pebble
diameter, which is larger than that in the vessel with a height of 1,000 mm.
It is noted that the head of the developing central region is comparatively flat in
Fig. 6.1, before t = 6 h, which means the pebble flow is relatively uniform in the upper
of the pebble packing. It is seldom influenced by the base cone of the vessel and can
be called plug flow. It is not the case in the lower part of the vessel due to the base
cone effects. In the physical calculations for two-region designed PBMR reactor, the
mixing zone was statistically estimated at 25 cm. Still, for the hot shutdown margin
with the control system, 33 cm was used, 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.
6 Applications: Two-Region Pebble Beds
Fig. 6.1 Establishment of the two-region arrangement
the pebble’s bouncing on the free surface. As a result, the maximum width of the
mixing zone was improved and was smaller on a scale of about 3–4 times the pebble
diameter, which is larger than that in the vessel with a height of 1,000 mm.
It is noted that the head of the developing central region is comparatively flat in
Fig. 6.1, before t = 6 h, which means the pebble flow is relatively uniform in the upper
of the pebble packing. It is seldom influenced by the base cone of the vessel and can
be called plug flow. It is not the case in the lower part of the vessel due to the base
cone effects. In the physical calculations for two-region designed PBMR reactor, the
mixing zone was statistically estimated at 25 cm. Still, for the hot shutdown margin
with the control system, 33 cm was used, 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.