6.4 Summary
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6.4 Summary
The concept of a two-region-designed pebble bed with the dynamic core is regarded
as a prospective type of future HTGR, which can significantly increase power output
and has good control of reactivity. According to this concept, the core is divided into
two distinct regions; a central region filled with graphite pebbles as moderator and
an outer annular region fully composed of fuel pebbles. As graphite pebbles do not
output power, this design can flatten the distribution of neutron flux and temperature
and consequently brings a significantly higher power output while maintaining the
required fuel safety margin. In addition, the two-region design is much safer for the
reason that the decay heat has to travel a shorter distance through pressure vessel to
the outside of the core when accidents occur.
For a two-region pebble-bed reactor, the first question is whether the two regions
can be established. Consequently, experiments are designed to investigate the establishment process of the two-region arrangement. During operation, the graphite pebbles represented by black pebbles are loaded at the top center while fuel pebbles,
which are colorless, are inserted at several positions on the periphery. It is shown that
a central region filled with black pebbles and side regions completely consisted of
colorless pebbles were shaped after some time of running. Relatively distinct interfaces had separated these 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 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 the pebble’s bouncing on the free
surface, reducing the maximum width of the mixing from 6–7 times to 3–4 times the
pebble diameter.
An attempt is made to determine whether the two-region arrangement is in an
equilibrium state. Three terms are considered as equilibrium conditions. First, the
amount of black and colorless pebbles remained in the vessel is approximately invariable. Second, the shape and size of the two regions mainly keep unchanged. Third,
the size of the mixing zone is invariable and acceptable. Besides, the strategy to eliminate the stagnant zone is suggested. In the case of unchanging the main geometric
design of the reactor core, the modification of the design of the bottom of the reactor
core maybe the most efficient strategy to eliminate the stagnant zone, such as the
increased base cone angle and smooth transition from bases to sides.
Moreover, numerical simulations based on the experiment facility using the DEM
method are conducted. For quantifying the mixing process of the two-region pebble
bed, the Particle-Scale Mixing Index (PSMI) and mixing entropy are used. The
influence of particle size and region width is investigated in different cases at a given
discharge rate. It can be found that
• The mixing region is more complicated in shape for the cases with larger particle
size differences. The fractal dimension increases quantitatively from 1.316 for R D
= 1.0 to 1.393 for R D = 2.0. Both the total particle volume and mixing entropy
439
6.4 Summary
The concept of a two-region-designed pebble bed with the dynamic core is regarded
as a prospective type of future HTGR, which can significantly increase power output
and has good control of reactivity. According to this concept, the core is divided into
two distinct regions; a central region filled with graphite pebbles as moderator and
an outer annular region fully composed of fuel pebbles. As graphite pebbles do not
output power, this design can flatten the distribution of neutron flux and temperature
and consequently brings a significantly higher power output while maintaining the
required fuel safety margin. In addition, the two-region design is much safer for the
reason that the decay heat has to travel a shorter distance through pressure vessel to
the outside of the core when accidents occur.
For a two-region pebble-bed reactor, the first question is whether the two regions
can be established. Consequently, experiments are designed to investigate the establishment process of the two-region arrangement. During operation, the graphite pebbles represented by black pebbles are loaded at the top center while fuel pebbles,
which are colorless, are inserted at several positions on the periphery. It is shown that
a central region filled with black pebbles and side regions completely consisted of
colorless pebbles were shaped after some time of running. Relatively distinct interfaces had separated these 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 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 the pebble’s bouncing on the free
surface, reducing the maximum width of the mixing from 6–7 times to 3–4 times the
pebble diameter.
An attempt is made to determine whether the two-region arrangement is in an
equilibrium state. Three terms are considered as equilibrium conditions. First, the
amount of black and colorless pebbles remained in the vessel is approximately invariable. Second, the shape and size of the two regions mainly keep unchanged. Third,
the size of the mixing zone is invariable and acceptable. Besides, the strategy to eliminate the stagnant zone is suggested. In the case of unchanging the main geometric
design of the reactor core, the modification of the design of the bottom of the reactor
core maybe the most efficient strategy to eliminate the stagnant zone, such as the
increased base cone angle and smooth transition from bases to sides.
Moreover, numerical simulations based on the experiment facility using the DEM
method are conducted. For quantifying the mixing process of the two-region pebble
bed, the Particle-Scale Mixing Index (PSMI) and mixing entropy are used. The
influence of particle size and region width is investigated in different cases at a given
discharge rate. It can be found that
• The mixing region is more complicated in shape for the cases with larger particle
size differences. The fractal dimension increases quantitatively from 1.316 for R D
= 1.0 to 1.393 for R D = 2.0. Both the total particle volume and mixing entropy