1.3 Pebble Flows
17
The concept of the two-region pebble-bed reactor with a dynamic core, which was
first developed by the International Atomic Energy Agency (2001), is expected as a
promising technology for the next generation of the pebble-bed HTGR [13]. According to this concept, the pebble-bed reactor core is subdivided into two distinct regions;
a central column region totally filled with graphite pebbles and an outer annular region
entirely consisted of the fuel pebbles. Graphite pebbles are inserted at a single central
spot to form the moderation region, and fuel spheres are loaded at several positions
on the periphery at the top of the core to construct the fuel region. Between the two
distinct regions, a mixing zone is vital supposed to develop. The scale of the mixing
zone is important for the two-region designed pebble-bed reactor, which must be
restricted in a specific range.
The two-region arrangement brings many advantages. It can significantly increase
power output and has the capability to control the reactivity. Because graphite pebbles do not generate 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 because the decay heat has to travel a shorter distance through pressure
vessel to the outside of the core when accidents occur [107].
However, the feasibility of the two-region-designed pebble bed concept has to be
further validated [119]. First of all, the primary problem is whether a stable tworegion pebble bed can be established and maintained. The two-region arrangement
must be able to achieve an equilibrium state during operation. Secondly, because of
the random dispersion of the pebbles, the mixing region caused by the radial diffusion
will appear on the two-region interface. As the neutron flux in that region is highly
thermalized, fuel pebbles in the mixing zone have a higher power output than those
in other fuel zones and need to withstand higher temperatures. As a consequence,
the size of the mixing zone plays a vital role in core safety and should be as small
as possible. Another crucial issue is the stagnant zone. Pebbles in the stagnant zone
will move extremely slow or even stay at rest. These pebbles spend excessive time
within the reactor, bearing more radiation, higher burn up, and becoming susceptible
to breaking up. Even excessive residence time could result in severe irradiation and
thermal damage to fuel pebbles with possible fission products escaping. It is possible
to avoid the formation of the stagnant zone or reduce the stagnant zone in the pebble
bed by specific design of the reactor core.
Therefore, to validate the feasibility of the two-region design, it is essential to take
into account the establishment and maintenance of the two-region arrangement, the
mixing zone, and the stagnant condition in the pebble bed. All these are fundamentally
determined by pebble flow characteristics in the dynamic core, which are intimately
related to the core physics and thermal-hydraulic engineering, laying the foundation
of the design and operation of the pebble-bed reactor.
Pebble flow is a fundamental problem and crucial for the design of the pebble
bed reactor. Pebble flow dynamics has become an important branch of granular
flow due to the essential in the development of the HTGR technology. As has been
mentioned above, the pebble flow in HTGR is the quasi-static dense granular flow. It
is an attractively simple and yet surprisingly complex subject. Fast, dilute flows are
17
The concept of the two-region pebble-bed reactor with a dynamic core, which was
first developed by the International Atomic Energy Agency (2001), is expected as a
promising technology for the next generation of the pebble-bed HTGR [13]. According to this concept, the pebble-bed reactor core is subdivided into two distinct regions;
a central column region totally filled with graphite pebbles and an outer annular region
entirely consisted of the fuel pebbles. Graphite pebbles are inserted at a single central
spot to form the moderation region, and fuel spheres are loaded at several positions
on the periphery at the top of the core to construct the fuel region. Between the two
distinct regions, a mixing zone is vital supposed to develop. The scale of the mixing
zone is important for the two-region designed pebble-bed reactor, which must be
restricted in a specific range.
The two-region arrangement brings many advantages. It can significantly increase
power output and has the capability to control the reactivity. Because graphite pebbles do not generate 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 because the decay heat has to travel a shorter distance through pressure
vessel to the outside of the core when accidents occur [107].
However, the feasibility of the two-region-designed pebble bed concept has to be
further validated [119]. First of all, the primary problem is whether a stable tworegion pebble bed can be established and maintained. The two-region arrangement
must be able to achieve an equilibrium state during operation. Secondly, because of
the random dispersion of the pebbles, the mixing region caused by the radial diffusion
will appear on the two-region interface. As the neutron flux in that region is highly
thermalized, fuel pebbles in the mixing zone have a higher power output than those
in other fuel zones and need to withstand higher temperatures. As a consequence,
the size of the mixing zone plays a vital role in core safety and should be as small
as possible. Another crucial issue is the stagnant zone. Pebbles in the stagnant zone
will move extremely slow or even stay at rest. These pebbles spend excessive time
within the reactor, bearing more radiation, higher burn up, and becoming susceptible
to breaking up. Even excessive residence time could result in severe irradiation and
thermal damage to fuel pebbles with possible fission products escaping. It is possible
to avoid the formation of the stagnant zone or reduce the stagnant zone in the pebble
bed by specific design of the reactor core.
Therefore, to validate the feasibility of the two-region design, it is essential to take
into account the establishment and maintenance of the two-region arrangement, the
mixing zone, and the stagnant condition in the pebble bed. All these are fundamentally
determined by pebble flow characteristics in the dynamic core, which are intimately
related to the core physics and thermal-hydraulic engineering, laying the foundation
of the design and operation of the pebble-bed reactor.
Pebble flow is a fundamental problem and crucial for the design of the pebble
bed reactor. Pebble flow dynamics has become an important branch of granular
flow due to the essential in the development of the HTGR technology. As has been
mentioned above, the pebble flow in HTGR is the quasi-static dense granular flow. It
is an attractively simple and yet surprisingly complex subject. Fast, dilute flows are