1.3 Pebble Flows
11
piston-pressured container are fairly independent of bed height unless the height to
diameter ratio becomes less than 10 [91].
Besides, the mechanical and thermal properties of pebble beds have coupled
effects, and the thermomechanical characteristics are critical for the reactor design
[92, 93]. For example, the compression force and gas pressure were tested[94], along
with the thermal conductivity at different values of temperature [95]. The strains
and stress analysis by finite element method, as well as their effects on the thermalhydraulics were also checked, although the coupled effects sometimes were not found
[96]. Moreover, the effective thermal conductivity was found not to be influenced
by the chemical composition of solid material whereas it could be affected by filling
gas such as helium clearly [97].
1.3.4 Importance of Flow Uniformity
In reactor engineering, the pebble path through the reactor core, the resident time
in the reactor core, and the optimization of fuel composition [98], are of crucial
importance. Excessive residence time inside the bed may cause severe irradiation
and thermal damage to the pebble, as well as possible escape of fission products. In
an extreme case, it may lead to permanent adhesion between pebbles [89]. On the
other hand, the residence time, as well as axial dispersion coefficient can be used
to characterize the axial dispersion and the mixing phenomenon of the coolant gas
flow, which are useful for the safe design and efficient operation of the reactors [99].
For example, the pebble flow dynamics were studied in a scaled-down test reactor
by using a non-invasive Radioactive Particle Tracking (RPT) technique [100]. They
used a Cobalt-60 based tracer to mimic pebbles in terms of shape, size, and density.
Across-correlation based position reconstruction algorithm and RPT calibration data
were used to obtain the Lagrangian trajectories, velocity field, and residence time
distributions. Moreover, the pebble size was found to strongly affect axial dispersion
and mixing in the packed pebble-bed reactor [99].
Moreover, the pebble motion in the reactor core is expected to follow “First-InFirst-Out" sequence to ensure each fuel pebble can reach almost the same burn-up
level when it is drained out [101]. In addition, Ref. [102], studied the scaling properties of granular flow in pebble-bed reactors via DEM simulation in a supercomputing
Centreon Rosa, a Cray XE6 system with 47,872 cores, where the pebbles are scaled
down by 3:1 and 6:1. The simulation results showed the feature of pebble stresses
and dust generation due to pebble wear. For example, the 6:1 simulation indicated a
very different feature with higher levels of wear, particularly pebble-wall wear.
Accurate flow characterization and description of the dynamics of pebbles in
pebble beds are essential with respect to the basic reactor design calculations, the
optimization of the fuel cycle and the burn-up calculations, as well as the monitoring
of fuel integrity over its lifetime [89]. In addition, the pebble recirculation in pebble
beds depends on the maintenance of satisfactory pebble flow through the vessel, its
outlet, and the pebble extractor [89]. In particular, the pebble flow pattern in the
11
piston-pressured container are fairly independent of bed height unless the height to
diameter ratio becomes less than 10 [91].
Besides, the mechanical and thermal properties of pebble beds have coupled
effects, and the thermomechanical characteristics are critical for the reactor design
[92, 93]. For example, the compression force and gas pressure were tested[94], along
with the thermal conductivity at different values of temperature [95]. The strains
and stress analysis by finite element method, as well as their effects on the thermalhydraulics were also checked, although the coupled effects sometimes were not found
[96]. Moreover, the effective thermal conductivity was found not to be influenced
by the chemical composition of solid material whereas it could be affected by filling
gas such as helium clearly [97].
1.3.4 Importance of Flow Uniformity
In reactor engineering, the pebble path through the reactor core, the resident time
in the reactor core, and the optimization of fuel composition [98], are of crucial
importance. Excessive residence time inside the bed may cause severe irradiation
and thermal damage to the pebble, as well as possible escape of fission products. In
an extreme case, it may lead to permanent adhesion between pebbles [89]. On the
other hand, the residence time, as well as axial dispersion coefficient can be used
to characterize the axial dispersion and the mixing phenomenon of the coolant gas
flow, which are useful for the safe design and efficient operation of the reactors [99].
For example, the pebble flow dynamics were studied in a scaled-down test reactor
by using a non-invasive Radioactive Particle Tracking (RPT) technique [100]. They
used a Cobalt-60 based tracer to mimic pebbles in terms of shape, size, and density.
Across-correlation based position reconstruction algorithm and RPT calibration data
were used to obtain the Lagrangian trajectories, velocity field, and residence time
distributions. Moreover, the pebble size was found to strongly affect axial dispersion
and mixing in the packed pebble-bed reactor [99].
Moreover, the pebble motion in the reactor core is expected to follow “First-InFirst-Out" sequence to ensure each fuel pebble can reach almost the same burn-up
level when it is drained out [101]. In addition, Ref. [102], studied the scaling properties of granular flow in pebble-bed reactors via DEM simulation in a supercomputing
Centreon Rosa, a Cray XE6 system with 47,872 cores, where the pebbles are scaled
down by 3:1 and 6:1. The simulation results showed the feature of pebble stresses
and dust generation due to pebble wear. For example, the 6:1 simulation indicated a
very different feature with higher levels of wear, particularly pebble-wall wear.
Accurate flow characterization and description of the dynamics of pebbles in
pebble beds are essential with respect to the basic reactor design calculations, the
optimization of the fuel cycle and the burn-up calculations, as well as the monitoring
of fuel integrity over its lifetime [89]. In addition, the pebble recirculation in pebble
beds depends on the maintenance of satisfactory pebble flow through the vessel, its
outlet, and the pebble extractor [89]. In particular, the pebble flow pattern in the