12
1 Introduction
reactor core is expected to be uniform and consistent (a mass flow regime). The
flooding of pebbles, channeling, and flow blockage phenomena should be prevented,
and the caking or crystallization of pebbles and the formation of stagnant regions
need to be avoided [101].
1.3.5 Optimization of Pebble Flow Design
The internal packing structure presents an important role and complex effects on the
flow dynamics of slow granular flow [69, 103]. Fast and dilute flows are known to
follow the classical hydrodynamics (with inelastic collisions). But slow and dense
flows pose a considerable challenge to theorists for the many-body interactions and
non-thermal fluctuations. In addition to their fundamental scientific interests, such
flows have vital engineering applications [46]. For example, the new pebble-bed
nuclear reactors have been recognized as a promising candidate of advanced nuclear
systems [48]. The efficiency and safety of the pebble-bed nuclear reactors depend
on the flow pattern of very slow fuel pebble flow in the pebble bed.
In addition, some kinds of wall structures are designed to affect the overall flow
field by avoiding or eliminating the crystallization of the pebbles in the near-wall
region. This can contribute to the elimination of stagnant regions and improve the
reactor’s safety capacity by reducing the probability of radiation leakage. Moreover,
such structures also enhance the dispersion degree of pebbles in the peripheral region
of the core, so that these fuel pebbles are more likely to be exposed to more neutron
flux, which would narrow the burn-up level in the radial direction [104].
For example, the two-region pebble-bed reactor core is expected as a promising
technique for pebble-bed HTGRs [105]. The two-region pebble bed is composed
of two distinct regions, i.e., a central column region consisting of graphite pebbles
(called graphite region) and an outer annular region consisting of fuel pebbles (called
fuel region). Therefore, the graphite pebbles are loaded into the core from the single
central hole and fuel pebbles are loaded from the annular periphery of the core. The
two-region pebble bed is an advantageous type as it flattens the neutron flux and
consequently allows a significantly higher power output without reducing the safety
margin. The decay heat is also transferred with a shorter distance from the core to
outside during accidents [106].
As aforementioned, the pebble flow characteristics are vital to the efficiency and
safety of pebble-bed HTGRs. The behavior of the pebbles should be ensured to
fulfill thermal-hydraulic and radiation safety requirements. Specifically speaking,
the expected flow pattern in the reactor core should be uniform and consistent (called
mass flow). Issues like flooding of powders, channeling, and hang-ups do not appear;
stagnant regions are eliminated, and caking or crystallization is minimized. Moreover,
pebbles circulated in the core should follow the “first-in-first-out” sequence, because
the excellent flow sequence makes sure that each fuel pebble reaches almost the same
burn-up level when it is drained out.
1 Introduction
reactor core is expected to be uniform and consistent (a mass flow regime). The
flooding of pebbles, channeling, and flow blockage phenomena should be prevented,
and the caking or crystallization of pebbles and the formation of stagnant regions
need to be avoided [101].
1.3.5 Optimization of Pebble Flow Design
The internal packing structure presents an important role and complex effects on the
flow dynamics of slow granular flow [69, 103]. Fast and dilute flows are known to
follow the classical hydrodynamics (with inelastic collisions). But slow and dense
flows pose a considerable challenge to theorists for the many-body interactions and
non-thermal fluctuations. In addition to their fundamental scientific interests, such
flows have vital engineering applications [46]. For example, the new pebble-bed
nuclear reactors have been recognized as a promising candidate of advanced nuclear
systems [48]. The efficiency and safety of the pebble-bed nuclear reactors depend
on the flow pattern of very slow fuel pebble flow in the pebble bed.
In addition, some kinds of wall structures are designed to affect the overall flow
field by avoiding or eliminating the crystallization of the pebbles in the near-wall
region. This can contribute to the elimination of stagnant regions and improve the
reactor’s safety capacity by reducing the probability of radiation leakage. Moreover,
such structures also enhance the dispersion degree of pebbles in the peripheral region
of the core, so that these fuel pebbles are more likely to be exposed to more neutron
flux, which would narrow the burn-up level in the radial direction [104].
For example, the two-region pebble-bed reactor core is expected as a promising
technique for pebble-bed HTGRs [105]. The two-region pebble bed is composed
of two distinct regions, i.e., a central column region consisting of graphite pebbles
(called graphite region) and an outer annular region consisting of fuel pebbles (called
fuel region). Therefore, the graphite pebbles are loaded into the core from the single
central hole and fuel pebbles are loaded from the annular periphery of the core. The
two-region pebble bed is an advantageous type as it flattens the neutron flux and
consequently allows a significantly higher power output without reducing the safety
margin. The decay heat is also transferred with a shorter distance from the core to
outside during accidents [106].
As aforementioned, the pebble flow characteristics are vital to the efficiency and
safety of pebble-bed HTGRs. The behavior of the pebbles should be ensured to
fulfill thermal-hydraulic and radiation safety requirements. Specifically speaking,
the expected flow pattern in the reactor core should be uniform and consistent (called
mass flow). Issues like flooding of powders, channeling, and hang-ups do not appear;
stagnant regions are eliminated, and caking or crystallization is minimized. Moreover,
pebbles circulated in the core should follow the “first-in-first-out” sequence, because
the excellent flow sequence makes sure that each fuel pebble reaches almost the same
burn-up level when it is drained out.