156
3 Experiments in Pebble Bed Heat Transfer
tests give repeatable results with different heating processes, and then proves the
repeatability and robustness of this facility, as well as the methodology of determining diffusivity and conductivity. The conductivity comparison with HTTU and
SANA also proves the availability of this facility and the methodology. With the
nominal positions, this experiment finds the wall effect that also exists in the independent experiment of HTTU [10]. However, the temperature variation produced
by the wall effect can be eliminated or alleviated by retrieving the proper positions
and using the high-order spline function in the improved method in Sect. (3.3.6).
The improved method shrinks the disparities of diffusivities and conductivities of
different azimuthal sets. It also identifies the interesting decrease of diffusivity of the
helium test within low or moderate temperatures.
The uncertainty analysis gives a reasonable expanded uncertainty of diffusivity
and conductivity. It is about 4 and 7% maximum expanded uncertainty for diffusivity
and conductivity, respectively.
This section presents reasonable results under 1,200
◦ C by the repeated tests and
comparison with the results of other experimental researches. The methodology and
facility can be used to measure the effective thermal diffusivity and conductivity of
a pebble bed up to the ultimate 1,600
◦ C in the future by only fitting the sheathed
tungsten-rhenium thermocouples. The full-range thermal properties of the pebble
bed will contribute to the safety analysis and future development of HTGRs with
attractive inherent safety.
3.4 Summary
It is essential to measure the effective thermal diffusivity and conductivity inside
the reactor core for the sound design and inherent safety of HTGR. An overview of
the full-radius-scale heat test facility and experimental process for measuring these
thermal properties of pebble bed under vacuum condition and temperature of pebble
bed up to about 1200
◦ C is presented. The feasibility of the facility is verified based
on the temperature distribution experiments, in which the temperature data prove to
be convincing, featuring both symmetry and stability.
Besides, the relationship between effective thermal diffusivity and conductivity is
explained. For avoiding the measurement of heat flow, an inverse method is described
and adopted to retrieve the thermal diffusivity from the measured temperatures. Then,
the effective thermal conductive is calculated though the effective thermal diffusivity,
bulk density, and specific heat capacity of graphite. The stable result is consistent
with previous researches in the effective thermal conductivity of a pebble bed. The
decrease in the effective thermal conductivity near the wall is also observed in this
test.
3 Experiments in Pebble Bed Heat Transfer
tests give repeatable results with different heating processes, and then proves the
repeatability and robustness of this facility, as well as the methodology of determining diffusivity and conductivity. The conductivity comparison with HTTU and
SANA also proves the availability of this facility and the methodology. With the
nominal positions, this experiment finds the wall effect that also exists in the independent experiment of HTTU [10]. However, the temperature variation produced
by the wall effect can be eliminated or alleviated by retrieving the proper positions
and using the high-order spline function in the improved method in Sect. (3.3.6).
The improved method shrinks the disparities of diffusivities and conductivities of
different azimuthal sets. It also identifies the interesting decrease of diffusivity of the
helium test within low or moderate temperatures.
The uncertainty analysis gives a reasonable expanded uncertainty of diffusivity
and conductivity. It is about 4 and 7% maximum expanded uncertainty for diffusivity
and conductivity, respectively.
This section presents reasonable results under 1,200
◦ C by the repeated tests and
comparison with the results of other experimental researches. The methodology and
facility can be used to measure the effective thermal diffusivity and conductivity of
a pebble bed up to the ultimate 1,600
◦ C in the future by only fitting the sheathed
tungsten-rhenium thermocouples. The full-range thermal properties of the pebble
bed will contribute to the safety analysis and future development of HTGRs with
attractive inherent safety.
3.4 Summary
It is essential to measure the effective thermal diffusivity and conductivity inside
the reactor core for the sound design and inherent safety of HTGR. An overview of
the full-radius-scale heat test facility and experimental process for measuring these
thermal properties of pebble bed under vacuum condition and temperature of pebble
bed up to about 1200
◦ C is presented. The feasibility of the facility is verified based
on the temperature distribution experiments, in which the temperature data prove to
be convincing, featuring both symmetry and stability.
Besides, the relationship between effective thermal diffusivity and conductivity is
explained. For avoiding the measurement of heat flow, an inverse method is described
and adopted to retrieve the thermal diffusivity from the measured temperatures. Then,
the effective thermal conductive is calculated though the effective thermal diffusivity,
bulk density, and specific heat capacity of graphite. The stable result is consistent
with previous researches in the effective thermal conductivity of a pebble bed. The
decrease in the effective thermal conductivity near the wall is also observed in this
test.