7.4 Conical Base Optimization
471
of the reactor operation, which should be taken into account carefully in the reactor
core design work.
Due to the conical-type contraction at the bottom of the pebble bed, the pebbles in
the stagnant region around the bed corner move extremely slowly or even stay at rest.
For example, the number rate of drainage of pebbles in the HTR-10 demonstration
reactor at Tsinghua University is about 100 pebbles per day. Under such a low flow
rate, the order of the mean velocity is only about O(10
−4
) m/hour inside the bed and
about O(10
−3
) m/hour inside the drainage orifice. In another experiment facility, it
is speeded up to 150 pebbles per minute [1], with the order of the velocities of about
O(10
−3
) m/min. Both of them are extremely slow pebble flows which are almost
quasi-static flows. On the other hand, the stagnated pebbles are not allowed in real
reactors since the residence time of the stagnated fuel pebbles can go beyond the burnup level. Excessive stay time could result in severe irradiation and thermal damage
to fuel pebbles with possible fission products escaping, which certainly increases the
risk of leakage of radiation [16]. Thus, the formation of the stagnant region is one
of the critical issues in the pebble-bed reactor design. In conclusion, the pebble flow
uniformity and stagnation characteristics are essential for the design of a pebble bed
high-temperature gas-cooled reactor.
However, a few studies have focused on the effect of the bed configuration, especially the contraction configuration at the bottom of the bed, on the flow uniformity,
and pebble stagnation characteristics. Due to the significance of this issue, this section
aims to show and demonstrate some typical results of the bed configuration effect.
For accomplishing this, some basic shapes are used for the configuration of the bed
contraction, and the discrete element method is employed to simulate the pebble
flows in these specifically designed bed configurations.
7.4.1 Simulation Setup and Bed Configuration
The simulation setup is sketched in Fig. 7.25. A bed with 800 × 1200 × 12 mm in
width, height, and depth directions, respectively, has a 120 mm wide drainage orifice
at the bottom center. About 7,345 pebbles with equal diameters of 12 mm are loaded
into the bed at the bed top uniformly and drained from the drainage orifice at a fixed
drainage rate (100 pebbles per second), which is the same as the reloading rate. The
properties and dimension parameters are listed in Table 7.4.
It is noticed that the depth of the bed is equal to the diameter of the pebbles. It
is because the present model is a three-dimensional model. This model has already
been validated and utilized in previous studies, such as particle flow study in a drum
[17, 18] and phenomenological study in a pebble bed [1, 19], whereas it is utilized
here as a pseudo-two-dimensional simulation. It means that the pebbles are spheres
moving on a vertical plane exactly. The interactions between the pebbles and the front
or rear walls are not considered. Thus, it is a monolayer pebble bed that shows the
pebble motion behavior in a two-dimensional plane. Since the emphasis of this study
is on the bed configuration effect, a pseudo-three-dimensional case is enough for
471
of the reactor operation, which should be taken into account carefully in the reactor
core design work.
Due to the conical-type contraction at the bottom of the pebble bed, the pebbles in
the stagnant region around the bed corner move extremely slowly or even stay at rest.
For example, the number rate of drainage of pebbles in the HTR-10 demonstration
reactor at Tsinghua University is about 100 pebbles per day. Under such a low flow
rate, the order of the mean velocity is only about O(10
−4
) m/hour inside the bed and
about O(10
−3
) m/hour inside the drainage orifice. In another experiment facility, it
is speeded up to 150 pebbles per minute [1], with the order of the velocities of about
O(10
−3
) m/min. Both of them are extremely slow pebble flows which are almost
quasi-static flows. On the other hand, the stagnated pebbles are not allowed in real
reactors since the residence time of the stagnated fuel pebbles can go beyond the burnup level. Excessive stay time could result in severe irradiation and thermal damage
to fuel pebbles with possible fission products escaping, which certainly increases the
risk of leakage of radiation [16]. Thus, the formation of the stagnant region is one
of the critical issues in the pebble-bed reactor design. In conclusion, the pebble flow
uniformity and stagnation characteristics are essential for the design of a pebble bed
high-temperature gas-cooled reactor.
However, a few studies have focused on the effect of the bed configuration, especially the contraction configuration at the bottom of the bed, on the flow uniformity,
and pebble stagnation characteristics. Due to the significance of this issue, this section
aims to show and demonstrate some typical results of the bed configuration effect.
For accomplishing this, some basic shapes are used for the configuration of the bed
contraction, and the discrete element method is employed to simulate the pebble
flows in these specifically designed bed configurations.
7.4.1 Simulation Setup and Bed Configuration
The simulation setup is sketched in Fig. 7.25. A bed with 800 × 1200 × 12 mm in
width, height, and depth directions, respectively, has a 120 mm wide drainage orifice
at the bottom center. About 7,345 pebbles with equal diameters of 12 mm are loaded
into the bed at the bed top uniformly and drained from the drainage orifice at a fixed
drainage rate (100 pebbles per second), which is the same as the reloading rate. The
properties and dimension parameters are listed in Table 7.4.
It is noticed that the depth of the bed is equal to the diameter of the pebbles. It
is because the present model is a three-dimensional model. This model has already
been validated and utilized in previous studies, such as particle flow study in a drum
[17, 18] and phenomenological study in a pebble bed [1, 19], whereas it is utilized
here as a pseudo-two-dimensional simulation. It means that the pebbles are spheres
moving on a vertical plane exactly. The interactions between the pebbles and the front
or rear walls are not considered. Thus, it is a monolayer pebble bed that shows the
pebble motion behavior in a two-dimensional plane. Since the emphasis of this study
is on the bed configuration effect, a pseudo-three-dimensional case is enough for