BISO-coated particle fuel

Figure 1. Photomicrographs of typical BISO coated fuel particles before and after irradiation (60X)...

In the THTR-300 the maximum fuel temperature of the fuel pebbles is 1 250 °C as the required design value and - at the same time - as a value for the commercial guarantee, fig 5, lit HKG-1969, Bd 1, S 4 16, tab 4 2 2-1 (THTR-300 Safety Report) For the release it is stated there "For these fuel elements the fraction of release for Xe-133 shall not exceed the value of 3 X 10 5 as the mean over the lifetime and as the mean over the core" The coated particles of the fuel elements of the THTR-300 had a BISO-coating... 

Different types of AVR fuel elements were exposed to Q-brine /8/at temperatures between 50 and 90°C and pressures up to 13 MPa. The fractional release of Cs is shown in FIG. 12. The release from fuel elements with BISO particles is less than lO", related to the inventory of one coated particle respectively the inventory generated by the U-contamination of the matrix graphite. After 200-300 days the release rate decreases to a stable equilibrium controlled by the diffusion of Cs from the pyrocarbon kernel into the matrix. Calculations based on a diffusion/adsorption model with the GETTER code match the experimental results very well. For the long time calculations the release of Cs can be described as a instantaneous release of about 20% of the matrix inventory, which is related to the amount of kernels with a defect in the coating, and a slow release of the remaining Cs in the graphite particles over a time period of several 100 years /4,6,9/. 

Both reactors have in total produced about 1 Million of spent fuel elements during their operating time. The typical fuel element is a tennis-ball sized sphere from graphite, containing up to twenty thousand pinhead-sized fuel particles containing oxide or carbide fuel each. The particles are surrounded by a high-porosity buffer layer to limit the internal pressure from swelling and gas production, and coated with a hi -density pyrocarbon layer (BISO) or with a combination of two pyrocarbon layers with a silicon carbide layer in between (TRISO) to retain radionuclides (see FIG. 1). 

---