Graphitization of Nuclear-Grade Graphite: Why Are the Requirements So S"&"tringent?
Nuclear-grade graphite is used in nuclear reactors such as high-temperature gas-cooled reactors as a neutron moderator and reflector material. Because it is directly related to nuclear safety, the requirements for nuclear-grade graphite"&" are much more stringent than those for ordinary graphite.
Extremely High Purity Requirements
Nuclear-grade graphite has extremely strict requirements for impurity content:
- Ash content: <20ppm (ordinary high-purity graphite only requires <50ppm)
- Boron equivalent: Extremely low (boron has a very large neutron absorption cross-section, affecting reactor performance)
- Other impurities: Rare earth elements, halogens, etc., all have strict upper limits
This requires the use of ultra-high purity protective gas (purity ≥99.9999%) during the graphitization process, and an extremely high level of cleanliness within the f"&"urnace environment.
Isotropic Requirements
Nuclear-grade graphite requires **isotropy (AGTR)**—physical properties must be essentially consistent in all directions. This requires:
- Isostatic press"&"ing process
- Extremely uniform temperature field during graphitization
- Avoiding any process factors that may lead to anisotropy in performance
Irradiation Stability
Nuclear-grade graphit"&"e is subjected to long-term neutron irradiation in reactors, requiring **extremely small rate of dimensional change due to irradiation**. This is closely related to the degree of graphitization and microstructure. The graphitization process needs precise "&"control to ensure the material has optimal radiation resistance.
Significance: Nuclear-grade graphite is the material basis for nuclear safety. Every det"&"ail of the graphitization process can affect the safe operation of a nuclear reactor. In making nuclear-grade graphite, there is no ""good enough,"" only ""zero defects.""
