One of the goals in the design of a nuclear reactor is the safe enclosure of all possible radioactive material not only during normal operation but especially during any possible accident. The High Temperature Reactor (HTR) achieves this goal by placing a small amount of fissile and/or fertile material surrounded by different layers of ceramics, effectively creating a miniature fission product containment vessel. This reactor relies on the ability of these ceramic coatings to retain all key radionuclides as long as a certain maximum fuel element temperature is not reached. Additionally, the reactor is designed in a way that in case the temperature of the reactor goes up above normal conditions the speed of the neutrons will increase, effectively shutting down the chain reaction necessary for fission to occur. Furthermore, because this reactor is constructed underground, natural heat conduction will help to reduce the temperature of the core.
The design of the coated fuel particle has changed since the early stages in the 1960’s. Although several variations of coating designs have been produced, only three designs have been used in a HTR. These three designs in chronological order are: (1) laminar, (2) BISO and (3) TRISO.
The laminar design was used briefly at the start of the HTR project. It contains a single layer of pyrolytic carbon (PyC). This design was used in the Peach Bottom, AVR and Dragon reactors. This design was abandoned after it was observed that fission product recoil damage produced early failure and release of fission products.
The BISO particle contains two types of material, a porous carbon coating (low density pyrolytic carbon, PyC), and a high density PyC coating. The porous carbon provides void volume to limit the fission gas pressure and protects the outer layer from recoil damage. The second dense isotropic coating layer is the containment to retain the fission products. The acronym BISO was derived from the fact that the two isotropic coating materials were used. BISO fuel particles are no longer part of any country’s HTR fuel design. The diffusional release of certain metallic fission products from BISO particles, particularly caesium, strontium, and silver, occurring at elevated temperatures, together with a high uranium contamination of matrix material during manufacture, led to only TRISO (Tistructural Isotropic) particles being used in all HTR modern designs.
The TRISO (Tristructural Isotropic) coated fuel particle was created in the UK as part of the DRAGON project. It consists of a microspherical fuel uranium kernel and coating layers of porous pyrolytic carbon (PyC) called the buffer, inner dense PyC (IPyC), silicon carbide (SiC) and an outer dense PyC (OPyC) (Fig. 1). The porous buffer layer provides free volume for gaseous fission products without causing excessive pressure build-ups, and isolates the structural coatings from the mechanical interactions caused by kernel swelling that accompanies fission product accumulation. It also protects the structural coatings by stopping fission fragment recoil atoms ejected from the fuel kernel. The inner dense PyC layer (IPyC) protects the SiC layer by stopping many fission products (i.e. rare earth elements) that might otherwise chemically attack it. It also prevents reaction between the UO2 kernel and chlorine containing materials released during the deposition of SiC. IPyC undertakes part of the internal pressure produced by CO2, CO and gaseous fission products. The silicon carbide layer (SiC) enhances the mechanical stability of the pressure vessel to retain gaseous fission products, and is the major containment barrier for fission products. The outer PyC (OPyC) layer gives the particle a higher temperature capability by preventing the vaporization of the SiC layer. It also protects the SiC from mechanical damage during fuel manufacture and is an additional barrier for gaseous fission products in case of disruption of the SiC layer.