atw - International Journal for Nuclear Power | 03.2024

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Fuel
The filling process of the graphite/TRISO powder must
be supported by vibrational compaction. The lid itself
represents a surface discontinuity which may disturb
the flow behaviour in pebble bed.
The manufacturing technology reveals as very ambitious
since the lost core in the injection moulding
process must be removed via a much smaller opening
respectively two half shells must be connected in green
state by thermoplastic melt bonding.
7 SSiC compact sphere (2016)
The compact SSiC sphere (Figure 6) offers the advantage
that no bonding process either for two halves or
for the lid is necessary. This seamless technology results
in a very robust and corrosion resistant solution,
because no second phase, the glass solder, is present.
Fig. 6.
Seamless compact SSiC sphere – accident tolerant,
disposal preconditioned
for which reason it may be difficult to achieve operating
temperatures of the reactor above 1.000 °C.
This temperature level is required for hydrogen
production.
8 SSiC sphere with coolant channels (2021)
Despite the advantages of the compact sphere described
above, the heat transfer from the core to the surface is
still a limiting factor regarding a possibly high outlet
temperature of the coolant. The temperature gradient
from core to surface of the 60 mm graphite pebble is as
high as ca. 400 K. A lower temperature gradient and
thus a higher coolant temperature can be achieved by
shortening the pathway for heat transfer.
The new technology of shape forming by 3D-printing
opens interesting alternatives in fuel pebble design.
Open channels and closed chambers are feasible,
as shown in Figure 7. Dual extrusion 3D-printing
technology (DEXPRINT @ ) allows to print simultaneously
the surrounding cladding of SSiC, and the uranium
oxide fuel diluted by SiC to the required concentration.
The separating walls between channels and chambers
exhibit a wall thickness of 0.9 – 1 mm, whereas the outer
shell is 2.5 mm thick. The cross section of chambers and
channels amounts to 4.9 × 4.9 mm2. Due to the high flexibility
of the 3D printing process all geometrical values
can be adjusted to the respective needs.
The technological challenges in case of the compact
sphere are higher than for the solutions described
above. A 2-step forming process is necessary: 1 st forming
and de-bonding of the fuel containing kernel and
2 nd injection moulding around of the pure SiC shell.
After de-bonding of the outer shell sintering of the
conglomerate component can be performed. It is
inevitable to adjust the shrinkage values in the sintering
process of kernel and surrounding shell. Otherwise, a
high risk of defect or crack formation is existing.
The SSiC compact sphere makes unnecessary the
manufacturing of TRISO-particles because its kernel
consists of a dispersion of SSiC and fuel. The technology
can be described in general as known ceramic
technology. A comparison of production costs reveals
that the ceramic technology is roughly 10 times less
expensive than the TRISO technology described by
A.T. Cisneros Jr [9] .
The porous kernel is capable to absorb gaseous fission
products. It is recommended to apply compact SSiC
spheres in molten salt reactors and small modular
reactors (SMR) in general.
One disadvantage of all solutions described so far,
including the compact sphere, is the high temperature
gradient from the centre of the sphere to the surface
Fig. 7.
DEXPRINT @ : SSiC sphere with coolant channels
The sphere with coolant channels exhibits an exchange
surface for heat transfer around 3 times higher than a
massive sphere of the same diameter.
Cylindrical and prismatic geometries are feasible as
well, but the sphere is considered as optimal due to its
flowability/moveability.
9 Conclusions
As demonstrated above substantial innovations in
nuclear technologies, especially for (V)HTR, molten salt
reactors and SMR in general are feasible, if progress in
material science and new concepts are combined. This
relates to improved safety in operation, lower costs and
better availability of nuclear fuel, higher yield in power
and hydrogen production, prevention of proliferation
and long-term safety for final deposition as well.
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