atw - International Journal for Nuclear Power | 03.2024

38
Fuel
2 Graphite sphere: Standard pebble
(since 1960’ties)
The graphite sphere with incorporated TRISO particles
represents the standard fuel design for pebble bed
reactors till today (Figure 1). AVR and THTR in Germany
are shut down since several decades and the project
PBMR in South Africa was cancelled in the year 2010.
Presently, China operates two HTR reactors with the
standard graphite pebbles.
Fig. 1.
Graphite sphere with TRISO particles in graphite matrix: Standard
pebble
Critics of this technology addressed different aspects
and explained why further development is required.
Due to the relatively low hardness of graphite abrasion
creates dust which contaminates the primary circuit.
Graphite is inflammable in case of air or water ingress.
The wall thickness of the SiC coating of TRISO-particles
for fission products retention is only 30 µm thick, thus
exhibiting a relatively low mechanical stability and a
short pathway for diffusion.
Safeguards of individual pebbles by engraved code are
not feasible due to the low abrasive resistance of
graphite and therefore the history of an individual
spent pebble is not traceable [6] .
Severe arguments against the use of TRISO-particles
concern as well the complicated process of production
in a high temperature fluidized bed reactor for
implementing the coatings around the uranium oxide
kernel. This technology of batch-wise production leads
to high overall costs and a low productivity [7] . It is the
aim of the presented work to overcome the deficiencies
mentioned thus leading to improved safety, lower fuel
costs and higher availability.
3 Basics: SiC for in-core-application
and laser bonding
The main disadvantages of the naked graphite fuel
elements are listed above in chapter 2. For applying
silicon carbide in the reactor core, a special feature of
SiC ceramics must first be addressed. Usually, silicon
carbide powder is sintered by the addition of carbon
and a small quantity of boron (carbide). For reasons of
neutron economy, the application of SiC components
in a reactor core requires a boron-free material. The
authors have developed such material.
SiCANA @ (Silicon Carbide for Advanced Nuclear
Applications) is a boron-free, pressure-less sintered
dense silicon carbide. Originally, it was intended to
encapsulate pebble fuel elements and to substitute
metallic fuel claddings in order to avoid hydrogen
generation during loss-of-coolant-accidents (LOCA) in
water-cooled reactors. The material is also suitable for
core components of SMR with molten salts or liquid
metals. The temperature-, thermal shock-, oxidationand
corrosion resistance are very high. Some properties
are listed below.
⁃ Density: > 3.10 g/cm3
⁃ Open porosity: none
⁃ Young`s Modulus: 420 GPa
⁃ Poisson ratio: 0.21
⁃ Bending strength: 400 MPa at 20 °C
450 MPa at 1,500 °C
⁃ Vickers hardness: 25 GPa at 5 N load
⁃ Coefficient of linear
thermal expansion: 4.5×10 -6 K -1
SiCANA @ is the basic material composition for injectionmoulding
feedstock and 3D-printing filaments. The
properties are identical to the standard SiC ceramics
except the neutron absorption. Figure 2 shows SiCANA @
microstructures in different scale.
Fig. 2.
SiCANA @ microstructures: polished cross section (left), polished/etched cross section (right)
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