Pac beads

Pac beads are a form of processing for nuclear fuel . The name is derived from the English expression sphere-pac (German "Kugelpackungs-") in sphere-pac fuel , "Kugelpackungsbrennstoff".

description

The fuel (usually a metal oxide or mixture of oxides) is poured into the fuel rod as a bed of approximately spherical particles and compacted by vibration. Sufficient bulk densities can be achieved with balls in 2 to 3 different sizes with diameters between about 1% and 10% of the inner diameter of the cladding tube. This process has various technical advantages over the conventional method, in which the material is stacked in the form of cylindrical pellets in the fuel rod.

use

The ball packing technique can be used for any nuclide composition of the fuel. It was initially developed for the production of thorium-containing fuel, but is also used for MOX fuel , i.e. uranium-plutonium mixed oxide, in which plutonium obtained from the reprocessing of used fuel elements is utilized. (This plutonium also contains small amounts of americium and curium as accompanying elements . This is probably the reason for the error that “Pac” is an abbreviation for “Plutonium-Americium-Curium”.)

Coated particles are a special form of spheres . This type of globule is encased in several layers of graphite , in some cases also silicon carbide , which are supposed to almost completely prevent the escape of fission products . However, recent studies show that certain radionuclides cannot be sufficiently retained due to diffusion. This type of fuel, embedded in graphite, was used in gas-cooled reactors, in particular pebble-bed reactors such as e.g. B. the nuclear power plant THTR-300 used.

Ball packing technology is generally economical when the respective material has to be handled remotely during processing because of its radiation and the risk of contamination . In this case, an additional advantage is stated that the balls are produced in a wet chemical process and thus without dust formation. Fresh uranium oxide that has not yet been irradiated with neutrons, on the other hand, can be handled directly by the production staff. In this case, conventional pellet technology is cheaper.

Questionable connection with leukemia cluster Elbmarsch

Small globules were found in the leukemia cluster Elbmarsch distributed in the ground and on the roofs of houses. The behavior of fuel pellets was theoretically researched at the GKSS , but there is no evidence for corresponding experiments. The assumption of concerned citizens and environmental groups that the particles found could be radioactive Pac-spheres is therefore controversial.

Individual evidence

1. ^ Luigi Massimo, Physics of High-Temperature Reactors . Oxford, etc .: Pergamon, 1976, p. Ix
2. ↑ Kurt Kugeler and Rudolf Schulten : Hoch Temperaturreaktortechnik , Berlin: Springer, 1989, p. 4
3. ↑ Rainer Moormann : A safety re-evaluation of the AVR pebble bed reactor operation and its consequences for future HTR concepts ( Memento of the original from May 24, 2012 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. , 2008. 
4. ^ "The condensation of cesium in coated particles", H. Luhleich, K. Penndorf and L. Sütterlin, Institute for Reactor Physics of the Society for Nuclear Energy Utilization in Shipbuilding and Shipping, Geesthacht-Tesperhude, Atomkernenergie Volume 13 (1968), pages 171-175
5. ↑ A. Gerdes, Elemental and U-Th-Pu isotope composition of soil and spherical particles from the Elbmarsch, Northern Germany . DMG, Hanover, 24.-26. September 2006, Beih. z. Eur. J. Mineralogy, 18.
6. ↑ Statement by the Radiation Protection Commission (SSK): Evaluation of measurements by ARGE PhAM on radioactivity in the Elbmarsch  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. , Pp. 48-56 (2003).  
7. ↑ Wolf Wetzel : The riddle of the globules . In: the Friday of May 11, 2007.

literature

• GD Del Cul et al .: Fuel Fabrication Development for the Surrogate Sphere-Pac Rodlet . Oak Ridge Nat. Lab., Oak Ridge, Tenn., USA, 2005. (PDF; 987 kB)
• KL Peddicord et al .: Analytical and Experimental Performance of Sphere-pac Nuclear Fuels . In: Progress in Nuclear Energy Vol. 18, pp. 265-299 (1986)